首页 > 代码库 > 设计模式-动态代理模式
设计模式-动态代理模式
代理模式是aop编程的基础,其主要作用是操作对象,并将你需要的新功能切入若干个你想要的切入点,静态代理模式比较简单,但是缺点比较大,这里就不上代码了,下面写上动态代理模式的代码(jdk方式,而不是采用cglib):
1 /** 2 * 主题接口 3 * @author TMAC-J 4 * 5 */ 6 interface Subject { 7 void write(); 8 } 9 10 11 12 13 public class SubjectImpl implements Subject{ 14 15 @Override 16 public void write() { 17 System.out.println("---write---"); 18 } 19 20 } 21 22 23 24 import java.lang.reflect.InvocationHandler; 25 import java.lang.reflect.Method; 26 27 public class MyInvocationHandler implements InvocationHandler{ 28 29 private Subject Subject = null; 30 31 public MyInvocationHandler(Subject subject) { 32 this.Subject = subject; 33 } 34 35 @Override 36 public Object invoke(Object object, Method method, Object[] objects) throws Throwable { 37 System.out.println("--before write--"); 38 Object result = method.invoke(Subject, objects); 39 System.out.println("--after write--"); 40 return result; 41 } 42 43 } 44 45 46 import java.lang.reflect.Proxy; 47 48 public class Main { 49 public static void main(String[] args) { 50 Subject subject = new SubjectImpl(); 51 MyInvocationHandler myInvocationHandler = new MyInvocationHandler(subject); 52 Subject subjectProxy = (Subject)Proxy.newProxyInstance(Subject.class.getClassLoader(), new Class[]{Subject.class}, myInvocationHandler); 53 subjectProxy.write(); 54 } 55 }
--before write--
---write---
--after write--
以上是运行结果
jdk动态代理实现的原理如下:
动态代理内部实现
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
public static Object newProxyInstance(ClassLoader loader, Class<?>[]interfaces,InvocationHandler h) throws IllegalArgumentException { // 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.*; 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();
总结
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))
美中不足
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。
本文参考:http://www.cnblogs.com/flyoung2008/archive/2013/08/11/3251148.html
http://blog.csdn.net/heyutao007/article/details/49738887
设计模式-动态代理模式