mybatis的懒加载是用到了javassist的动态代理，所以想先简单说一下这个，顺便带上cglib动态代理。

javassist动态代理

这里我用的依赖

<dependency>
    <groupId>org.javassist</groupId>
    <artifactId>javassist</artifactId>
    <version>3.21.0-GA</version>
</dependency>

测试代码：

package lnstark.test.testjavassistproxy;

import javassist.NotFoundException;
import javassist.util.proxy.MethodHandler;
import javassist.util.proxy.ProxyFactory;

import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

/**
 * by Lnstark
 * 2021/12/18
 */
public class MyProxyFactory implements MethodHandler {

    private Object target;

    public MyProxyFactory(Object target) {
        this.target = target;
    }

    /**
     * 增强方法
     *
     * @param self 代理后的类
     * @param thisMethod 代理类的tweet方法
     * @param proceed 代理类的_d8tweet方法
     * @param args 方法参数
     * @return 执行结果
     */
    @Override
    public Object invoke(Object self, Method thisMethod, Method proceed, Object[] args) throws Throwable {
        System.out.println("before invoke");
        // 还记得jdk动态代理这里的传参会是target
        Object ret = proceed.invoke(self, args);
        System.out.println("after invoke");
        return ret;
    }

    static class Bird {
        private String name;
        public Bird(String name) {
            this.name = name;
        }
        void tweet() {
            System.out.println("my name is: " + name);
        }
    }

    public static void main(String[] args) throws IOException, NotFoundException {
        Bird bird = new Bird("百灵");
        Bird proxy = null;
        ProxyFactory enhancer = new ProxyFactory();
        // 设置代理类输出路径，实际的目录还要带上包名
        // 如此类的包名为lnstark.test.testjavassistproxy
        // 那生成的.class文件所在目录为 d:/lnstark/test/testjavassistproxy
        enhancer.writeDirectory = "d:/";
        // 需要代理的类
        enhancer.setSuperclass(Bird.class);
        try {
            // 生成代理对象
            proxy = (Bird) enhancer.create(new Class[]{String.class}, new Object[]{"布谷"}, new MyProxyFactory(bird));
        } catch (NoSuchMethodException | InstantiationException
                | IllegalAccessException | InvocationTargetException e) {
            e.printStackTrace();
        }
        
        proxy.tweet();
    }
}

执行结果：

before invoke
my name is: 布谷
after invoke

编译后的动态代理.class文件丢到idea里：

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

package lnstark.test.testjavassistproxy;

import java.io.ObjectStreamException;
import java.lang.reflect.Method;
import javassist.util.proxy.MethodHandler;
import javassist.util.proxy.ProxyObject;
import javassist.util.proxy.RuntimeSupport;
import lnstark.test.testjavassistproxy.MyProxyFactory.Bird;

public class MyProxyFactory$Bird_$$_jvstae2_0 extends Bird implements ProxyObject {
    private MethodHandler handler;
    public static byte[] _filter_signature;
    public static final long serialVersionUID;
    private static Method[] _methods_;

    public MyProxyFactory$Bird_$$_jvstae2_0(String var1) {
        this.handler = RuntimeSupport.default_interceptor;
        super(var1);
    }

    public final Object _d0clone() throws CloneNotSupportedException {
        return super.clone();
    }

    protected final Object clone() throws CloneNotSupportedException {
        Method[] var1 = _methods_;
        return (Object)this.handler.invoke(this, var1[0], var1[1], new Object[0]);
    }

    public final boolean _d1equals(Object var1) {
        return super.equals(var1);
    }

    public final boolean equals(Object var1) {
        Method[] var2 = _methods_;
        return (Boolean)this.handler.invoke(this, var2[2], var2[3], new Object[]{var1});
    }

    public final void _d2finalize() throws Throwable {
        super.finalize();
    }

    protected final void finalize() throws Throwable {
        Method[] var1 = _methods_;
        this.handler.invoke(this, var1[4], var1[5], new Object[0]);
    }

    public final int _d4hashCode() {
        return super.hashCode();
    }

    public final int hashCode() {
        Method[] var1 = _methods_;
        return (Integer)this.handler.invoke(this, var1[8], var1[9], new Object[0]);
    }

    public final String _d7toString() {
        return super.toString();
    }

    public final String toString() {
        Method[] var1 = _methods_;
        return (String)this.handler.invoke(this, var1[14], var1[15], new Object[0]);
    }

    public final void _d8tweet() {
        super.tweet();
    }

    final void tweet() {
        Method[] var1 = _methods_;
        this.handler.invoke(this, var1[16], var1[17], new Object[0]);
    }

    static {
        Method[] var0 = new Method[24];
        Class var1 = Class.forName("lnstark.test.testjavassistproxy.MyProxyFactory$Bird_$$_jvstae2_0");
        RuntimeSupport.find2Methods(var1, "clone", "_d0clone", 0, "()Ljava/lang/Object;", var0);
        RuntimeSupport.find2Methods(var1, "equals", "_d1equals", 2, "(Ljava/lang/Object;)Z", var0);
        RuntimeSupport.find2Methods(var1, "finalize", "_d2finalize", 4, "()V", var0);
        RuntimeSupport.find2Methods(var1, "hashCode", "_d4hashCode", 8, "()I", var0);
        RuntimeSupport.find2Methods(var1, "toString", "_d7toString", 14, "()Ljava/lang/String;", var0);
        RuntimeSupport.find2Methods(var1, "tweet", "_d8tweet", 16, "()V", var0);
        _methods_ = var0;
        serialVersionUID = -1L;
    }

    public void setHandler(MethodHandler var1) {
        this.handler = var1;
    }

    public MethodHandler getHandler() {
        return this.handler;
    }

    Object writeReplace() throws ObjectStreamException {
        return RuntimeSupport.makeSerializedProxy(this);
    }
}

find2Methods方法：

public static void find2Methods(Class clazz, String superMethod,
                                String thisMethod, int index,
                                String desc, java.lang.reflect.Method[] methods)
{
    methods[index + 1] = thisMethod == null ? null
                                            : findMethod(clazz, thisMethod, desc);
    methods[index] = findSuperClassMethod(clazz, superMethod, desc);
}

可以看出tweet是_methods_[16]，_d8tweet是_methods_[17]。
代理类调用tweet方法，里面调用handler.invoke（即我们实现的方法）。
传入参数：代理类本身，tweet方法，_d8tweet方法和方法参数。

cglib动态代理

引入包：

<dependency>
    <groupId>cglib</groupId>
    <artifactId>cglib</artifactId>
    <version>3.3.0</version>
</dependency>

测试代码：

package lnstark.test.testcglibproxy;

import net.sf.cglib.core.DebuggingClassWriter;
import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;

import java.lang.reflect.Method;

/**
 * by Lnstark
 * 2021/12/19
 */
public class CglibProxy {

    static class Bird {
        private String name;
        public Bird(String name) {
            this.name = name;
        }
        void tweet() {
            System.out.println("my name is: " + name);
        }
    }
    static class MyInterceptor implements MethodInterceptor {

        /**
         *
         * @param o 代理后的对象
         * @param method Bird的原方法
         * @param objects 方法参数
         * @param methodProxy
         * @return 方法返回
         */
        @Override
        public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {

            System.out.println("before invoke " + method.getName());
            // 这里用methodProxy而不是原始方法method
            Object ret = methodProxy.invokeSuper(o, objects);
            System.out.println("after invoke " + method.getName());
            return ret;
        }
    }

    public static void main(String[] args) {
        Bird bird = new Bird("百灵");
        // 指定生成的.class文件目录
        System.setProperty(DebuggingClassWriter.DEBUG_LOCATION_PROPERTY, "D:/");
        // 生成的步骤类似javassist
        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(Bird.class);
        enhancer.setCallback(new MyInterceptor());
        Bird proxy = (Bird) enhancer.create(new Class[]{String.class}, new Object[]{"布谷"});
        proxy.tweet();
    }
}

结果：

before invoke tweet
my name is: 布谷
after invoke tweet

代理类里会生成下面两个方法，tweet方法里调用我们实现的intercept方法，里面执行了methodProxy.invokeSuper(o, objects);底层是调用的这里的CGLIB$tweet$0方法。

final void CGLIB$tweet$0() {
    super.tweet();
}

final void tweet() {
    MethodInterceptor var10000 = this.CGLIB$CALLBACK_0;
    if (var10000 == null) {
        CGLIB$BIND_CALLBACKS(this);
        var10000 = this.CGLIB$CALLBACK_0;
    }

    if (var10000 != null) {
        var10000.intercept(this, CGLIB$tweet$0$Method, CGLIB$emptyArgs, CGLIB$tweet$0$Proxy);
    } else {
        super.tweet();
    }
}