1.引言

2.Java中的常见注解

JDK自带注解：<br>
@Override @Deprecated @Suppvisewarnings
@override 下面的方法肯定覆盖了父类的方法
@Deprecated 下面的方法表示已经过时
@SuppressWarnings("deprecation") 忽视@Deprecation的警告

3.注解的分类

注解的注解就叫做元注解

4.自定义注解

5.解析注解

RetentionPolicy.SOURCE

RetentionPolicy.CLASS

RetentionPolicy.RUNTIME

6.注解实战

package com.myAnnotation;

@Table("user")
public class Filter {
    
    @Column("id")
    private int id;
    @Column("user_name")
    private String userName;
    @Column("nick_name")
    private String nickName;
    @Column("age")
    private int age;
    @Column("city")
    private String city;
    @Column("email")
    private String email;
    @Column("mobile")
    private String mobile;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getUserName() {
        return userName;
    }
    public void setUserName(String name) {
        this.userName = name;
    }
    public String getNickName() {
        return nickName;
    }
    public void setNickName(String nickName) {
        this.nickName = nickName;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getCity() {
        return city;
    }
    public void setCity(String city) {
        this.city = city;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }
    public String getMobile() {
        return mobile;
    }
    public void setMobile(String mobile) {
        this.mobile = mobile;
    }
    @Override
    public String toString() {
        return "Filter [id=" + id + ", name=" + userName + ", nickName=" + nickName
                + ", age=" + age + ", city=" + city + ", email=" + email
                + ", mobile=" + mobile + "]";
    }
    
    
}

package com.myAnnotation;

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

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

package com.myAnnotation;

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

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

package com.myAnnotation;

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

public class Test {
    
    public static void main(String[] args) {
        
        Filter f1 = new Filter();
        
        f1.setId(10);//查询id为10的用户
        
        Filter f2 = new Filter();
        
        f2.setUserName("lucy");//模糊查询用户名为lucy的用户
    
        Filter f3 = new Filter();
        
        //查询邮箱为其中任意一个的
        f3.setEmail("liu@sina.com,zh@163.com,2222@qq.com");
    
    
        String sql1 = query(f1);
        
        String sql2 = query(f2);
        
        String sql3 = query(f3);
        
        System.out.println(sql1);
        System.out.println(sql2);
        System.out.println(sql3);
    }
    
    //查询语句 可以用于各种语句
    @SuppressWarnings("unchecked")
    private static String query(Filter f){
        
        StringBuilder sb = new StringBuilder();
        //1.获取到class
        @SuppressWarnings("rawtypes")
        Class c = f.getClass();
        //2.获取到table的名字
        boolean texists = c.isAnnotationPresent(Table.class);
    
        if(!texists){
            
            return null;
        }
        
        Table t = (Table)c.getAnnotation(Table.class);
        
        String tableName = t.value();
        
        sb.append("SELECT * FROM ").append(tableName).append(" WHERE 1=1");
        
        //3.遍历所有的字段
        Field[] fArray = c.getDeclaredFields();
        
        for(Field field:fArray){
            //4.处理每个字段对应的sql
            //4.1拿到字段名
            boolean cexists = field.isAnnotationPresent(Column.class);
            if(!cexists){
                continue;
            }
            Column column = field.getAnnotation(Column.class);
            String columnName = column.value();
            //4.2拿到字段值
            String fieldName = field.getName();
            String getMethodName = "get"+ fieldName.substring(0,1).toUpperCase()+fieldName.substring(1);
            
            Object fieldValue = null;
            
            try {

                Method getMethod = c.getMethod(getMethodName);

                fieldValue = getMethod.invoke(f);
                
            } catch (NoSuchMethodException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (SecurityException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            catch (IllegalAccessException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (IllegalArgumentException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (InvocationTargetException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            //4.3拼装sql
            if(fieldValue==null||(fieldValue instanceof Integer)&&((Integer)fieldValue==0)){
                
                continue;
            }
            
            sb.append(" AND ").append(columnName);
            
            if(fieldValue instanceof String){
                
                if(((String)fieldValue).contains(",")){
                    
                    String[] values = ((String)fieldValue).split(",");
                    
                    sb.append(" in(");
                    
                    for(String value:values){
                        
                        sb.append("'").append(value).append("'").append(",");
                        
                    }
                    sb.deleteCharAt(sb.length() - 1);
                    
                    sb.append(")");
                    
                }else{
                    
                    sb.append(" = ").append("'").append(fieldValue).append("'");
                    
                }
                
            }else if(fieldValue instanceof Integer){
                
                sb.append(" = ").append(fieldValue);
            }
            
        }
        
        return sb.toString();
    }
}