jdk动态代理原理解析

jdk动态代理原理解析

上篇文章研究了retrofit的源码,提到了一个重要的概念动态代理,这是一个很重要的知识点,打算详细梳理一下。 相比于静态代理,动态代理的好处是可以让接口里的方法统一做一些处理,而不必要手动添加多个代理类。 先来看一个使用例子,首先建一个接口类,带2个方法:...

上篇文章研究了retrofit的源码,提到了一个重要的概念动态代理,这是一个很重要的知识点,打算详细梳理一下。
相比于静态代理,动态代理的好处是可以让接口里的方法统一做一些处理,而不必要手动添加多个代理类。
先来看一个使用例子,首先建一个接口类,带2个方法:

public interface ServiceApi {
    void login();
    void loginOut();
}

来一个实现类:

public class ServiceApiImp implements ServiceApi{
    @Override
    public void login() {
        Log.d("TAG","登录了");
    }

    @Override
    public void loginOut() {
        Log.d("TAG","退出登录了");
    }
}

再来一个InvocationHandler的子类:

public class ServiceInvocationHandler implements InvocationHandler {
    private ServiceApi serviceApi;
    public ServiceInvocationHandler(ServiceApi serviceApi){
        this.serviceApi = serviceApi;
    }
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        Log.d("TAG","执行了invoke");
        //执行传入的serviceApi的method,
        return method.invoke(serviceApi,args);
    }
}

使用:

//新建ServiceApiImp对象
ServiceApiImp serviceApiImp = new ServiceApiImp();
//新建ServiceInvocationHandler 
ServiceInvocationHandler invocationHandler = new ServiceInvocationHandler(serviceApiImp);
//调用Proxy的newProxyInstance方法,传入接口的classLoader和interface类
//还有invocationHandler 
//这时候得到的是一个ServiceApi 实例
ServiceApi proxyServiceApi = (ServiceApi) Proxy.newProxyInstance(ServiceApi.class.getClassLoader(), new Class>[]{ServiceApi.class},invocationHandler);
//调用它的两个方法
proxyServiceApi.login();
proxyServiceApi.loginOut();

得到打印信息:

2021-07-18 18:12:55.592 5991-5991/com.example.rxjavaex2 D/TAG: 执行了invoke
2021-07-18 18:12:55.592 5991-5991/com.example.rxjavaex2 D/TAG: 登录了
2021-07-18 18:12:55.592 5991-5991/com.example.rxjavaex2 D/TAG: 执行了invoke
2021-07-18 18:12:55.592 5991-5991/com.example.rxjavaex2 D/TAG: 退出登录了

从效果来看,利用动态代理生成的那个ServiceApi实例在调用自身方法的时候看来实际执行了是传入的那个InvocationHandler里的那个invoke()方法:

  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        Log.d("TAG","执行了invoke");
        //执行传入的serviceApi的method,
        return method.invoke(serviceApi,args);
    }

即invoke()方法既执行了代理的ServiceApi实例的方法,同时也执行了自身的代码,而且这个规则对所有的ServiceApi里的方法都是有效的。

带着这个疑问,我们来看看它是怎么做到的,其实从那个Proxy的newProxyInstance()方法我们可以肯定这里构建了一个ServiceApi的实现类,也就是我们要的代理类,并且new了一个它的实例对象,至于这个实现类长什么样子,我们继续看newProxyInstance(ClassLoader loader,Class>[] interfaces,InvocationHandler h)方法:
 public static Object newProxyInstance(ClassLoader loader,Class>[] interfaces,InvocationHandler h)throws IllegalArgumentException{
        Objects.requireNonNull(h);

        final Class>[] intfs = interfaces.clone();
        /*
         * Look up or generate the designated proxy class.
          查找或者生成指定的代理类
         */
        //这个是关键的步骤,注意,就是在这里生成的代理类
        Class> cl = getProxyClass0(loader, intfs);

        try {
            //拿到代理类的构造方法,这是一个入参为InvocationHandler的构造方法
            final Constructor> cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            //判断权限
            if (!Modifier.isPublic(cl.getModifiers())) {
                cons.setAccessible(true);
            }
            //最终通过这个构造方法传入InvocationHandler创建了实例
            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);
        }
    }
看到这里,能猜到那个生成的代理类有一个入参为InvocationHandler数组的构造方法,并且就是用这个构造方法创建了实例。

接下来继续追踪那个构建代理类的方法:

  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
//翻译上面的注释:如果这个通过实现了给定的接口的给定的classLoader定义的代理类是存
//在的,则会返回一个缓存复制品,否则会通过ProxyClassFactory来创建它
        return proxyClassCache.get(loader, interfaces);
    }

proxyClassCache是个缓存工具,这里不深究它,直接看看代理类工厂ProxyClassFactory:

    /**
     * A factory function that generates, defines and returns the proxy class given
     * the ClassLoader and array of interfaces.
     *一个生成,定义,返回给定的classLoader和接口数组的代理类的工厂
     */
//这是Proxy的内部类,主要看apply()方法

    private static final class ProxyClassFactory
        implements BiFunction[], Class>>
    {
        // prefix for all proxy class names
        private static final String proxyClassNamePrefix = "$Proxy";

        // next number to use for generation of unique proxy class names
        private static final AtomicLong nextUniqueNumber = new AtomicLong();
        //搜集接口名的方法,最后拿去生成类
        @Override
        public Class> apply(ClassLoader loader, Class>[] interfaces) {

            Map, 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 the default package.
                proxyPkg = "";
            }

            {
                // Android-changed: Generate the proxy directly instead of calling
                // through to ProxyGenerator.
                List methods = getMethods(interfaces);
                Collections.sort(methods, ORDER_BY_SIGNATURE_AND_SUBTYPE);
                validateReturnTypes(methods);
                List[]> exceptions = deduplicateAndGetExceptions(methods);

                Method[] methodsArray = methods.toArray(new Method[methods.size()]);
                Class>[][] exceptionsArray = exceptions.toArray(new Class>[exceptions.size()][]);

                /*
                 * Choose a name for the proxy class to generate.
                 */
                long num = nextUniqueNumber.getAndIncrement();
                String proxyName = proxyPkg + proxyClassNamePrefix + num;
                //最终把这些参数丢到这个方法里
                return generateProxy(proxyName, interfaces, loader, methodsArray,
                                     exceptionsArray);
            }
        }
    }

//我这个版本是个本地方法,看不到具体实现了- -尴尬了
 private static native Class> generateProxy(String name, Class>[] interfaces,
                                                 ClassLoader loader, Method[] methods,
                                                 Class>[][] exceptions);

很不幸,分析到这里看不下去了,看来只能去网上找找看生成的那个代理类长什么样了,找了一圈发现有很多,结合自己的例子改了改:


// 继承了Proxy类
public final class $Proxy0 extends Proxy implements ServiceApi {
    private static Method m1;
    private static Method m8;
    private static Method m2;
    private static Method m3;
    private static Method m5;
    private static Method m4;
    private static Method m7;
    private static Method m9;
    private static Method m0;
    private static Method m6;
    //构造函数,入参是InvocationHandler 
    public $Proxy0(InvocationHandler var1) throws  {
        super(var1);
    }

....
....

/**
* 这里是代理类实现的被代理对象的接口的相同方法
*/
    public final void login() throws  {
        try {
            // super.h 对应的是父类的h变量,他就是Proxy.nexInstance方法中的InvocationHandler参数
           // 所以这里实际上就是使用了我们自己写的InvocationHandler实现类的invoke方法
            super.h.invoke(this, m3, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }
    public final void loginOut() throws  {
        try {
            //可见logOut跟login都是一样的实现
         
            super.h.invoke(this, m3, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

   

   public final Class getClass() throws  {
        try {
            return (Class)super.h.invoke(this, m7, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

....
....
// 在静态构造块中,代理类通过反射获取了被代理类的详细信息,比如各种方法
    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m8 = Class.forName("com.gwf.jdkproxy.ProductServiceImpl").getMethod("notify");
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m3 = Class.forName("com.gwf.jdkproxy.ProductServiceImpl").getMethod("login");
m4= Class.forName("com.gwf.jdkproxy.ProductServiceImpl").getMethod("loginOut");
            m5 = Class.forName("com.gwf.jdkproxy.ProductServiceImpl").getMethod("wait", Long.TYPE);
         
            m7 = Class.forName("com.gwf.jdkproxy.ProductServiceImpl").getMethod("getClass");
            m9 = Class.forName("com.gwf.jdkproxy.ProductServiceImpl").getMethod("notifyAll");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
            m6 = Class.forName("com.gwf.jdkproxy.ProductServiceImpl").getMethod("wait");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

看到这里应该豁然开朗了吧,其实很多听起来牛逼的技术术语,归根结底都是很原始的实现。

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