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
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 $proxy
在运行的时候动态生成的类
代理分发 InvocationHandler
the invocation handler to dispatch method invocations to
InvocationHandler
接口创建自己的调用处理器 Proxy
类指定 ClassLoader
对象和一组 interface 来创建动态代理类 // InvocationHandlerImpl 实现了 InvocationHandler 接口,并能实现方法调用从代理类到委托类的分派转发 // 其内部通常包含指向委托类实例的引用,用于真正执行分派转发过来的方法调用 InvocationHandler handler = new InvocationHandlerImpl(..); // 通过 Proxy 为包括 Interface 接口在内的一组接口动态创建代理类的类对象 Class clazz = Proxy.getProxyClass(classLoader, new Class[] { Interface.class, ... }); // 通过反射从生成的类对象获得构造函数对象 Constructor constructor = clazz.getConstructor(new Class[] { InvocationHandler.class }); // 通过构造函数对象创建动态代理类实例 Interface Proxy = (Interface)constructor.newInstance(new Object[] { handler });
简化一下就是
InvocationHandler handler = new InvocationHandlerImpl(..); // 通过 Proxy 直接创建动态代理类实例 Interface proxy = (Interface)Proxy.newProxyInstance( classLoader, new Class[] { Interface.class }, handler );
public interface IInterface { void function(); }
public class Impl implements IInterface { public void function() { System.out.println("function method invoke"); } }
public class InvocationHandlerImpl implements InvocationHandler { // delegate 为委托类对象; private Object delegate; public InvocationHandlerImpl(Object delegate) { this.delegate = delegate; } @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { System.out.println("before"); Object result = method.invoke(delegate, args); System.out.println("after"); return result; } }
public class ProxyMain { public static void main(String[] args) { InvocationHandlerImpl invocationHandlerImpl = new InvocationHandlerImpl(new Impl()); IInterface iInterface = (IInterface) (Proxy.newProxyInstance( IInterface.class.getClassLoader(), new Class[]{IInterface.class}, invocationHandlerImpl)); System.out.println(iInterface.getClass().getName()); // 打印代理类名字 iInterface.function(); } }
com.sun.proxy.$Proxy0 before function method invoke after
$Proxy0
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
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 { 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. */ Class<?> cl = getProxyClass0(loader, intfs); /* * 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 */ 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. */ 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