JDK動態代理案例實現：實現 InvocationHandler 介面重寫 invoke 方法，其中包含一個物件變數和提供一個包含物件的構造方法；
public class MyInvocationHandler implements InvocationHandler { Object target;//目標物件 public MyInvocationHandler(Object target){ this.target=target; } /** * @param proxy 代理物件 * @param method 目標物件的目標方法 * @param args 目標方法的引數 */ @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { System.out.println("log"); return method.invoke(target,args); } }
public class MyInvocationHandlerTest { public static void main(String[] args) { //引數： 當前類的classLoader（保證MyInvocationHandlerTest當前類可用） // 介面陣列：通過介面反射得到介面裡面的方法，對介面裡面的所有方法都進行代理 // 實現的InvocationHandler：引數是目標物件 UserDao jdkproxy = (UserDao) Proxy.newProxyInstance(MyInvocationHandlerTest.class.getClassLoader(), new Class[]{UserDao.class},new MyInvocationHandler(new UserService())); jdkproxy.query("query"); //-----結果------ //log //query } }
接下來檢視 Proxy.newProxyInstance 原始碼探究它的實現過程：
public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) throws IllegalArgumentException { //檢查InvocationHandler是否為空，為空丟擲空指標異常 Objects.requireNonNull(h); //克隆拿到介面 final Class<?>[] intfs = interfaces.clone(); //進行安全校驗 final SecurityManager sm = System.getSecurityManager(); //檢查是否能被代理 if (sm != null) { checkProxyAccess(Reflection.getCallerClass(), loader, intfs); } /* * Look up or generate the designated proxy class. * 得到代理類 */ Class<?> cl = getProxyClass0(loader, intfs); /* * Invoke its constructor with the designated invocation handler. */ try { if (sm != null) { checkNewProxyPermission(Reflection.getCallerClass(), cl); } //通過構造方法得到代理類的物件 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; } }); } //new得到代理物件 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); } }
從上面可以看出 Class<?> cl = getProxyClass0(loader, intfs); 這行程式碼最重要，它可以得到一個代理物件，下面是 Proxy#getProxyClass0 的原始碼：
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 //如果介面的代理類已經存在快取中了，直接從快取中取出來返回，如果不存在則通過ProxyClassFactory建立一個並放入快取中供下次使用 return proxyClassCache.get(loader, interfaces); }
proxyClassCache.get() 的實現在 WeakCache#get() 中：
public V get(K key, P parameter) { Objects.requireNonNull(parameter); expungeStaleEntries(); //從快取中取出代理類的key值，因為代理類的生成需要耗時間和效能，所有快取起來方便下次直接使用 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 獲取到介面資訊 Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter)); Supplier<V> supplier = valuesMap.get(subKey); Factory factory = null; //開始進來supplier是null，所以執行後面的程式碼建立一個Factory然後賦值給supplier while (true) { if (supplier != null) { // supplier might be a Factory or a CacheValue<V> instance //獲取到代理類 V value = supplier.get(); if (value != null) { //返回代理類 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);//通過介面資訊和classLoader建立一個工廠 } 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;//將工廠賦值給supplier } else { // retry with current supplier supplier = valuesMap.get(subKey); } } } }
下面檢視 supplier.get()：WeakCache.Factory#get()
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 { //建立獲取到value 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);//根據代理類建立一個快取物件 // put into reverseMap reverseMap.put(cacheValue, Boolean.TRUE);//將代理放入快取中 // 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; }
接下來檢視 Objects.requireNonNull() 中的 apply方法：Proxy.ProxyClassFactory#apply()
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) { //獲取到所有實現介面 Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length); //遍歷所有介面 for (Class<?> intf : interfaces) { /* * Verify that the class loader resolves the name of this * interface to the same Class object. */ Class<?> interfaceClass = null; try { //對介面再次裝載，相當classLoader.loadClass interfaceClass = Class.forName(intf.getName(), false, loader); } catch (ClassNotFoundException e) { } //判斷兩個介面是不是同一個介面（同一個類載入器載入的）：判斷物件是否是同一個 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()); } } //代理物件的包名 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. */ for (Class<?> intf : interfaces) { int flags = intf.getModifiers(); //判斷介面的型別是否是public，如果不是public，將代理類放到目標物件同一目錄下 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 //預設定義包名 //public static final String PROXY_PACKAGE = "com.sun.proxy"; proxyPkg = ReflectUtil.PROXY_PACKAGE + "."; } /* * Choose a name for the proxy class to generate. */ long num = nextUniqueNumber.getAndIncrement(); //標識：private static final String proxyClassNamePrefix = "$Proxy"; //隨機數（防止多執行緒測試相同的類名）：long num = nextUniqueNumber.getAndIncrement(); //代理類的名字：代理包名+ 標識+ 隨機數 String proxyName = proxyPkg + proxyClassNamePrefix + num; /* * Generate the specified proxy class. * 產生指定的代理類資訊，是二進位制資訊，可以使用IO流輸出代理類的Java檔案（可檢視前文有介紹） * ProxyGenerator.generateProxyClass()是一個靜態方法，所以可以外部直接呼叫 */ byte[] proxyClassFile = ProxyGenerator.generateProxyClass( proxyName, interfaces, accessFlags); try { //產生代理類，返回一個class：將byte位元組碼轉換成class //defineClass0是一個native方法，由JVM實現的：private static native Class<?> defineClass0(); 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()); } }