动态代理（JDK 动态代理）

小咪咪 2022-02-02 14:09 417阅读 0赞

**动态代理**：** **代理类在程序运行时被创建，这种代理方式称为动态代理

动态代理实现有两种方式，JDK 动态代理和 CGlib 动态代理，本文介绍JDK 动态代理

**JDK 动态代理**：创建动态代理类，实现 InvocationHandler 接口，重写 invoke 方法，在 invoke 方法中通过反射调用被代理类的方法，实现代理。jdk 动态代理只能代理接口，即只能代理接口的实现类

JDK 动态代理示例代码

定义一个接口

package com.jdkdynamicproxy.service;
    
    public interface Subject {
    	
    	void proxyMethod();
    
    }

被代理类，实现接口

package com.jdkdynamicproxy.service.impl;
    
    import com.jdkdynamicproxy.service.Subject;
    
    /**
     * 被代理类
    * @author Administrator
    */
    public class AgentTarget implements Subject {
    
    	@Override
    	public void proxyMethod() {
    		System.out.println("被代理的方法执行了");
    	}
    }

动态代理类,实现 InvocationHandler 接口

package com.jdkdynamicproxy.jdkproxy;
    
    import java.lang.reflect.InvocationHandler;
    import java.lang.reflect.Method;
    import com.jdkdynamicproxy.service.impl.AgentTarget;
    
    /**
       * 动态代理类,实现 InvocationHandler接口
     * @author Administrator
     */
    public class JdkDynamicProxySubject implements InvocationHandler {
    	
    	private AgentTarget agentTarget;
    
    	public JdkDynamicProxySubject(AgentTarget agentTarget) {
    		this.agentTarget = agentTarget;
    	}
    
    	@Override
    	public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    		    //动态代理逻辑
                System.out.println("before");
                Object res = null;
                try {
            	     //用反射调用被代理类的方法
                     res = method.invoke(agentTarget, args);
                }catch (Exception ex){
                    System.out.println("ex:" + ex.getMessage());
                    throw ex;
                }finally {
                    System.out.println("after");
                }
                return res;
    	}
    }

测试类

package com.jdkdynamicproxy;
    
    import java.lang.reflect.Proxy;
    import com.jdkdynamicproxy.jdkproxy.JdkDynamicProxySubject;
    import com.jdkdynamicproxy.service.Subject;
    import com.jdkdynamicproxy.service.impl.AgentTarget;
    
    public class JdkDynamicProxyMain {
    
    	public static void main(String[] args) {
    		
    	      //配置系统属性sun.misc.ProxyGenerator.saveGeneratedFile为true，代理类生成时会自动将生成的代理类写入硬盘
                  System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
    
                  Subject subject = (Subject) Proxy.newProxyInstance(
        		          JdkDynamicProxyMain.class.getClassLoader(), //类加载器
                              new Class[]{Subject.class} ,  //代理接口
                              new JdkDynamicProxySubject(new AgentTarget()) //InvocationHandler对象
                  );
          
                  subject.proxyMethod();
    	}
    }

运行如下

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dzanp6Y2Jx_size_16_color_FFFFFF_t_70][]

程序运行后，在项目目录下生成了代理类的 class 文件

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dzanp6Y2Jx_size_16_color_FFFFFF_t_70 1][]

对生成的 $Proxy0.class 文件进行反编译

package com.sun.proxy;
    
    import com.jdkdynamicproxy.service.Subject;
    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 Subject {
    	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});
          } 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 proxyMethod() throws  {
          try {
             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);
          } 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"));
    			m2 = Class.forName("java.lang.Object").getMethod("toString");
    			m3 = Class.forName("com.jdkdynamicproxy.service.Subject").getMethod("proxyMethod");
    			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 $Proxy0.class 实现了 Subject 接口，重写 proxyMethod 方法。在重写的 proxyMethod 方法中，去调用 JdkDynamicProxySubject 的 invoke(Object proxy, Method method, Object\[\] args) 方法，实现代理

**整个程序的运行流程**：Proxy.newProxyInstance(JdkDynamicProxyMain.class.getClassLoader(), new Class\[\]\{Subject.class\} , new JdkDynamicProxySubject(new AgentTarget()) )  创建对象 subject 时，生成了代理类 $Proxy0.class；subject 对象调用proxyMethod 方法时，proxyMethod 方法调用 JdkDynamicProxySubject 的 invoke 方法，invoke 方法中通过反射调用被代理方法，实现动态代理

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dzanp6Y2Jx_size_16_color_FFFFFF_t_70]: /images/20220202/127d52e838344700a856c343773d017e.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dzanp6Y2Jx_size_16_color_FFFFFF_t_70 1]: /images/20220202/f45d6e88f53d4742bdce55a9312cdb96.png