注解：java 自定义注解应用实例

本例子旨在使用自定义注解为实体打上标记，为自动生成 sql 提供依据，模拟 hibernate 的注解，至于注解的原理自己搜吧

1.定义 Table 注解

package test;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Inherited
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Table {
    String value() default "";
}

2.定义 Column 注解

package test;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Inherited
@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Column {
    String value() default "";
}

3.定义使用注解的实体

package test;


@Table("tb_test")
public class TestDto {

    @Deprecated
    private String tt;

    @Column("_id")
    private String id;

    @Column("username")
    private String name;

    public TestDto(String id, String name) {
        super();
        this.id = id;
        this.name = name;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }


}

4.测试注解

package test;

import java.lang.reflect.Field;
import java.lang.reflect.Method;

public class Test {
    public static void main(String[] args) {
        TestDto testDto = new TestDto("123", "34");
        TestDto testDto1 = new TestDto("123", "test1");
        TestDto testDto2 = new TestDto("", "test1,test2,test3,test4");
        String sql = assembleSqlFromObj(testDto);
        String sql1 = assembleSqlFromObj(testDto1);
        String sql2 = assembleSqlFromObj(testDto2);
        System.out.println(sql);
        System.out.println(sql1);
        System.out.println(sql2);
    }

    /**
     * 通过注解来组装查询条件，生成查询语句
     *
     * @param obj
     * @return
     */
    public static String assembleSqlFromObj(Object obj) {
        Table table = obj.getClass().getAnnotation(Table.class);
        StringBuffer sbSql = new StringBuffer();
        String tableName = table.value();
        sbSql.append("select * from " + tableName + " where 1=1 ");
        Field[] fileds = obj.getClass().getDeclaredFields();
        for (Field f : fileds) {
            String fieldName = f.getName();
            String methodName = "get" + fieldName.substring(0, 1).toUpperCase()
                    + fieldName.substring(1);
            try {
                Column column = f.getAnnotation(Column.class);
                if (column != null) {
                    Method method = obj.getClass().getMethod(methodName);
                    String value = (String) method.invoke(obj);
                    if (value != null && !value.equals("")) {
                        if (!isNum(column.value()) && !isNum(value)) {
                            // 判断参数是不是 in 类型参数 1,2,3
                            if (value.contains(",")) {
                                sbSql.append(" and " + column.value() + " in (" + value + ") ");
                            } else {
                                sbSql.append(" and " + column.value() + " like '%" + value + "%' ");
                            }
                        } else {
                            sbSql.append(" and " + column.value() + "=" + value + " ");
                        }
                    }
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return sbSql.toString();
    }

    /**
     * 检查给定的值是不是 id 类型 1.检查字段名称 2.检查字段值
     *
     * @param target
     * @return
     */
    public static boolean isNum(String target) {
        boolean isNum = false;
        if (target.toLowerCase().contains("id")) {
            isNum = true;
        }
        if (target.matches("\d+")) {
            isNum = true;
        }
        return isNum;
    }
}

测试结果：

select * from tb_test where 1=1  and _id=123  and username=34 
select * from tb_test where 1=1  and _id=123  and username like '%test1%' 
select * from tb_test where 1=1  and username in (test1,test2,test3,test4)