深入浅析java1.8中动态代理的原理

class proxyed implements pro{
 @Override
 public void text() {
  System.err.println("本方法");
 }
}

interface pro {
 void text();
}

public class JavaProxy implements InvocationHandler {
  private Object source;
  public JavaProxy(Object source) {
   super();
   this.source = source;
  }
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
   System.out.println("before");
   Object invoke = method.invoke(source, args);
   System.out.println("after");
   return invoke;
  }
  public Object getProxy(){
   return Proxy.newProxyInstance(getClass().getClassLoader(), source.getClass().getInterfaces(), this);
  }
  public static void main(String[] args) throws IllegalAccessException, InvocationTargetException, InstantiationException, NoSuchMethodException {
   //第一种，自己写
   //1.设置saveGeneratedFiles值为true则生成 class字节码文件方便分析
   System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
   //2.获取动态代理类
   Class proxyClazz = Proxy.getProxyClass(pro.class.getClassLoader(),pro.class);
   //3.获得代理类的构造函数，并传入参数类型InvocationHandler.class
   Constructor constructor = proxyClazz.getConstructor(InvocationHandler.class);
   //4.通过构造函数来创建动态代理对象，将自定义的InvocationHandler实例传入
   pro iHello = (pro) constructor.newInstance(new JavaProxy(new proxyed()));
   //5.通过代理对象调用目标方法
   iHello.text();
   //第二种，调用JDK提供的方法，实现了2~4步
   Proxy.newProxyInstance(JavaProxy.class.getClassLoader(),proxyed.class.getInterfaces(),new JavaProxy(new proxyed()));
  }
}

public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) throws IllegalArgumentException {
  //Objects.requireNonNull 判空方法，之后所有的单纯的判断null并抛异常，都是此方法
  Objects.requireNonNull(h);
  //clone 类实现的所有接口
  final Class<?>[] intfs = interfaces.clone();
  //获取当前系统安全接口
  final SecurityManager sm = System.getSecurityManager();
  if (sm != null) {
   //Reflection.getCallerClass返回调用该方法的方法的调用类;loader：接口的类加载器
   //进行包访问权限、类加载器权限等检查
   checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
  }

  /*
   * Look up or generate the designated proxy class.
   * 查找或生成代理类
   */
  Class<?> cl = getProxyClass0(loader, intfs);

  /*
   * Invoke its constructor with the designated invocation handler.
   * 使用指定的调用处理程序调用它的构造函数
   */
  try {
   if (sm != null) {
    checkNewProxyPermission(Reflection.getCallerClass(), cl);
   }
   //获取构造
   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;
     }
    });
   }
   //返回 代理对象
   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);
  }
 }

/**
  * a cache of proxy classes：动态代理类的弱缓存容器
  * KeyFactory:根据接口的数量，映射一个最佳的key生成函数,其中表示接口的类对象被弱引用；也就是key对象被弱引用继承自WeakReference（key0、key1、key2、keyX），保存接口密钥(hash值)
  * ProxyClassFactory：生成动态类的工厂
  * 注意，两个都实现了BiFunction[], Object>接口
  */
 private static final WeakCache[], Class<?>> proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

 /**
  * Generate a proxy class. Must call the checkProxyAccess method
  * to perform permission checks before calling this.
  * 生成代理类，调用前必须进行 checkProxyAccess权限检查，所以newProxyInstance进行了权限检查
  */
 private static Class<?> getProxyClass0(ClassLoader loader, Class<?>... interfaces) {
  //实现接口的最大数量<65535;谁写的类能实现这么多接口
  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
  // 如果缓存中有，就直接返回，否则会生成
  return proxyClassCache.get(loader, interfaces);
 }

public V get(K key, P parameter) {
  //key：类加载器；parameter：接口数组
  Objects.requireNonNull(parameter);
  //清除已经被GC回收的弱引用
  expungeStaleEntries();

  //CacheKey弱引用类，refQueue已经被回收的弱引用队列；构建一个CacheKey
  Object cacheKey = CacheKey.valueOf(key, refQueue);
  
  //map一级缓存，获取valuesMap二级缓存
  ConcurrentMap> valuesMap = map.get(cacheKey);
  if (valuesMap == null) {
   ConcurrentMap> oldValuesMap
     = map.putIfAbsent(cacheKey,
     valuesMap = new ConcurrentHashMap<>());
   if (oldValuesMap != null) {
    valuesMap = oldValuesMap;
   }
  }

  // subKeyFactory类型是KeyFactory，apply返回表示接口的key
  Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
  //Factory 实现了supplier，我们实际是获取缓存中的Factory，调用其get方法
  Supplier supplier = valuesMap.get(subKey);
  Factory factory = null;
  
  //下面用到了 CAS+重试 实现的多线程安全的 非阻塞算法
  while (true) {
   if (supplier != null) {
    // 只需要知道，最终会调用get方法，此supplier可能是缓存中取出来的，也可能是Factory新new出来的
    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);
    }
   }
  }
 }

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.
     * 类加载器和接口名解析出的是同一个
     */
   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.
     * 确保是一个接口
     */
   if (!interfaceClass.isInterface()) {
    throw new IllegalArgumentException( interfaceClass.getName() + " is not an interface");
   }
    /*
     * Verify that this interface is not a duplicate.
     * 确保接口没重复
     */
   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.
    * 验证所有非公共的接口在同一个包内；公共的就无需处理
    */
  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.
    * proxyClassNamePrefix = $Proxy
    * nextUniqueNumber 是一个原子类，确保多线程安全，防止类名重复，类似于：$Proxy0，$Proxy1......
    */
  long num = nextUniqueNumber.getAndIncrement();
  String proxyName = proxyPkg + proxyClassNamePrefix + num;
   /*
    * Generate the specified proxy class.
    * 生成类字节码的方法:重点
    */
  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());
  }
 }

public static byte[] generateProxyClass(final String name, Class<?>[] interfaces, int accessFlags) {
  ProxyGenerator gen = new ProxyGenerator(name, interfaces, accessFlags);
  //真正生成字节码的方法
  final byte[] classFile = gen.generateClassFile();
  //如果saveGeneratedFiles为true 则生成字节码文件，所以在开始我们要设置这个参数
  //当然，也可以通过返回的bytes自己输出
  if (saveGeneratedFiles) {
   java.security.AccessController.doPrivileged( new java.security.PrivilegedAction() {
      public Void run() {
       try {
        int i = name.lastIndexOf('.');
        Path path;
        if (i > 0) {
         Path dir = Paths.get(name.substring(0, i).replace('.', File.separatorChar));
         Files.createDirectories(dir);
         path = dir.resolve(name.substring(i+1, name.length()) + ".class");
        } else {
         path = Paths.get(name + ".class");
        }
        Files.write(path, classFile);
        return null;
       } catch (IOException e) {
        throw new InternalError( "I/O exception saving generated file: " + e);
       }
      }
     });
  }
  return classFile;
 }

private byte[] generateClassFile() {
  /* ============================================================
   * Step 1: Assemble ProxyMethod objects for all methods to generate proxy dispatching code for.
   * 步骤1：为所有方法生成代理调度代码，将代理方法对象集合起来。
   */
  //增加 hashcode、equals、toString方法
  addProxyMethod(hashCodeMethod, Object.class);
  addProxyMethod(equalsMethod, Object.class);
  addProxyMethod(toStringMethod, Object.class);
  //增加接口方法
  for (Class<?> intf : interfaces) {
   for (Method m : intf.getMethods()) {
    addProxyMethod(m, intf);
   }
  }

  /*
   * 验证方法签名相同的一组方法，返回值类型是否相同；意思就是重写方法要方法签名和返回值一样
   */
  for (List sigmethods : proxyMethods.values()) {
   checkReturnTypes(sigmethods);
  }

  /* ============================================================
   * Step 2: Assemble FieldInfo and MethodInfo structs for all of fields and methods in the class we are generating.
   * 为类中的方法生成字段信息和方法信息
   */
  try {
   //增加构造方法
   methods.add(generateConstructor());
   for (List sigmethods : proxyMethods.values()) {
    for (ProxyMethod pm : sigmethods) {
     // add static field for method's Method object
     fields.add(new FieldInfo(pm.methodFieldName,
       "Ljava/lang/reflect/Method;",
       ACC_PRIVATE | ACC_STATIC));
     // generate code for proxy method and add it
     methods.add(pm.generateMethod());
    }
   }
   //增加静态初始化信息
   methods.add(generateStaticInitializer());
  } catch (IOException e) {
   throw new InternalError("unexpected I/O Exception", e);
  }

  if (methods.size() > 65535) {
   throw new IllegalArgumentException("method limit exceeded");
  }
  if (fields.size() > 65535) {
   throw new IllegalArgumentException("field limit exceeded");
  }

  /* ============================================================
   * Step 3: Write the final class file.
   * 步骤3：编写最终类文件
   */
  /*
   * Make sure that constant pool indexes are reserved for the following items before starting to write the final class file.
   * 在开始编写最终类文件之前，确保为下面的项目保留常量池索引。
   */
  cp.getClass(dotToSlash(className));
  cp.getClass(superclassName);
  for (Class<?> intf: interfaces) {
   cp.getClass(dotToSlash(intf.getName()));
  }

  /*
   * Disallow new constant pool additions beyond this point, since we are about to write the final constant pool table.
   * 设置只读，在这之前不允许在常量池中增加信息，因为要写常量池表
   */
  cp.setReadOnly();

  ByteArrayOutputStream bout = new ByteArrayOutputStream();
  DataOutputStream dout = new DataOutputStream(bout);

  try {
   // u4 magic;
   dout.writeInt(0xCAFEBABE);
   // u2 次要版本;
   dout.writeShort(CLASSFILE_MINOR_VERSION);
   // u2 主版本
   dout.writeShort(CLASSFILE_MAJOR_VERSION);

   cp.write(dout);    // (write constant pool)

   // u2 访问标识;
   dout.writeShort(accessFlags);
   // u2 本类名;
   dout.writeShort(cp.getClass(dotToSlash(className)));
   // u2 父类名;
   dout.writeShort(cp.getClass(superclassName));
   // u2 接口;
   dout.writeShort(interfaces.length);
   // u2 interfaces[interfaces_count];
   for (Class<?> intf : interfaces) {
    dout.writeShort(cp.getClass(
      dotToSlash(intf.getName())));
   }
   // u2 字段;
   dout.writeShort(fields.size());
   // field_info fields[fields_count];
   for (FieldInfo f : fields) {
    f.write(dout);
   }
   // u2 方法;
   dout.writeShort(methods.size());
   // method_info methods[methods_count];
   for (MethodInfo m : methods) {
    m.write(dout);
   }
   // u2 类文件属性：对于代理类来说没有类文件属性;
   dout.writeShort(0); // (no ClassFile attributes for proxy classes)

  } catch (IOException e) {
   throw new InternalError("unexpected I/O Exception", e);
  }

  return bout.toByteArray();
 }

final class $Proxy0 extends Proxy implements pro {
  //fields 
  private static Method m1;
  private static Method m2;
  private static Method m3;
  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})).booleanValue();
   } catch (RuntimeException | Error var3) {
    throw var3;
   } catch (Throwable var4) {
    throw new UndeclaredThrowableException(var4);
   }
  }

  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 text() throws {
   try {
    //实际就是调用代理类的invoke方法 
    super.h.invoke(this, m3, (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)).intValue();
   } catch (RuntimeException | Error var2) {
    throw var2;
   } catch (Throwable var3) {
    throw new UndeclaredThrowableException(var3);
   }
  }

  static {
   try {
    //这里每个方法对象 和类的实际方法绑定
    m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[]{Class.forName("java.lang.Object")});
    m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
    m3 = Class.forName("spring.commons.api.study.CreateModel.pro").getMethod("text", new Class[0]);
    m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
   } catch (NoSuchMethodException var2) {
    throw new NoSuchMethodError(var2.getMessage());
   } catch (ClassNotFoundException var3) {
    throw new NoClassDefFoundError(var3.getMessage());
   }
  }
 }