代理模式模式

代理模式模式

  1. 动态代理：代理类可以在被代理类调用方法执行前后穿插其他操作;
  2. Jdk的Proxy实现动态代理：代理类实现InvocationHandler,使用Proxy.newProxyInstance(ClassLoader loader,
    Class<?>[] interfaces, InvocationHandler h)创建代理对象,使用代理对象调用代理方法;
  3. Cglib实现动态代理：创建Enhancer.create()创建代理对象调用代理方法对象,为Enhancer设置代理类,设置MethodInterceptor,这里的MethodInterceptor相当于proxy的InvocationHandler,Enhancer.create()创建代理对象调用代理方法;

JDK动态代理的原理

理解：在我们实现JDK动态代理时的核心代码是调用Proxy类newProxyInstance方法,传入被代理类的ClassLoader,它实现的接口及InvocationHandler,这里我们可以通过Proxy这个核心类了解动态代理的实现原理,它内部首先会为生成代理类的class文件做好准备工作,例如包名为com.sun.proxy,类名为$Proxy加上从零开始的数字,然后会根据传入的代理类的接口中方法再加上hashCode,equals,toString及以InvocationHandler为参数的构造器生成代理类的字节码数组,然后判断是否需要保存文件,保存文件的话创建文件夹,将字节码写入文件,然后返回代理类的class对象,然后根据代理类的class对象获取构造器,然后以传入的InvocationHandler为参数调用构造器的newInstance方法,返回代理对象实例;当使用代理对象执行被代理对象的方法时,通过生成的代理对象的class文件可以发现,它内部其实是调用的是我们传入的InvocationHandler的invoke方法,而我们传入的InvovationHandler的invoke方法的实现逻辑可以由我们自己编写,由此我们就实现了创建代理对象,增强
被代理对象方法的目的;

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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);
    }
}
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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);
}

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);
            }
        }
    }
}
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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());
        }
    }
}
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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;
}
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private static native Class<?> defineClass0(ClassLoader loader, String name, 
                                            byte[] b, int off, int len);

Jdk的Proxy实现动态代理

理解：由Proxy的内部实现决定了jdk反射生成代理必须面向接口，通过ASM动态创建代理

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public class Tank implements Movable {

    @Override
    public void move() {
        System.out.println("Tank moving claclacla...");
        try {
            Thread.sleep(new Random().nextInt(10000));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
        Tank tank = new Tank();

        System.getProperties().put("jdk.proxy.ProxyGenerator.saveGeneratedFiles","true");

        // 参数1：代理类的类加载器
        // 参数2：代理类的接口数组
        // 参数3：InvocationHandler
        Movable m = (Movable)Proxy.newProxyInstance(Tank.class.getClassLoader(),
                new Class[]{Movable.class}, //tank.class.getInterfaces()
                new TimeProxy(tank)
        );

        m.move();

    }
}

class TimeProxy implements InvocationHandler {
    Movable m;

    public TimeProxy(Movable m) {
        this.m = m;
    }

    public void before() {
        System.out.println("method start..");
    }

    public void after() {
        System.out.println("method stop..");
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        //Arrays.stream(proxy.getClass().getMethods()).map(Method::getName)
        //					      .forEach(System.out::println);
        before();
        Object o = method.invoke(m, args);
        after();
        return o;
    }

}

interface Movable {
    void move();
}

Cglib动态代理的原理

理解:Cglib动态代理在我们创建Enhancer对象的时候就会准备好一个缓存,它包含一个类加载器和ClassLoaderData对象,ClassLoaderData里面包含两个Function函数的处理逻辑,一个是为了获取类生成器的key值,一个是为了获取字节码文件,当我们使用Enhancer创建代理对象时,会根据设置进Enhancer的被代理类的Class对象及实现了MethodInterceptor的Callback函数及解析被代理的所有方法,JDk的版本,以及构造方法等信息来构建代理类的字节码,然后根据字节码生成代理对象的实例.

理解:我们在使用Cglib创建代理对象时,首先需要创建一个Enhancer对象,在创建时需要创建一个KEY_FACTORY的内部对象,这个KEY_FACTORY
比较特殊,它是Enhancer的内部接口EnhanerKey类型,没有具体的实现类,需要通过动态代理的方式来创建它的代理实现类,创建Enhancer时还会准备好一个缓存,它包含一个类加载器和ClassLoaderData对象,ClassLoaderData里面包含两个Function函数的处理逻辑,一个是为了获取类生成器的key值,一个是为了获取字节码文件,当我们使用Enhancer创建代理对象时,会通过EnhancerKey接口的代理对象将被代理类的class对象,回调函数等值设置进去,然后再根据被代理类的Class对象及实现了MethodInterceptor的Callback函数及解析被代理的所有方法,JDk的版本,以及构造方法等信息来构建代理类的字节码,然后根据字节码生成代理对象的实例.

Cglib实现动态代理

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// CGLIB实现动态代理代理类不需要实现接口
public class Main {
    public static void main(String[] args) {
        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(Tank.class);
        enhancer.setCallback(new TimeMethodInterceptor());
        Tank tank = (Tank)enhancer.create();
        tank.move();
    }
}

class TimeMethodInterceptor implements MethodInterceptor {

    @Override
    public Object intercept(Object o, Method method, Object[] objects, 
    						MethodProxy methodProxy) throws Throwable {
        System.out.println(o.getClass().getSuperclass().getName());
        System.out.println("before");
        Object result = null;
        result = methodProxy.invokeSuper(o, objects);
        System.out.println("after");
        return result;
    }
}

class Tank {
    public void move() {
        System.out.println("Tank moving claclacla...");
        try {
            Thread.sleep(new Random().nextInt(10000));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}