Spring温故而知新 – AOP代理

AOP的概念

AOP：Aspect-Oriented Programming（面向切面编程），维基百科的解释如下：Aspect是一种新的模块化机制，用来描述分散在对象、类或者函数中的横切关注点，从关注点中分离出横切关注点是面向切面的程序设计的核心概念。分离关注点使解决特定领域问题的代码从业务逻辑中独立出来，业务逻辑的代码中不在含有针对特定领域问题的代码的调用，业务逻辑同特定领域问题的关系通过切面来封装、维护，这样原本分散在整个应用程序中的变动就可以很好地管理起来。从AOP的角度，应用可以分为横切关注点和业务逻辑代码，实际开发中，这些横切关注点往往会直接嵌入到业务逻辑代码中，面向切面编程就是要解决把横切关注点与业务逻辑相分离

实现方式：

Spring默认使用 JDK 动态代理作为AOP的代理，缺陷是目标类的类必须实现接口，否则不能使用JDK动态代理。如果需要代理的是类而不是接口，那么Spring会默认使用CGLIB代理，关于两者的区别：jdk动态代理是通过java的反射机制来实现的,目标类必须要实现接口，cglib是针对类来实现代理的，他的原理是动态的为指定的目标类生成一个子类，并覆盖其中方法实现增强，但因为采用的是继承，所以不能对final修饰的类进行代理。

JDK动态代理

Jdk动态代理是在程序运行过程中，根据目标类实现的接口来动态生成代理类的class文件,使用主要涉及两个类：

InvocationHandler接口： 它提供了一个invoke(Object obj,Method method, Object[] args)方法供实现者提供相应的代理逻辑的实现。可以对实际的实现进行一些特殊的处理其中参数

　　　　　　　　　　　　Object obj ：被代理的目标类

　　　　　　　　　　　　Method method： 需要执行的目标类的方法

　　　　　　　　　　　　Object[] args ：目标方法的参数

Proxy类：提供一个方法newProxyInstance (ClassLoader loader, Class[] interfaces, InvocationHandler h)来获得动态代理类

示例代码：

public interface OrderService {

    public void createOrder(); 
}

public class OrderServiceImpl implements OrderService {
    
    @Override
    public void createOrder() {
        System.out.println("creating order");
    }
}

public class OrderLogger {
    
    public void beforeCreateOrder(){
        System.out.println("before create order");
    }

    public void afterCreateOrder(){
        System.out.println("after create order");
    }
}

package com.sl.aop;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class ServiceProxy implements InvocationHandler {
    private Object targetClass;
    private OrderLogger orderLogger;

    public ServiceProxy(Object targetClass,OrderLogger orderLogger) {
        this.targetClass = targetClass;
        this.orderLogger = orderLogger;
    }
    
    //获取代理
    public Object GetDynamicProxy()
    {
        return Proxy.newProxyInstance(targetClass.getClass().getClassLoader(), //通过这个ClassLoader生成代理对象
                targetClass.getClass().getInterfaces(),//代理类已实现的接口
                this);  //动态代理调用方法是关联的InvocationHandler,最终通过此InvocationHandler的invoke方法执行真正的方法
    }
    
    //实现相应的代理逻辑
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        this.orderLogger.beforeCreateOrder();
        Object result= method.invoke(targetClass, args);
        this.orderLogger.afterCreateOrder();
        
        return result;
    }

}

测试类：

package com.sl.aop;
import org.junit.Test;
public class AopTest {
    @Test
    public void Testdynamicproxy() {

        OrderServiceImpl serviceImpl = new OrderServiceImpl();
        OrderLogger logger = new OrderLogger();
        OrderService service = (OrderService) new ServiceProxy(serviceImpl, logger).GetDynamicProxy();
        service.createOrder();
    }
}

运行结果：

到这个其实还是有点困惑，Proxy.newProxyInstance（）这个返回的是什么？ Invoke方法在哪里调用的？我们看一下JDK源码：看看DK动态代理的过程是什么样的：

根据源码内部的函数调用Proxy.newProxyInstance()->Proxy.getProxyClass0()->WeakCache.get() ，先定位到

WeakCache.class:

public V get(K key, P parameter) {
        Objects.requireNonNull(parameter);

        expungeStaleEntries();

        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
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;

        while (true) {
            if (supplier != null) {
                // supplier might be a Factory or a CacheValue<V> instance
                V value = supplier.get();
                if (value != null) {
                    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);
            }

            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;
                } else {
                    // retry with current supplier
                    supplier = valuesMap.get(subKey);
                }
            }
        }
    }

可以看到函数return value;  而 V value = supplier.get();   继续往下读可以发现 supper=factory,实际上是一个Factory对象,那么继续查看Factory.get()方法

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 {
                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);

            // try replacing us with CacheValue (this should always succeed)
            if (valuesMap.replace(subKey, this, cacheValue)) {
                // put also in reverseMap
                reverseMap.put(cacheValue, Boolean.TRUE);
            } else {
                throw new AssertionError("Should not reach here");
            }

            // successfully replaced us with new CacheValue -> return the value
            // wrapped by it
            return value;
        }

Return value；那么直接查看赋值语句：value = Objects.requireNonNull(valueFactory.apply(key, parameter)); valueFactory又什么鬼？

public WeakCache(BiFunction<K, P, ?> subKeyFactory,
                     BiFunction<K, P, V> valueFactory) {
        this.subKeyFactory = Objects.requireNonNull(subKeyFactory);
        this.valueFactory = Objects.requireNonNull(valueFactory);
    }

private static final WeakCache<ClassLoader, Class<?>[], Class<?>>  proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

可以知道valueFactory是ProxyClassFactory类型对象，直接查看ProxyClassFactory. Apply()方法

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());
            }
        }
}

直接画重点：

byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);

return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);

调用ProxyGenerator.generateProxyClass最终动态生成一个代理类，但是似乎并未找到何处调用了invoke方法；参考CSDN: https://blog.csdn.net/jiankunking/article/details/52143504这边文章，尝试将这个动态生成的二进制字节码输出到本地，并反编译出来一看究竟，测试代码如下：

public class AopTest {
    @Test
    public void Testdynamicproxy() {

        OrderServiceImpl serviceImpl = new OrderServiceImpl();
        OrderLogger logger = new OrderLogger();
        OrderService service = (OrderService) new ServiceProxy(serviceImpl, logger).GetDynamicProxy();
        service.createOrder();
        //输出动态代理类字节码
        createProxyClassFile();
    }

    private static void createProxyClassFile(){  
        String name = "ProxyObject";  
        byte[] data = ProxyGenerator.generateProxyClass(name,new Class[]{OrderService.class});  
        FileOutputStream out =null;  
        try {  
            out = new FileOutputStream(name+".class");  
            System.out.println((new File("hello")).getAbsolutePath());  
            out.write(data);  
        } catch (FileNotFoundException e) {  
            e.printStackTrace();  
        } catch (IOException e) {  
            e.printStackTrace();  
        }finally {  
            if(null!=out) try {  
                out.close();  
            } catch (IOException e) {  
                e.printStackTrace();  
            }  
        }  
    } 
}

使用java Decompiler工具将这个二进制class文件反编译查看：

具体动态代理类ProxyObject.java：

import com.sl.aop.OrderService;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class ProxyObject
  extends Proxy
  implements OrderService
{
  private static Method m1;
  private static Method m2;
  private static Method m3;
  private static Method m0;
  
  public ProxyObject(InvocationHandler paramInvocationHandler)
  {
    super(paramInvocationHandler);
  }
  
  public final boolean equals(Object paramObject)
  {
    try
    {
      return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
    }
    catch (Error|RuntimeException localError)
    {
      throw localError;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }
  
  public final String toString()
  {
    try
    {
      return (String)this.h.invoke(this, m2, null);
    }
    catch (Error|RuntimeException localError)
    {
      throw localError;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }
  
  public final void createOrder()
  {
    try
    {
      this.h.invoke(this, m3, null);
      return;
    }
    catch (Error|RuntimeException localError)
    {
      throw localError;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }
  
  public final int hashCode()
  {
    try
    {
      return ((Integer)this.h.invoke(this, m0, null)).intValue();
    }
    catch (Error|RuntimeException localError)
    {
      throw localError;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }
  
  static
  {
    try
    {
      m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
      m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
      m3 = Class.forName("com.sl.aop.OrderService").getMethod("createOrder", new Class[0]);
      m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
      return;
    }
    catch (NoSuchMethodException localNoSuchMethodException)
    {
      throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
    }
    catch (ClassNotFoundException localClassNotFoundException)
    {
      throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
    }
  }

终于看到关于invoke的部分了：

public final void createOrder()
  {
    try
    {
      this.h.invoke(this, m3, null);
      return;
    }
    catch (Error|RuntimeException localError)
    {
      throw localError;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }

实际上动态代理类继承自Proxy，并且实现了目标类继承的接口，在createOrder方法中调用了invoke方法，实现了切面逻辑的植入，这里也回答了一个问题，为什么JDK动态代理的目标类必须是实现接口的，因为代理类其实是针对接口代理，而不是针对类来代理的，动态代理类自己继承自Proxy,Java也不允许多重继承。动态代理类和目标类其实是各自实现了接口，代理类通过InvocationHandler.invoke实现对目标类方法的调用。

CGLIB动态代理

CGLIB代理是通过使用一个字节码处理框架ASM，来转换字节码并生成新的类，并在子类中采用方法拦截的技术拦截所有父类方法的调用，实现织如如横切逻辑 ，效率上比使用反射技术的JDK动态代理要高，但是由于CGLIB的原理是动态为目标类生成子类代理类，所以不能为声明为final的方法进行代理。其使用主要涉及两个类：

MethodInterceptor接口：该接口提供一个方法intercept(Object arg0, Method arg1, Object[] arg2, MethodProxy arg3)主要用于拦截目标类方法的调用

　　　　　　　　Object arg0, ：被代理的目标类

 　　　　　　　   Method arg1, 委托方法

　　　　　　　　Object[] arg2, 方法参数

　　　　　　　　MethodProxy arg3 ：代理方法的MethodProxy对象

Enhancer类：用于创建代理类

示例：

实现MethodInterceptor接口

package com.sl.aop;
import java.lang.reflect.Method;
import org.springframework.cglib.proxy.Enhancer;
import org.springframework.cglib.proxy.MethodInterceptor;
import org.springframework.cglib.proxy.MethodProxy;

public class CglibServiceProxy implements MethodInterceptor {

    private Object targetClass;
    private OrderLogger orderLogger;
    
    
    public CglibServiceProxy(Object targetClass,OrderLogger orderLogger) {
        this.targetClass = targetClass;
        this.orderLogger = orderLogger;
    }
    /** 
     * 创建代理对象 
     *
     */  
    public Object getInstance() { 
        Enhancer enhancer = new Enhancer();  
        
        //设置目标类（需要被代理的类）  
        enhancer.setSuperclass(this.targetClass.getClass());
        
        // 回调方法  
        enhancer.setCallback(this);
        
        // 创建代理对象  
        return enhancer.create();  
    }  
  
    /** 
     * 拦截所有目标类方法的调用 
     *
     */
    @Override
    public Object intercept(Object arg0, Method arg1, Object[] arg2, MethodProxy arg3) throws Throwable {
        orderLogger.beforeCreateOrder();  
        
        Object o1 = arg3.invokeSuper(arg0, arg2);
        
        orderLogger.afterCreateOrder();  
        return o1;  
    }
}

测试方法：

public void Testdynamicproxy() {
        System.setProperty(DebuggingClassWriter.DEBUG_LOCATION_PROPERTY, "D:\\class");
        OrderServiceImpl serviceImpl = new OrderServiceImpl();
        OrderLogger logger = new OrderLogger();
        CglibServiceProxy proxy = new CglibServiceProxy(serviceImpl,logger);  
          //通过生成子类的方式创建代理类  
        OrderServiceImpl proxyImp = (OrderServiceImpl)proxy.getInstance();
        proxyImp.createOrder();
        
    }

结果：

最后两种代理对比一下：

JDK动态代理：代理类与委托类实现同一接口，主要是通过代理类实现InvocationHandler并重写invoke方法来进行动态代理的，在invoke方法中将对方法进行增强处理  优点：不需要硬编码接口，代码复用率高，缺点：只能够代理实现了接口的委托类 

CGLIB动态代理：

代理类将委托类作为自己的父类并为其中的非final委托方法创建两个方法，一个是与委托方法签名相同的方法，它在方法中会通过super调用委托方法；另一个是代理类独有的方法。在代理方法中，它会判断是否存在实现了MethodInterceptor接口的对象，若存在则将调用intercept方法对委托方法进行代理  优点：可以在运行时对类或者是接口进行增强操作，且委托类无需实现接口，缺点：不能对final类以及final方法进行代理