Java代理2动态代理的实现原理分析
上篇Java代理(一):代理和动态代理的基础与使用中得知通过调用静态方法Proxy.newProxyInstance()可以创建动态代理,之后会调用InvocationHandler接口的一个实现DynamicProxyHandler对象的invoker方法。
故这里我们主要探寻这两个方法的源码实现。
本篇所有源码基于JDK1.8.
上篇中也提到该方法存在三个参数
public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) throws IllegalArgumentException { // 检测指定的对象引用不为空(即h不为空),若为空则抛出NullPointerException异常。 Objects.requireNonNull(h); // 拷贝代理类要实现的接口列表,之后的操作均是使用该拷贝intfs,而不会涉及原列表interfaces。 final Class<?>[] intfs = interfaces.clone(); // 获取一个安全管理器对象security,这个对象所属的目录为java.lang.SecurityManager final SecurityManager sm = System.getSecurityManager(); if (sm != null) { // 检测创建代理类需要的权限 checkProxyAccess(Reflection.getCallerClass(), loader, intfs); } // 关键代码1:获得与传入的指定类装载器(loader)和接口列表(intfs)相关的代理类类型对象 /* * Look up or generate the designated proxy class. */ Class<?> cl = getProxyClass0(loader, intfs); // 通过反射获取构造函数对象并生成代理类cl的实例 /* * Invoke its constructor with the designated invocation handler. */ try { // 检测权限 if (sm != null) { checkNewProxyPermission(Reflection.getCallerClass(), cl); } // 关键代码2:通过反射获取该代理类的构造函数 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; } }); } // 关键代码3:返回这个新的代理类的一个实例 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); } }
检测创建代理类需要的权限,此处涉及到的SecurityManager安全管理器内容,后期单开篇细究。
private static void checkProxyAccess(Class<?> caller, ClassLoader loader, Class<?>... interfaces) { SecurityManager sm = System.getSecurityManager(); if (sm != null) { ClassLoader ccl = caller.getClassLoader(); if (VM.isSystemDomainLoader(loader) && !VM.isSystemDomainLoader(ccl)) { sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION); } ReflectUtil.checkProxyPackageAccess(ccl, interfaces); } }
关键代码1,用于从缓存获取或重新生成所需要这个代理类。
通过下面具体实现可见,这方法会根据类加载器与接口类型到缓存中寻找一个代理类的Class对象,如果没有就通过ProxyClassFactory创建一个新的。
由此可见,在传入loader参数的时候,需要跟传入的interface相关,所以比较常见的做法就是用接口或其实现类getClass().getClassLoader()方法(或如上篇的Animal.class.getClassLoader())获得一个类加载器
/** * Generate a proxy class. Must call the checkProxyAccess method * to perform permission checks before calling this. */ private static Class<?> getProxyClass0(ClassLoader loader, Class<?>... interfaces) { // 校验接口数量 if (interfaces.length > 65535) { throw new IllegalArgumentException("interface limit exceeded"); } // 从缓存中获取,如果没有就通过ProxyClassFactory创建 // 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); }
继续跟入proxyClassCache.get(loader, interfaces);
该方法是java.lang.reflect.WeakCache的get方法。
此处不深究里面提到的缓存,后期单开篇细究。
方法中最终通过 V value = supplier.get();获取动态代理类,其中supplier是Factory,这个类定义在WeakCach的内部。
public V get(K key, P parameter) { Objects.requireNonNull(parameter); expungeStaleEntries(); // 通过类加载器classLoader生成以及一级缓存key Object cacheKey = CacheKey.valueOf(key, refQueue); // lazily install the 2nd level valuesMap for the particular cacheKey // 获取二级缓存,不存在则新建 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 // 生成二级缓存key Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter)); // 通过key获取二级缓存value,即缓存的代理类。不存在则新建代理类并加入缓存。 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); } } } }
现在进入上面supplier.get()内部一探究竟。
发现内部关键语句 value = Objects.requireNonNull(valueFactory.apply(key, parameter));
其中,valueFactory是ProxyClassFactory类型
@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 { // 这里又通过valueFactory.apply(key, parameter)得到value进行返回 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); // put into reverseMap reverseMap.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 it return value; }
进入java.lang.reflect.Proxy.ProxyClassFactory#apply 在这里面可以找到生成字节码(即代理类)的语句。
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) { Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length); for (Class<?> intf : interfaces) { // 确保该loader加载的此类(intf) /* * 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"); } // 确保是一个接口 /* * Verify that the Class object actually represents an * interface. */ if (!interfaceClass.isInterface()) { throw new IllegalArgumentException( interfaceClass.getName() + " is not an interface"); } // 确保接口没重复 /* * 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; // 验证所有非公共的接口在同一个包内;公共的就无需处理. /* * 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 + "."; } // 为代理类生成一个名字,防止重复 /* * Choose a name for the proxy class to generate. */ long num = nextUniqueNumber.getAndIncrement(); String proxyName = proxyPkg + proxyClassNamePrefix + num; // 生成指定代理类 /* * Generate the specified proxy class. */ 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()); } }
关键语句有:
byte[] proxyClassFile = ProxyGenerator.generateProxyClass( proxyName, interfaces, accessFlags);
进入sun.misc.ProxyGenerator#generateProxyClass(java.lang.String, java.lang.Class<?>[])。
这里面便是得到class文件byte内容的逻辑。
public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) { ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2); //动态生成代理类的字节码,非常复杂 final byte[] var4 = var3.generateClassFile(); //根据配置,判断class文件是否存储在本地 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); } } }); } //返回代理类的字节码 return var4; }
可以自定义存储硬盘位置 调用sun.misc.ProxyGenerator#generateProxyClass(java.lang.String, java.lang.Class<?>[])函数即可
public static void showProxyClass() { String path = "./$Proxy0.class"; byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy0", Dog.class.getInterfaces()); FileOutputStream out = null; try { out = new FileOutputStream(path); out.write(classFile); out.flush(); } catch (Exception e) { e.printStackTrace(); } finally { try { out.close(); } catch (IOException e) { e.printStackTrace(); } } }
在mian函数中调用,即可获取到生成的$Proxy0.class类
public final class $Proxy0 extends Proxy implements Animal { private static Method m1; private static Method m3; private static Method m2; private static Method m4; 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 doBark() 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 void somethingElse(String var1) throws { try { super.h.invoke(this, m4, new Object[]{var1}); } catch (RuntimeException | Error var3) { throw var3; } catch (Throwable var4) { throw new UndeclaredThrowableException(var4); } } 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("Others.base.SimpleProxy.Animal").getMethod("doBark"); m2 = Class.forName("java.lang.Object").getMethod("toString"); m4 = Class.forName("Others.base.SimpleProxy.Animal").getMethod("somethingElse", Class.forName("java.lang.String")); m0 = Class.forName("java.lang.Object").getMethod("hashCode"); } catch (NoSuchMethodException var2) { throw new NoSuchMethodError(var2.getMessage()); } catch (ClassNotFoundException var3) { throw new NoClassDefFoundError(var3.getMessage()); } } }
可以看最上面java.lang.reflect.Proxy#newProxyInstance中的详情,里面便是生成方式。
简单说是生成了代理对象的字节码文件后,根据构造函数生成对象
//生成字节码,获取类文件 Class<?> cl = getProxyClass0(loader, intfs); //省略 //反射,根据构造函数,生成对象,构造参数为InvocationHandler h return cons.newInstance(new Object[]{h});
对应的$Proxy0构造函数为:
public $Proxy0(InvocationHandler var1) throws { super(var1); }
其会调用父级的,即:
protected Proxy(InvocationHandler h) { Objects.requireNonNull(h); this.h = h; }
此处的h即为之前传入的InvocationHandler实现类DynamicProxyHandler
由方法doBark()分析可见,代理类实现了委托类的方法,也就是说 代理对象触发doBark方法时,调用InvocationHandler的m3方法
public final void doBark() throws { try { super.h.invoke(this, m3, (Object[])null); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } }
其m3的定义在下面
static { try { m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object")); m3 = Class.forName("Others.base.SimpleProxy.Animal").getMethod("doBark"); m2 = Class.forName("java.lang.Object").getMethod("toString"); m4 = Class.forName("Others.base.SimpleProxy.Animal").getMethod("somethingElse", Class.forName("java.lang.String")); m0 = Class.forName("java.lang.Object").getMethod("hashCode"); } catch (NoSuchMethodException var2) { throw new NoSuchMethodError(var2.getMessage()); } catch (ClassNotFoundException var3) { throw new NoClassDefFoundError(var3.getMessage()); } }
可见m3是通过反射获取到的Animal的doBark方法。
m3 = Class.forName("Others.base.SimpleProxy.Animal").getMethod("doBark");
然后看我们自定义的实现Others.base.SimpleProxy.DynamicProxyDemo.DynamicProxyHandler#invoke
三个参数分别对应:
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { System.out.println("*** proxy: " + proxy.getClass() + " , method: " + method + " , args: " + args); if (args != null){ for (Object arg: args){ PrintUtill.println(" " + arg); } } //通过反射,把proxy的代理类 转交给 实现类上 return method.invoke(proxied, args); }
代理类调用doBark方法,实际是利用父类InvocationHandler属性h,传入doBark这个method到h实现类DynamicProxyHandler的invoke中, 在自定义的InvocationHandler即DynamicProxyHandler中,再利用反射,利用实现类Dog完成doBark操作,最终完成了代理类 到 实现类的调用。
关于AccessController.doPrivileged
对AccessController.doPrivileged一点了解