jdk 动态生成代理类

1.代理模式:给用户对象提供一个代理对象,由代理对象来控制对委托类的引用,就像生活中的中介:

2.使用代理模式的好处

1.由于代理对象和委托类实现的接口相同.当用户对象不能或者不方便持有委托类对象时,可以通过代理对象来完成委托类方法的调用,代理对象起到一个中介的作用.也可用来隔离用户和服务

2.符合开闭原则,可以拓展委托类方法.当委托类方法不满足用户对象时,可以通过代理类来进行委托类的方法拓展而不必直接修改委托类.符合开闭原则.代理类主要进行消息预处理,消息的过滤,转发消息给委托类.代理类并不真正实现服务内容.真正的业务是由委托类来实现的,代理类是依靠调用委托类方法来完成服务的调用,因此代理类在调用委托类方法的前后可以拓展公共方法.比如增加日志或者上报信息等

代理模式的实现:

1.静态代理:每一个委托类对应一个代理类,由程序员手动编写,弊端:多个委托类需要编写多个代理类,接口有变化的时候需要修改多个代理类

2.动态代理:可以由 Jdk 通过反射动态生成代理类对象,减少了代码量,同时减少了对业务接口的依赖性.降低了耦合度, 弊端:反射生成代理类对象相较静态代理更耗时,只能支持对接口的代理.动态生成的代理类均有同一个父类为 Proxy.由于 java 的单继承的特性.导致了动态代理不能支持对 Class 的代理

jdk 动态生成代理对象的解析:

时序图如下:

代码分析:

Proxy类中:

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);//获取代理类的Class对象

        /*
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }
             
            //获取代理类的构造方法对象
            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});//用反射通过构造方法生成代理类的对象
        } 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);
        }
    }

方法中 Class<?> cl = getProxyClass0(loader, intfs) 获取到代理类的 Class 对象.跟进 getProxyClass()方法中:

 private static Class<?> getProxyClass0(ClassLoader loader,
                                           Class<?>... interfaces) {
        if (interfaces.length > 65535) { //校验接口数量不超过 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);//获取 Class 对象
    }

其中 ProxyClassCache 是 WeakCache<K, P, V> 类的对象.跟进 get 方法中:

 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
       //获取代理类对象
        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);
                }
            }
        }
    }

get()方法中由subKeyFactory.apply(key, parameter) 获取代理类实例对象.其中 subKeyFactory 变量为BiFunction<K, P, ?>实现类的实例.查看实现类可以追踪到Proxy 的内部类 ProxyClassFactory 类.由此查看 ProxyClassFactory 内的 apply () 方法:

  @Override
        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();//生成自增数
            String proxyName = proxyPkg + proxyClassNamePrefix + num;//此为代理类对象名称

            /*
             * Generate the specified proxy class.
             *生成代理类 Class类的 class 文件的二进制数组
             */
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags); 
            try {
               //defineClass0() 此为 native 方法,由本地 C语言方法根据字节数组生成代理类对象
                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.byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName,interfaces,accessFlags),该方法生成代理类的.class 文件的字节数组返回,并将.class二进制文件保存至本地或者内存中.java 类实例对象加载过程如下:

下面可以用工具类将动态生成的代理类的.class 文件保存至本地目录.将其反编译为 java 文件看一下jdk 生成代理类的代码:

工具方法为:

    public static void generateClassFile(Class clazz,String proxyName){
        /*ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);*/
        byte[] proxyClassFile =ProxyGenerator.generateProxyClass(
                proxyName, new Class[]{clazz});
        String paths = clazz.getResource(".").getPath();
        System.out.println(paths);
        FileOutputStream out = null;

        try {
            out = new FileOutputStream(paths+proxyName+".class");
            out.write(proxyClassFile);
            out.flush();
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            try {
                out.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

生成的.class 文件反编译后的代码如下:

import exce.DzCarFactory;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class dz extends Proxy implements IFactory {
  private static Method m1;
  
  private static Method m3;
  
  private static Method m8;
  
  private static Method m2;
  
  private static Method m5;
  
  private static Method m4;
  
  private static Method m7;
  
  private static Method m9;
  
  private static Method m0;
  
  private static Method m6;
  
  public dz(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 error) {
      throw null;
    } catch (Throwable throwable) {
      throw new UndeclaredThrowableException(throwable);
    } 
  }
  
  public final void product(String paramString) {
    try {
      this.h.invoke(this, m3, new Object[] { paramString });
      return;
    } catch (Error|RuntimeException error) {
      throw null;
    } catch (Throwable throwable) {
      throw new UndeclaredThrowableException(throwable);
    } 
  }
  
  public final void notify() {
    try {
      this.h.invoke(this, m8, null);
      return;
    } catch (Error|RuntimeException error) {
      throw null;
    } catch (Throwable throwable) {
      throw new UndeclaredThrowableException(throwable);
    } 
  }
  
  public final String toString() {
    try {
      return (String)this.h.invoke(this, m2, null);
    } catch (Error|RuntimeException error) {
      throw null;
    } catch (Throwable throwable) {
      throw new UndeclaredThrowableException(throwable);
    } 
  }
  
  public final void wait(long paramLong) throws InterruptedException {
    try {
      this.h.invoke(this, m5, new Object[] { Long.valueOf(paramLong) });
      return;
    } catch (Error|RuntimeException|InterruptedException error) {
      throw null;
    } catch (Throwable throwable) {
      throw new UndeclaredThrowableException(throwable);
    } 
  }
  
  public final void wait(long paramLong, int paramInt) throws InterruptedException {
    try {
      this.h.invoke(this, m4, new Object[] { Long.valueOf(paramLong), Integer.valueOf(paramInt) });
      return;
    } catch (Error|RuntimeException|InterruptedException error) {
      throw null;
    } catch (Throwable throwable) {
      throw new UndeclaredThrowableException(throwable);
    } 
  }
  
  public final Class getClass() {
    try {
      return (Class)this.h.invoke(this, m7, null);
    } catch (Error|RuntimeException error) {
      throw null;
    } catch (Throwable throwable) {
      throw new UndeclaredThrowableException(throwable);
    } 
  }
  
  public final void notifyAll() {
    try {
      this.h.invoke(this, m9, null);
      return;
    } catch (Error|RuntimeException error) {
      throw null;
    } catch (Throwable throwable) {
      throw new UndeclaredThrowableException(throwable);
    } 
  }
  
  public final int hashCode() {
    try {
      return ((Integer)this.h.invoke(this, m0, null)).intValue();
    } catch (Error|RuntimeException error) {
      throw null;
    } catch (Throwable throwable) {
      throw new UndeclaredThrowableException(throwable);
    } 
  }
  
  public final void wait() throws InterruptedException {
    try {
      this.h.invoke(this, m6, null);
      return;
    } catch (Error|RuntimeException|InterruptedException error) {
      throw null;
    } catch (Throwable throwable) {
      throw new UndeclaredThrowableException(throwable);
    } 
  }
  
  static {
    try {
      m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
      m3 = Class.forName("exce.DzCarFactory").getMethod("product", new Class[] { Class.forName("java.lang.String") });
      m8 = Class.forName("exce.DzCarFactory").getMethod("notify", new Class[0]);
      m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
      m5 = Class.forName("exce.DzCarFactory").getMethod("wait", new Class[] { long.class });
      m4 = Class.forName("exce.DzCarFactory").getMethod("wait", new Class[] { long.class, int.class });
      m7 = Class.forName("exce.DzCarFactory").getMethod("getClass", new Class[0]);
      m9 = Class.forName("exce.DzCarFactory").getMethod("notifyAll", new Class[0]);
      m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
      m6 = Class.forName("exce.DzCarFactory").getMethod("wait", new Class[0]);
      return;
    } catch (NoSuchMethodException noSuchMethodException) {
      throw new NoSuchMethodError(noSuchMethodException.getMessage());
    } catch (ClassNotFoundException classNotFoundException) {
      throw new NoClassDefFoundError(classNotFoundException.getMessage());
    } 
  }
}

由反编译出的自动生成的代理类代码可以看出:

1.自动生成的代理类均由一个固定的父类为 Proxy,这决定了 jdk 只能动态代理接口;

2.代理类方法的实现是调用 InvocationHandler 实例的 invoke()方法,如 product()方法中:this.h.invoke(this, m3, new Object[] { paramString });其中参数 m3是利用反射的方式根据委托类的包路径+类名,方法名以及参数类型,拿到的委托类对应方法的 Method 对象,this.h 为 InvocationHandler 实例,由 Proxy 类中 newProxyInstance()方法中获取代理类构造方法时传入.因此在使用 jdk 动态代理 api 时,必须实现 InvocationHandler 的 invoke 方法.动态代理对象才能完成对委托类方法的调用

以上为 jdk 动态生成代理类对象的分析.