JDK动态代理之ProxyGenerator生成代理类的字节码文件解析

private byte[] generateClassFile() {
  //第一步, 将所有的方法组装成ProxyMethod对象
  //首先为代理类生成toString, hashCode, equals等代理方法
  addProxyMethod(hashCodeMethod, Object.class);
  addProxyMethod(equalsMethod, Object.class);
  addProxyMethod(toStringMethod, Object.class);
  //遍历每一个接口的每一个方法, 并且为其生成ProxyMethod对象
  for (int i = 0; i < interfaces.length; i++) {
    Method[] methods = interfaces[i].getMethods();
    for (int j = 0; j < methods.length; j++) {
      addProxyMethod(methods[j], interfaces[i]);
    }
  }
  //对于具有相同签名的代理方法, 检验方法的返回值是否兼容
  for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
    checkReturnTypes(sigmethods);
  }
  
  //第二步, 组装要生成的class文件的所有的字段信息和方法信息
  try {
    //添加构造器方法
    methods.add(generateConstructor());
    //遍历缓存中的代理方法
    for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
      for (ProxyMethod pm : sigmethods) {
        //添加代理类的静态字段, 例如:private static Method m1;
        fields.add(new FieldInfo(pm.methodFieldName,
            "Ljava/lang/reflect/Method;", ACC_PRIVATE | ACC_STATIC));
        //添加代理类的代理方法
        methods.add(pm.generateMethod());
      }
    }
    //添加代理类的静态字段初始化方法
    methods.add(generateStaticInitializer());
  } catch (IOException e) {
    throw new InternalError("unexpected I/O Exception");
  }
  
  //验证方法和字段集合不能大于65535
  if (methods.size() > 65535) {
    throw new IllegalArgumentException("method limit exceeded");
  }
  if (fields.size() > 65535) {
    throw new IllegalArgumentException("field limit exceeded");
  }

  //第三步, 写入最终的class文件
  //验证常量池中存在代理类的全限定名
  cp.getClass(dotToSlash(className));
  //验证常量池中存在代理类父类的全限定名, 父类名为:"java/lang/reflect/Proxy"
  cp.getClass(superclassName);
  //验证常量池存在代理类接口的全限定名
  for (int i = 0; i < interfaces.length; i++) {
    cp.getClass(dotToSlash(interfaces[i].getName()));
  }
  //接下来要开始写入文件了,设置常量池只读
  cp.setReadOnly();
  
  ByteArrayOutputStream bout = new ByteArrayOutputStream();
  DataOutputStream dout = new DataOutputStream(bout);
  try {
    //1.写入魔数
    dout.writeInt(0xCAFEBABE);
    //2.写入次版本号
    dout.writeShort(CLASSFILE_MINOR_VERSION);
    //3.写入主版本号
    dout.writeShort(CLASSFILE_MAJOR_VERSION);
    //4.写入常量池
    cp.write(dout);
    //5.写入访问修饰符
    dout.writeShort(ACC_PUBLIC | ACC_FINAL | ACC_SUPER);
    //6.写入类索引
    dout.writeShort(cp.getClass(dotToSlash(className)));
    //7.写入父类索引, 生成的代理类都继承自Proxy
    dout.writeShort(cp.getClass(superclassName));
    //8.写入接口计数值
    dout.writeShort(interfaces.length);
    //9.写入接口集合
    for (int i = 0; i < interfaces.length; i++) {
      dout.writeShort(cp.getClass(dotToSlash(interfaces[i].getName())));
    }
    //10.写入字段计数值
    dout.writeShort(fields.size());
    //11.写入字段集合 
    for (FieldInfo f : fields) {
      f.write(dout);
    }
    //12.写入方法计数值
    dout.writeShort(methods.size());
    //13.写入方法集合
    for (MethodInfo m : methods) {
      m.write(dout);
    }
    //14.写入属性计数值, 代理类class文件没有属性所以为0
    dout.writeShort(0);
  } catch (IOException e) {
    throw new InternalError("unexpected I/O Exception");
  }
  //转换成二进制数组输出
  return bout.toByteArray();
}

public class Proxy0 extends Proxy implements UserDao {

  //第一步, 生成构造器
  protected Proxy0(InvocationHandler h) {
    super(h);
  }

  //第二步, 生成静态域
  private static Method m1;  //hashCode方法
  private static Method m2;  //equals方法
  private static Method m3;  //toString方法
  private static Method m4;  //...
  
  //第三步, 生成代理方法
  @Override
  public int hashCode() {
    try {
      return (int) h.invoke(this, m1, null);
    } catch (Throwable e) {
      throw new UndeclaredThrowableException(e);
    }
  }
  
  @Override
  public boolean equals(Object obj) {
    try {
      Object[] args = new Object[] {obj};
      return (boolean) h.invoke(this, m2, args);
    } catch (Throwable e) {
      throw new UndeclaredThrowableException(e);
    }
  }
  
  @Override
  public String toString() {
    try {
      return (String) h.invoke(this, m3, null);
    } catch (Throwable e) {
      throw new UndeclaredThrowableException(e);
    }
  }
  
  @Override
  public void save(User user) {
    try {
      //构造参数数组, 如果有多个参数往后面添加就行了
      Object[] args = new Object[] {user};
      h.invoke(this, m4, args);
    } catch (Throwable e) {
      throw new UndeclaredThrowableException(e);
    }
  }
  
  //第四步, 生成静态初始化方法
  static {
    try {
      Class c1 = Class.forName(Object.class.getName());
      Class c2 = Class.forName(UserDao.class.getName());  
      m1 = c1.getMethod("hashCode", null);
      m2 = c1.getMethod("equals", new Class[]{Object.class});
      m3 = c1.getMethod("toString", null);
      m4 = c2.getMethod("save", new Class[]{User.class});
      //...
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
  
}