SpringAop 之JDK动态代理原理

一、JDK动态代理实现

接口

public interface UserService {
    void findAll();
}

实现类

public class UserServiceImpl implements UserService {

    public void findAll() {
        System.out.println("findAll。。。");
    }
}

定义一个代理类，并实现 InvocationHandler 接口

public class MyJdkProxy implements InvocationHandler {
    private Object target;

    /**
     * 绑定业务对象并返回一个代理类
     */
    public Object createProxy(Object target) {
        this.target = target;
        // 通过反射机制，创建一个代理类对象实例并返回。用户进行方法调用时使用
        // 创建代理对象时，需要传递该业务类的类加载器（用来获取业务实现类的元数据，在包装方法是调用真正的业务方法）、接口、handler实现类
        return Proxy.newProxyInstance(this.getClass().getClassLoader(), target.getClass().getInterfaces(), this);
    }

    /**
     * 包装调用方法：进行预处理、调用后处理
     */
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        Object result = null;
        System.out.println("预处理操作——————");
        // 调用真正的业务方法
        result = method.invoke(target, args);

        System.out.println("调用后处理——————");
        return result;
    }

}

测试类

public class Test {
    public static void main(String[] args) {
        MyJdkProxy jdkProxy = new MyJdkProxy();
        UserService userService = (UserService) jdkProxy.createProxy(new UserServiceImpl());
        userService.findAll();
    }
}

 

二、JDK动态代理，在什么时候执行invoke 方法？

我们再来一个测试，通过生成的代理类直接执行toString() 方法，发现invoke 方法也被调用了一次。那么这是为什么？是什么时候调用了invoke？

public static void main(String[] args) {
    MyJdkProxy jdkProxy = new MyJdkProxy();
    UserService userService = (UserService) jdkProxy.createProxy(new UserServiceImpl());
    userService.toString(); 
}

在MyJdkProxy 类中，生成代理类有一个核心方法 Proxy.newProxyInstance 方法

public Object createProxy(Object target) {
    this.target = target;
    // 通过反射机制，创建一个代理类对象实例并返回。用户进行方法调用时使用
    // 创建代理对象时，需要传递该业务类的类加载器（用来获取业务实现类的元数据，在包装方法是调用真正的业务方法）、接口、handler实现类
    return Proxy.newProxyInstance(target.getClass().getClassLoader(), target.getClass().getInterfaces(), this);
}

我们再来看一下这个 newProxyInstance 方法

public static Object newProxyInstance(ClassLoader loader,
                                      Class<?>[] interfaces,
                                      InvocationHandler h)
    throws IllegalArgumentException  {
    // 如果目标代理类为空，抛出异常
    if (h == null) {
        throw new NullPointerException();
    }

    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.
     */
    // 关键代码：生成接口的代理类的字节码文件
    Class<?> cl = getProxyClass0(loader, intfs);

    /*
     * Invoke its constructor with the designated invocation handler.
     * 通过反射获取构造函数对象并生成代理类实例
     */
    try {
        // 获取代理对象的构造方法（也就是$Proxy0(InvocationHandler h)）
        final Constructor<?> cons = cl.getConstructor(constructorParams);
        final InvocationHandler ih = h;
        if (sm != null && ProxyAccessHelper.needsNewInstanceCheck(cl)) {
            // create proxy instance with doPrivilege as the proxy class may
            // implement non-public interfaces that requires a special permission
            return AccessController.doPrivileged(new PrivilegedAction<Object>() {
                public Object run() {
                    return newInstance(cons, ih);
                }
            });
        } else {
            return newInstance(cons, ih);
        }
    } catch (NoSuchMethodException e) {
        throw new InternalError(e.toString());
    }
}

找到关键代码：Class<?> cl = getProxyClass0(loader, intfs);

private static Class<?> getProxyClass0(ClassLoader loader,
                                       Class<?>... interfaces) {
    // 限定代理的接口不能超过65535个
    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
    // 关键代码：如果缓存中已经存在相应接口的代理类，直接返回；否则，使用ProxyClassFactory创建代理类
    return proxyClassCache.get(loader, interfaces);
}

这里 proxyClassCache 中一个 WeakCache 类型

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

再来看一下这个WeakCache 中的 get 方法

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
    // 核心代码：当WeakCache中没有缓存相应接口的代理类，则会调用ProxyClassFactory类的apply方法来创建代理类
    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);
            }
        }
    }
}

ProxyClassFactory 是Proxy的一个内部类，主要有一个apply 方法

    private static final class ProxyClassFactory
        implements BiFunction<ClassLoader, Class<?>[], Class<?>>
    {
        // 代理类前缀
        private static final String proxyClassNamePrefix = "$Proxy";

        // 生成代理类名称的计数器
        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) {
                /*
                 * Verify that the class loader resolves the name of this
                 * interface to the same Class object.
                 * 校验类加载器是否能通过接口名称加载该类
                 */
                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.
                 * 校验该类是否是接口类型
                 */
                if (!interfaceClass.isInterface()) {
                    throw new IllegalArgumentException(
                        interfaceClass.getName() + " is not an interface");
                }
                /*
                 * Verify that this interface is not a duplicate.
                 * 校验接口是否重复
                 */
                if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
                    throw new IllegalArgumentException(
                        "repeated interface: " + interfaceClass.getName());
                }
            }
            // 代理类包名
            String proxyPkg = null;     // package to define proxy class in

            /*
             * 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.
             * 非public接口，代理类的包名与接口的包名相同
             */
            for (Class<?> intf : interfaces) {
                int flags = intf.getModifiers();
                if (!Modifier.isPublic(flags)) {
                    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
                // public代理接口，使用com.sun.proxy包名
                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
            }

            /*
             * Choose a name for the proxy class to generate.
             * 为代理类生成名字
             */
            long num = nextUniqueNumber.getAndIncrement();
            String proxyName = proxyPkg + proxyClassNamePrefix + num;

            /*
             * Generate the specified proxy class.
             */
             // 核心代码：真正生成代理类的字节码文件的地方
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces);
            try {
                // 使用类加载器将代理类的字节码文件加载到JVM中
                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());
            }
        }
    }

仔细看这段代码，终于找到了其最核心的代码：

byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces);　　// 生成字节码文件

　　

编写测试类，并生成一个代理类的class文件

public class Test {
    public static void main(String[] args) {
        jdkTest();
        createProxyClassFile();
    }

    public static void jdkTest() {
        MyJdkProxy jdkProxy = new MyJdkProxy();
        UserService userService = (UserService) jdkProxy.createProxy(new UserServiceImpl());
        userService.hashCode();
    }
    /**
     * 生成代理类.class文件
     */
    private static void createProxyClassFile() {
        // 设置生成.class文件名称
        String name = "UserServiceProxy";
        byte[] data = ProxyGenerator.generateProxyClass(name, new Class[] { UserService.class });
        FileOutputStream out = null;
        try {
            out = new FileOutputStream(name + ".class");
            // 打印hello文件所在目录，方便找到生成的 UserServiceProxy.class 文件
            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();
                }
        }
    }
}

用jd-jui 工具将生成的字节码反编译：

import com.service.UserService;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class UserServiceProxy extends Proxy implements UserService {
  private static Method m3;
  
  private static Method m1;
  
  private static Method m0;
  
  private static Method m2;
  
  public ProxySubject(InvocationHandler paramInvocationHandler) {
    super(paramInvocationHandler);
  }
  // UserService 中的 findAll() 方法
  public final void findAll() {
    try {
      this.h.invoke(this, m3, null);
      return;
    } catch (Error|RuntimeException error) {
      throw null;
    } catch (Throwable throwable) {
      throw new UndeclaredThrowableException(throwable);
    } 
  }
  
  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 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 String toString() {
    try {
      return (String)this.h.invoke(this, m2, null);
    } catch (Error|RuntimeException error) {
      throw null;
    } catch (Throwable throwable) {
      throw new UndeclaredThrowableException(throwable);
    } 
  }
  
  static {
    try {
      m3 = Class.forName("com.service.UserService").getMethod("findAll", new Class[0]);
      m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
      m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
      m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
      return;
    } catch (NoSuchMethodException noSuchMethodException) {
      throw new NoSuchMethodError(noSuchMethodException.getMessage());
    } catch (ClassNotFoundException classNotFoundException) {
      throw new NoClassDefFoundError(classNotFoundException.getMessage());
    } 
  }
}

我们发现这个类的 toString()、hashCode()、equals 以及 findAll() 方法都是通过  this.h.invoke(this, m1, null) 调用。其中 h 为 InvocationHandler 类型。

由此可总结：在使用jdk动态代理时，相当于会生成一个代理类 ***Proxy.class 文件（实际上并不生成文件，而是以字节码的形式直接加载到JVM中）