JDK代理与Cglib代理底层实现

1. 引入：

Q ：为什么Spring项目里推荐写 Interface Service 然后再实现 Impl ？

A ：解耦，测试方便，面向接口编程，代理方式不同（代理细节）。Spring @Autowired 的对象是通过接口的话会使用JDK动态代理，而JDK动态代理只能对实现了接口的类生成代理，而不能针对类来生成代理；而cglib是针对类来实现代理。

对于这个问题我们可能都知道，jdk代理与cglib代理的区别，但是对其底层的实现却知之甚少。

2. JDK代理与源码实现

2.1 JDK代理实现

  1. 编写代理接口

    public interface PrintService {
    void print(String message);
}

  2. 编写接口实现类

    public class PrintServiceImpl implements PrintService {
    @Override
    public void print(String message) {
        System.out.println("message : "+message);
    }
}

  3. 编写Proxy的调用处理程序 InvocationHandler

    public class MyInvocationHandler implements InvocationHandler {
    

    private Object proxyTarget;//要代理的真实对象

    public MyInvocationHandler(Object proxyTarget) {
        this.proxyTarget = proxyTarget;
    }

    //为PrintService 生成代理对象
    public Object getProxy(){
        return Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(), proxyTarget.getClass().getInterfaces(), this);
    }


    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("before proxy invoke ================"+method.getName());
        Object invoke = method.invoke(proxyTarget, args);
        System.out.println("after proxy invoke ================="+method.getName());
        return invoke;
    }
}

  4. 编写测试代码

    public static void main(String[] args) {
        PrintServiceImpl printService = new PrintServiceImpl();
        MyInvocationHandler invokeHandler = new MyInvocationHandler(printService);
        PrintService proxy =(PrintService) invokeHandler.getProxy();

        System.out.println("proxy class : "+proxy.getClass().getName());
        proxy.print("my proxy test");
    }
/******************************************************************************************/
/**
proxy class : com.sun.proxy.$Proxy0
before proxy invoke ================print
message : my proxy test
after proxy invoke =================print
*/

2.2 JDK代理源码分析

​ 通过上述的示例，我们可以发现Proxy类以及接口 InvocationHandler是实现JDK动态代理的核心。

2.2.1.InvocationHandler：
package java.lang.reflect;

/**
 * {@code InvocationHandler} is the interface implemented by
 * the <i>invocation handler</i> of a proxy instance.
 *
 * <p>Each proxy instance has an associated invocation handler.
 * When a method is invoked on a proxy instance, the method
 * invocation is encoded and dispatched to the {@code invoke}
 * method of its invocation handler.
 *
 * @author      Peter Jones
 * @see         Proxy
 * @since       1.3
 */
public interface InvocationHandler {

    /**
     * Processes a method invocation on a proxy instance and returns
     * the result.  This method will be invoked on an invocation handler
     * when a method is invoked on a proxy instance that it is
     * associated with.
     *
     * @param   proxy the proxy instance that the method was invoked on
     *
     * @param   method the {@code Method} instance corresponding to
     * the interface method invoked on the proxy instance.  The declaring
     * class of the {@code Method} object will be the interface that
     * the method was declared in, which may be a superinterface of the
     * proxy interface that the proxy class inherits the method through.
     *
     * @param   args an array of objects containing the values of the
     * arguments passed in the method invocation on the proxy instance,
     * or {@code null} if interface method takes no arguments.
     * Arguments of primitive types are wrapped in instances of the
     * appropriate primitive wrapper class, such as
     * {@code java.lang.Integer} or {@code java.lang.Boolean}.
     *
     * @return  the value to return from the method invocation on the
     * proxy instance.  If the declared return type of the interface
     * method is a primitive type, then the value returned by
     * this method must be an instance of the corresponding primitive
     * wrapper class; otherwise, it must be a type assignable to the
     * declared return type.  If the value returned by this method is
     * {@code null} and the interface method's return type is
     * primitive, then a {@code NullPointerException} will be
     * thrown by the method invocation on the proxy instance.  If the
     * value returned by this method is otherwise not compatible with
     * the interface method's declared return type as described above,
     * a {@code ClassCastException} will be thrown by the method
     * invocation on the proxy instance.
     *
     * @throws  Throwable the exception to throw from the method
     * invocation on the proxy instance.  The exception's type must be
     * assignable either to any of the exception types declared in the
     * {@code throws} clause of the interface method or to the
     * unchecked exception types {@code java.lang.RuntimeException}
     * or {@code java.lang.Error}.  If a checked exception is
     * thrown by this method that is not assignable to any of the
     * exception types declared in the {@code throws} clause of
     * the interface method, then an
     * {@link UndeclaredThrowableException} containing the
     * exception that was thrown by this method will be thrown by the
     * method invocation on the proxy instance.
     *
     * @see     UndeclaredThrowableException
     */
    public Object invoke(Object proxy, Method method, Object[] args)
        throws Throwable;
}

通过InvocationHandler接口上的注释我们可以了解到，每一个动态代理类的调用处理类都必须实现此接口，并且每个调用处理器类里都要关联代理类实例，当我们通过代理对象去调用一个方法的时候，方法调用就会被转发到InvocationHandler接口实现类中的invoke方法里。

2.2.2. Proxy类
2.2.2.1 newProxyInstance
/**
 * Returns an instance of a proxy class for the specified interfaces
 * that dispatches method invocations to the specified invocation
 * handler.
 */
@CallerSensitive
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);
    }
}

首先我们会调用Proxy.newProxyInstance来获取代理对象，这个方法接收三个参数：

  1. loader : 类加载器classLoader
  2. interfaces : 代理类所要实现的接口
  3. h ： InvocationHandler对象，当前动态代理对象调用方法时所关联的InvocationHandler对象

精简后的代码

public static Object newProxyInstance(ClassLoader loader,
                                      Class<?>[] interfaces,
                                      InvocationHandler h)
    throws IllegalArgumentException
{
    /*
     * Look up or generate the designated proxy class.
     */
    Class<?> cl = getProxyClass0(loader, intfs);
    final Constructor<?> cons = cl.getConstructor(constructorParams);
    return cons.newInstance(new Object[]{h});
}
2.2.2.2 getProxyClass0（loader, intfs）获取代理类的字节码
private static Class<?> getProxyClass0(ClassLoader loader,
                                           Class<?>... interfaces) {
        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
        return proxyClassCache.get(loader, interfaces);
    }

get方法涉及到了代理类的缓存对象ProxyClassCache ，该对象采用WeakReference 实现，GC回收的时候会被回收掉，这里不做过多讲解。

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);
                }
            }
        }
    }

接着看get方法里的subKeyFactory.apply(key, parameter)方法，该方法最终调到ProxyClassFactory.apply方法中

2.2.2.3 ProxyClassFactory.apply(生成代理类的主要方法)

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());
            }
        }
    }

apply方法中，首先加载了要实现的接口，然后做了一系列配置，接着通过ProxyGenerator.generateProxyClass来生成Class字节码文件，再调用defineClass0(native方法)对其进行加载。

public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
        ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
        final byte[] var4 = var3.generateClassFile();
        if (saveGeneratedFiles) {
            AccessController.doPrivileged(new PrivilegedAction<Void>() {
                public Void run() {
                    try {
                        int var1 = var0.lastIndexOf(46);
                        Path var2;
                        if (var1 > 0) {
                            Path var3 = Paths.get(var0.substring(0, var1).replace('.', File.separatorChar));
                            Files.createDirectories(var3);
                            var2 = var3.resolve(var0.substring(var1 + 1, var0.length()) + ".class");
                        } else {
                            var2 = Paths.get(var0 + ".class");
                        }

                        Files.write(var2, var4, new OpenOption[0]);
                        return null;
                    } catch (IOException var4x) {
                        throw new InternalError("I/O exception saving generated file: " + var4x);
                    }
                }
            });
        }

        return var4;
    }
2.2.2.4 接着看回newProxyInstance中的最后两行代码
final Constructor<?> cons = cl.getConstructor(constructorParams);
return cons.newInstance(new Object[]{h});

最后通过代理类获取构造对象，将InvocationHanlder h 作为参数，创建实例对象，所有constructorParams的类型肯定是InvocationHandler.class。至此整代理类的生成过程已经全部完成，可以看出 JDK生成代理类时是基于接口生成代理类把目标类 当做属性放在代理类里面。

3. Cglib代理与底层实现

3.1 Cglib代理实现

  1. 编写代理对象(不需要接口)

    public class PrintServiceImpl {
   
    public void print(String message) {
        System.out.println("message : "+message);
    }
}

  2. 编写MethodInterceptor

    public class MyMethodInterceptor implements MethodInterceptor {

    @Override
    public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
        System.out.println("proxy by cglib ....."+method.getName());
        Object res = methodProxy.invokeSuper(o, objects);//此处一定要是methodProxy来调用，要不会死循环
        return res;
    }
}

    1. Object o ：所生成的代理对象

    2. method ：要被拦截的方法

    3. args ： 方法参数

    4. methodProxy ： 要触发的父类的方法对象(即父类方法代理)

  3. 编写测试方法

     public static void main(String[] args) {
        System.setProperty(DebuggingClassWriter.DEBUG_LOCATION_PROPERTY, "D:\\Cglib");//将生成的代理class写入到文件夹下
        Enhancer enhancer = new Enhancer();
        //设置代理类的对象
        enhancer.setSuperclass(PrintServiceImpl.class);
        //设置代理
        enhancer.setCallback(new MyMethodInterceptor());

        PrintServiceImpl printServiceProxy = (PrintServiceImpl)enhancer.create();
        System.out.println(printServiceProxy.getClass().getName());
     
        printServiceProxy.print("print aaa ");

    }
/*************************************************************************************/
/**
leonardo.ezio.boot.demo.proxy.PrintServiceImpl$$EnhancerByCGLIB$$f8e5363
proxy by cglib .....print
message : print aaa 
*/

3.2 Cglib代理实现原理

3.2.1 create() 

public Object create() {
    this.classOnly = false;
    this.argumentTypes = null;
    return this.createHelper();
}


private Object createHelper() {
    this.preValidate();
    Object key = KEY_FACTORY.newInstance(this.superclass != null ? this.superclass.getName() : null, ReflectUtils.getNames(this.interfaces), this.filter == ALL_ZERO ? null : new WeakCacheKey(this.filter), this.callbackTypes, this.useFactory, this.interceptDuringConstruction, this.serialVersionUID);
    this.currentKey = key;
    Object result = super.create(key);
    return result;
}

createHelper中通过newInstance创建了一个EnhancerKey对象，并以其为参数调用分类的create()方法。

protected Object create(Object key) {
        try {
            ClassLoader loader = this.getClassLoader();
            Map<ClassLoader, AbstractClassGenerator.ClassLoaderData> cache = CACHE;
            AbstractClassGenerator.ClassLoaderData data = (AbstractClassGenerator.ClassLoaderData)cache.get(loader);
            if (data == null) {
                Class var5 = AbstractClassGenerator.class;
                synchronized(AbstractClassGenerator.class) {
                    cache = CACHE;
                    data = (AbstractClassGenerator.ClassLoaderData)cache.get(loader);
                    if (data == null) {
                        Map<ClassLoader, AbstractClassGenerator.ClassLoaderData> newCache = new WeakHashMap(cache);
                        data = new AbstractClassGenerator.ClassLoaderData(loader);
                        newCache.put(loader, data);
                        CACHE = newCache;
                    }
                }
            }

            this.key = key;
            Object obj = data.get(this, this.getUseCache());
            return obj instanceof Class ? this.firstInstance((Class)obj) : this.nextInstance(obj);
        } catch (Error | RuntimeException var9) {
            throw var9;
        } catch (Exception var10) {
            throw new CodeGenerationException(var10);
        }
    }

3.2.2 newInstance() 

/**Enhance类中*/
protected Object nextInstance(Object instance) {
    Enhancer.EnhancerFactoryData data = (Enhancer.EnhancerFactoryData)instance;
    if (this.classOnly) {
        return data.generatedClass;
    } else {
        Class[] argumentTypes = this.argumentTypes;
        Object[] arguments = this.arguments;
        if (argumentTypes == null) {
            argumentTypes = Constants.EMPTY_CLASS_ARRAY;
            arguments = null;
        }

        return data.newInstance(argumentTypes, arguments, this.callbacks);
    }
}

public Object newInstance(Class[] argumentTypes, Object[] arguments, Callback[] callbacks) {
    this.setThreadCallbacks(callbacks);

    Object var4;
    try {
        if (this.primaryConstructorArgTypes == argumentTypes || Arrays.equals(this.primaryConstructorArgTypes, argumentTypes)) {
            var4 = ReflectUtils.newInstance(this.primaryConstructor, arguments);
            return var4;
        }

        var4 = ReflectUtils.newInstance(this.generatedClass, argumentTypes, arguments);
    } finally {
        this.setThreadCallbacks((Callback[])null);
    }

    return var4;
}

最后，通过反射来生成代理对象，阅读源码的过程中我们可以发现cglib生成动态代理的过程中使用了WeakHashMap做为缓存，与jdk中的ProxyClassCache有相似之处，都是采用弱引用实现的。

最后我们可以总结出：

  • JDK基于接口生成代理类 把目标类当做属性放在代理类里面

  • cglib基于class生成实现类 继承目标对象加强

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