1. 代理模式
代理模式
是常用的设计模式之一,其特征是代理类与被代理类有相同的接口,代理类可以为被代理类方法执行进行前置后置处理,增强被代理类方法
代理模式的类结构通常如上图所示,代理类与被代理类之间会存在关联关系,一个代理类的对象持有一个被代理类的对象。代理类的对象本身并不真正实现服务,而是通过调用被代理类对象的相关方法来提供特定的服务
2. 动态代理使用
代理类并不是在 Java 代码中定义,而是在运行时根据在 Java 代码中的“指示”动态生成(字节码由JVM在运行时动态生成而非预存在任何一个 .class 文件中
), 这种在程序运行时创建代理类的代理方式被称为动态代理
,它的优势在于可以方便地对代理类的函数进行统一处理。这是因为所有被代理执行的方法,都是通过InvocationHandler#invoke()
方法调用,相当于给被代理类所有方法套了一层壳,所以只要在这个方法中统一处理,就可以对所有被代理的方法进行相同的操作了
以下代码展示了动态代理的简单使用,其基本步骤如下:
- 定义一个公共接口,本例中为
IHello
,接口中有一个抽象方法- 定义一个实现了公共接口的实体类作为被代理类,本例中被代理类
Hello
实现了IHello
接口,重写了接口中的抽象方法- 定义一个实现了
InvocationHandler
接口的方法拦截类,重写invoke()
方法实现拦截到被代理类方法执行时候的处理逻辑- 通过
Proxy.newProxyInstance()
方法生成代理对象,持有代理对象之后执行接口方法即可
public class ServiceProxy {
public interface IHello {
String sayHi();
}
public static class Hello implements IHello {
@Override
public String sayHi() {
return "Hello";
}
}
// 动态代理类
public static class ProxyHandler<T> implements InvocationHandler {
private T origin;
public ProxyHandler(T origin) {
this.origin = origin;
}
/**
* @param o 代理对象引用
* @param method 正在执行目标的方法
* @param objects 目标方法执行时的入参
*/
@Override
public Object invoke(Object o, Method method, Object[] objects) throws Throwable {
String s = "proxy";
s += method.invoke(origin, objects);
return s;
}
}
public static void main(String[] args) {
IHello IHello = (IHello) getInstance(IHello.class, new ProxyHandler<>(new Hello()));
System.out.println(IHello.toString());
generateProxyClass();
}
// 创建代理对象
public static <T> Object getInstance(Class<T> clazz, ProxyHandler<T> handler) {
return Proxy.newProxyInstance(clazz.getClassLoader(), new Class[]{clazz}, handler);
}
private static void generateProxyClass() {
byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy0", Hello.class.getInterfaces());
String path = "/Users/nathan.yang/workspace/algorithm_Java/out/StuProxy.class";
try (FileOutputStream fos = new FileOutputStream(path)) {
fos.write(classFile);
fos.flush();
System.out.println("代理类文件写入成功");
} catch (Exception e) {
System.out.println("写文件错误");
}
}
}
3. 动态代理原理
-
Proxy#newProxyInstance()
方法是动态代理的入口,其生成动态代理对象主要有以下几个步骤:getProxyClass0()
方法生成代理类- 获取到代理类后将
InvocationHandler
对象入参,反射调用构造方法生成动态代理对象
public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) throws IllegalArgumentException { Objects.requireNonNull(h); final Class<?>[] intfs = interfaces.clone(); final SecurityManager sm = System.getSecurityManager(); if (sm != null) { checkProxyAccess(Reflection.getCallerClass(), loader, intfs); } /* * Look up or generate the designated proxy class. */ Class<?> cl = getProxyClass0(loader, intfs); /* * Invoke its constructor with the designated invocation handler. */ try { if (sm != null) { checkNewProxyPermission(Reflection.getCallerClass(), cl); } final Constructor<?> cons = cl.getConstructor(constructorParams); final InvocationHandler ih = h; if (!Modifier.isPublic(cl.getModifiers())) { AccessController.doPrivileged(new PrivilegedAction<Void>() { public Void run() { cons.setAccessible(true); return null; } }); } return cons.newInstance(new Object[]{h}); } catch (IllegalAccessException|InstantiationException e) { throw new InternalError(e.toString(), e); } catch (InvocationTargetException e) { Throwable t = e.getCause(); if (t instanceof RuntimeException) { throw (RuntimeException) t; } else { throw new InternalError(t.toString(), t); } } catch (NoSuchMethodException e) { throw new InternalError(e.toString(), e); } }
-
Proxy#getProxyClass0()
方法其实是从一个WeakCache
中去获取代理类,其获取逻辑是如果缓存类中没有代理类的话就调用ProxyClassFactory#apply()
,通过代理类工厂去即时生成一个代理类,其步骤如下:- 首先通过指定的类加载器去验证目标接口是否可被其加载
- 通过接口所在包等条件决定代理类所在包及代理类的全限定名称,代理类名称是
包名+$Proxy+id
- 通过
ProxyGenerator.generateProxyClass()
生成字节码数组,将其动态生成代理类并加载
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()); } }
-
反射获取到代理类参数为
InvocationHandler.class
的构造器,其实也就是Proxy
的带参构造器,调用构造器cons.newInstance(new Object[]{h})
生成代理对象protected Proxy(InvocationHandler h) { Objects.requireNonNull(h); this.h = h; }
-
通过以下代码可以将 JVM 中加载的代理类输出成 class 文件,之后就可以使用反编译工具查看代理类的源码
private static void generateProxyClass() { byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy0", Hello.class.getInterfaces()); String path = "/Users/nathan/workspace/algorithm_Java/out/StuProxy.class"; try (FileOutputStream fos = new FileOutputStream(path)) { fos.write(classFile); fos.flush(); System.out.println("代理类文件写入成功"); } catch (Exception e) { System.out.println("写文件错误"); } }
-
生成的代理类源码如下,很明显可以看到该类实现动态代理的原理:
- 通过
static
代码块将被代理类中每一个方法封装为Method
对象,生成方法表 - 代理类对象执行被代理类同名方法时,通过其父类
Proxy
保留的指向InvocationHandler
对象的引用调用InvocationHandler#invoke()
方法,完成动态代理
public final class $Proxy0 extends Proxy implements IHello { private static Method m1; private static Method m3; private static Method m2; private static Method m0; public $Proxy0(InvocationHandler var1) throws { super(var1); } public final String sayHi() throws { try { // 父类 Proxy 保留的指向 InvocationHandler 对象的引用调用 invoke() 方法 return (String)super.h.invoke(this, m3, (Object[])null); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } } public final String toString() throws { try { return (String)super.h.invoke(this, m2, (Object[])null); } catch (RuntimeException | Error var2) { throw var2; } catch (Throwable var3) { throw new UndeclaredThrowableException(var3); } } ...... // 方法表 static { try { m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object")); m3 = Class.forName("ServiceProxy$IHello").getMethod("sayHi"); 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()); } } }
- 通过