SpringRetry重试机制实现原理

JavaWeb项目中如果想对某个接口设置重试机制，可用Spring的@Retryable注解实现接口重试。本文通过一个简单的例子演示Spring Retry的实现原理，例子中定义的注解只包含重试次数属性，实际上Spring Retry中注解可设置属性要多很多。

一、注解定义

package retry.annotation;

import java.lang.annotation.*;


@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Retryable {

    int maxAttemps() default 0;

}

二、代理实现

以Cglib作为代理工具，先来写个Callback实现，这也是重试的实现的核心逻辑

package retry.interceptor;

import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;
import retry.annotation.Retryable;

import java.lang.reflect.Method;


public class AnnotationAwareRetryOperationsInterceptor implements MethodInterceptor{

    //记录重试次数
    private int times = 0;

    @Override
    public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
        //获取拦截的方法中的Retryable注解
        Retryable retryable = method.getAnnotation(Retryable.class);
        if(retryable == null){
            return proxy.invokeSuper(obj,args);
        }else{ //有Retryable注解，加入异常重试逻辑
            int maxAttemps = retryable.maxAttemps();
            try {
                return proxy.invokeSuper(obj,args);
            } catch (Throwable e) {
                if(times++ == maxAttemps){
                    System.out.println("已达最大重试次数：" + maxAttemps + ",不再重试！");
                }else{
                    System.out.println("调用" + method.getName() + "方法异常，开始第" + times +"次重试。。。");
                    //注意这里不是invokeSuper方法，invokeSuper会退出当前interceptor的处理
                    proxy.invoke(obj,args);
                }
            }
        }
        return null;
    }
}

然后是写个代理类，使用AnnotationAwareRetryOperationsInterceptor作为拦截器

package retry.core;

import net.sf.cglib.proxy.Enhancer;
import retry.interceptor.AnnotationAwareRetryOperationsInterceptor;

/**
 * Created by Jack.wu on 2016/9/30.
 */
public class SpringRetryProxy {

    public Object newProxyInstance(Object target){
        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(target.getClass());
        enhancer.setCallback(new AnnotationAwareRetryOperationsInterceptor());
        return enhancer.create();
    }
}

三、测试
通过一个用户相关的业务方法来测试上面的代码

接口定义：

package facade;

/**
 * Created by Jack.wu on 2016/9/26.
 */
public interface UserFacade {

    void add() throws Exception;

    void query() throws Exception;
}

接口实现：

package facade.impl;

import facade.UserFacade;
import retry.annotation.Retryable;

/**
 * Created by Jack.wu on 2016/9/26.
 */
public class UserFacadeImpl implements UserFacade {

    @Override
    public void add() throws Exception {
        System.out.println("添加用户。。。");
        throw new RuntimeException();
    }

    @Override
    @Retryable(maxAttemps = 3)
    public void query() {
        System.out.println("查询用户。。。");
        throw new RuntimeException();
    }
}

测试：

public class Main {

    public static void main(String[] args) throws Exception{
        UserFacadeImpl user = new UserFacadeImpl();
        //SpringRetry代理测试
        SpringRetryProxy springRetryProxy = new SpringRetryProxy();
        UserFacade u = (UserFacade)springRetryProxy.newProxyInstance(user);
        //u.add();//失败不重试
        u.query();//失败重试
    }
}