动态代理源码解析

//Java 实现

@Proxy.java
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() {
                    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);
        }
    }

@Proxy.java
//可以看到,实现的接口数量不能大于 65535 的限制
 private static Class getProxyClass0(ClassLoader loader,
                                           Class... interfaces) {
        if (interfaces.length > 65535) {
            throw new IllegalArgumentException("interface limit exceeded");
        }
        return proxyClassCache.get(loader, interfaces);
    }
@Proxy.java
 private static final WeakCache[], Class>
        proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

//来看 Wakcache 的申明<泛型>
final class WeakCache{
   private final BiFunction subKeyFactory;
    private final BiFunction valueFactory;
  public WeakCache(BiFunction subKeyFactory,
                     BiFunction valueFactory) {
        this.subKeyFactory = Objects.requireNonNull(subKeyFactory);
        this.valueFactory = Objects.requireNonNull(valueFactory);
    }
}

关键方法

 public V get(K key, P parameter) {
        Objects.requireNonNull(parameter);

        expungeStaleEntries();

        Object cacheKey = CacheKey.valueOf(key, refQueue);
        ConcurrentMap> valuesMap = map.get(cacheKey);
        if (valuesMap == null) {
            ConcurrentMap> oldValuesMap
                = map.putIfAbsent(cacheKey,
                                  valuesMap = new ConcurrentHashMap<>());
            if (oldValuesMap != null) {
                valuesMap = oldValuesMap;
            }
        }
        //subKeyFacoty在此处是Proxy$KeyFactory,根据 paramter 的个数返回
        //Proxy$Key1, Proxy$Key2, Proxy$KeyX
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        Supplier supplier = valuesMap.get(subKey);
        Factory factory = null;

        while (true) {
            if (supplier != null) {
                //第一次是查询缓存,发现不存在。第二次进来的 suppiler 被赋值为 Factory
                V value = supplier.get();
                if (value != null) {
                    return value;
                }
            }
       
            if (factory == null) {
                //Factory 包装了参数,还有 valueFactory
                factory = new Factory(key, parameter, subKey, valuesMap);
            }

            if (supplier == null) {
                supplier = valuesMap.putIfAbsent(subKey, factory);
                if (supplier == null) {
                    supplier = factory;
                }
                // else retry with winning supplier
            } else {
                if (valuesMap.replace(subKey, supplier, factory)) {
                    supplier = factory;
                } else {
                    // retry with current supplier
                    supplier = valuesMap.get(subKey);
                }
            }
        }
    }
@WeakCache$Factory
 @Override
        public synchronized V get() { 
            Supplier supplier = valuesMap.get(subKey);
            if (supplier != this) {
                return null;
            }
            V value = null;
            try {
                //关键方法,这里的 valueFactory 就是 Proxy$ProxyClassFactory
                value = Objects.requireNonNull(valueFactory.apply(key, parameter));
            } finally {
                if (value == null) { 
                    valuesMap.remove(subKey, this);
                }
            }
            // the only path to reach here is with non-null value
            assert value != null;

            // wrap value with CacheValue (WeakReference)
            CacheValue cacheValue = new CacheValue<>(value);

            // put into reverseMap
            reverseMap.put(cacheValue, Boolean.TRUE);

            // try replacing us with CacheValue (this should always succeed)
            if (!valuesMap.replace(subKey, this, cacheValue)) {
                throw new AssertionError("Should not reach here");
            }

            // successfully replaced us with new CacheValue -> return the value
            // wrapped by it
            return value;
        }
    }
@Proxy$ProxyClassFactory
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) {
              proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
            }
            long num = nextUniqueNumber.getAndIncrement();
            String proxyName = proxyPkg + proxyClassNamePrefix + num;

            //最最关键
            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());
            }
        }
    }

ProxyGenerator.generateProxyClass(final String name,Class[] interfaces)会动态生成一个类的二进制文件,其中类名为 Proxy$name,实现了 interfaces[]接口,附上动态生成的类的文件

interface Goal {
  void doSomething();
}

interface Goall {
  void doS1();
}
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class $proxy1 extends Proxy implements Goal, Goall {
  private static Method m1;
  private static Method m3;
  private static Method m2;
  private static Method m4;
  private static Method m0;

  public $proxy1(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 void doS1() throws  {
    try {
      super.h.invoke(this, m4, (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("Goal").getMethod("doSomething");
      m2 = Class.forName("java.lang.Object").getMethod("toString");
      m4 = Class.forName("Goall").getMethod("doS1");
      m0 = Class.forName("java.lang.Object").getMethod("hashCode");
    } catch (NoSuchMethodException var2) {
      throw new NoSuchMethodError(var2.getMessage());
    } catch (ClassNotFoundException var3) {
      throw new NoClassDefFoundError(var3.getMessage());
    }
  }
}

可以看到,生成的类是 Java.lang.reflect.Proxy 的子类,并且实现了传入的接口,重写了 equals,toString,hasCode 方法,因此我们在使用的时候,都是将生成的动态代理下向上转型为希望的接口,然后调用方法。因为 Java 的单继承,多接口,造成了直接动态代理的只能是接口。同时可以看到,在调用接口的方法时,都会转到传入的 InvocationHandler去执行。

在 newProxyInstance()@Proxy 中,根据反射生成这个 Proxy 类,传入 InvocationHandler 作为其构造函数。其他的 Cache 之类,都是对传入的class[]是否是接口之类的权限校验,并且进行缓存。InvocationHandler 内部进行 invoke 的时候已经获得了对应方法的 Method,因此只有 Method 的 invoke 的性能损耗,而没有 findMethod 的性能耗损。

关于动态代理缓存的介绍,参见https://www.cnblogs.com/liuyun1995/p/8144676.html

其关于动态代理类的生成,是严格按照 class 文件的格式进行拼接生成的。

Android 中动态代理的实现与 Java 相比,区别在于 generateProxy( )方法的实现。Java 的generateProxy( )是生成对应的二进制字节码文件,然后加载到 JVM 内存。Android 中,是依赖于虚拟机,譬如在 Dalvik 虚拟机中,

//4.0.4
.//dalvik/vm/native/java_lang_reflect_Proxy.cpp
static void Dalvik_java_lang_reflect_Proxy_generateProxy(const u4* args, JValue* pResult){
  StringObject* str = (StringObject*)args[0];
  ArrayObject* interfaces = (ArrayObject*) args[1];
  Object* loader = (Object*) args[2];
  ClassObject* result;
  result = dvmGenerateProxyClass(str, interfaces, loader);
  RETURN_PTR(result);
 }

dvmGenerateProxyClass( )的实现在 Proxy.cpp 中,就是利用虚拟机直接在内存创建一个 Java 层 Class 类在虚拟机对应的 ClassObject 对象,并且为 ClassObject 赋予必要的字段
Dalvik 动态代理的实现

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