Java动态代理原理

分析版本： JDK1.8

代理类在程序运行时创建的代理方式被成为动态代理

机制

• 需要代理的接口

  interfaces for the proxy class to implement

• 需要代理的具体实现类

  实现于 需要代理的接口

• 代理类 $proxy

  在运行的时候动态生成的类

• 代理分发 InvocationHandler

  the invocation handler to dispatch method invocations to

步骤

• 通过实现 InvocationHandler 接口创建自己的调用处理器
• 通过为 Proxy 类指定 ClassLoader 对象和一组 interface 来创建动态代理类
• 通过反射机制获得动态代理类的构造函数，其唯一参数类型是调用处理器接口类型
• 通过构造函数创建动态代理类实例，构造时调用处理器对象作为参数被传入

// InvocationHandlerImpl 实现了 InvocationHandler 接口，并能实现方法调用从代理类到委托类的分派转发
// 其内部通常包含指向委托类实例的引用，用于真正执行分派转发过来的方法调用
InvocationHandler handler = new InvocationHandlerImpl(..); 
 
// 通过 Proxy 为包括 Interface 接口在内的一组接口动态创建代理类的类对象
Class clazz = Proxy.getProxyClass(classLoader, new Class[] { Interface.class, ... }); 
 
// 通过反射从生成的类对象获得构造函数对象
Constructor constructor = clazz.getConstructor(new Class[] { InvocationHandler.class }); 
 
// 通过构造函数对象创建动态代理类实例
Interface Proxy = (Interface)constructor.newInstance(new Object[] { handler });

简化一下就是

InvocationHandler handler = new InvocationHandlerImpl(..); 
 
// 通过 Proxy 直接创建动态代理类实例
Interface proxy = (Interface)Proxy.newProxyInstance( classLoader, 
     new Class[] { Interface.class }, 
     handler );

实现

需要代理的接口

public interface IInterface {
    void function();
}

需要代理的具体实现类

public class Impl implements IInterface {
    public void function() {
        System.out.println("function method invoke");
    }
}

代理分发

public class InvocationHandlerImpl implements InvocationHandler {
    // delegate 为委托类对象;
    private Object delegate;

    public InvocationHandlerImpl(Object delegate) {
        this.delegate = delegate;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("before");
        Object result = method.invoke(delegate, args);
        System.out.println("after");
        return result;
    }
}

Main

public class ProxyMain {
    public static void main(String[] args) {
        InvocationHandlerImpl invocationHandlerImpl = new InvocationHandlerImpl(new Impl());
        IInterface iInterface = (IInterface) (Proxy.newProxyInstance(
                IInterface.class.getClassLoader(),
                new Class[]{IInterface.class},
                invocationHandlerImpl));
        System.out.println(iInterface.getClass().getName()); // 打印代理类名字
        iInterface.function();
    }
}

输出

com.sun.proxy.$Proxy0
before
function method invoke
after

• 输出的代理类类名为 $Proxy0
• 当调用 iInterface.function(); 时， InvocationHandler 中的 invoke() 被调用，并且 method.invoke 方法会调用实现类中的具体实现方法

原理

从 Proxy 的 newProxyInstance() 入手

public class Proxy implements java.io.Serializable {
    public static Object newProxyInstance(ClassLoader loader,                                          Class<?>[] interfaces,                                          InvocationHandler h)        throws IllegalArgumentException    {
                                          Class<?>[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException
    
        Objects.requireNonNull(h);

      	// 准备一份所有被实现的业务接口
        final Class<?>[] intfs = interfaces.clone();
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }

      	// 寻找或生成指定的代理类
        /*         * Look up or generate the designated proxy class.         */
         * Look up or generate the designated proxy class.
         */
        Class<?> cl = getProxyClass0(loader, intfs);

        /*         * Invoke its constructor with the designated invocation handler.         */
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }

          	// 通过反射类中的 Constructor 获取其所有构造方法
            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;
                    }
                });
            }
          	// 通过Constructor返回代理类的实例
            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);
        }
    }
}

• classLoader 的作用是将字节码文件加载进虚拟机并生成相应的 class
• interfaces 就是被实现的那些接口
• h 就是 InvocationHandler

再来看下 getProxyClass0()

public class Proxy implements java.io.Serializable {
  
    /**     * a cache of proxy classes     */
     * a cache of proxy classes
     */
    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.     */
     * Generate a proxy class.  Must call the checkProxyAccess method
     * to perform permission checks before calling this.
     */
    private static Class<?> getProxyClass0(ClassLoader loader,
                                           Class<?>... interfaces) {
        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
        return proxyClassCache.get(loader, interfaces);
    }
}

proxyClassCache 是 WeakCache 对象，构造函数中需要传两个 BiFunction 的实现类，分别是 KeyFactory() 和 ProxyClassFactory，对应着 subKeyFactory 和 valueFactory

这里从 proxyClassCache 中获取对应的 class，进 WeakCache#get() 看下

final class WeakCache<K, P, V> {
    public V get(K key, P parameter) {
        Objects.requireNonNull(parameter);

        expungeStaleEntries();

        Object cacheKey = CacheKey.valueOf(key, refQueue);

        // lazily install the 2nd level valuesMap for the particular cacheKey
        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;
            }
        }

        // create subKey and retrieve the possible Supplier<V> stored by that
        // subKey from valuesMap
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;

        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);
                }
            }
        }
    }
}

梳理一下，通过 factory = new Factory(key, parameter, subKey, valuesMap) 得到 factory，在通过 supplier = valuesMap.putIfAbsent(subKey, factory) 获得 supplier，通过 supplier.get() 调用到 valueFactory.apply(key, parameter)

final class WeakCache<K, P, V> {
    private final class Factory implements Supplier<V> {

        private final K key;
        private final P parameter;
        private final Object subKey;
        private final ConcurrentMap<Object, Supplier<V>> valuesMap;

        Factory(K key, P parameter, Object subKey,
                ConcurrentMap<Object, Supplier<V>> valuesMap) {
            this.key = key;
            this.parameter = parameter;
            this.subKey = subKey;
            this.valuesMap = valuesMap;
        }

        @Override
        public synchronized V get() { // serialize access
            // re-check
            Supplier<V> supplier = valuesMap.get(subKey);
            if (supplier != this) {
                // something changed while we were waiting:
                // might be that we were replaced by a CacheValue
                // or were removed because of failure ->
                // return null to signal WeakCache.get() to retry
                // the loop
                return null;
            }
            // else still us (supplier == this)

            // create new value
            V value = null;
            try {
                value = Objects.requireNonNull(valueFactory.apply(key, parameter));
            } finally {
                if (value == null) { // remove us on failure
                    valuesMap.remove(subKey, this);
                }
            }
            // the only path to reach here is with non-null value
            assert value != null;

            // wrap value with CacheValue (WeakReference)
            CacheValue<V> cacheValue = new CacheValue<>(value);

            // try replacing us with CacheValue (this should always succeed)
            if (valuesMap.replace(subKey, this, cacheValue)) {
                // put also in reverseMap
                reverseMap.put(cacheValue, Boolean.TRUE);
            } else {
                throw new AssertionError("Should not reach here");
            }

            // successfully replaced us with new CacheValue -> return the value
            // wrapped by it
            return value;
        }
    }
}

生成 class 是由 valueFactory.apply(key, parameter) 得到的，valueFactory 是 ProxyClassFactory 对象

public class Proxy implements java.io.Serializable {
    private static final class ProxyClassFactory
        implements BiFunction<ClassLoader, Class<?>[], Class<?>>
    {
      	// 这两个常量就是代理类名字的由来
        // prefix for all proxy class names
        private static final String proxyClassNamePrefix = "$Proxy";

        // next number to use for generation of unique proxy class names
        private static final AtomicLong nextUniqueNumber = new AtomicLong();

        @Override
        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.
                 */
                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());
                }
            }

            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.
             */
            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.
             */
            long num = nextUniqueNumber.getAndIncrement();
            String proxyName = proxyPkg + proxyClassNamePrefix + num;

          	// 这里生成了我们要的字节码形式的代理类
            /*
             * Generate the specified proxy class.
             */
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);
          	// defineClass0是个native方法，丢给 classLoader 去加载
            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());
            }
        }
    }
}

对 byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags) 进行获取

public class ProxyMain {
    public static void main(String[] args) {
        byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                ReflectUtil.PROXY_PACKAGE + "." + "$Proxy" + "0",
                new Class[]{IInterface.class}, Modifier.PUBLIC | Modifier.FINAL);
        File file = new File("/Users/yuyidong/Downloads/proxy.class");
        try (FileOutputStream fileOutputStream = new FileOutputStream(file)) {
            fileOutputStream.write(proxyClassFile);
            fileOutputStream.flush();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

反编译后

package com.sun.proxy;

import com.yydcdut.proxy.IInterface;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class $Proxy0 extends Proxy implements IInterface {
    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 function() throws  {
        try {
            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 {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m3 = Class.forName("com.yydcdut.proxy.IInterface").getMethod("function");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

当我们将接口 IInterface 和代理分发类 InvocationHandlerImpl 传入 Proxy 中后会为我们生成一个实现了 IInterface 接口并继承了 Proxy 的代理类 $Proxy0

在我们具体调用方法 iInterface.function() 时它其实是调用了 Proxy0 中的 function()，然后再调用 InvocationHandler#invoke() ，所以 InvocationHandlerImpl 中的 invoke 方法才是最终执行的方法