源码之：java动态代理

源码系列

关于代理模式这里就不再详述了。

在编写简易的RPC以及AOP框架过程中，涉及到了使用java动态代理（Proxy）。
我们来首先来看java动态代理的简易demo：
1.定义接口Subject

package com.lxy.dynamicproxy;
/**
 * Created by coldxiangyu on 2017/6/15.
 */
public interface Subject {
    public void request();
}

2.定义真实角色类

package com.lxy.dynamicproxy;
/**
 * Created by coldxiangyu on 2017/6/15.
 */
public class RealSubject implements Subject {
    @Override
    public void request() {
       System.out.println("this is real subject");
    }
}

3.定义代理类DynamicSubject实现InvocationHandler接口，重写invoke方法：

import java.lang.reflect.Method;
/**
 * Created by coldxiangyu on 2017/6/15.
 */
public class DynamicSubject implements InvocationHandler{
    private Object obj;
    public DynamicSubject(Object obj){
        this.obj = obj;
    }
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("before calling:" + method);
        System.out.println("before calling:" + method);
        return method.invoke(obj, args);
    }
}

4.客户端通过Proxy.newProxyInstance调用：

package com.lxy.dynamicproxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Proxy;
/**
 * Created by coldxiangyu on 2017/6/15.
 */
public class Client {
    public static void main(String[] args){
        Subject rs = new RealSubject();
        InvocationHandler ds = new DynamicSubject(rs);
        Class cls = rs.getClass();
        Subject subject = (Subject) Proxy.newProxyInstance(cls.getClassLoader(),cls.getInterfaces(),ds);
        subject.request();
    }
}

调用结果：

before calling:public abstract void com.lxy.dynamicproxy.Subject.request()
before calling:public abstract void com.lxy.dynamicproxy.Subject.request()
this is real subject
进程已结束,退出代码0

我们来看一下源码（我本地JDK1.8）是怎样的，一个是InvocationHandler接口，另一个就是Proxy类。
InvocationHandler接口比较简单，只定义了一个返回类型是Object的方法：public Object invoke(Object proxy, Method method, Object[] args)，那么这个方法必然是在Proxy类中存在调用的，带着这个猜测我们去看Proxy类：
Proxy类就复杂多了，我们挑主要的地方看，首先看我们调用的Proxy.newProxyInstance方法：

public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) throws IllegalArgumentException {
        //Objects.requireNonNull 判空方法，之后所有的单纯的判断null并抛异常，都是此方法
        Objects.requireNonNull(h);
        //clone 类实现的所有接口
        final Class<?>[] intfs = interfaces.clone();
        //获取当前系统安全接口
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            //Reflection.getCallerClass返回调用该方法的方法的调用类;loader：接口的类加载器
            //进行包访问权限、类加载器权限等检查
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }
        /*
         * Look up or generate the designated proxy class.
         *  查找或生成代理类
         */
        Class<?> cl = getProxyClass0(loader, intfs);
        /*
         * Invoke its constructor with the designated invocation handler.
         * 使用指定的调用处理程序调用它的构造函数
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }
            //获取构造
            final Constructor<?> cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            if (!Modifier.isPublic(cl.getModifiers())) {
                AccessController.doPrivileged(new PrivilegedAction<Void>() {
                    public Void run() {
                        cons.setAccessible(true);
                        return null;
                    }
                });
            }
            //返回 代理对象
            return cons.newInstance(new Object[]{h});
        } catch (IllegalAccessException|InstantiationException e) {
            throw new InternalError(e.toString(), e);
        } catch (InvocationTargetException e) {
            Throwable t = e.getCause();
            if (t instanceof RuntimeException) {
                throw (RuntimeException) t;
            } else {
                throw new InternalError(t.toString(), t);
            }
        } catch (NoSuchMethodException e) {
            throw new InternalError(e.toString(), e);
        }
    }

从上面的源码可以看出，newProxyInstance主要通过getProxyClass0(loader, intfs)方法生成代理类，并通过getConstructor(constructorParams)获取构造方法，最后通过cons.newInstance(new Object[]{h})返回代理类对象。
我们再来看看getProxyClass0是如何获取代理类的：

/**
     * a cache of proxy classes：动态代理类的弱缓存容器
     * KeyFactory:根据接口的数量，映射一个最佳的key生成函数,其中表示接口的类对象被弱引用；也就是key对象被弱引用继承自WeakReference（key0、key1、key2、keyX），保存接口密钥(hash值)
     * ProxyClassFactory：生成动态类的工厂
     * 注意，两个都实现了BiFunction<ClassLoader, Class<?>[], Object>接口
     */
    private static final WeakCache<ClassLoader, Class<?>[], Class<?>> proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
    /**
     * Generate a proxy class.  Must call the checkProxyAccess method
     * to perform permission checks before calling this.
     * 生成代理类，调用前必须进行 checkProxyAccess权限检查，所以newProxyInstance进行了权限检查
     */
    private static Class<?> getProxyClass0(ClassLoader loader, Class<?>... interfaces) {
        //实现接口的最大数量<65535;
        if (interfaces.length > 65535) {
            throw new IllegalArgumentException("interface limit exceeded");
        }
        // If the proxy class defined by the given loader implementing
        // the given interfaces exists, this will simply return the cached copy;
        // otherwise, it will create the proxy class via the ProxyClassFactory
        // 如果缓存中有，就直接返回，否则会生成
        return proxyClassCache.get(loader, interfaces);
    }

这个方法并没有太多有效信息，我们继续跳转proxyClassCache.get(loader, interfaces)方法：

public V get(K key, P parameter) {
        //key：类加载器；parameter：接口数组
        Objects.requireNonNull(parameter);
        //清除已经被GC回收的弱引用
        expungeStaleEntries();
        //CacheKey弱引用类，refQueue已经被回收的弱引用队列；构建一个CacheKey
        Object cacheKey = CacheKey.valueOf(key, refQueue);
        //map一级缓存，获取valuesMap二级缓存
        ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
        if (valuesMap == null) {
            ConcurrentMap<Object, Supplier<V>> oldValuesMap
                    = map.putIfAbsent(cacheKey,
                    valuesMap = new ConcurrentHashMap<>());
            if (oldValuesMap != null) {
                valuesMap = oldValuesMap;
            }
        }
        // subKeyFactory类型是KeyFactory，apply返回表示接口的key
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        //Factory 实现了supplier，我们实际是获取缓存中的Factory，调用其get方法
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;
        //下面用到了 CAS+重试 实现的多线程安全的 非阻塞算法
        while (true) {
            if (supplier != null) {
                // 只需要知道，最终会调用get方法，此supplier可能是缓存中取出来的，也可能是Factory新new出来的
                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);
                }
            }
        }
    }

我们看到这个方法的返回值是通过supplier.get()获取的，这个方法调用ProxyClassFactory的apply()方法：

public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
        Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
        for (Class<?> intf : interfaces) {
                /*
                 * Verify that the class loader resolves the name of this interface to the same Class object.
                 * 类加载器和接口名解析出的是同一个
                 */
            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.
                 * 确保是一个接口
                 */
            if (!interfaceClass.isInterface()) {
                throw new IllegalArgumentException( interfaceClass.getName() + " is not an interface");
            }
                /*
                 * Verify that this interface is not a duplicate.
                 * 确保接口没重复
                 */
            if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
                throw new IllegalArgumentException( "repeated interface: " + interfaceClass.getName());
            }
        }
        String proxyPkg = null;     // package to define proxy class in
        int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
            /*
             * Record the package of a non-public proxy interface so that the proxy class will be defined in the same package.
             * Verify that all non-public proxy interfaces are in the same package.
             * 验证所有非公共的接口在同一个包内；公共的就无需处理
             */
        for (Class<?> intf : interfaces) {
            int flags = intf.getModifiers();
            if (!Modifier.isPublic(flags)) {
                accessFlags = Modifier.FINAL;
                String name = intf.getName();
                int n = name.lastIndexOf('.');
                String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
                if (proxyPkg == null) {
                    proxyPkg = pkg;
                } else if (!pkg.equals(proxyPkg)) {
                    throw new IllegalArgumentException(  "non-public interfaces from different packages");
                }
            }
        }
        if (proxyPkg == null) {
            // if no non-public proxy interfaces, use com.sun.proxy package
            proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
        }
            /*
             * Choose a name for the proxy class to generate.
             * proxyClassNamePrefix = $Proxy
             * nextUniqueNumber 是一个原子类，确保多线程安全，防止类名重复，类似于：$Proxy0，$Proxy1......
             */
        long num = nextUniqueNumber.getAndIncrement();
        String proxyName = proxyPkg + proxyClassNamePrefix + num;
            /*
             * Generate the specified proxy class.
             * 生成类字节码的方法:重点
             */
        byte[] proxyClassFile = ProxyGenerator.generateProxyClass( proxyName, interfaces, accessFlags);
        try {
            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());
        }
    }

由此ProxyGenerator.generateProxyClass( proxyName, interfaces, accessFlags)，生成类的字节码：

public static byte[] generateProxyClass(final String name, Class<?>[] interfaces, int accessFlags) {
        ProxyGenerator gen = new ProxyGenerator(name, interfaces, accessFlags);
        //真正生成字节码的方法
        final byte[] classFile = gen.generateClassFile();
        //如果saveGeneratedFiles为true 则生成字节码文件，所以在开始我们要设置这个参数
        //当然，也可以通过返回的bytes自己输出
        if (saveGeneratedFiles) {
            java.security.AccessController.doPrivileged( new java.security.PrivilegedAction<Void>() {
                        public Void run() {
                            try {
                                int i = name.lastIndexOf('.');
                                Path path;
                                if (i > 0) {
                                    Path dir = Paths.get(name.substring(0, i).replace('.', File.separatorChar));
                                    Files.createDirectories(dir);
                                    path = dir.resolve(name.substring(i+1, name.length()) + ".class");
                                } else {
                                    path = Paths.get(name + ".class");
                                }
                                Files.write(path, classFile);
                                return null;
                            } catch (IOException e) {
                                throw new InternalError( "I/O exception saving generated file: " + e);
                            }
                        }
                    });
        }
        return classFile;
    }

我们看到 final byte[] classFile = gen.generateClassFile();是真正获取字节码的方法。限于篇幅，这里就不贴如何生成字节码类文件了，有兴趣的可以自行去看。我们真正关心的是生成的字节码文件是怎样的，我们将字节码类文件反编译查看一下：

final class $Proxy0 extends Proxy implements pro {
        //fields    
        private static Method m1;
        private static Method m2;
        private static Method m3;
        private static Method m0;
        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})).booleanValue();
            } 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 void text() throws  {
            try {
                //实际就是调用代理类的invoke方法 
                super.h.invoke(this, m3, (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)).intValue();
            } catch (RuntimeException | Error var2) {
                throw var2;
            } catch (Throwable var3) {
                throw new UndeclaredThrowableException(var3);
            }
        }
        static {
            try {
                //这里每个方法对象 和类的实际方法绑定
                m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[]{Class.forName("java.lang.Object")});
                m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
                m3 = Class.forName("spring.commons.api.study.CreateModel.pro").getMethod("text", new Class[0]);
                m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
            } catch (NoSuchMethodException var2) {
                throw new NoSuchMethodError(var2.getMessage());
            } catch (ClassNotFoundException var3) {
                throw new NoClassDefFoundError(var3.getMessage());
            }
        }
    }

这时候，我们就看得很清楚了，每个方法的实现其实都是通过调用InvocationHandler的invoke方法实现的。
这时候总结一下Proxy.newProxyInstance(cls.getClassLoader(),cls.getInterfaces(),ds)的实现过程就是首先通过classLoader和interfaces生成字节码代理类，再进行重新构造，生成代理对象的过程。