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 8  */
@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
 * 
 * @author10  */
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：

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