参考1
参考2
参考3

一：基本概念

代理：在某些情况下，我们不希望或是不能直接访问对象 A，而是通过访问一个中介对象 B，由 B 去访问 A 达成目的，这种方式我们就称为代理。
静态代理：代理类在程序运行前已经存在的代理方式称为静态代理。由开发人员编写或是编译器生成代理类的方式都属于静态代理；静态代理中代理类和委托类也常常继承同一父类或实现同一接口。
动态代理： 代理类在程序运行前不存在、运行时由程序动态生成的代理方式称为动态代理。Java 提供了动态代理的实现方式，可以在运行时刻动态生成代理类。

二：基本原理

实现动态代理包括三步：
(1) 新建委托类及接口。
(2) 实现 InvocationHandler 接口，这是连接委托类和代理类的中间接口。
(3) 通过Proxy.newProxyInstance() 新建代理类。
(1)新建委托类(省略)
(2)实现 InvocationHandler 接口

new InvocationHandler() {
          private final Platform platform = Platform.get();

          @Override 
           public Object invoke(Object proxy, Method method, Object... args)
              throws Throwable {
            // If the method is a method from Object then defer to normal invocation.
            if (method.getDeclaringClass() == Object.class) {
              return method.invoke(this, args);
            }
            if (platform.isDefaultMethod(method)) {
              return platform.invokeDefaultMethod(method, service, proxy, args);
            }
            ServiceMethod serviceMethod = loadServiceMethod(method);
            OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
            return serviceMethod.callAdapter.adapt(okHttpCall);
          }
        }

该接口唯一的方法

public Object invoke(Object proxy, Method method, Object[] args)

proxy：Proxy.newProxyInstance() 生成的代理类对象。
method：表示代理对象被调用的函数。
args：args表示代理对象被调用的函数的参数。
调用代理对象的每个函数实际最终都是调用了InvocationHandler
的invoke函数。

(3) 通过 Proxy 类静态函数生成代理对象

public class Main {
    public static void main(String[] args) {
        // create proxy instance
        TimingInvocationHandler timingInvocationHandler = new TimingInvocationHandler(new OperateImpl());
        Operate operate = (Operate)(Proxy.newProxyInstance(Operate.class.getClassLoader(), new Class[] {Operate.class},
                timingInvocationHandler));

        // call method of proxy instance
        operate.operateMethod1();
        System.out.println();
        operate.operateMethod2();
        System.out.println();
        operate.operateMethod3();
    }
}

Proxy.newProxyInstance()

public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h)

loader：表示类加载器
interfaces：表示委托类的接口，生成代理类时需要实现这些接口
h：是InvocationHandler实现类对象，负责连接代理类和委托类的中间类。

动态代理实现实际是双层的静态代理，开发者提供了委托类 B，程序动态生成了代理类 A。开发者还需要提供一个实现了InvocationHandler
的子类 C，子类 C 连接代理类 A 和委托类 B，它是代理类 A 的委托类，委托类 B 的代理类。用户直接调用代理类 A 的对象，A 将调用转发给委托类 C，委托类 C 再将调用转发给它的委托类 B。
生成的动态代理类代码(生成代理时用到了反射的原理)

import com.codekk.java.test.dynamicproxy.Operate;
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 Operate
{
  private static Method m4;
  private static Method m1;
  private static Method m5;
  private static Method m0;
  private static Method m3;
  private static Method m2;

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

  public final void operateMethod1()
    throws 
  {
    try
    {
      h.invoke(this, m4, null);
      return;
    }
    catch (Error|RuntimeException localError)
    {
      throw localError;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }

  public final boolean equals(Object paramObject)
    throws 
  {
    try
    {
      return ((Boolean)h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
    }
    catch (Error|RuntimeException localError)
    {
      throw localError;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }

  public final void operateMethod2()
    throws 
  {
    try
    {
      h.invoke(this, m5, null);
      return;
    }
    catch (Error|RuntimeException localError)
    {
      throw localError;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }

  public final int hashCode()
    throws 
  {
    try
    {
      return ((Integer)h.invoke(this, m0, null)).intValue();
    }
    catch (Error|RuntimeException localError)
    {
      throw localError;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }

  public final void operateMethod3()
    throws 
  {
    try
    {
      h.invoke(this, m3, null);
      return;
    }
    catch (Error|RuntimeException localError)
    {
      throw localError;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }

  public final String toString()
    throws 
  {
    try
    {
      return (String)h.invoke(this, m2, null);
    }
    catch (Error|RuntimeException localError)
    {
      throw localError;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }

  static
  {
    try
    {
      m4 = Class.forName("com.codekk.java.test.dynamicproxy.Operate").getMethod("operateMethod1", new Class[0]);
      m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
      m5 = Class.forName("com.codekk.java.test.dynamicproxy.Operate").getMethod("operateMethod2", new Class[0]);
      m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
      m3 = Class.forName("com.codekk.java.test.dynamicproxy.Operate").getMethod("operateMethod3", new Class[0]);
      m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
      return;
    }
    catch (NoSuchMethodException localNoSuchMethodException)
    {
      throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
    }
    catch (ClassNotFoundException localClassNotFoundException)
    {
      throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
    }
  }
}

Proxy.newProxyInstance():动态生成代理类。先获取代理类的Class对象，在通过反射获取到代理类的构造方法，最后通过newInstance()构造代理对象。

 return getProxyClass(loader, interfaces) 
.getConstructor(InvocationHandler.class) 
.newInstance(invocationHandler);

public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces,
                                          InvocationHandler invocationHandler)
            throws IllegalArgumentException {

        if (invocationHandler == null) {
            throw new NullPointerException("invocationHandler == null");
        }
        Exception cause;
        try {
            return getProxyClass(loader, interfaces)
                    .getConstructor(InvocationHandler.class)
                    .newInstance(invocationHandler);
        } catch (NoSuchMethodException e) {
            cause = e;
        } catch (IllegalAccessException e) {
            cause = e;
        } catch (InstantiationException e) {
            cause = e;
        } catch (InvocationTargetException e) {
            cause = e;
        }
        AssertionError error = new AssertionError();
        error.initCause(cause);
        throw error;
    }

Proxy.getProxyClass()：如果这个代理类已经创建过，直接从ClassLoader的缓存获取，否则调用native方法generateProxy方法创建代理类对应的class。

 public static Class<?> getProxyClass(ClassLoader loader, Class<?>... interfaces)
            throws IllegalArgumentException {
        if (loader == null) {
            loader = ClassLoader.getSystemClassLoader();
        }

        if (interfaces == null) {
            throw new NullPointerException("interfaces == null");
        }

        // Make a copy of the list early on because we're using the list as a
        // cache key and we don't want it changing under us.
        final List<Class<?>> interfaceList = new ArrayList<Class<?>>(interfaces.length);
        Collections.addAll(interfaceList, interfaces);

        // We use a HashSet *only* for detecting duplicates and null entries. We
        // can't use it as our cache key because we need to preserve the order in
        // which these interfaces were specified. (Different orders should define
        // different proxies.)
        final Set<Class<?>> interfaceSet = new HashSet<Class<?>>(interfaceList);
        if (interfaceSet.contains(null)) {
            throw new NullPointerException("interface list contains null: " + interfaceList);
        }

        if (interfaceSet.size() != interfaces.length) {
            throw new IllegalArgumentException("duplicate interface in list: " + interfaceList);
        }

        synchronized (loader.proxyCache) {
            Class<?> proxy = loader.proxyCache.get(interfaceList);
            if (proxy != null) {
                return proxy;
            }
        }

        String commonPackageName = null;
        for (Class<?> c : interfaces) {
            if (!c.isInterface()) {
                throw new IllegalArgumentException(c + " is not an interface");
            }
            if (!isVisibleToClassLoader(loader, c)) {
                throw new IllegalArgumentException(c + " is not visible from class loader");
            }
            if (!Modifier.isPublic(c.getModifiers())) {
                String packageName = c.getPackageName$();
                if (packageName == null) {
                    packageName = "";
                }
                if (commonPackageName != null && !commonPackageName.equals(packageName)) {
                    throw new IllegalArgumentException(
                            "non-public interfaces must be in the same package");
                }
                commonPackageName = packageName;
            }
        }

        List<Method> methods = getMethods(interfaces);
        Collections.sort(methods, ORDER_BY_SIGNATURE_AND_SUBTYPE);
        validateReturnTypes(methods);
        List<Class<?>[]> exceptions = deduplicateAndGetExceptions(methods);
        Method[] methodsArray = methods.toArray(new Method[methods.size()]);
        Class<?>[][] exceptionsArray = exceptions.toArray(new Class<?>[exceptions.size()][]);

        String baseName = commonPackageName != null && !commonPackageName.isEmpty()
                ? commonPackageName + ".$Proxy"
                : "$Proxy";

        Class<?> result;
        synchronized (loader.proxyCache) {
            result = loader.proxyCache.get(interfaceList);
            if (result == null) {
                String name = baseName + nextClassNameIndex++;
                result = generateProxy(name, interfaces, loader, methodsArray, exceptionsArray);
                loader.proxyCache.put(interfaceList, result);
            }
        }

        return result;
    }