Spring MVC -- 验证器

输入验证是Spring处理的最重要Web开发任务之一。在Spring MVC中，有两种方式可以验证输入，即利用Spring自带的验证框架，或者利用JSR 303实现。本篇博客将介绍这两种输入验证方法。

本篇博客用两个不同的示例分别介绍这两种方式：spring-validator和jsr303-validator。

一 验证概览

Converter和Formatter作用于字段级。在MVC Web应用中，它们将String类型转换或格式化成另一种Java类型，如java.time.LocalDate。验证器则作用于对象级。它决定某一个对象中的所有字段是否均是有效的，以及是否遵循某些规则。一个典型的Spring MVC应用会同时应用到Formatter(或Converter)和Validator。

如果一个应用程序既使用了Formatter，又有了Validator，那么，应用中的事件顺序是这样的：在调用Controller的请求处理方式时，将会有一个或者多个Formatter，试图将输入字符串转换成domain对象中的属性（或者说字段）值，一旦格式化成功，验证器就会介入。

例如：Order对象有一个shippingDate属性（其类型为LocalDate），它的值绝对不可能早于今天的日期。当调用OrderController时，LocalDateFormatter会将字符串转换成LocalDate，并将它赋予Order对象的shippingDate属性。如果转换失败，用户就会被转回到前一个表单；如果转换成功，则会调用验证器，查看shippingDate是否早于今天的日期。

现在，你或许会问，将验证逻辑转移到LocalDateFormatter中是否更加明智？

因为比较一下日期并非难事，但答案却是肯定的。首先，LocalDateFormatter还可以用于将其它字符串格式化成日期，如birthDate或者purchaseDate。这两个日期的规则都不同于shippingDate，事实上，比如，员工的出生日期绝对不可能晚于今日。

其次，校验器可以检查两个或更多字段之间的关系，各字段均受不同的Formatter支持。例如，假设Employee对象有birthDate属性和startDate属性，验证器就可以设定规则，使任何员工的入职日期均不可能早于他的出生日期。因此，有效的Employee对象必须让它的birthDate属性值早于其startDate值，这就是验证器的任务。

二 Spring验证器

从一开始，Spring就设计了输入验证，甚至早于JSR 303（Java验证规范）。因此，Spring的Validation框架至今都很普遍，对于新项目，一般建议使用JSR 303验证器。

为了创建Spring验证器，要实现org.springframework.validation.Validator接口，这个接口的源码如下：

/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.validation;

/**
 * A validator for application-specific objects.
 *
 * <p>This interface is totally divorced from any infrastructure
 * or context; that is to say it is not coupled to validating
 * only objects in the web tier, the data-access tier, or the
 * whatever-tier. As such it is amenable to being used in any layer
 * of an application, and supports the encapsulation of validation
 * logic as a first-class citizen in its own right.
 *
 * <p>Find below a simple but complete {@code Validator}
 * implementation, which validates that the various {@link String}
 * properties of a {@code UserLogin} instance are not empty
 * (that is they are not {@code null} and do not consist
 * wholly of whitespace), and that any password that is present is
 * at least {@code 'MINIMUM_PASSWORD_LENGTH'} characters in length.
 *
 * <pre class="code"> public class UserLoginValidator implements Validator {
 *
 *    private static final int MINIMUM_PASSWORD_LENGTH = 6;
 *
 *    public boolean supports(Class clazz) {
 *       return UserLogin.class.isAssignableFrom(clazz);
 *    }
 *
 *    public void validate(Object target, Errors errors) {
 *       ValidationUtils.rejectIfEmptyOrWhitespace(errors, "userName", "field.required");
 *       ValidationUtils.rejectIfEmptyOrWhitespace(errors, "password", "field.required");
 *       UserLogin login = (UserLogin) target;
 *       if (login.getPassword() != null
 *             && login.getPassword().trim().length() < MINIMUM_PASSWORD_LENGTH) {
 *          errors.rejectValue("password", "field.min.length",
 *                new Object[]{Integer.valueOf(MINIMUM_PASSWORD_LENGTH)},
 *                "The password must be at least [" + MINIMUM_PASSWORD_LENGTH + "] characters in length.");
 *       }
 *    }
 * }</pre>
 *
 * <p>See also the Spring reference manual for a fuller discussion of
 * the {@code Validator} interface and its role in an enterprise
 * application.
 *
 * @author Rod Johnson
 * @see SmartValidator
 * @see Errors
 * @see ValidationUtils
 */
public interface Validator {

    /**
     * Can this {@link Validator} {@link #validate(Object, Errors) validate}
     * instances of the supplied {@code clazz}?
     * <p>This method is <i>typically</i> implemented like so:
     * <pre class="code">return Foo.class.isAssignableFrom(clazz);</pre>
     * (Where {@code Foo} is the class (or superclass) of the actual
     * object instance that is to be {@link #validate(Object, Errors) validated}.)
     * @param clazz the {@link Class} that this {@link Validator} is
     * being asked if it can {@link #validate(Object, Errors) validate}
     * @return {@code true} if this {@link Validator} can indeed
     * {@link #validate(Object, Errors) validate} instances of the
     * supplied {@code clazz}
     */
    boolean supports(Class<?> clazz);

    /**
     * Validate the supplied {@code target} object, which must be
     * of a {@link Class} for which the {@link #supports(Class)} method
     * typically has (or would) return {@code true}.
     * <p>The supplied {@link Errors errors} instance can be used to report
     * any resulting validation errors.
     * @param target the object that is to be validated
     * @param errors contextual state about the validation process
     * @see ValidationUtils
     */
    void validate(Object target, Errors errors);

}

这个接口需要实现两个方法：supports()和validate()。

如果验证器可以处理指定的Class，supports()方法将返回true。只有当supports()方法的返回结果为true的时候，validate()方法才会被调用来验证目标对象，并将验证错误填入Errors对象。

Errors对象是org.springframework.validation.Errors接口的一个实现类。Errors接口的源代码如下：

/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.validation;

import java.util.List;

import org.springframework.beans.PropertyAccessor;
import org.springframework.lang.Nullable;

/**
 * Stores and exposes information about data-binding and validation
 * errors for a specific object.
 *
 * <p>Field names can be properties of the target object (e.g. "name"
 * when binding to a customer object), or nested fields in case of
 * subobjects (e.g. "address.street"). Supports subtree navigation
 * via {@link #setNestedPath(String)}: for example, an
 * {@code AddressValidator} validates "address", not being aware
 * that this is a subobject of customer.
 *
 * <p>Note: {@code Errors} objects are single-threaded.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @see #setNestedPath
 * @see BindException
 * @see DataBinder
 * @see ValidationUtils
 */
public interface Errors {

    /**
     * The separator between path elements in a nested path,
     * for example in "customer.name" or "customer.address.street".
     * <p>"." = same as the
     * {@link org.springframework.beans.PropertyAccessor#NESTED_PROPERTY_SEPARATOR nested property separator}
     * in the beans package.
     */
    String NESTED_PATH_SEPARATOR = PropertyAccessor.NESTED_PROPERTY_SEPARATOR;


    /**
     * Return the name of the bound root object.
     */
    String getObjectName();

    /**
     * Allow context to be changed so that standard validators can validate
     * subtrees. Reject calls prepend the given path to the field names.
     * <p>For example, an address validator could validate the subobject
     * "address" of a customer object.
     * @param nestedPath nested path within this object,
     * e.g. "address" (defaults to "", {@code null} is also acceptable).
     * Can end with a dot: both "address" and "address." are valid.
     */
    void setNestedPath(String nestedPath);

    /**
     * Return the current nested path of this {@link Errors} object.
     * <p>Returns a nested path with a dot, i.e. "address.", for easy
     * building of concatenated paths. Default is an empty String.
     */
    String getNestedPath();

    /**
     * Push the given sub path onto the nested path stack.
     * <p>A {@link #popNestedPath()} call will reset the original
     * nested path before the corresponding
     * {@code pushNestedPath(String)} call.
     * <p>Using the nested path stack allows to set temporary nested paths
     * for subobjects without having to worry about a temporary path holder.
     * <p>For example: current path "spouse.", pushNestedPath("child") ->
     * result path "spouse.child."; popNestedPath() -> "spouse." again.
     * @param subPath the sub path to push onto the nested path stack
     * @see #popNestedPath
     */
    void pushNestedPath(String subPath);

    /**
     * Pop the former nested path from the nested path stack.
     * @throws IllegalStateException if there is no former nested path on the stack
     * @see #pushNestedPath
     */
    void popNestedPath() throws IllegalStateException;

    /**
     * Register a global error for the entire target object,
     * using the given error description.
     * @param errorCode error code, interpretable as a message key
     */
    void reject(String errorCode);

    /**
     * Register a global error for the entire target object,
     * using the given error description.
     * @param errorCode error code, interpretable as a message key
     * @param defaultMessage fallback default message
     */
    void reject(String errorCode, String defaultMessage);

    /**
     * Register a global error for the entire target object,
     * using the given error description.
     * @param errorCode error code, interpretable as a message key
     * @param errorArgs error arguments, for argument binding via MessageFormat
     * (can be {@code null})
     * @param defaultMessage fallback default message
     */
    void reject(String errorCode, @Nullable Object[] errorArgs, @Nullable String defaultMessage);

    /**
     * Register a field error for the specified field of the current object
     * (respecting the current nested path, if any), using the given error
     * description.
     * <p>The field name may be {@code null} or empty String to indicate
     * the current object itself rather than a field of it. This may result
     * in a corresponding field error within the nested object graph or a
     * global error if the current object is the top object.
     * @param field the field name (may be {@code null} or empty String)
     * @param errorCode error code, interpretable as a message key
     * @see #getNestedPath()
     */
    void rejectValue(@Nullable String field, String errorCode);

    /**
     * Register a field error for the specified field of the current object
     * (respecting the current nested path, if any), using the given error
     * description.
     * <p>The field name may be {@code null} or empty String to indicate
     * the current object itself rather than a field of it. This may result
     * in a corresponding field error within the nested object graph or a
     * global error if the current object is the top object.
     * @param field the field name (may be {@code null} or empty String)
     * @param errorCode error code, interpretable as a message key
     * @param defaultMessage fallback default message
     * @see #getNestedPath()
     */
    void rejectValue(@Nullable String field, String errorCode, String defaultMessage);

    /**
     * Register a field error for the specified field of the current object
     * (respecting the current nested path, if any), using the given error
     * description.
     * <p>The field name may be {@code null} or empty String to indicate
     * the current object itself rather than a field of it. This may result
     * in a corresponding field error within the nested object graph or a
     * global error if the current object is the top object.
     * @param field the field name (may be {@code null} or empty String)
     * @param errorCode error code, interpretable as a message key
     * @param errorArgs error arguments, for argument binding via MessageFormat
     * (can be {@code null})
     * @param defaultMessage fallback default message
     * @see #getNestedPath()
     */
    void rejectValue(@Nullable String field, String errorCode,
            @Nullable Object[] errorArgs, @Nullable String defaultMessage);

    /**
     * Add all errors from the given {@code Errors} instance to this
     * {@code Errors} instance.
     * <p>This is a convenience method to avoid repeated {@code reject(..)}
     * calls for merging an {@code Errors} instance into another
     * {@code Errors} instance.
     * <p>Note that the passed-in {@code Errors} instance is supposed
     * to refer to the same target object, or at least contain compatible errors
     * that apply to the target object of this {@code Errors} instance.
     * @param errors the {@code Errors} instance to merge in
     */
    void addAllErrors(Errors errors);

    /**
     * Return if there were any errors.
     */
    boolean hasErrors();

    /**
     * Return the total number of errors.
     */
    int getErrorCount();

    /**
     * Get all errors, both global and field ones.
     * @return a list of {@link ObjectError} instances
     */
    List<ObjectError> getAllErrors();

    /**
     * Are there any global errors?
     * @return {@code true} if there are any global errors
     * @see #hasFieldErrors()
     */
    boolean hasGlobalErrors();

    /**
     * Return the number of global errors.
     * @return the number of global errors
     * @see #getFieldErrorCount()
     */
    int getGlobalErrorCount();

    /**
     * Get all global errors.
     * @return a list of {@link ObjectError} instances
     */
    List<ObjectError> getGlobalErrors();

    /**
     * Get the <i>first</i> global error, if any.
     * @return the global error, or {@code null}
     */
    @Nullable
    ObjectError getGlobalError();

    /**
     * Are there any field errors?
     * @return {@code true} if there are any errors associated with a field
     * @see #hasGlobalErrors()
     */
    boolean hasFieldErrors();

    /**
     * Return the number of errors associated with a field.
     * @return the number of errors associated with a field
     * @see #getGlobalErrorCount()
     */
    int getFieldErrorCount();

    /**
     * Get all errors associated with a field.
     * @return a List of {@link FieldError} instances
     */
    List<FieldError> getFieldErrors();

    /**
     * Get the <i>first</i> error associated with a field, if any.
     * @return the field-specific error, or {@code null}
     */
    @Nullable
    FieldError getFieldError();

    /**
     * Are there any errors associated with the given field?
     * @param field the field name
     * @return {@code true} if there were any errors associated with the given field
     */
    boolean hasFieldErrors(String field);

    /**
     * Return the number of errors associated with the given field.
     * @param field the field name
     * @return the number of errors associated with the given field
     */
    int getFieldErrorCount(String field);

    /**
     * Get all errors associated with the given field.
     * <p>Implementations should support not only full field names like
     * "name" but also pattern matches like "na*" or "address.*".
     * @param field the field name
     * @return a List of {@link FieldError} instances
     */
    List<FieldError> getFieldErrors(String field);

    /**
     * Get the first error associated with the given field, if any.
     * @param field the field name
     * @return the field-specific error, or {@code null}
     */
    @Nullable
    FieldError getFieldError(String field);

    /**
     * Return the current value of the given field, either the current
     * bean property value or a rejected update from the last binding.
     * <p>Allows for convenient access to user-specified field values,
     * even if there were type mismatches.
     * @param field the field name
     * @return the current value of the given field
     */
    @Nullable
    Object getFieldValue(String field);

    /**
     * Return the type of a given field.
     * <p>Implementations should be able to determine the type even
     * when the field value is {@code null}, for example from some
     * associated descriptor.
     * @param field the field name
     * @return the type of the field, or {@code null} if not determinable
     */
    @Nullable
    Class<?> getFieldType(String field);

}

Errors对象中包含了一个FieldError类型的集合和一个ObjectError类型的集合：

• FieldError对象表示与被验证对象中的某个属性相关的一个错误；
• ObjectError对象表示与被验证对象相关的一个错误；其中FieldError继承自ObjectError类。

FieldError类的源码如下：

/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.validation;

import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ObjectUtils;

/**
 * Encapsulates a field error, that is, a reason for rejecting a specific
 * field value.
 *
 * <p>See the {@link DefaultMessageCodesResolver} javadoc for details on
 * how a message code list is built for a {@code FieldError}.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @since 10.03.2003
 * @see DefaultMessageCodesResolver
 */
@SuppressWarnings("serial")
public class FieldError extends ObjectError {

    private final String field;

    @Nullable
    private final Object rejectedValue;

    private final boolean bindingFailure;


    /**
     * Create a new FieldError instance.
     * @param objectName the name of the affected object
     * @param field the affected field of the object
     * @param defaultMessage the default message to be used to resolve this message
     */
    public FieldError(String objectName, String field, String defaultMessage) {
        this(objectName, field, null, false, null, null, defaultMessage);
    }

    /**
     * Create a new FieldError instance.
     * @param objectName the name of the affected object
     * @param field the affected field of the object
     * @param rejectedValue the rejected field value
     * @param bindingFailure whether this error represents a binding failure
     * (like a type mismatch); else, it is a validation failure
     * @param codes the codes to be used to resolve this message
     * @param arguments the array of arguments to be used to resolve this message
     * @param defaultMessage the default message to be used to resolve this message
     */
    public FieldError(String objectName, String field, @Nullable Object rejectedValue, boolean bindingFailure,
            @Nullable String[] codes, @Nullable Object[] arguments, @Nullable String defaultMessage) {

        super(objectName, codes, arguments, defaultMessage);
        Assert.notNull(field, "Field must not be null");
        this.field = field;
        this.rejectedValue = rejectedValue;
        this.bindingFailure = bindingFailure;
    }


    /**
     * Return the affected field of the object.
     */
    public String getField() {
        return this.field;
    }

    /**
     * Return the rejected field value.
     */
    @Nullable
    public Object getRejectedValue() {
        return this.rejectedValue;
    }

    /**
     * Return whether this error represents a binding failure
     * (like a type mismatch); otherwise it is a validation failure.
     */
    public boolean isBindingFailure() {
        return this.bindingFailure;
    }


    @Override
    public boolean equals(@Nullable Object other) {
        if (this == other) {
            return true;
        }
        if (!super.equals(other)) {
            return false;
        }
        FieldError otherError = (FieldError) other;
        return (getField().equals(otherError.getField()) &&
                ObjectUtils.nullSafeEquals(getRejectedValue(), otherError.getRejectedValue()) &&
                isBindingFailure() == otherError.isBindingFailure());
    }

    @Override
    public int hashCode() {
        int hashCode = super.hashCode();
        hashCode = 29 * hashCode + getField().hashCode();
        hashCode = 29 * hashCode + ObjectUtils.nullSafeHashCode(getRejectedValue());
        hashCode = 29 * hashCode + (isBindingFailure() ? 1 : 0);
        return hashCode;
    }

    @Override
    public String toString() {
        return "Field error in object '" + getObjectName() + "' on field '" + this.field +
                "': rejected value [" + ObjectUtils.nullSafeToString(this.rejectedValue) + "]; " +
                resolvableToString();
    }

}

ObjectError类的源码如下：

/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.validation;

import org.springframework.context.support.DefaultMessageSourceResolvable;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;

/**
 * Encapsulates an object error, that is, a global reason for rejecting
 * an object.
 *
 * <p>See the {@link DefaultMessageCodesResolver} javadoc for details on
 * how a message code list is built for an {@code ObjectError}.
 *
 * @author Juergen Hoeller
 * @since 10.03.2003
 * @see FieldError
 * @see DefaultMessageCodesResolver
 */
@SuppressWarnings("serial")
public class ObjectError extends DefaultMessageSourceResolvable {

    private final String objectName;

    @Nullable
    private transient Object source;


    /**
     * Create a new instance of the ObjectError class.
     * @param objectName the name of the affected object
     * @param defaultMessage the default message to be used to resolve this message
     */
    public ObjectError(String objectName, String defaultMessage) {
        this(objectName, null, null, defaultMessage);
    }

    /**
     * Create a new instance of the ObjectError class.
     * @param objectName the name of the affected object
     * @param codes the codes to be used to resolve this message
     * @param arguments    the array of arguments to be used to resolve this message
     * @param defaultMessage the default message to be used to resolve this message
     */
    public ObjectError(
            String objectName, @Nullable String[] codes, @Nullable Object[] arguments, @Nullable String defaultMessage) {

        super(codes, arguments, defaultMessage);
        Assert.notNull(objectName, "Object name must not be null");
        this.objectName = objectName;
    }


    /**
     * Return the name of the affected object.
     */
    public String getObjectName() {
        return this.objectName;
    }

    /**
     * Preserve the source behind this error: possibly an {@link Exception}
     * (typically {@link org.springframework.beans.PropertyAccessException})
     * or a Bean Validation {@link javax.validation.ConstraintViolation}.
     * <p>Note that any such source object is being stored as transient:
     * that is, it won't be part of a serialized error representation.
     * @param source the source object
     * @since 5.0.4
     */
    public void wrap(Object source) {
        if (this.source != null) {
            throw new IllegalStateException("Already wrapping " + this.source);
        }
        this.source = source;
    }

    /**
     * Unwrap the source behind this error: possibly an {@link Exception}
     * (typically {@link org.springframework.beans.PropertyAccessException})
     * or a Bean Validation {@link javax.validation.ConstraintViolation}.
     * <p>The cause of the outermost exception will be introspected as well,
     * e.g. the underlying conversion exception or exception thrown from a setter
     * (instead of having to unwrap the {@code PropertyAccessException} in turn).
     * @return the source object of the given type
     * @throws IllegalArgumentException if no such source object is available
     * (i.e. none specified or not available anymore after deserialization)
     * @since 5.0.4
     */
    public <T> T unwrap(Class<T> sourceType) {
        if (sourceType.isInstance(this.source)) {
            return sourceType.cast(this.source);
        }
        else if (this.source instanceof Throwable) {
            Throwable cause = ((Throwable) this.source).getCause();
            if (sourceType.isInstance(cause)) {
                return sourceType.cast(cause);
            }
        }
        throw new IllegalArgumentException("No source object of the given type available: " + sourceType);
    }

    /**
     * Check the source behind this error: possibly an {@link Exception}
     * (typically {@link org.springframework.beans.PropertyAccessException})
     * or a Bean Validation {@link javax.validation.ConstraintViolation}.
     * <p>The cause of the outermost exception will be introspected as well,
     * e.g. the underlying conversion exception or exception thrown from a setter
     * (instead of having to unwrap the {@code PropertyAccessException} in turn).
     * @return whether this error has been caused by a source object of the given type
     * @since 5.0.4
     */
    public boolean contains(Class<?> sourceType) {
        return (sourceType.isInstance(this.source) ||
                (this.source instanceof Throwable && sourceType.isInstance(((Throwable) this.source).getCause())));
    }


    @Override
    public boolean equals(@Nullable Object other) {
        if (this == other) {
            return true;
        }
        if (other == null || other.getClass() != getClass() || !super.equals(other)) {
            return false;
        }
        ObjectError otherError = (ObjectError) other;
        return getObjectName().equals(otherError.getObjectName());
    }

    @Override
    public int hashCode() {
        return super.hashCode() * 29 + getObjectName().hashCode();
    }

    @Override
    public String toString() {
        return "Error in object '" + this.objectName + "': " + resolvableToString();
    }

}

编写验证器时，不需要直接创建Error对象，并且实例化FieldError和ObjectError花费了大量的编程精力。这是因为ObjectError类有两个构造函数，其中一个需要2个参数，另一个需要4个参数：FieldError类的构造器也有2个，其中一个需要3个参数，另一个需要7个参数。

但是我们可以通过调用rejectValue()方法向Errors对象中添加被验证对象的字段错误信息，该方法实际上是创建一个FieldError对象，并添加Errors该对象的List<FieldError>集合中。

通过调用reject()方法向Errors对象中添加被验证对象错误信息，该方法实际上是创建一个ObjectError对象，并添加到Errors对象的List<ObjectError>中。

下面是reject()和rejectValue的部分方法重载：

    void reject(String errorCode);
    void reject(String errorCode, String defaultMessage);
    void reject(String errorCode, @Nullable Object[] errorArgs, @Nullable String defaultMessage);
    void rejectValue(@Nullable String field, String errorCode);
    void rejectValue(@Nullable String field, String errorCode, String defaultMessage);
    void rejectValue(@Nullable String field, String errorCode,
            @Nullable Object[] errorArgs, @Nullable String defaultMessage);

大多数时候，只给reject()或者rejectValue()方法传入一个错误码，Spring就会在属性文件中查找错误码，获取相应的错误消息。还可以传入一个默认消息，当没有找到指定的错误码时，就会使用默认消息。

Errors对象中的错误消息，可以利用表单标签库的Errors标签显示在HTML页面中，错误消息可以通过Spring支持的国际化特性本地化。

三 ValidationUtils类 

org.springframework.validation.ValidationUtils类是一个工具，源码如下：

/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.validation;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ObjectUtils;
import org.springframework.util.StringUtils;

/**
 * Utility class offering convenient methods for invoking a {@link Validator}
 * and for rejecting empty fields.
 *
 * <p>Checks for an empty field in {@code Validator} implementations can become
 * one-liners when using {@link #rejectIfEmpty} or {@link #rejectIfEmptyOrWhitespace}.
 *
 * @author Juergen Hoeller
 * @author Dmitriy Kopylenko
 * @since 06.05.2003
 * @see Validator
 * @see Errors
 */
public abstract class ValidationUtils {

    private static final Log logger = LogFactory.getLog(ValidationUtils.class);


    /**
     * Invoke the given {@link Validator} for the supplied object and
     * {@link Errors} instance.
     * @param validator the {@code Validator} to be invoked
     * @param target the object to bind the parameters to
     * @param errors the {@link Errors} instance that should store the errors
     * @throws IllegalArgumentException if either of the {@code Validator} or {@code Errors}
     * arguments is {@code null}, or if the supplied {@code Validator} does not
     * {@link Validator#supports(Class) support} the validation of the supplied object's type
     */
    public static void invokeValidator(Validator validator, Object target, Errors errors) {
        invokeValidator(validator, target, errors, (Object[]) null);
    }

    /**
     * Invoke the given {@link Validator}/{@link SmartValidator} for the supplied object and
     * {@link Errors} instance.
     * @param validator the {@code Validator} to be invoked
     * @param target the object to bind the parameters to
     * @param errors the {@link Errors} instance that should store the errors
     * @param validationHints one or more hint objects to be passed to the validation engine
     * @throws IllegalArgumentException if either of the {@code Validator} or {@code Errors}
     * arguments is {@code null}, or if the supplied {@code Validator} does not
     * {@link Validator#supports(Class) support} the validation of the supplied object's type
     */
    public static void invokeValidator(
            Validator validator, Object target, Errors errors, @Nullable Object... validationHints) {

        Assert.notNull(validator, "Validator must not be null");
        Assert.notNull(target, "Target object must not be null");
        Assert.notNull(errors, "Errors object must not be null");

        if (logger.isDebugEnabled()) {
            logger.debug("Invoking validator [" + validator + "]");
        }
        if (!validator.supports(target.getClass())) {
            throw new IllegalArgumentException(
                    "Validator [" + validator.getClass() + "] does not support [" + target.getClass() + "]");
        }

        if (!ObjectUtils.isEmpty(validationHints) && validator instanceof SmartValidator) {
            ((SmartValidator) validator).validate(target, errors, validationHints);
        }
        else {
            validator.validate(target, errors);
        }

        if (logger.isDebugEnabled()) {
            if (errors.hasErrors()) {
                logger.debug("Validator found " + errors.getErrorCount() + " errors");
            }
            else {
                logger.debug("Validator found no errors");
            }
        }
    }


    /**
     * Reject the given field with the given error code if the value is empty.
     * <p>An 'empty' value in this context means either {@code null} or
     * the empty string "".
     * <p>The object whose field is being validated does not need to be passed
     * in because the {@link Errors} instance can resolve field values by itself
     * (it will usually hold an internal reference to the target object).
     * @param errors the {@code Errors} instance to register errors on
     * @param field the field name to check
     * @param errorCode the error code, interpretable as message key
     */
    public static void rejectIfEmpty(Errors errors, String field, String errorCode) {
        rejectIfEmpty(errors, field, errorCode, null, null);
    }

    /**
     * Reject the given field with the given error code and default message
     * if the value is empty.
     * <p>An 'empty' value in this context means either {@code null} or
     * the empty string "".
     * <p>The object whose field is being validated does not need to be passed
     * in because the {@link Errors} instance can resolve field values by itself
     * (it will usually hold an internal reference to the target object).
     * @param errors the {@code Errors} instance to register errors on
     * @param field the field name to check
     * @param errorCode error code, interpretable as message key
     * @param defaultMessage fallback default message
     */
    public static void rejectIfEmpty(Errors errors, String field, String errorCode, String defaultMessage) {
        rejectIfEmpty(errors, field, errorCode, null, defaultMessage);
    }

    /**
     * Reject the given field with the given error code and error arguments
     * if the value is empty.
     * <p>An 'empty' value in this context means either {@code null} or
     * the empty string "".
     * <p>The object whose field is being validated does not need to be passed
     * in because the {@link Errors} instance can resolve field values by itself
     * (it will usually hold an internal reference to the target object).
     * @param errors the {@code Errors} instance to register errors on
     * @param field the field name to check
     * @param errorCode the error code, interpretable as message key
     * @param errorArgs the error arguments, for argument binding via MessageFormat
     * (can be {@code null})
     */
    public static void rejectIfEmpty(Errors errors, String field, String errorCode, Object[] errorArgs) {
        rejectIfEmpty(errors, field, errorCode, errorArgs, null);
    }

    /**
     * Reject the given field with the given error code, error arguments
     * and default message if the value is empty.
     * <p>An 'empty' value in this context means either {@code null} or
     * the empty string "".
     * <p>The object whose field is being validated does not need to be passed
     * in because the {@link Errors} instance can resolve field values by itself
     * (it will usually hold an internal reference to the target object).
     * @param errors the {@code Errors} instance to register errors on
     * @param field the field name to check
     * @param errorCode the error code, interpretable as message key
     * @param errorArgs the error arguments, for argument binding via MessageFormat
     * (can be {@code null})
     * @param defaultMessage fallback default message
     */
    public static void rejectIfEmpty(Errors errors, String field, String errorCode,
            @Nullable Object[] errorArgs, @Nullable String defaultMessage) {

        Assert.notNull(errors, "Errors object must not be null");
        Object value = errors.getFieldValue(field);
        if (value == null || !StringUtils.hasLength(value.toString())) {
            errors.rejectValue(field, errorCode, errorArgs, defaultMessage);
        }
    }

    /**
     * Reject the given field with the given error code if the value is empty
     * or just contains whitespace.
     * <p>An 'empty' value in this context means either {@code null},
     * the empty string "", or consisting wholly of whitespace.
     * <p>The object whose field is being validated does not need to be passed
     * in because the {@link Errors} instance can resolve field values by itself
     * (it will usually hold an internal reference to the target object).
     * @param errors the {@code Errors} instance to register errors on
     * @param field the field name to check
     * @param errorCode the error code, interpretable as message key
     */
    public static void rejectIfEmptyOrWhitespace(Errors errors, String field, String errorCode) {
        rejectIfEmptyOrWhitespace(errors, field, errorCode, null, null);
    }

    /**
     * Reject the given field with the given error code and default message
     * if the value is empty or just contains whitespace.
     * <p>An 'empty' value in this context means either {@code null},
     * the empty string "", or consisting wholly of whitespace.
     * <p>The object whose field is being validated does not need to be passed
     * in because the {@link Errors} instance can resolve field values by itself
     * (it will usually hold an internal reference to the target object).
     * @param errors the {@code Errors} instance to register errors on
     * @param field the field name to check
     * @param errorCode the error code, interpretable as message key
     * @param defaultMessage fallback default message
     */
    public static void rejectIfEmptyOrWhitespace(
            Errors errors, String field, String errorCode, String defaultMessage) {

        rejectIfEmptyOrWhitespace(errors, field, errorCode, null, defaultMessage);
    }

    /**
     * Reject the given field with the given error code and error arguments
     * if the value is empty or just contains whitespace.
     * <p>An 'empty' value in this context means either {@code null},
     * the empty string "", or consisting wholly of whitespace.
     * <p>The object whose field is being validated does not need to be passed
     * in because the {@link Errors} instance can resolve field values by itself
     * (it will usually hold an internal reference to the target object).
     * @param errors the {@code Errors} instance to register errors on
     * @param field the field name to check
     * @param errorCode the error code, interpretable as message key
     * @param errorArgs the error arguments, for argument binding via MessageFormat
     * (can be {@code null})
     */
    public static void rejectIfEmptyOrWhitespace(
            Errors errors, String field, String errorCode, @Nullable Object[] errorArgs) {

        rejectIfEmptyOrWhitespace(errors, field, errorCode, errorArgs, null);
    }

    /**
     * Reject the given field with the given error code, error arguments
     * and default message if the value is empty or just contains whitespace.
     * <p>An 'empty' value in this context means either {@code null},
     * the empty string "", or consisting wholly of whitespace.
     * <p>The object whose field is being validated does not need to be passed
     * in because the {@link Errors} instance can resolve field values by itself
     * (it will usually hold an internal reference to the target object).
     * @param errors the {@code Errors} instance to register errors on
     * @param field the field name to check
     * @param errorCode the error code, interpretable as message key
     * @param errorArgs the error arguments, for argument binding via MessageFormat
     * (can be {@code null})
     * @param defaultMessage fallback default message
     */
    public static void rejectIfEmptyOrWhitespace(
            Errors errors, String field, String errorCode, @Nullable Object[] errorArgs, @Nullable String defaultMessage) {

        Assert.notNull(errors, "Errors object must not be null");
        Object value = errors.getFieldValue(field);
        if (value == null ||!StringUtils.hasText(value.toString())) {
            errors.rejectValue(field, errorCode, errorArgs, defaultMessage);
        }
    }

}

ValidationUtils类有助于编写Spring验证器，比如检测一个name属性是否为null或者是“”字符串，不需要像下面这样编写：

if(name== null || name.isEmpty()){
   errors.rejectValue("name"，"name.required");
}

而是可以利用类的rejectIfEmpty()方法，像下面这样：

ValidationUtils.rejectIfEmpty(errors,"name","name.required");  

其中，"name"是属性名，"name.required"是错误代码。

或者下面这样的代码：

if(name== null || name.trim().isEmpty()){
   errors.rejectValue("name","name.required");  
}

可以编写成：

ValidationUtils.rejectIfEmptyOrWhitespace("name");

下面是ValidationUtils中rejectIfEmpty()和rejectIfEmptyOrWhitespace()方法的方法重载：

    public static void rejectIfEmpty(Errors errors, String field, String errorCode) {
        rejectIfEmpty(errors, field, errorCode, null, null);
    }

    public static void rejectIfEmpty(Errors errors, String field, String errorCode, String defaultMessage) {
        rejectIfEmpty(errors, field, errorCode, null, defaultMessage);
    }

    public static void rejectIfEmpty(Errors errors, String field, String errorCode, Object[] errorArgs) {
        rejectIfEmpty(errors, field, errorCode, errorArgs, null);
    }

    public static void rejectIfEmpty(Errors errors, String field, String errorCode,
            @Nullable Object[] errorArgs, @Nullable String defaultMessage) {

        Assert.notNull(errors, "Errors object must not be null");
        Object value = errors.getFieldValue(field);
        if (value == null || !StringUtils.hasLength(value.toString())) {
            errors.rejectValue(field, errorCode, errorArgs, defaultMessage);
        }
    }

    public static void rejectIfEmptyOrWhitespace(Errors errors, String field, String errorCode) {
        rejectIfEmptyOrWhitespace(errors, field, errorCode, null, null);
    }
    public static void rejectIfEmptyOrWhitespace(
            Errors errors, String field, String errorCode, String defaultMessage) {

        rejectIfEmptyOrWhitespace(errors, field, errorCode, null, defaultMessage);
    }
    public static void rejectIfEmptyOrWhitespace(
            Errors errors, String field, String errorCode, @Nullable Object[] errorArgs) {

        rejectIfEmptyOrWhitespace(errors, field, errorCode, errorArgs, null);
    }

    public static void rejectIfEmptyOrWhitespace(
            Errors errors, String field, String errorCode, @Nullable Object[] errorArgs, @Nullable String defaultMessage) {

        Assert.notNull(errors, "Errors object must not be null");
        Object value = errors.getFieldValue(field);
        if (value == null ||!StringUtils.hasText(value.toString())) {
            errors.rejectValue(field, errorCode, errorArgs, defaultMessage);
        }
    }

此外，ValidationUtils还有一个invokeValidator()方法，用来代用验证器：

    public static void invokeValidator(
            Validator validator, Object target, Errors errors, @Nullable Object... validationHints) {

        Assert.notNull(validator, "Validator must not be null");
        Assert.notNull(target, "Target object must not be null");
        Assert.notNull(errors, "Errors object must not be null");

        if (logger.isDebugEnabled()) {
            logger.debug("Invoking validator [" + validator + "]");
        }
        if (!validator.supports(target.getClass())) {
            throw new IllegalArgumentException(
                    "Validator [" + validator.getClass() + "] does not support [" + target.getClass() + "]");
        }

        if (!ObjectUtils.isEmpty(validationHints) && validator instanceof SmartValidator) {
            ((SmartValidator) validator).validate(target, errors, validationHints);
        }
        else {
            validator.validate(target, errors);
        }

        if (logger.isDebugEnabled()) {
            if (errors.hasErrors()) {
                logger.debug("Validator found " + errors.getErrorCount() + " errors");
            }
            else {
                logger.debug("Validator found no errors");
            }
        }
    }

接下来将通过范例来介绍如何使用这个工具。

四 Spring的Validator范例

本节将会创建一个spring-validator应用，该应用包含一个名为ProductValidator的验证器，用于验证Product对象。

1、目录结构

2、Product类

package domain;
import java.io.Serializable;
import java.math.BigDecimal;
import java.time.LocalDate;

public class Product implements Serializable {
    private static final long serialVersionUID = 1L;
    private String name;
    private String description;
    private BigDecimal price;
    private LocalDate productionDate;

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getDescription() {
        return description;
    }
    public void setDescription(String description) {
        this.description = description;
    }
    public BigDecimal getPrice() {
        return price;
    }
    public void setPrice(BigDecimal price) {
        this.price = price;
    }
    public LocalDate getProductionDate() {
        return productionDate;
    }
    public void setProductionDate(LocalDate productionDate) {
        this.productionDate = productionDate;
    }
    
}

3、Formatter

为了使ProductForm.jsp页面中表单输入的日期可以使用不同于当前语言区域的日期样式，，我们创建了一个LocalDateFormatter 类：

package formatter;

import java.text.ParseException;
import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeParseException;
import java.util.Locale;

import org.springframework.format.Formatter;

public class LocalDateFormatter implements Formatter<LocalDate> {

    private DateTimeFormatter formatter;
    private String datePattern;

    // 设定日期样式
    public LocalDateFormatter(String datePattern) {
        this.datePattern = datePattern;
        formatter = DateTimeFormatter.ofPattern(datePattern);
    }

    //利用指定的Locale将一个LocalDate解析成String类型
    @Override
    public String print(LocalDate date, Locale locale) {
        return date.format(formatter);
    }

    //利用指定的Locale将一个String解析成LocalDate类型
    @Override
    public LocalDate parse(String s, Locale locale) throws ParseException {
        try {
            //使用指定的formatter从字符串中获取一个LocalDate对象  如果字符串不符合formatter指定的样式要求，转换会失败
            return LocalDate.parse(s, DateTimeFormatter.ofPattern(datePattern));
        } catch (DateTimeParseException e) {
            // the error message will be displayed in <form:errors>
            throw new IllegalArgumentException(
                    "invalid date format. Please use this pattern""
                            + datePattern + """);
        }
    }
}

LocalDateFormatter 类的parse()方法，它利用传给构造器的日期样式，将一个String转换成LocalDate。

如果输入的日期格式有问题，将会抛出IllegalArgumentException异常，这表明以下代码中input标签绑定到表单支持对象的birthDate属性出现错误：

        <p>
            <label for="productionDate">*Production Date (MM-dd-yyyy): </label>
            <form:input id="productionDate" path="productionDate" tabindex="4"/>
        </p>

在/save-product页面对应的请求处理方法saveEmployee()中，bindingResult参数将会记录到这个绑定错误，即类型转换错误。

4、Validator

该应用包含一个名为ProductValidator的验证器：

package validator;

import java.math.BigDecimal;
import java.time.LocalDate;

import org.springframework.validation.Errors;
import org.springframework.validation.ValidationUtils;
import org.springframework.validation.Validator;

import domain.Product;

public class ProductValidator implements Validator {

    @Override
    public boolean supports(Class<?> klass) {
        //支持Product类？
        return Product.class.isAssignableFrom(klass);
    }

    //将目标对象target的错误注册到errors对象中
    @Override
    public void validate(Object target, Errors errors) {
        //强制类型转换
        Product product = (Product) target;
        
        //如果目标对象的name属性为null，或者为""字符串，则将错误注册到errors对象
        ValidationUtils.rejectIfEmpty(errors, "name", "productName.required");
        //如果目标对象的price属性为null，或者为""字符串，则将错误注册到errors对象中
        ValidationUtils.rejectIfEmpty(errors, "price", "price.required");
        //如果目标对象的productionDate属性为null，或者为""字符串，则将错误注册到errors对象中
        ValidationUtils.rejectIfEmpty(errors, "productionDate", "productionDate.required");
        
        BigDecimal price = product.getPrice();
        
        //如果价格为负数 则将错误注册到errors对象中
        if (price != null && price.compareTo(BigDecimal.ZERO) < 0) {
            errors.rejectValue("price", "price.negative");
        }
        //如果产品日期在今天之后 则将错误注册到errors对象中
        LocalDate productionDate = product.getProductionDate();
        if (productionDate != null) {
            if (productionDate.isAfter(LocalDate.now())) {
                errors.rejectValue("productionDate", "productionDate.invalid");
            }
        }
    }
}

ProductValidator验证器是一个非常简单的验证器。它的validate()方法会检验Product是否有名称和价格，并且价格是否不为负数，它还会确保生产日期不晚于今天。

5、Controller类

在Controller类中通过实例化validator类，可以使用Spring验证器。

package controller;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.validation.BindingResult;
import org.springframework.validation.FieldError;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.RequestMapping;

import domain.Product;
import validator.ProductValidator;

@Controller
public class ProductController {

    private static final Log logger = LogFactory
            .getLog(ProductController.class);

    @RequestMapping(value = "/add-product")
    public String inputProduct(Model model) {
        model.addAttribute("product", new Product());
        return "ProductForm";
    }

    @RequestMapping(value = "/save-product")
    public String saveProduct(@ModelAttribute Product product,
            BindingResult bindingResult, Model model) {
        //创建一个ProductValidator，并调用其validate()方法校验Product对象，并将验证错误填入bindingResult中。
        ProductValidator productValidator = new ProductValidator();
        productValidator.validate(product, bindingResult);

        if (bindingResult.hasErrors()) {
            FieldError fieldError = bindingResult.getFieldError();
            logger.debug("Code:" + fieldError.getCode() + ", field:"
                    + fieldError.getField());
            return "ProductForm";
        }

        // save product here
        //model.addAttribute("product", product);
        return "ProductDetails";
    }
}    

ProductController类的saveProduct()方法，有三个参数：

• 第一个参数product，使用了注解@ModelAttribute，该对象的各个属性被用来接受表单的各个字段信息，并且将"product"属性添加到Model对象中；
• 第二个参数bindingResult中设置了Spring所有的绑定错误（主要是类型转换问题，例如将表单String转换为LocalDate类型）；
• 第三个参数是Model。

注意：BindingResult接口是Errors接口的子类，在请求处理方法的签名中使用了BindingResult参数，就是告诉Spring关于表单对象数据校验的错误将由我们自己来处理，否则Spring会直接抛出异常。

该方法创建一个ProductValidator，并调用其validate()方法校验Product对象，并将验证错误填入bindingResult中。

ProductValidator productValidator = new ProductValidator();
productValidator.validate(product, bindingResult);

为了检验该验证器是否生成错误消息，需要在BindingResult中调用hasErrors()方法：

 if (bindingResult.hasErrors()) {
            FieldError fieldError = bindingResult.getFieldError();
            logger.debug("Code:" + fieldError.getCode() + ", field:"
                    + fieldError.getField());
            return "ProductForm";
        }

如果存在表单绑定错误或者是输入验证错误，将会打印出错误相关的字段，并重定位到ProductForm.jsp页面。

如果表单输入的数据均合法，则会重定位到ProductDetails.jsp页面。

使用Spring验证器的另一种方法是：在Controller中编写initBinder()方法，并将验证器传到WebDataBinder，并调用其validate()方法：

@org.springframework.web.bind.annotation.InitBinder
public void initBinder(WebDataBinder binder){
    //this will apply the Validator to all request-handling methods
    binder.setValidator(new ProductValidator)();
    binder.validate();  
}

将验证器传到WebDataBinder，会使该验证器应用于Controller类中的所有请求处理的方法。

或者利用@javax.validation.Valid对要验证的对象参数进行注解，例如：

  public String saveProduct(@Valid @ModelAttribute Product product,
            BindingResult bindingResult, Model model) {

注意：这种写法不需要编写validator，但是需要使用JSR 303注解类型进行字段校验，此外，Valid注解类型也是在JSR 303中定义的，关于JSR 303的相关信息，后面介绍。

6、视图

spring-validator应用包含三个视图文件：

ProductForm.jsp：

<%@ taglib prefix="form" uri="http://www.springframework.org/tags/form" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE HTML>
<html>
<head>
<title>Add Product Form</title>
<style type="text/css">@import url("<c:url value="/css/main.css"/>");</style>
</head>
<body>

<div id="global">
<form:form modelAttribute="product" action="save-product" method="post">
    <fieldset>
        <legend>Add a product</legend>
        <p class="errorLine">
            <form:errors path="name" cssClass="error"/>
        </p>
        <p>
            <label for="name">*Product Name: </label>
            <form:input id="name" path="name" tabindex="1"/>
        </p>
        <p>
            <label for="description">Description: </label>
            <form:input id="description" path="description" tabindex="2"/>
        </p>
        <p class="errorLine">
            <form:errors path="price" cssClass="error"/>
        </p>
        <p>
            <label for="price">*Price: </label>
            <form:input id="price" path="price" tabindex="3"/>
        </p>
        <p class="errorLine">
            <form:errors path="productionDate" cssClass="error"/>
        </p>
        <p>
            <label for="productionDate">*Production Date (MM-dd-yyyy): </label>
            <form:input id="productionDate" path="productionDate" tabindex="4"/>
        </p>
        <p id="buttons">
            <input id="reset" type="reset" tabindex="5">
            <input id="submit" type="submit" tabindex="6" 
                value="Add Product">
        </p>
    </fieldset>
</form:form>
</div>
</body>
</html>

在ProductForm.jsp视图中我们使用到了表单标签，并且使用了errors标签。下面详细介绍errors的用途：

1. 当通过浏览器访问http://localhost:8008/spring-validator/add-product，将会调用Controller类的请求处理方法inputProduct()，返回ProductForm.jsp视图；
2. 当表单中输入有非法数据时，提交数据到save-product，将会发生表单绑定错误或者是输入验证错误，这些信息都会被填入请求处理方法saveProduct()方法的bindingResult参数中；
3. saveProduct()方法将请求转发到ProductForm.jsp页面时，然后就可以利用erros标签（可以把其看做bindingResult参数）将path指定的属性的错误消息显示出来。

如果想要从某个属性文件中获取错误消息，则需要通过声明messageSource.bean。告诉Spring要去哪里查找这个文件。下面是springmvc-config.xml中的messageSource.bean：

    <bean id="messageSource" 
            class="org.springframework.context.support.ReloadableResourceBundleMessageSource">
        <property name="basename" value="/WEB-INF/resource/messages" />
    </bean>

配置中需要注意的地方：

•  ReloadableResourceBundleMessageSource ：spring中提供的信息源配置类，支持proerties和xml文件，更改配置无需重启服务，basename指定文件位置和名称(可使用classpath前缀)，spring中首先查找.properties后缀文件，找不到再查找.xml后缀文件。

这个bean实际上是说，错误码和错误信息可以在/WEB-INF/resource/messages.properties文件中找到：

productname.required=Please enter a product name
price.required=Please enter a price
price.negative=Price cannot be less than 0
productionDate.required=Please enter a production date
productionDate.invalid=Please ensure the production date is not later than today
typeMismatch.productionDate=Invalid production date

每一行代表一个错误，格式为：

errorCode=defaultMessage

如果是验证错误（validator），错误码一般就是errors.rejectValue()方法中errorCode参数；如果是类型转换错误，错误码一般就是：typeMismatch.属性名。

ProductDetails.jsp：

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE HTML>
<html>
<head>
<title>Save Product</title>
<style type="text/css">@import url("<c:url value="/css/main.css"/>");</style>
</head>
<body>
<div id="global">
    <h4>The product has been saved.</h4>
    <p>
        <h5>Details:</h5>
        Product Name: ${product.name}<br/>
        Description: ${product.description}<br/>
        Price: $${product.price}
    </p>
</div>
</body>
</html>

ProductView.jsp(没用到)：

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE HTML>
<html>
<head>
<title>View Product</title>
<style type="text/css">@import url("<c:url value="/css/main.css"/>");</style>
</head>
<body>
<div id="global">
    <h4>${message}</h4>
    <p>
        <h5>Details:</h5>
        Product Name: ${product.name}<br/>
        Description: ${product.description}<br/>
        Price: $${product.price}
    </p>
</div>
</body>
</html>

 main.css：

#global {
    text-align: left;
    border: 1px solid #dedede;
    background: #efefef;
    width: 560px;
    padding: 20px;
    margin: 100px auto;
}

form {
  font:100% verdana;
  min-width: 500px;
  max-width: 600px;
  width: 560px;
}

form fieldset {
  border-color: #bdbebf;
  border-width: 3px;
  margin: 0;
}

legend {
    font-size: 1.3em;
}

form label { 
    width: 250px;
    display: block;
    float: left;
    text-align: right;
    padding: 2px;
}

#buttons {
    text-align: right;
}
#errors, li {
    color: red;
}
.error {
    color: red;
    font-size: 9pt;    
}
.errorLine {
    text-align: center;
}

7、配置文件

下面给出springmvc-config.xml文件的所有内容：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p"
    xmlns:mvc="http://www.springframework.org/schema/mvc" xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/mvc
        http://www.springframework.org/schema/mvc/spring-mvc.xsd     
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context.xsd">

    <context:component-scan base-package="controller" />
    <context:component-scan base-package="formatter" />
    <mvc:annotation-driven conversion-service="conversionService" />
    <mvc:resources mapping="/css/**" location="/css/" />
    <mvc:resources mapping="/*.html" location="/" />

    <bean id="viewResolver"
        class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix" value="/WEB-INF/jsp/" />
        <property name="suffix" value=".jsp" />
    </bean>
    
    <bean id="messageSource" 
            class="org.springframework.context.support.ReloadableResourceBundleMessageSource">
        <property name="basename" value="/WEB-INF/resource/messages" />
    </bean>
    
    <bean id="conversionService"
        class="org.springframework.format.support.FormattingConversionServiceFactoryBean">
        <property name="formatters">
            <set>
                <bean class="formatter.LocalDateFormatter">
                    <constructor-arg type="java.lang.String" value="MM-dd-yyyy" />
                </bean>
            </set>
        </property>
    </bean>
</beans>

部署描述符(web.xml文件)：

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.1" 
        xmlns="http://xmlns.jcp.org/xml/ns/javaee" 
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
        xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee 
            http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"> 
    <servlet>
        <servlet-name>springmvc</servlet-name>
        <servlet-class>
            org.springframework.web.servlet.DispatcherServlet
        </servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>/WEB-INF/config/springmvc-config.xml</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>    
    </servlet>

    <servlet-mapping>
        <servlet-name>springmvc</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>
</web-app>

8、测试

部署项目，并在浏览器输入：

http://localhost:8008/spring-validator/add-product

试着输入一个无效的日期，将会跳转到/save--product，但是表单内容不会丢失，并且会在表单中看到错误的消息：

可以看到由于输入的日期没有按照MM-dd-yyyy的格式，所以将String转换LocalDate类型时，将会发生typeMismatch.productionDate错误，Errors标签将会将messages.properties中设置的errorCode对应的错误消息显示出来。

但是如果将Spring MVC配置文件中声明的messageSource bean删除，将会提示错误码typeMismatch.productionDate对应的默认系统错误消息。

但是针对验证器中我们自己设定的errorCode，我们必须在messages.properties指定其对应的错误消息，并配置messageSource bean.

 

五 JSR303验证

JSR 303 是 JAVA EE 6 中的一项子规范，叫做 Bean Validation。 JSR 303 用于对 Java Bean 中的字段的值进行验证。

当前，JSR只是一个规范文档，本身用处不大，除非编写了它的实现。用于实现JSR Bean Validation，目前有两个实现：

• 第一个实现是Hibernate Validator（官方参考实现）;
• 第二个实现是Apache BVal，可以从以下网站下载：http://bval.apache.org/downloads.html。

JSR 303不需要编写验证器，但要利用JSR 303注解类型嵌入约束。JSR 303约束见表：

属性	描述	范例
@AssertFalse	验证 Boolean 对象是否为 false	@AssertFalse boolean hasChildren;
@AssertTrue	验证 Boolean 对象是否为 true	@AssertTrue boolean isEmpty;
@DecimalMax	被标注的值必须不大于约束中指定的最大值. 这个约束的参数是一个通过BigDecimal定义的最大值的字符串表示.小数存在精度	@DecimalMax("1.1") BigDecimal price;
@DecimalMin	被标注的值必须不小于约束中指定的最小值. 这个约束的参数是一个通过BigDecimal定义的最小值的字符串表示.小数存在精度	@DecimalMin("0.04") BigDecimal price;
@Digits(integer=,fraction=)	验证字符串是否是符合指定格式的数字，interger指定整数精度，fraction指定小数精度。	@Digits(integer=5,fraction=2) BigDecimal price;
@Future	验证 Date 和 Calendar 对象是否在当前时间之后	@Future Date shippingDate;
@Max	验证 Number 和 String 对象是否小等于指定的值	@MAX(150) int age;
@Min	验证 Number 和 String 对象是否大等于指定的值	@Min(30) int age;
@NotNull	验证对象是否不为null, 无法查检长度为0的字符串	@NotNull String testName;
@Null	验证对象是否为null	@Null String testString;
@Past	验证 Date 和 Calendar 对象是否在当前时间之前	@Past Date birthDate;
@Pattern	验证 String 对象是否符合正则表达式的规则	@Pattern(regext="\d{3}") String areaCode;
@Size	验证对象（Array,Collection,Map,String）长度是否在给定的范围之内	@Size(min=2,max=140) String description;

更多JSR303定义的校验类型可以参考：使用JSR-303进行校验 @Valid。

一旦了解了JSR 303 validation的使用方法，使用起来会比Spring验证器还要容易。像使用Spring验证器一样，可以在属性文件中以下列格式来使用property键，覆盖来自JSR 303验证器的错误消息：

constraint.object.property

例如，为了覆盖以@Size注解约束的Product对象的name，可以在属性文件中使用下面这个键：

Size.Product.name

为了覆盖以@Past注解约束的Product对象的productionDate属性，可以在属性文件中使用下面这个键：

Past.Product.productionDate

六 JSR 303 Validator范例

jsr303-validator应用展示了JSR 303输入验证的例子。这个例子是对spring-validator进行修改之后的版本，与之前版本有一些区别。首先，它没有ProductValidator类。

其次，我们使用JSR Bean Validation实现是Hiberanate Validator，需要引入以下4个jar包：

• hibernate-validator-6.0.16.Final.jar 
• validation-api-2.1.0.Final.jar
• classmate-1.5.0.jar
• jboss-logging-3.4.0.Final

JSR规范定义的注解类型在validation-api下javax.validation.constraints包下，有兴趣可以自己查看。

下面我们主要给出jsr303-validator应用与spring-validator应用的不同之处，相同部分代码不再重复，可以参考spring-validator应用。

1、目录结构

其中lib库文件如下：

2、Product类

Product类的name和productionDate字段已经用JSR 303注解类型进行了注解：

package domain;
import java.io.Serializable;
import java.math.BigDecimal;
import java.time.LocalDate;

import javax.validation.constraints.Past;
import javax.validation.constraints.Size;

public class Product implements Serializable {
    private static final long serialVersionUID = 78L;

    @Size(min=1, max=10)
    private String name;
    
    private String description;
    private BigDecimal price;
    
    @Past
    private LocalDate productionDate;

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getDescription() {
        return description;
    }
    public void setDescription(String description) {
        this.description = description;
    }
    public BigDecimal getPrice() {
        return price;
    }
    public void setPrice(BigDecimal price) {
        this.price = price;
    }
    public LocalDate getProductionDate() {
        return productionDate;
    }
    public void setProductionDate(LocalDate productionDate) {
        this.productionDate = productionDate;
    }

}

3、ProductController类

在ProductController类的saveProduct()方法中，必须用@Valid对Product参数进行注解：

package controller;

import javax.validation.Valid;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.validation.BindingResult;
import org.springframework.validation.FieldError;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.RequestMapping;

import domain.Product;

@Controller
public class ProductController {

    private static final Log logger = LogFactory
            .getLog(ProductController.class);

    @RequestMapping(value = "/add-product")
    public String inputProduct(Model model) {
        model.addAttribute("product", new Product());
        return "ProductForm";
    }

    @RequestMapping(value = "/save-product")
    public String saveProduct(@Valid @ModelAttribute Product product,
            BindingResult bindingResult, Model model) {

        if (bindingResult.hasErrors()) {
            FieldError fieldError = bindingResult.getFieldError();
            logger.info("Code:" + fieldError.getCode() + ", object:"
                    + fieldError.getObjectName() + ", field:"
                    + fieldError.getField());
            return "ProductForm";
        }

        // save product here

        model.addAttribute("product", product);
        return "ProductDetails";
    }

}

为了定制来自验证器的错误消息，要在messages.properties文件中使用两个键：

typeMismatch.productionDate=Invalid production date
Past.product.productionDate=Production date must be a past date
Size.product.name=Product name's size  must be  between 1 and 10  

4、测试

想要测试jsr303-validator中的验证器，可以在浏览器中打开以下网址：

http://localhost:8008/jsr303-validator/add-product

输入以下内容，并提交，可以看到页面中提示了错误信息：

如果数据输入合法：

参考文章

[1]从源码分析java.lang.String.isEmpty()

[2]SpringMVC介绍之Validation

[3]Spring MVC学习指南

[4]使用JSR-303进行校验 @Valid

[5]JSR 303 - Bean Validation 介绍及最佳实践

[6]spring MVC 使用 hibernate validator验证框架,国际化配置