别紧张，你只是缺少这个demo
关注我，一个脑回路氢气的程序员，总有一些神奇的想法，最近有问题的老铁，可以下方评论区回复我或者私信我，想要学习什么知识，大家一起努力 概念 JDK动态代理与静态代理是相对的，静态代理是在编译期就确定了代理类和被代理类的关系，且生成代理类。而且静态代理要求被代理的接口必须有其实现，因为代理类实现接口方法实际上是调用接口实现类的方法实现的。而动态代理是在运行期利用JVM的反射机制生成代理类,这里是直接生成类的字节码，然后通过类加载器载入JAVA虚拟机执行。 demo 不做赘述，直接上demo。 被代理类接口 public interface ISubject { void doSomething(); } 被代理类实现 public class SubjectImpl implements ISubject { @Override public void doSomething() { System.out.println("doSomething..."); } } InvocationHandler工具类 import java.lang.reflect.InvocationHandler; import java.lang.reflect.Method; import java.lang.reflect.Proxy; public class JDKDynamicProxy implements InvocationHandler{ private Object target; public JDKDynamicProxy(){ } public JDKDynamicProxy(Object target){ this.target=target; } public <T>T getProxy(){ System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true"); // return (T) Proxy.newProxyInstance(target.getClass().getClassLoader(),target.getClass().getInterfaces(),this); return (T) Proxy.newProxyInstance(target.getClass().getClassLoader(),target.getClass().getInterfaces(),this); } @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { System.out.println("before..."); Object result=method.invoke(target,args); System.out.println("after..."); return result; } } 测试类 public class TestProxy { public static void main(String[] args) { System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles","true"); ISubject subject=new JDKDynamicProxy(new SubjectImpl()).getProxy(); //ISubject subject= (ISubject) Proxy.newProxyInstance(subjectImpl.getClass().getClassLoader(),subjectImpl.getClass().getInterfaces(),new JDKDynamicProxy(subjectImpl)); //ISubject subject= (ISubject) Proxy.newProxyInstance(ISubject.class.getClassLoader(),new Class[]{ISubject.class},new JDKDynamicProxy(subjectImpl)); subject.doSomething(); } } 运行结果 before... doSomething... after... 生成的代理类文件 // // Source code recreated from a .class file by IntelliJ IDEA // (powered by Fernflower decompiler) // package com.sun.proxy; 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 ISubject { private static Method m1; private static Method m3; private static Method m2; private static Method m0; public $Proxy0(InvocationHandler var1) throws { super(var1); } public final boolean equals(Object var1) throws { try { return (Boolean)super.h.invoke(this, m1, new Object[]{var1}); } catch (RuntimeException | Error var3) { throw var3; } catch (Throwable var4) { throw new UndeclaredThrowableException(var4); } } public final void doSomething() throws { try { super.h.invoke(this, m3, (Object[])null); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } } public final String toString() throws { try { return (String)super.h.invoke(this, m2, (Object[])null); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } } public final int hashCode() throws { try { return (Integer)super.h.invoke(this, m0, (Object[])null); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } } static { try { m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object")); m3 = Class.forName("ISubject").getMethod("doSomething"); m2 = Class.forName("java.lang.Object").getMethod("toString"); m0 = Class.forName("java.lang.Object").getMethod("hashCode"); } catch (NoSuchMethodException var2) { throw new NoSuchMethodError(var2.getMessage()); } catch (ClassNotFoundException var3) { throw new NoClassDefFoundError(var3.getMessage()); } } } 原理 流程 获取代理类——调用Proxy.newProxyInstance获取代理类的Class实例getProxyClass0(loader, intfs)获取代理类实例的构造方法，并确保其访问权限final Constructor cons = cl.getConstructor(constructorParams)利用反射机制调用构造方法，返回代理类实例，参数是调用Proxy.newProxyInstance时传进来的InvocationHandler实例h 。cons.newInstance(new Object[]{h}) 方法实现前面利用反射机制调用代理类的构造方法时传入了InvocationHandler实例h ，代理类$Proxy0的构造方法如下： public $Proxy0(InvocationHandler var1) throws { super(var1); } 这是调用父类的构造函数，并传入参数h。所以在获取到代理类实例并执行方法时如测试类的subject.doSomething()，实际调用的是代理类内的doSomething()，如下所示。 public final void doSomething() throws { try { super.h.invoke(this, m3, (Object[])null); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } } 所以JDK动态代理的接口方法实现逻辑是完全由InvocationHandler实例的invoke方法决定的 。 获取代理类的Class实例探究 getProxyClass0(loader, intfs)方法 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); } 在Proxy类getProxyClass0(loader, intfs)方法中并无核心代码，主要是方法最后一行去缓存对象中获取代理类实例。 java.lang.reflect.WeakCache 类private static final WeakCache<ClassLoader, Class<?>[], Class<?>> proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory()); proxyClassCache是Proxy类的静态变量，调用WeakCache类的构造函数初始化时，传入keyFactory和proxyClassFactory两个实例。注意WeakCache对象的数据结构包含两级键值。接下来就进入WeakCache类的get方法查看一番。 WeakCache类get方法代码 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); } } } } get方法核心代码解释获取一级和二级键。 //根据类加载器计算一级键 cacheKey=f(loader); Object cacheKey = CacheKey.valueOf(key, refQueue); //根据接口计算二级键 subKey=g(interfaces); Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter)); 注意这里的subKeyFactory是Proxy初始化WeakCache类对象proxyClassCache时的入参new KeyFactory()；KeyFactory是Proxy的内部类。不妨进入apply简单看一下：@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); } } 循环 接下来是一个while循环，第一次循环前factory和supplier都为null ，所以循环体内只是做了一些初始化的工作，并将factory赋值给了supplier（Factory类是WeakCache的内部类，且继承了Supplier类）。第二次循环时，执行V value = supplier.get()，实际上就是调用factory.get()方法获取动态代理类的Class实例 。factory.get()方法方法完整代码如下： @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; } 第4行代码Supplier supplier = valuesMap.get(subKey)通过subKey获取方才的supplier也就是factory。然后在第18行调用了valueFactory.apply方法创建动态代理类并将结果赋值给变量value。 valueFactory是初始化WeakCache对象调用构造方法时传入的参数 即Prxoy的内部类ProxyClassFactory对象。 ProxyClassFactory类进入apply方法 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; /* * 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()); } } 该方法前边包括一些校验、计算出访问标志和包名、代理类定名 ，然后在73行创建代理类的字节码文件，并defineClass0将字节码文件加载到虚拟机和返回代理类的Class实例。 如上就是第一次创建代理类的大致流程。 个人公众号：Java架构师联盟，每日更新技术好文
