JDK动态代理源码解析

代理模式是个结构性模式，主要是在原基础上添加了一个代理类来控制访问目标类的方式，这样做的好处是双方可以不用直接通信，实现了松耦合的效果使真实对象可以更专心做自己的事（单一职责，最少知识职责）更独立，维护成本更低！！

JDK动态代理是基于接口实现的，有想过为什么要用实现，不用继承吗？？

测试案例

被代理对象实现

public interface Student {
    void running();
}
public class OneStudent implements Student {
    @Override
    public void running() {
        System.out.println("走路");
    }
}

代理拦截方法

public class StudentProxy implements InvocationHandler {
    public OneStudent student;
 
    public StudentProxy(OneStudent student) {
        this.student = student;
    }
 
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("动态代理实现了");
        return method.invoke(student, args); // 实现被代理类的方法
    }
}

 

public class Demo {
    public static void main(String[] args) {
        Student student = (Student)Proxy.newProxyInstance(OneStudent.class.getClassLoader(), OneStudent.class.getInterfaces(), new StudentProxy(new OneStudent()));
        student.running();
    }
}

可以看到实现被代理对象方法时被StudentProxy的Invoke给拦截了

怎么实现的呢？

Proxy.newProxyInstance源码

public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h) {
    // 不重要的省略
    Class<?> cl = getProxyClass0(loader, intfs);
    final Constructor<?> cons = cl.getConstructor(constructorParams);
    // 这里创建被代理类的实例并且传入代理对象
    return cons.newInstance(new Object[]{h});
 }

进去getProxyClass0看看

private static Class<?> getProxyClass0(ClassLoader loader,
                                           Class<?>... interfaces) {
        // 如果设置的代理类缓存有拿到就返回缓存副本，如果没有就ProxyClassFactory去创建
        return proxyClassCache.get(loader, interfaces);
    }

get方法解析

public V get(K key, P parameter) {
        // 不重要的省略。。。
        
        // 解析真实类，将被代理类转换成字节码，通过输出流方式进行修改
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;
 
        // 代理类缓存的处理
        while (true) {
            if (supplier != null) {
                // supplier might be a Factory or a CacheValue<V> instance
                V value = supplier.get();
                if (value != null) {
                    return value;
                }
            }
            // else no supplier in cache
            // or a supplier that returned null (could be a cleared CacheValue
            // or a Factory that wasn't successful in installing the CacheValue)
 
            // lazily construct a Factory
            if (factory == null) {
                factory = new Factory(key, parameter, subKey, valuesMap);
            }
 
            if (supplier == null) {
                supplier = valuesMap.putIfAbsent(subKey, factory);
                if (supplier == null) {
                    // successfully installed Factory
                    supplier = factory;
                }
                // else retry with winning supplier
            } else {
                if (valuesMap.replace(subKey, supplier, factory)) {
                    // successfully replaced
                    // cleared CacheEntry / unsuccessful Factory
                    // with our Factory
                    supplier = factory;
                } else {
                    // retry with current supplier
                    supplier = valuesMap.get(subKey);
                }
            }
        }
    }

subKeyFactory.apply(key, parameter)

 

public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
    
    Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
    // 检查传过来的类加载器是否拥有当前被代理类接口下的所有实现类
    for (Class<?> intf : interfaces) {
        /*
        * Verify that the class loader resolves the name of this
        * interface to the same Class object.
        */
        Class<?> interfaceClass = null;
        try {
            interfaceClass = Class.forName(intf.getName(), false, loader);
        } catch (ClassNotFoundException e) {
        }
        if (interfaceClass != intf) {
            throw new IllegalArgumentException(
                intf + " is not visible from class loader");
        }
        /*
        * Verify that the Class object actually represents an
        * interface.
        */
        if (!interfaceClass.isInterface()) {
            throw new IllegalArgumentException(
                interfaceClass.getName() + " is not an interface");
        }
        /*
        * Verify that this interface is not a duplicate.
        */
        if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
            throw new IllegalArgumentException(
                "repeated interface: " + interfaceClass.getName());
        }
    }
 
    // 代理类的名称xxx$ProxyXX,是唯一不重复的，重复的话num++;
    String proxyName = proxyPkg + proxyClassNamePrefix + num;
    // 修改被代理类的字节码！！返回byte流
    byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
        proxyName, interfaces, accessFlags);
    try {
        // 调用native底层方法将byte流转换成类返回
        return defineClass0(loader, proxyName,
                            proxyClassFile, 0, proxyClassFile.length);
    } catch (ClassFormatError e) {
        /*
        * A ClassFormatError here means that (barring bugs in the
        * proxy class generation code) there was some other
        * invalid aspect of the arguments supplied to the proxy
        * class creation (such as virtual machine limitations
        * exceeded).
        */
        throw new IllegalArgumentException(e.toString());
    }
}

到这里基本主流程就走完了，看下被修改后的被代理类对象

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//
 
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
import 代理模式.Student;
 
// 可以看到为什么JDK动态代理必须实现接口了，因为它有个很重要的属性必须由子类去定义那就是InvocationHandler（接口是完全抽象的它的属性都是常量，不能被子类修改） 
public final class $Proxy0 extends Proxy implements Student {
    // 父类的关键属性
    protected InvocationHandler h;
    private static Method m1;
    private static Method m3;
    private static Method m2;
    private static Method m0;
    
    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m3 = Class.forName("代理模式.Student").getMethod("running");
            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());
        }
    }
 
    public $Proxy0(InvocationHandler var1) throws  {
        super(var1);
    }
 
    public final boolean equals(Object var1) throws  {
        try {
            return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }
 
    // 被代理对象的方法全部委托的致代理对象的invoker方法
    public final void running() throws  {
        try {
            super.h.invoke(this, m3, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }
 
}

总结

通过Proxy的newProxyInstance方法生成代理类，会先查询代理类缓存，如果没有就通过ProxyFactory工厂创建代理对象。创建的代理类的过程是将被代理类的字节码进行修改，让它继承自Proxy类定义Proxy类的InvocationHandler的属性，最后将每个被代理对象的方法委托致InvocationHandler的invoke当中，这样就可以支配这个被代理类的访问方式了。