设计模式学习——JAVA动态代理原理分析
一、JDK动态代理执行过程
上一篇我们讲了JDK动态代理的简单使用，今天我们就来研究一下它的原理。
首先我们回忆下上一篇的代码：
public class Main { public static void main(String[] args) { IPaymentService paymentService = new WatchPaymentService(); PaymentIH paymentIH = new PaymentIH(paymentService); IPaymentService proxy = (IPaymentService) Proxy.newProxyInstance( paymentService.getClass().getClassLoader(), new Class[] {IPaymentService.class}, paymentIH); proxy.pay(); } }
我们通过Proxy.newProxyInstance方法创建了代理对象，我们通过Debug看下这个proxy到底是什么：
我们看到proxy的类是$Proxy0，很显然这是一个自动生成的类，我们使用如下工具类将此动态类保存下来看看：
public class ProxyUtils { /** * 将动态类的二进制字节码保存到硬盘中，默认的是clazz目录下 * params: clazz 需要生成动态代理类的类 * proxyName: 为动态生成的代理类的名称 */ public static void generateClassFile(Class clazz, String proxyName) { // 根据类信息和提供的代理类名称，生成字节码 byte[] classFile = ProxyGenerator.generateProxyClass(proxyName, clazz.getInterfaces()); String paths = clazz.getResource(".").getPath(); System.out.println(paths); FileOutputStream out = null; try { //保留到硬盘中 out = new FileOutputStream(paths + proxyName + ".class"); out.write(classFile); out.flush(); } catch (Exception e) { e.printStackTrace(); } finally { try { out.close(); } catch (IOException e) { e.printStackTrace(); } } } }
在main方法中执行下面这段代码，我们便可以在target的classes下找到生成的动态类。
ProxyUtils.generateClassFile(paymentService.getClass(), "PaymentServiceProxy");
我们通过IDEA查看这个动态类的内容如下：
public final class PaymentServiceProxy extends Proxy implements IPaymentService { private static Method m1; private static Method m3; private static Method m2; private static Method m0; public PaymentServiceProxy(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); } } public final void pay() throws { try { super.h.invoke(this, m3, (Object[])null); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } } 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 int hashCode() throws { try { return (Integer)super.h.invoke(this, m0, (Object[])null); } 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")); m3 = Class.forName("proxy.IPaymentService").getMethod("pay"); 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()); } } }
从 PaymentServiceProxy 的代码中我们可以发现：
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PaymentServiceProxy 继承了 Proxy 类，并且实现了被代理的所有接口，以及equals、hashCode、toString等方法；
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由于 PaymentServiceProxy继承了 Proxy 类，所以每个代理类都会关联一个 InvocationHandler 方法调用处理器；
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类和所有方法都被 public final 修饰，所以代理类只可被使用，不可以再被继承；
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每个方法都有一个 Method 对象来描述，Method 对象在static静态代码块中创建，以 m + 数字 的格式命名；
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被代理对象方法的调用是通过super.h.invoke(this, m1, (Object[])null); 完成的，其中的 super.h.invoke 实际上是在创建代理的时候传递给 Proxy.newProxyInstance 的 PaymentIH 对象，即 InvocationHandler的实现类，负责实际的调用处理逻辑；
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PaymentIH的invoke方法接收到method、args等参数后，通过反射机制让被代理对象执行对应的方法。
综上，JDK的动态代理执行流程如下：
那么这个类是如何生成的呢？这个类与InvocationHandler又是如何关联起来的？带着这两个问题我们深入研究下Proxy的源码。
二、JDK动态代理源码解读
@CallerSensitive public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) throws IllegalArgumentException{ // null检查，h为null就抛出NullPointerException Objects.requireNonNull(h); // 将接口类对象数组clone一份。 final Class<?>[] intfs = interfaces.clone(); //执行权限检查 final SecurityManager sm = System.getSecurityManager(); if (sm != null) { checkProxyAccess(Reflection.getCallerClass(), loader, intfs); } /* * Look up or generate the designated proxy class. */ // 查找或者是生成一个特定的代理类对象 Class<?> cl = getProxyClass0(loader, intfs); /* * Invoke its constructor with the designated invocation handler. */ try { if (sm != null) { checkNewProxyPermission(Reflection.getCallerClass(), cl); } // 从代理类对象中查找参数为InvocationHandler的构造器 final Constructor<?> cons = cl.getConstructor(constructorParams); final InvocationHandler ih = h; // 判断构造器是否是Public，如果不是则将其设置为可以访问的。 if (!Modifier.isPublic(cl.getModifiers())) { AccessController.doPrivileged(new PrivilegedAction<Void>() { public Void run() { cons.setAccessible(true); return null; } }); } // 通过反射，将h作为参数，实例化代理类，返回代理类实例。 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); } }
上面的代码中最重要的就是第21行和第44行，这两行实现了代理类的生成与实例化代理对象。
首先我们看下getProxyClass0(loader, intfs)的实现逻辑：
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 // 如果代理类被指定的类加载器定义了，并实现了给定的接口， // 那么就返回缓存的代理类对象，否则使用ProxyClassFactory创建代理类。 return proxyClassCache.get(loader, interfaces); }
根据注释分析，proxyClassCache.get方法是获取代理类的入口，那我们接下来首先看看这个proxyClassCache是什么东东：
private static final WeakCache<ClassLoader, Class<?>[], Class<?>> proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
proxyClassCache是一个WeakCache的对象，我们看看他的定义：（由于代码篇幅较大，这里只展示出私有变量定义与构造函数定义）
final class WeakCache<K, P, V> { private final ReferenceQueue<K> refQueue = new ReferenceQueue<>(); // the key type is Object for supporting null key private final ConcurrentMap<Object, ConcurrentMap<Object, Supplier<V>>> map = new ConcurrentHashMap<>(); private final ConcurrentMap<Supplier<V>, Boolean> reverseMap = new ConcurrentHashMap<>(); private final BiFunction<K, P, ?> subKeyFactory; private final BiFunction<K, P, V> valueFactory; /** * Construct an instance of {@code WeakCache} * * @param subKeyFactory a function mapping a pair of * {@code (key, parameter) -> sub-key} * @param valueFactory a function mapping a pair of * {@code (key, parameter) -> value} * @throws NullPointerException if {@code subKeyFactory} or * {@code valueFactory} is null. */ public WeakCache(BiFunction<K, P, ?> subKeyFactory, BiFunction<K, P, V> valueFactory) { this.subKeyFactory = Objects.requireNonNull(subKeyFactory); this.valueFactory = Objects.requireNonNull(valueFactory); }
其中map变量是实现缓存的核心变量，它是一个双重的Map结构: (key, subKey) -> value。其中key是传进来的Classloader进行包装后的对象，subKey是由WeakCache构造函数传人的KeyFactory()生成的。value就是产生代理类的对象由WeakCache构造函数传人的ProxyClassFactory()生成，这个可以从proxyClassCache的初始化能看出来。
产生subKey的KeyFactory代码如下：
private static final class KeyFactory implements BiFunction<ClassLoader, Class<?>[], Object> { @Override public Object apply(ClassLoader classLoader, Class<?>[] interfaces) { switch (interfaces.length) { case 1: return new Key1(interfaces[0]); // the most frequent case 2: return new Key2(interfaces[0], interfaces[1]); case 0: return key0; default: return new KeyX(interfaces); } } }
这部分代码没有必要深究，我们只需要知道它是根据传进去的interface生成subKey就行了，我们接着来看WeakCache.get方法：
public V get(K key, P parameter) { // 校验parameter不为空 Objects.requireNonNull(parameter); // 清除无效缓存 expungeStaleEntries(); // cacheKey就是缓存的一级键 Object cacheKey = CacheKey.valueOf(key, refQueue); // 根据一级键得到ConcurrentMap<Object, Supplier<V>>，如果不存在则创建之 // 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 // 根据classloader和interfaces获取二级键，即KeyFactory的apply方法 Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter)); // 根据下面的代码可以得知这个supplier就是factory，这里同样也是先看缓存中有没有，没有就重新创建， // 这也是此处代码使用一个while循环的原因 Supplier<V> supplier = valuesMap.get(subKey); Factory factory = null; while (true) { if (supplier != null) { // supplier might be a Factory or a CacheValue<V> instance // 这里调用的就是Factory.get,得到代理类并返回 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); } } } }
上面的代码稍微比较绕，总之就是先尝试从key获取（subKey -> value）如果不存在则创建（subKey -> value），然后再根据subKey获取value，同样的不存在则创建新的value。最终通过Factory.get方法获取代理类，接下来我们来看看Factory的代码：
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); // 检测得到的supplier是不是当前对象 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 { // 调用ProxyClassFactory创建代理类 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) // 把value包装成弱引用 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; } }
终于我们来到了ProxyClassFactory类了，这个类就是用来创建代理类的工厂类：
private static final class ProxyClassFactory implements BiFunction<ClassLoader, Class<?>[], Class<?>> { // prefix for all proxy class names // 代理类的类名前缀 private static final String proxyClassNamePrefix = "$Proxy"; // next number to use for generation of unique proxy class names // 代理类类名编号，即$Proxy0,$Proxy1,$Proxy2...... private static final AtomicLong nextUniqueNumber = new AtomicLong(); @Override public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) { Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length); // 校验接口是否能被当前classloader加载以及其是否是接口类 for (Class<?> intf : interfaces) { /* * Verify that the class loader resolves the name of this * interface to the same Class object. */ Class<?> interfaceClass = null; try { interfaceClass = Class.forName(intf.getName(), false, loader); } catch (ClassNotFoundException e) { } if (interfaceClass != intf) { throw new IllegalArgumentException( intf + " is not visible from class loader"); } /* * Verify that the Class object actually represents an * interface. */ if (!interfaceClass.isInterface()) { throw new IllegalArgumentException( interfaceClass.getName() + " is not an interface"); } /* * Verify that this interface is not a duplicate. */ if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) { throw new IllegalArgumentException( "repeated interface: " + interfaceClass.getName()); } } // 生成代理类包名 String proxyPkg = null; // package to define proxy class in int accessFlags = Modifier.PUBLIC | Modifier.FINAL; /* * Record the package of a non-public proxy interface so that the * proxy class will be defined in the same package. Verify that * all non-public proxy interfaces are in the same package. */ //验证所有非公共的接口在同一个包内；公共的就无需处理 //生成包名和类名的逻辑，包名默认是com.sun.proxy，类名默认是$Proxy 加上一个自增的整数值 //如果被代理类是 non-public proxy interface ，则用和被代理类接口一样的包名 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(); // 代理类的完全限定名，如com.sun.proxy.$Proxy0.calss String proxyName = proxyPkg + proxyClassNamePrefix + num; /* * Generate the specified proxy class. */ // 代理类字节码生成 byte[] proxyClassFile = ProxyGenerator.generateProxyClass( proxyName, interfaces, accessFlags); try { //把代理类加载到JVM中，至此动态代理过程基本结束了 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()); } } }
在ProxyGenerator.generateProxyClass()方法中完成代理类的字节码的组装，最终就生成了本文一开始保存出来的代理类的内容，感兴趣的读者可以自行研究，值得注意的是在创建代理类的构造函数时，此处设定需要传入InvocationHandler对象，所以这样就能确保我们代理类能够通过我们实现的InvocationHandler接口去调用被代理类的方法。
private ProxyGenerator.MethodInfo generateConstructor() throws IOException { ProxyGenerator.MethodInfo var1 = new ProxyGenerator.MethodInfo("<init>", "(Ljava/lang/reflect/InvocationHandler;)V", 1); DataOutputStream var2 = new DataOutputStream(var1.code); this.code_aload(0, var2); this.code_aload(1, var2); var2.writeByte(183); var2.writeShort(this.cp.getMethodRef("java/lang/reflect/Proxy", "<init>", "(Ljava/lang/reflect/InvocationHandler;)V")); var2.writeByte(177); var1.maxStack = 10; var1.maxLocals = 2; var1.declaredExceptions = new short[0]; return var1; }
