Java 動態代理

#Java 動態代理

---

##1.代理模式 首先了解下設計模式中的代理模式 定義：爲其餘對象提供一種代理以控制對這個對象的訪問。在某些狀況下，一個對象不適合或者不能直接引用另外一個對象，而代理對象能夠在客戶端和目標對象之間起到中介的做用。 UML類圖：

代碼：

//Subject 
public abstract class Subject {
public void methodA() {
}
public void methodB() {
}
}

//RealSubject
public class RealSubject extends Subject{

@Override
public void methodA() {
	System.out.println("this is methodA");
}

@Override
public void methodB() {
	System.out.println("this is methodB");
}
}

//Proxy
public class Proxy extends Subject{

private Subject subject;
public Proxy(Subject subject){
	this.subject=subject;
}
@Override
public void methodA() {
	System.out.println("before this methodA");
	subject.methodA();
}

@Override
public void methodB() {
	System.out.println("before this methodB");
	subject.methodB();
}
}

//Main
public class Main {
public static void main(String[] args){
	Proxy proxy =new Proxy(new RealSubject());
	proxy.methodA();
	proxy.methodB();
}
}

輸出爲： before this methodA

this is methodA

before this methodB

this is methodB

##2.JDK動態代理

###2.1瞭解 java.lang.reflect.Proxy類

//代理類，它提供了一組靜態方法來爲一組接口動態地生成代理類及其對象。
		//靜態變量
		//代理緩存
		 private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
    proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
    
    
		// newProxyInstance 該方法用於爲指定類裝載器、一組接口及調用處理器生成動態代理類實例
	  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.
     */
     //生成代理class的地方
    Class<?> cl = getProxyClass0(loader, intfs);

    /*
     * Invoke its constructor with the designated invocation handler.
     */
     
     //利用反射，使用實現的InvocationHandler做爲參數調用構造方法來得到代理類的實例
    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);
    }
}
	  
	  // getProxyClass 該方法用於得到Proxy的Class對象
	  public static Class<?> getProxyClass(ClassLoader loader,Class<?>... interfaces)    throws IllegalArgumentException
{
	//接口clone
    final Class<?>[] intfs = interfaces.clone();
    //安全管理器
    final SecurityManager sm = System.getSecurityManager();
    //檢查可否獲取Class對象
    if (sm != null) {
        checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
    }
	//調用 getProxyClass0方法
    return getProxyClass0(loader, intfs);
}


// getProxyClass0

 private static Class<?> getProxyClass0(ClassLoader loader,Class<?>... interfaces) {
 
 	//首先接口的數目不能超過 65535
    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
    
    //JDK對代理進行了緩存，若是已經存在相應的代理類，則直接返回，不然纔會經過ProxyClassFactory來建立代理
    return proxyClassCache.get(loader, interfaces);
}

//靜態內部類
//ProxyClassFactory

 private static final class ProxyClassFactory
    implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
    // 全部代理類名字的前綴
    private static final String proxyClassNamePrefix = "$Proxy";

   	// 用於生成代理類名字的計數器
    private static final AtomicLong nextUniqueNumber = new AtomicLong();

    @Override
    public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {

        Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
        for (Class<?> intf : interfaces) {
            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");
            }
            
            if (!interfaceClass.isInterface()) {
                throw new IllegalArgumentException(
                    interfaceClass.getName() + " is not an interface");
            }
                           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;
		 
		 // 對於非公共接口，代理類的包名與接口的相同
        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");
                }
            }
        }
        
        // 對於公共接口的包名，默認爲com.sun.proxy
        if (proxyPkg == null) {
            proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
        }

        long num = nextUniqueNumber.getAndIncrement();
        String proxyName = proxyPkg + proxyClassNamePrefix + num;
        
		// 真正的生成代理類的字節碼的地方
        byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
            proxyName, interfaces, accessFlags);
        try {
        	// 根據二進制字節碼返回相應的Class實例
            return defineClass0(loader, proxyName,
                                proxyClassFile, 0, proxyClassFile.length);
        } catch (ClassFormatError e) 
                        throw new IllegalArgumentException(e.toString());
        }
    }
}

###2.2瞭解java.lang.reflect.InvocationHandler類

//處理器接口，自定義invoke方法，集中處理在動態代理對象上的調用，一般在該方法中實現對委託類的代理訪問。
	//第一個參數爲動態代理對象，第二個參數爲委託對象，第三個對象爲調用參數。
	public Object invoke(Object proxy, Method method, Object[] args)

###2.3代碼實現

//HelloWord
	public interface HelloWord {
public void sayHelloWorld();
}

	//HelloWordImpl
	public class HelloWordImpl implements HelloWord {
 public void sayHelloWorld() {
	         System.out.println("HelloWorld!");
	      }
}

//HelloWorldHandler
public class HelloWorldHandler implements InvocationHandler {

// 要代理的原始對象
private Object obj;

public HelloWorldHandler(Object obj) {
	super();
	this.obj = obj;
}

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
	Object result = null;
	// 調用以前
	doBefore();
	// 調用原始對象的方法
	result = method.invoke(obj, args);
	// 調用以後
	doAfter();
	return result;
}

private void doBefore() {
	System.out.println("before method invoke");
}

private void doAfter() {
	System.out.println("after method invoke");
}
}

//Client
public class HelloWorldTest {
public static void main(String[] args) {
    HelloWord helloWorld=new HelloWordImpl();
    InvocationHandler handler=newHelloWorldHandler(helloWorld);
    
    //建立動態代理對象
    HelloWord proxy=(HelloWord)Proxy.newProxyInstance(
            helloWorld.getClass().getClassLoader(), 
            helloWorld.getClass().getInterfaces(), 
            handler);
    proxy.sayHelloWorld();
}
}

輸出結果爲:before method invoke

HelloWorld!

after method invoke

##3.CGLib動態代理

###3.1介紹： CGLib是經過生成java字節碼從而動態的產生代理對象，所以須要字節碼解析處理的依賴asm類庫，字節碼動態生成的代理對象其實是繼承了真實委託類的。這種實現方式須要導入cglib和asm的類庫。 ###3.2原理： 經過字節碼技術爲一個類建立子類，並在子類中採用方法攔截的技術攔截全部父類方法的調用。 ###3.2代碼實現：

//Tiger
public class Tiger{

public void run() {
	System.out.println("Tiger is running");
}
}

//CglibProxy

import java.lang.reflect.Method;

import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;

public class CglibProxy implements MethodInterceptor {

public Object getProxyInstance(Class target) {
	// 聲明加強類實例
	Enhancer enhancer = new Enhancer();
	   // 設置須要建立子類的類
    enhancer.setSuperclass(target);
    enhancer.setCallback(this);
    // 經過字節碼技術動態建立子類實例
    return enhancer.create();
}

public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
	System.out.println("before");
	proxy.invokeSuper(obj, args);
	System.out.println("after");
	return null;
}
}

//Client
public class Client {

public static void main(String[] args) {  
	CglibProxy proxy = new CglibProxy();
    Tiger tiger = (Tiger)proxy.getProxyInstance(Tiger.class);
    tiger.run();
}  
}

輸出爲：before

Tiger is running

after

##4.JDK動態代理和CgLib動態代理比較

* JDK動態代理對有實現接口的對象作代理

* CgLib動態代理對對沒有實現接口的普通類作代理