Java JDK动态代理的基本原理详细介绍

package com.yao.proxy;
/**
 * Created by robin
 */
public interface Helloworld {
  void sayHello();
}

package com.yao.proxy;
import com.yao.HelloWorld;
/**
 * Created by robin
 */
public class HelloworldImpl implements HelloWorld {
  public void sayHello() {
    System.out.print("hello world");
  }
}

package com.yao.proxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
/**
 * Created by robin
 */
public class MyInvocationHandler implements InvocationHandler{
  private Object target;
  public MyInvocationHandler(Object target) {
    this.target=target;
  }
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    System.out.println("method :"+ method.getName()+" is invoked!");
    return method.invoke(target,args);
  }
}

package com.yao.proxy;
import com.yao.HelloWorld;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Proxy;
/**
 * Created by robin
 */
public class JDKProxyTest {
  public static void main(String[]args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
    //这里有两种写法，我们采用略微复杂的一种写法，这样更有助于大家理解。
    Class<?> proxyClass= Proxy.getProxyClass(JDKProxyTest.class.getClassLoader(),HelloWorld.class);
    final Constructor<?> cons = proxyClass.getConstructor(InvocationHandler.class);
    final InvocationHandler ih = new MyInvocationHandler(new HelloworldImpl());
    HelloWorld helloWorld= (HelloWorld)cons.newInstance(ih);
    helloWorld.sayHello();
    //下面是更简单的一种写法，本质上和上面是一样的
    /* 
    HelloWorld helloWorld=(HelloWorld)Proxy.
         newProxyInstance(JDKProxyTest.class.getClassLoader(),
            new Class<?>[]{HelloWorld.class},
            new MyInvocationHandler(new HelloworldImpl()));
    helloWorld.sayHello();
    */
  }
}

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

 private static final class ProxyClassFactory
    implements BiFunction<ClassLoader, Class<?>[], Class<?>>
  {
    // prefix for all proxy class names
    private static final String proxyClassNamePrefix = "$Proxy";
    // next number to use for generation of unique proxy class names
    private static final AtomicLong nextUniqueNumber = new AtomicLong();
    @Override
    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.
       */
      //生成包名和类名逻辑
      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. 生成代理类的字节码
       * -Dsun.misc.ProxyGenerator.saveGeneratedFiles=true 在该部起作用
       */
      byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
        proxyName, interfaces, accessFlags);
      try {
        //加载到JVM中
        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());
      }
    }
  }

package com.sun.proxy;
import com.yao.HelloWorld;
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 HelloWorld {
  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})).booleanValue();
    } catch (RuntimeException | Error var3) {
      throw var3;
    } catch (Throwable var4) {
      throw new UndeclaredThrowableException(var4);
    }
  }
  public final void sayHello() 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)).intValue();
    } catch (RuntimeException | Error var2) {
      throw var2;
    } catch (Throwable var3) {
      throw new UndeclaredThrowableException(var3);
    }
  }
  static {
    try {
      m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[]{Class.forName("java.lang.Object")});
      m3 = Class.forName("com.yao.HelloWorld").getMethod("sayHello", new Class[0]);
      m2 = Class.forName("java.lang.Object").getMethod("toString", 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());
    }
  }
}