定义:为其他对象提供一种代理以控制对这个对象的访问。
java动态代理机制以巧妙的方式实现了代理模式的设计理念。
代理模式示例代码
- public interface Subject
- {
- public void doSomething();
- }
- public class RealSubject implements Subject
- {
- public void doSomething()
- {
- System.out.println( "call doSomething()" );
- }
- }
- public class ProxyHandler implements InvocationHandler
- {
- private Object proxied;
- public ProxyHandler( Object proxied )
- {
- this.proxied = proxied;
- }
- public Object invoke( Object proxy, Method method, Object[] args ) throws Throwable
- {
- //在转调具体目标对象之前,可以执行一些功能处理
- //转调具体目标对象的方法
- return method.invoke( proxied, args);
- //在转调具体目标对象之后,可以执行一些功能处理
- }
- }
- import java.lang.reflect.InvocationHandler;
- import java.lang.reflect.Method;
- import java.lang.reflect.Proxy;
- import sun.misc.ProxyGenerator;
- import java.io. * ;
- /**
- * Java学习交流QQ群:589809992 我们一起学Java!
- */
- public class DynamicProxy {
- public static void main(String args[]) {
- RealSubject real = new RealSubject();
- Subject proxySubject = (Subject) Proxy.newProxyInstance(Subject.class.getClassLoader(), new Class[] {
- Subject.class
- },
- new ProxyHandler(real));
- proxySubject.doSomething();
- //write proxySubject class binary data to file
- createProxyClassFile();
- }
- public static void createProxyClassFile() {
- String name = "ProxySubject";
- byte[] data = ProxyGenerator.generateProxyClass(name, new Class[] {
- Subject.class
- });
- try {
- FileOutputStream out = new FileOutputStream(name + ".class");
- out.write(data);
- out.close();
- } catch(Exception e) {
- e.printStackTrace();
- }
- }
- }
首先来看看类Proxy的代码实现 Proxy的主要静态变量
- // 映射表:用于维护类装载器对象到其对应的代理类缓存
- private static Map loaderToCache = new WeakHashMap();
- // 标记:用于标记一个动态代理类正在被创建中
- private static Object pendingGenerationMarker = new Object();
- // 同步表:记录已经被创建的动态代理类类型,主要被方法 isProxyClass 进行相关的判断
- private static Map proxyClasses = Collections.synchronizedMap(new WeakHashMap());
- // 关联的调用处理器引用
- protected InvocationHandler h;
Proxy的构造方法
- // 由于 Proxy 内部从不直接调用构造函数,所以 private 类型意味着禁止任何调用
- private Proxy() {}
- // 由于 Proxy 内部从不直接调用构造函数,所以 protected 意味着只有子类可以调用
- protected Proxy(InvocationHandler h) {this.h = h;}
Proxy静态方法newProxyInstance
- // 检查 h 不为空,否则抛异常
- if (h == null) {
- throw new NullPointerException();
- }
- // 获得与指定类装载器和一组接口相关的代理类类型对象
- Class cl = getProxyClass(loader, interfaces);
- // 通过反射获取构造函数对象并生成代理类实例
- try {
- Constructor cons = cl.getConstructor(constructorParams);
- return (Object) cons.newInstance(new Object[] { h });
- } catch (NoSuchMethodException e) { throw new InternalError(e.toString());
- } catch (IllegalAccessException e) { throw new InternalError(e.toString());
- } catch (InstantiationException e) { throw new InternalError(e.toString());
- } catch (InvocationTargetException e) { throw new InternalError(e.toString());
- }
- }
类Proxy的getProxyClass方法调用ProxyGenerator的 generateProxyClass方法产生ProxySubject.class的二进制数据:
- public
- static
- byte
- []
- generateProxyClass
- (
- final
- String name, Class[] interfaces)
我们可以import sun.misc.ProxyGenerator,调用 generateProxyClass方法产生binary data,然后写入文件,最后通过反编译工具来查看内部实现原理。 反编译后的ProxySubject.java Proxy静态方法newProxyInstance
- import java.lang.reflect.*;
- /**
- * Java学习交流QQ群:589809992 我们一起学Java!
- */
- public final class ProxySubject extends Proxy
- implements Subject
- {
- private static Method m1;
- private static Method m0;
- private static Method m3;
- private static Method m2;
- public ProxySubject(InvocationHandler invocationhandler)
- {
- super(invocationhandler);
- }
- public final boolean equals(Object obj)
- {
- try
- {
- return ((Boolean)super.h.invoke(this, m1, new Object[] {
- obj
- })).booleanValue();
- }
- catch(Error _ex) { }
- catch(Throwable throwable)
- {
- throw new UndeclaredThrowableException(throwable);
- }
- }
- public final int hashCode()
- {
- try
- {
- return ((Integer)super.h.invoke(this, m0, null)).intValue();
- }
- catch(Error _ex) { }
- catch(Throwable throwable)
- {
- throw new UndeclaredThrowableException(throwable);
- }
- }
- public final void doSomething()
- {
- try
- {
- super.h.invoke(this, m3, null);
- return;
- }
- catch(Error _ex) { }
- catch(Throwable throwable)
- {
- throw new UndeclaredThrowableException(throwable);
- }
- }
- public final String toString()
- {
- try
- {
- return (String)super.h.invoke(this, m2, null);
- }
- catch(Error _ex) { }
- catch(Throwable throwable)
- {
- throw new UndeclaredThrowableException(throwable);
- }
- }
- static
- {
- try
- {
- m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] {
- Class.forName("java.lang.Object")
- });
- m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
- m3 = Class.forName("Subject").getMethod("doSomething", new Class[0]);
- m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
- }
- catch(NoSuchMethodException nosuchmethodexception)
- {
- throw new NoSuchMethodError(nosuchmethodexception.getMessage());
- }
- catch(ClassNotFoundException classnotfoundexception)
- {
- throw new NoClassDefFoundError(classnotfoundexception.getMessage());
- }
- }
- }
ProxyGenerator内部是如何生成class二进制数据,可以参考源代码。
- private byte[] generateClassFile() {
- /*
- * Record that proxy methods are needed for the hashCode, equals,
- * and toString methods of java.lang.Object. This is done before
- * the methods from the proxy interfaces so that the methods from
- * java.lang.Object take precedence over duplicate methods in the
- * proxy interfaces.
- */
- addProxyMethod(hashCodeMethod, Object.class);
- addProxyMethod(equalsMethod, Object.class);
- addProxyMethod(toStringMethod, Object.class);
- /*
- * Now record all of the methods from the proxy interfaces, giving
- * earlier interfaces precedence over later ones with duplicate
- * methods.
- */
- for (int i = 0; i < interfaces.length; i++) {
- Method[] methods = interfaces[i].getMethods();
- for (int j = 0; j < methods.length; j++) {
- addProxyMethod(methods[j], interfaces[i]);
- }
- }
- /*
- * For each set of proxy methods with the same signature,
- * verify that the methods' return types are compatible.
- */
- for (List < ProxyMethod > sigmethods: proxyMethods.values()) {
- checkReturnTypes(sigmethods);
- }
- /* ============================================================
- * Step 2: Assemble FieldInfo and MethodInfo structs for all of
- * fields and methods in the class we are generating.
- */
- try {
- methods.add(generateConstructor());
- for (List < ProxyMethod > sigmethods: proxyMethods.values()) {
- for (ProxyMethod pm: sigmethods) {
- // add static field for method's Method object
- fields.add(new FieldInfo(pm.methodFieldName, "Ljava/lang/reflect/Method;", ACC_PRIVATE | ACC_STATIC));
- // generate code for proxy method and add it
- methods.add(pm.generateMethod());
- }
- }
- methods.add(generateStaticInitializer());
- } catch(IOException e) {
- throw new InternalError("unexpected I/O Exception");
- }
- /* ============================================================
- * Step 3: Write the final class file.
- */
- /*
- * Make sure that constant pool indexes are reserved for the
- * following items before starting to write the final class file.
- */
- cp.getClass(dotToSlash(className));
- cp.getClass(superclassName);
- for (int i = 0; i < interfaces.length; i++) {
- cp.getClass(dotToSlash(interfaces[i].getName()));
- }
- /*
- * Disallow new constant pool additions beyond this point, since
- * we are about to write the final constant pool table.
- */
- cp.setReadOnly();
- ByteArrayOutputStream bout = new ByteArrayOutputStream();
- DataOutputStream dout = new DataOutputStream(bout);
- try {
- /*
- * Write all the items of the "ClassFile" structure.
- * See JVMS section 4.1.
- */
- // u4 magic;
- dout.writeInt(0xCAFEBABE);
- // u2 minor_version;
- dout.writeShort(CLASSFILE_MINOR_VERSION);
- // u2 major_version;
- dout.writeShort(CLASSFILE_MAJOR_VERSION);
- cp.write(dout); // (write constant pool)
- // u2 access_flags;
- dout.writeShort(ACC_PUBLIC | ACC_FINAL | ACC_SUPER);
- // u2 this_class;
- dout.writeShort(cp.getClass(dotToSlash(className)));
- // u2 super_class;
- dout.writeShort(cp.getClass(superclassName));
- // u2 interfaces_count;
- dout.writeShort(interfaces.length);
- // u2 interfaces[interfaces_count];
- for (int i = 0; i < interfaces.length; i++) {
- dout.writeShort(cp.getClass(dotToSlash(interfaces[i].getName())));
- }
- // u2 fields_count;
- dout.writeShort(fields.size());
- // field_info fields[fields_count];
- for (FieldInfo f: fields) {
- f.write(dout);
- }
- // u2 methods_count;
- dout.writeShort(methods.size());
- // method_info methods[methods_count];
- for (MethodInfo m: methods) {
- m.write(dout);
- }
- // u2 attributes_count;
- dout.writeShort(0); // (no ClassFile attributes for proxy classes)
- } catch(IOException e) {
- throw new InternalError("unexpected I/O Exception");
- }
- return bout.toByteArray();
一个典型的动态代理创建对象过程可分为以下四个步骤:
1、通过实现InvocationHandler接口创建自己的调用处理器 IvocationHandler handler = new InvocationHandlerImpl(…);
2、通过为Proxy类指定ClassLoader对象和一组interface创建动态代理类
Class clazz = Proxy.getProxyClass(classLoader,new Class[]{…});
3、通过反射机制获取动态代理类的构造函数,其参数类型是调用处理器接口类型
Constructor constructor = clazz.getConstructor(new Class[]{InvocationHandler.class});
4、通过构造函数创建代理类实例,此时需将调用处理器对象作为参数被传入
Interface Proxy = (Interface)constructor.newInstance(new Object[] (handler));
为了简化对象创建过程,Proxy类中的newInstance方法封装了2~4,只需两步即可完成代理对象的创建。
生成的ProxySubject继承Proxy类实现Subject接口,实现的Subject的方法实际调用处理器的invoke方法,而invoke方法利用反射调用的是被代理对象的的方法(Object result=method.invoke(proxied,args))
诚然,Proxy已经设计得非常优美,但是还是有一点点小小的遗憾之处,那就是它始终无法摆脱仅支持interface代理的桎梏,因为它的设计注定了这个遗憾。回想一下那些动态生成的代理类的继承关系图,它们已经注定有一个共同的父类叫Proxy。Java的继承机制注定了这些动态代理类们无法实现对class的动态代理,原因是多继承在Java中本质上就行不通。有很多条理由,人们可以否定对 class代理的必要性,但是同样有一些理由,相信支持class动态代理会更美好。接口和类的划分,本就不是很明显,只是到了Java中才变得如此的细化。如果只从方法的声明及是否被定义来考量,有一种两者的混合体,它的名字叫抽象类。实现对抽象类的动态代理,相信也有其内在的价值。此外,还有一些历史遗留的类,它们将因为没有实现任何接口而从此与动态代理永世无缘。如此种种,不得不说是一个小小的遗憾。但是,不完美并不等于不伟大,伟大是一种本质,Java动态代理就是佐例。
来源: http://blog.csdn.net/qq_37267015/article/details/78709785