探索Mybatis之JDK动态代理：探究Proxy.newProxyInstance()生成的代理类解析

Mybatis的Mapper接口UserMapper

package com.safin.Mapper;

import com.safin.Pojo.User;

import java.util.List;

public interface UserMapper {
    int insertUser(User user);
    User getUser(Long id);
    List<User> findUser(String userName);
}

我使用的jdk版本是12的，在java.lang.reflect包下的ProxyGenerator是生成代理类的工具，这是用来生成运行时代理类（$proxy为前缀）。注意其中一个属性，这是用来保存那些构建的代理类的开关saveGeneratedFiles，其默认赋值是false，当saveGeneratedFiles的值为true时，那些运行时生成的代理类将会以.class文件保存下来。

/** debugging flag for saving generated class files */
    private static final boolean saveGeneratedFiles =
        java.security.AccessController.doPrivileged(
            new GetBooleanAction(
                "jdk.proxy.ProxyGenerator.saveGeneratedFiles")).booleanValue();

在测试主函数开始第一句先添加如下代码，把saveGeneratedFiles的开关打开。jdk12中的"jdk.proxy.ProxyGenerator.saveGeneratedFiles"这个值与jdk8中的稍微不同，在jdk8的这个值是"sun.misc.ProxyGenerator.saveGeneratedFiles"。说起misc，他的全称应该是minimal instruction set computer，最小指令集计算机是一种处理器体系结构，具有非常少量的基本操作和相应的操作码。

还有jdk8与jdk12的Proxy的实现发生了很大的变化，以后有时间再去研究研究。

System.getProperties().put("jdk.proxy.ProxyGenerator.saveGeneratedFiles", "true");

测试的Main函数如下：

public static void main(String[] args) throws IOException {
        SqlSession sqlSession = null;
        System.getProperties().put("jdk.proxy.ProxyGenerator.saveGeneratedFiles", "true"); // 打开保存生成的代理类

        try {
            sqlSession = SqlSessionFactoryUtil.openSqlSession();
            UserMapper userMapper = (UserMapper)sqlSession.getMapper(UserMapper.class);
            User user = null;
            user = userMapper.getUser(30L);
            sqlSession.commit();
        } catch (Exception var9) {
            System.err.println(var9.getMessage());
            sqlSession.rollback();
        } finally {
            if (sqlSession != null) {
                sqlSession.close();
            }

        }

    }

运行后保存了许多代理类，找出与Mapper有关的的代理类$proxy，代理类经过IDEA的反编译后，代码如下：

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package com.sun.proxy;

import com.safin.Mapper.UserMapper;
import com.safin.Pojo.User;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
import java.util.List;

public final class $Proxy19 extends Proxy implements UserMapper {
    private static Method m1;
    private static Method m3;
    private static Method m4;
    private static Method m5;
    private static Method m2;
    private static Method m0;
　　 
    public $Proxy19(InvocationHandler var1) throws  {
        super(var1); 　　// 传入构造方法的参数是实现了InvocationHandler接口的MapperProxy
    }

    public final boolean equals(Object var1) throws  {
        try {
            return (Boolean)super.h.invoke(this, m1, new Object[]{var1}); // 从InvocationHandler的invoke方法
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final User getUser(Long var1) throws  {
        try {
            return (User)super.h.invoke(this, m3, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final int insertUser(User var1) throws  {
        try {
            return (Integer)super.h.invoke(this, m4, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final List findUser(String var1) throws  {
        try {
            return (List)super.h.invoke(this, m5, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final String toString() throws  {
        try {
            return (String)super.h.invoke(this, m2, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final int hashCode() throws  {
        try {
            return (Integer)super.h.invoke(this, m0, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    static { // 拿到了反射API的Method
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m3 = Class.forName("com.safin.Mapper.UserMapper").getMethod("getUser", Class.forName("java.lang.Long"));
            m4 = Class.forName("com.safin.Mapper.UserMapper").getMethod("insertUser", Class.forName("com.safin.Pojo.User"));
            m5 = Class.forName("com.safin.Mapper.UserMapper").getMethod("findUser", Class.forName("java.lang.String"));
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

可以看见生成的代理类 $Proxy19 继承了 Proxy 类，实现了Mybatis的 UserMapper 接口，调用InvocationHandler的invoke方法来利用MapperMethod来对sqlSession的操作。

如果上面的main方法配置行不通，还有一种是通过配置JVM参数 -Dsun.misc.ProxyGenerator.saveGeneratedFiles=true 来把JDK生成的代理类持久化到本地。