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动态代理玩不明白?别紧张,你只是缺少这个 demo

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发布于: 2020 年 10 月 15 日

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概念

JDK动态代理与静态代理是相对的,静态代理是在编译期就确定了代理类和被代理类的关系,且生成代理类。而且静态代理要求被代理的接口必须有其实现,因为代理类实现接口方法实际上是调用接口实现类的方法实现的。而动态代理是在运行期利用JVM的反射机制生成代理类,这里是直接生成类的字节码,然后通过类加载器载入JAVA虚拟机执行。

demo

不做赘述,直接上demo。

被代理类接口

public interface ISubject {
   void doSomething();
}

被代理类实现

public class SubjectImpl implements ISubject {

   @Override
   public void doSomething() {
       System.out.println("doSomething...");
  }
}

InvocationHandler工具类

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
public class JDKDynamicProxy implements InvocationHandler{
   private Object target;
   public JDKDynamicProxy(){
  }
   public JDKDynamicProxy(Object target){
       this.target=target;
  }
   public <T>T getProxy(){
       System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
       // return (T) Proxy.newProxyInstance(target.getClass().getClassLoader(),target.getClass().getInterfaces(),this);
       return (T) Proxy.newProxyInstance(target.getClass().getClassLoader(),target.getClass().getInterfaces(),this);
  }
   @Override
   public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
       System.out.println("before...");
       Object result=method.invoke(target,args);
       System.out.println("after...");
       return result;
  }
}

测试类

public class TestProxy {
   public static void main(String[] args) {
       System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles","true");
       ISubject subject=new JDKDynamicProxy(new SubjectImpl()).getProxy();
       //ISubject subject= (ISubject) Proxy.newProxyInstance(subjectImpl.getClass().getClassLoader(),subjectImpl.getClass().getInterfaces(),new JDKDynamicProxy(subjectImpl));
       //ISubject subject= (ISubject) Proxy.newProxyInstance(ISubject.class.getClassLoader(),new Class[]{ISubject.class},new JDKDynamicProxy(subjectImpl));
       subject.doSomething();
  }
}

运行结果

before...doSomething...after...

生成的代理类文件

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
package com.sun.proxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
public final class $Proxy0 extends Proxy implements ISubject {
   private static Method m1;
   private static Method m3;
   private static Method m2;
   private static Method m0;
   public $Proxy0(InvocationHandler var1) throws {
       super(var1);
  }
   public final boolean equals(Object var1) throws {
       try {
           return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
      } catch (RuntimeException | Error var3) {
           throw var3;
      } catch (Throwable var4) {
           throw new UndeclaredThrowableException(var4);
      }
  }
   public final void doSomething() throws {
       try {
           super.h.invoke(this, m3, (Object[])null);
      } catch (RuntimeException | Error var2) {
           throw var2;
      } catch (Throwable var3) {
           throw new UndeclaredThrowableException(var3);
      }
  }
   public final String toString() throws {
       try {
           return (String)super.h.invoke(this, m2, (Object[])null);
      } catch (RuntimeException | Error var2) {
           throw var2;
      } catch (Throwable var3) {
           throw new UndeclaredThrowableException(var3);
      }
  }
   public final int hashCode() throws {
       try {
           return (Integer)super.h.invoke(this, m0, (Object[])null);
      } catch (RuntimeException | Error var2) {
           throw var2;
      } catch (Throwable var3) {
           throw new UndeclaredThrowableException(var3);
      }
  }
   static {
       try {
           m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
           m3 = Class.forName("ISubject").getMethod("doSomething");
           m2 = Class.forName("java.lang.Object").getMethod("toString");
           m0 = Class.forName("java.lang.Object").getMethod("hashCode");
      } catch (NoSuchMethodException var2) {
           throw new NoSuchMethodError(var2.getMessage());
      } catch (ClassNotFoundException var3) {
           throw new NoClassDefFoundError(var3.getMessage());
      }
  }
}

原理

流程

获取代理类——调用Proxy.newProxyInstance获取代理类的Class实例getProxyClass0(loader, intfs)获取代理类实例的构造方法,并确保其访问权限final Constructor cons = cl.getConstructor(constructorParams)利用反射机制调用构造方法,返回代理类实例,参数是调用Proxy.newProxyInstance时传进来的InvocationHandler实例h 。cons.newInstance(new Object[]{h})

方法实现前面利用反射机制调用代理类的构造方法时传入了InvocationHandler实例h ,代理类$Proxy0的构造方法如下:

public $Proxy0(InvocationHandler var1) throws {
   super(var1);
}



这是调用父类的构造函数,并传入参数h。所以在获取到代理类实例并执行方法时如测试类的subject.doSomething(),实际调用的是代理类内的doSomething(),如下所示。

public final void doSomething() throws {
   try {
       super.h.invoke(this, m3, (Object[])null);
  } catch (RuntimeException | Error var2) {
       throw var2;
  } catch (Throwable var3) {
       throw new UndeclaredThrowableException(var3);
  }
}



所以JDK动态代理的接口方法实现逻辑是完全由InvocationHandler实例的invoke方法决定的 。

获取代理类的Class实例探究

getProxyClass0(loader, intfs)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
   return proxyClassCache.get(loader, interfaces);
}



在Proxy类getProxyClass0(loader, intfs)方法中并无核心代码,主要是方法最后一行去缓存对象中获取代理类实例。

java.lang.reflect.WeakCache 类private static final WeakCache<ClassLoader, Class<?>[], Class<?>>

   proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

proxyClassCache是Proxy类的静态变量,调用WeakCache类的构造函数初始化时,传入keyFactoryproxyClassFactory两个实例。注意WeakCache对象的数据结构包含两级键值。接下来就进入WeakCache类的get方法查看一番。

WeakCache类get方法代码

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);
          }
      }
  }
}

​get方法核心代码解释获取一级和二级键。

//根据类加载器计算一级键 cacheKey=f(loader);
Object cacheKey = CacheKey.valueOf(key, refQueue);
//根据接口计算二级键 subKey=g(interfaces);
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
​注意这里的subKeyFactory是Proxy初始化WeakCache类对象proxyClassCache时的入参new KeyFactory();KeyFactory是Proxy的内部类。不妨进入apply简单看一下:@Override
public Object apply(ClassLoader classLoader, Class<?>[] interfaces) {
   switch (interfaces.length) {
       case 1: return new Key1(interfaces[0]); // the most frequent
       case 2: return new Key2(interfaces[0], interfaces[1]);
       case 0: return key0;
       default: return new KeyX(interfaces);
  }
}

​循环 接下来是一个while循环,第一次循环前factory和supplier都为null ,所以循环体内只是做了一些初始化的工作,并将factory赋值给了supplier(Factory类是WeakCache的内部类,且继承了Supplier类)。第二次循环时,执行V value = supplier.get(),实际上就是调用factory.get()方法获取动态代理类的Class实例 。factory.get()方法方法完整代码如下:

@Override
public synchronized V get() { // serialize access
   // re-check
   Supplier<V> supplier = valuesMap.get(subKey);
   if (supplier != this) {
       // something changed while we were waiting:
       // might be that we were replaced by a CacheValue
       // or were removed because of failure ->
       // return null to signal WeakCache.get() to retry
       // the loop
       return null;
  }
   // else still us (supplier == this)
   // create new value
   V value = null;
   try {
       value = Objects.requireNonNull(valueFactory.apply(key, parameter));
  } finally {
       if (value == null) { // remove us on failure
           valuesMap.remove(subKey, this);
      }
  }
   // the only path to reach here is with non-null value
   assert value != null;
   // wrap value with CacheValue (WeakReference)
   CacheValue<V> cacheValue = new CacheValue<>(value);
   // try replacing us with CacheValue (this should always succeed)
   if (valuesMap.replace(subKey, this, cacheValue)) {
       // put also in reverseMap
       reverseMap.put(cacheValue, Boolean.TRUE);
  } else {
       throw new AssertionError("Should not reach here");
  }
   // successfully replaced us with new CacheValue -> return the value
   // wrapped by it
   return value;
}

​第4行代码Supplier supplier = valuesMap.get(subKey)通过subKey获取方才的supplier也就是factory。然后在第18行调用了valueFactory.apply方法创建动态代理类并将结果赋值给变量value。 valueFactory是初始化WeakCache对象调用构造方法时传入的参数 即Prxoy的内部类ProxyClassFactory对象。

ProxyClassFactory类进入apply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());
  }
}

​该方法前边包括一些校验、计算出访问标志和包名、代理类定名 ,然后在73行创建代理类的字节码文件,并defineClass0将字节码文件加载到虚拟机和返回代理类的Class实例。

如上就是第一次创建代理类的大致流程。

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发布于: 2020 年 10 月 15 日阅读数: 28
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动态代理玩不明白?别紧张,你只是缺少这个demo