java注解实例-反射生成sql

定义描述用户表的注解：

package dao;

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

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

定义描述用户属性的注解：

 package dao;

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

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

定义映射Bean类User：

 package dao;

 /**
  *
  * 用户表，字段包括：用户ID、用户名、昵称、年龄、性别、所在城市、邮箱、手机号：
  *
  * @author  */
 @Table("user")
 public class User {
     @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 userName) {
         this.userName = userName;
     }

     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;
     }
 }

根据参数动态返回查询语句：

 package dao;

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

 /**
  * 根据参数动态返回查询语句
  *
  * @author  */
 public class ReturnQuery {

     public static String query(User u1) {
         StringBuilder str = new StringBuilder();
         // 1.获取一个类class
         Class c = u1.getClass();
         // 2.获取Table的名字
         boolean exists = c.isAnnotationPresent(Table.class);
         if (!exists) {
             return null;
         }
         Table t = (Table) c.getAnnotation(Table.class);
         String tableName = t.value();
         str.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取到字段名
             boolean fExists = field.isAnnotationPresent(Column.class);// 判断是否包含Column类型的注解
             if (!fExists) {
                 continue;
             }
             Column column = field.getAnnotation(Column.class);
             String columnName = column.value();
             // 4.2取到字段的值
             String fieldName = field.getName();
             // 获取相应字段的getXXX()方法
             String getMethodName = "get" + fieldName.substring(0, 1).toUpperCase() + fieldName.substring(1);
             Object fieldValue=null;
             try {
                 Method getMethod = c.getMethod(getMethodName);
                 fieldValue = getMethod.invoke(u1);
             } catch (Exception e) {
                 e.printStackTrace();
             }
             //4.3拼接Sql
             if (fieldValue==null||fieldValue instanceof Integer &&(Integer)fieldValue==0) {
                 continue;
             }
             str.append(" and ").append(fieldName);
             if (fieldValue instanceof String) {
                 if (((String) fieldValue).contains(",")) {
                     String[] values=((String) fieldValue).split(",");
                     str.append(" in (");
                     for (String s : values) {
                         str.append("'").append(s).append("'").append(",");
                     }
                     str.deleteCharAt(str.length()-1);
                     str.append(")");
                 }else{
                     str.append("=").append("'").append(fieldValue).append("' ");
                 }
             }else {
                 str.append("=").append(fieldValue);
             }
         }
         return str.toString();
     }

 }

测试类：

 package dao;

 public class Test {
     public static void main(String[] args) {
         User u1 = new User();
         u1.setId(10); // 查询id

         User u2 = new User();
         u2.setUserName("JSFei"); // 模糊查询用户名
         u2.setAge(21);

         User u3 = new User();
         u3.setEmail("123@163.com,123@qq.com"); // 查询邮箱有任意一个的用户

         String sql1 = ReturnQuery.query(u1);
         String sql2 = ReturnQuery.query(u2);
         String sql3 = ReturnQuery.query(u3);

         System.out.println(sql1);
         System.out.println(sql2);
         System.out.println(sql3);
     }
 }

输出结果：

...............................................