jdk动态代理源码分析
//test.java
public class test {public static void main(String[] args) {MyService myService = new MyServiceImpl();MyInvocationHandler myInvocationHandler = new MyInvocationHandler(myService);System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles","true");MyService instance = (MyService) Proxy.newProxyInstance(myService.getClass().getClassLoader(), myService.getClass().getInterfaces(), myInvocationHandler);instance.myMethod();}
}
class MyInvocationHandler implements InvocationHandler{Object target;public MyInvocationHandler(Object target){this.target = target;}@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {System.out.println("before run myMethod()");Object result = method.invoke(target, args);System.out.println("after run myMethod()");return result;}
}
interface MyService{void myMethod();
}
class MyServiceImpl implements MyService{@Overridepublic void myMethod() {System.out.println("run myMethod()");}
}
这段代码是一个使用jdk动态代理的例子，接下来根据源码看看动态代理是怎么实现的，进入newProxyInstance()方法。
//Proxy.java
public static Object newProxyInstance(ClassLoader loader,Class<?>[] interfaces,InvocationHandler h)throws IllegalArgumentException
{Objects.requireNonNull(h);//克隆接口类final Class<?>[] intfs = interfaces.clone();final SecurityManager sm = System.getSecurityManager();if (sm != null) {checkProxyAccess(Reflection.getCallerClass(), loader, intfs);}/** Look up or generate the designated proxy class.*///这个是重点，获取代理类Class<?> cl = getProxyClass0(loader, intfs);/** Invoke its constructor with the designated invocation handler.*/try {if (sm != null) {checkNewProxyPermission(Reflection.getCallerClass(), cl);}//获取代理类的构造器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;}});}//反射生成代理类实例对象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);}
}
重点就是这三行代码：
//获取代理类
Class<?> cl = getProxyClass0(loader, intfs);
//获取代理类构造器
final Constructor<?> cons = cl.getConstructor(constructorParams);
//生成代理类实例对象
return cons.newInstance(new Object[]{h});
进入getProxyClass0()方法，get()方法涉及到一个缓存的概念，先从缓存中获取代理类，如果缓存没命中，则创建一个工厂，从工厂中去获取一个代理类。我们先不考虑缓存，看看怎么从工厂中获取代理类。重点是这句代码：
V value = supplier.get();
//Proxy.java
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 ProxyClassFactoryreturn proxyClassCache.get(loader, interfaces);
}
//WeakCache.java
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 cacheKeyConcurrentMap<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 valuesMapObject 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> instanceV 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 Factoryif (factory == null) {factory = new Factory(key, parameter, subKey, valuesMap);}if (supplier == null) {supplier = valuesMap.putIfAbsent(subKey, factory);if (supplier == null) {// successfully installed Factorysupplier = factory;}// else retry with winning supplier} else {if (valuesMap.replace(subKey, supplier, factory)) {// successfully replaced// cleared CacheEntry / unsuccessful Factory// with our Factorysupplier = factory;} else {// retry with current suppliersupplier = valuesMap.get(subKey);}}}
}
看一下上面这段get()方法的代码逻辑，在while循环中，如果supplier是空的，因为缓存中没有，那么就会先执行下面的new Factory()方法创建一个工厂，并让supplier = factory，这样在下次进入循环的时候，就会执行Factory的get()方法，进入get()方法看看。
//WeakCache.javapublic synchronized V get() { // serialize access// re-checkSupplier<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 loopreturn null;}// else still us (supplier == this)// create new valueV value = null;try {value = Objects.requireNonNull(valueFactory.apply(key, parameter));} finally {if (value == null) { // remove us on failurevaluesMap.remove(subKey, this);}}// the only path to reach here is with non-null valueassert value != null;// wrap value with CacheValue (WeakReference)CacheValue<V> cacheValue = new CacheValue<>(value);// put into reverseMapreverseMap.put(cacheValue, Boolean.TRUE);// try replacing us with CacheValue (this should always succeed)if (!valuesMap.replace(subKey, this, cacheValue)) {throw new AssertionError("Should not reach here");}// successfully replaced us with new CacheValue -> return the value// wrapped by itreturn value;}
}
这段代码返回一个value，也就是代理类，而value是在这里被赋值的，更准确的说是调用valueFactory的apply方法生成的代理类。
value = Objects.requireNonNull(valueFactory.apply(key, parameter));
这个valueFactory是什么，看一下这个类（WeakCache.java）的构造方法：
//WeakCache.java
public WeakCache(BiFunction<K, P, ?> subKeyFactory,BiFunction<K, P, V> valueFactory) {this.subKeyFactory = Objects.requireNonNull(subKeyFactory);this.valueFactory = Objects.requireNonNull(valueFactory);
}
在回到Proxy.java，可以看到这个valueFactory就是这个ProxyClassFactory。
//Proxy.java
public class Proxy implements java.io.Serializable {...private static final WeakCache<ClassLoader, Class<?>[], Class<?>>proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());...
}
继续跟进代码，看看这个apply()方法做了什么。
//ProxyGenerator.javapublic 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 inint 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 packageproxyPkg = ReflectUtil.PROXY_PACKAGE + ".";}/** Choose a name for the proxy class to generate.*/long num = nextUniqueNumber.getAndIncrement();String proxyName = proxyPkg + proxyClassNamePrefix + num;/** Generate the specified proxy class.*///获取代理类文件字节码二进制流byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags);try {//定义这个代理类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());}}
}
重点是generateProxyClass()，调用generateClassFile()生成代理类文件字节码，这个方法会组装要生成的代理类class文件的所有字段和方法。
//ProxyGenerator.java
public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {//创建一个代理类生成器ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);//得到代理类文件字节码二进制流final byte[] var4 = var3.generateClassFile();//这个值为true时，会在项目根目录下生成Proxy代理类，这个我们前面已经设了。if (saveGeneratedFiles) {AccessController.doPrivileged(new PrivilegedAction<Void>() {public Void run() {try {int var1 = var0.lastIndexOf(46);Path var2;if (var1 > 0) {Path var3 = Paths.get(var0.substring(0, var1).replace('.', File.separatorChar));Files.createDirectories(var3);var2 = var3.resolve(var0.substring(var1 + 1, var0.length()) + ".class");} else {var2 = Paths.get(var0 + ".class");}Files.write(var2, var4, new OpenOption[0]);return null;} catch (IOException var4x) {throw new InternalError("I/O exception saving generated file: " + var4x);}}});}return var4;
}
//ProxyGenerator.java
private byte[] generateClassFile() {//添加方法this.addProxyMethod(hashCodeMethod, Object.class);this.addProxyMethod(equalsMethod, Object.class);this.addProxyMethod(toStringMethod, Object.class);Class[] var1 = this.interfaces;int var2 = var1.length;int var3;Class var4;//遍历接口类，添加每个接口中的每个方法for(var3 = 0; var3 < var2; ++var3) {var4 = var1[var3];Method[] var5 = var4.getMethods();int var6 = var5.length;for(int var7 = 0; var7 < var6; ++var7) {Method var8 = var5[var7];this.addProxyMethod(var8, var4);}}Iterator var11 = this.proxyMethods.values().iterator();List var12;while(var11.hasNext()) {var12 = (List)var11.next();checkReturnTypes(var12);}Iterator var15;try {//添加构造器this.methods.add(this.generateConstructor());var11 = this.proxyMethods.values().iterator();while(var11.hasNext()) {var12 = (List)var11.next();var15 = var12.iterator();while(var15.hasNext()) {ProxyMethod var16 = (ProxyMethod)var15.next();//添加字段this.fields.add(new FieldInfo(var16.methodFieldName, "Ljava/lang/reflect/Method;", 10));//添加代理方法this.methods.add(var16.generateMethod());}}//添加静态字段初始化方法this.methods.add(this.generateStaticInitializer());} catch (IOException var10) {throw new InternalError("unexpected I/O Exception", var10);}if (this.methods.size() > 65535) {throw new IllegalArgumentException("method limit exceeded");} else if (this.fields.size() > 65535) {throw new IllegalArgumentException("field limit exceeded");} else {this.cp.getClass(dotToSlash(this.className));this.cp.getClass("java/lang/reflect/Proxy");var1 = this.interfaces;var2 = var1.length;for(var3 = 0; var3 < var2; ++var3) {var4 = var1[var3];this.cp.getClass(dotToSlash(var4.getName()));}this.cp.setReadOnly();ByteArrayOutputStream var13 = new ByteArrayOutputStream();DataOutputStream var14 = new DataOutputStream(var13);//把上面的信息写入class文件try {var14.writeInt(-889275714);var14.writeShort(0);var14.writeShort(49);this.cp.write(var14);var14.writeShort(this.accessFlags);var14.writeShort(this.cp.getClass(dotToSlash(this.className)));var14.writeShort(this.cp.getClass("java/lang/reflect/Proxy"));var14.writeShort(this.interfaces.length);Class[] var17 = this.interfaces;int var18 = var17.length;for(int var19 = 0; var19 < var18; ++var19) {Class var22 = var17[var19];var14.writeShort(this.cp.getClass(dotToSlash(var22.getName())));}var14.writeShort(this.fields.size());var15 = this.fields.iterator();while(var15.hasNext()) {FieldInfo var20 = (FieldInfo)var15.next();var20.write(var14);}var14.writeShort(this.methods.size());var15 = this.methods.iterator();while(var15.hasNext()) {MethodInfo var21 = (MethodInfo)var15.next();var21.write(var14);}var14.writeShort(0);return var13.toByteArray();} catch (IOException var9) {throw new InternalError("unexpected I/O Exception", var9);}}
}
再回到ProxyGenerator的apply方法，会接下来调用defineClass0来定义这个类，到这里就已经生成了一个代理类$Proxy0.java。我们在开头加入了这样一行代码，它可以在项目路径中生成那个代理类。
System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles","true");
进入$Proxy0.java，通过invoke()方法来调用目标类的方法。
final class $Proxy0 extends Proxy implements MyService {private static Method m1;private static Method m3;private static Method m2;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});} catch (RuntimeException | Error var3) {throw var3;} catch (Throwable var4) {throw new UndeclaredThrowableException(var4);}}public final void myMethod() 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("com.lian.xhs.utils.MyService").getMethod("myMethod");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());}}
}
到这里，下面这第一句代码就分析完了，有个代理类，就可以通过反射获取构造器，创建对象实例了。
//Proxy.java
//获取代理类Class对象
Class<?> cl = getProxyClass0(loader, intfs);
//获取代理类构造器
final Constructor<?> cons = cl.getConstructor(constructorParams);
//生成代理类实例对象
return cons.newInstance(new Object[]{h});
总结一下：JDK 动态代理的作用是在不修改目标对象的前提下，提供额外的功能扩展。代理对象的生成主要分为两步：首先生成代理类的 Class 对象，然后通过反射创建代理类的实例。在生成代理类 Class 对象的过程中，可以概括为三个步骤。第一步，获取目标接口的所有方法，并将它们封装成 ProxyMethod 对象，注册到一个 Map 集合中。第二步，填充代理类的所有字段和方法信息。第三步，将这些信息写入代理类的字节码文件，并返回对应的二进制流。
