通过反射与注解拼装sql查询的小例子

package com.imooc.ann;

import java.lang.annotation.ElementType;

import java.lang.annotation.Retention;

import java.lang.annotation.RetentionPolicy;

import java.lang.annotation.Target;

import java.lang.reflect.Field;

import java.lang.reflect.Method;

public class Tests {

public static void main(String[] args) {

Users u1 = new Users();

u1.setUserName("张三");

Users u2 = new Users();

u2.setId(4);

u2.setAddress("济南");

String sql1 = query(u1);

String sql2 = query(u2);

//输出sql查询语句

System.out.println(sql1);

System.out.println(sql2);

}

@SuppressWarnings({ "unchecked", "rawtypes" })

public static String query(Object obj){

//字符串缓冲区

StringBuilder sb = new StringBuilder();

Class u = obj.getClass();

//拼装sql语句

boolean isExist = u.isAnnotationPresent(Table.class);

if(isExist){

sb.append("select * from ");

//得到表名

Table t = (Table)u.getAnnotation(Table.class);

sb.append(t.value()+" where 1=1 ");

}

//得到字段名和值

Field[] f = u.getDeclaredFields();

for (Field field : f) {

boolean fExists = field.isAnnotationPresent(Column.class);

Column c = null;

if(fExists){

//拿到字段

c = (Column)field.getAnnotation(Column.class);

}

//拿到字段值

String fieldName = field.getName();

Object methodValue =null;

try {

//拼装方法名

String methodName = "get"+fieldName.substring(0,1).toUpperCase()+fieldName.substring(1);

//通过反射获取方法

Method m = u.getMethod(methodName);

//通过反射获取方法内的值

methodValue = (Object)m.invoke(obj);

} catch (Exception e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

//判断值是否为空或如果是Integer类型数据值是否为0

if(methodValue==null||(methodValue instanceof Integer&&(Integer)methodValue==0)){

continue;

}

//添加字段名

sb.append(" and "+c.value());

//添加字段值

if(methodValue instanceof Integer){

sb.append(" = "+methodValue);

}

if(methodValue instanceof String){

sb.append(" = '"+methodValue+"'");

}

}

return sb.toString();

}

}

//自定义注解

@Target({ElementType.TYPE})

@Retention(RetentionPolicy.RUNTIME)

@interface Table{

String value();

}

@Target({ElementType.FIELD})

@Retention(RetentionPolicy.RUNTIME)

@interface Column{

String value();

}

//关联数据库表

@Table("users")

class Users{

@Column("id")

private Integer id;

@Column("user_name")

private String userName;

@Column("email")

private String email;

@Column("address")

private String address;

public Integer getId() {

return id;

}

public void setId(Integer id) {

this.id = id;

}

public String getUserName() {

return userName;

}

public void setUserName(String userName) {

this.userName = userName;

}

public String getEmail() {

return email;

}

public void setEmail(String email) {

this.email = email;

}

public String getAddress() {

return address;

}

public void setAddress(String address) {

this.address = address;

}

}