guava-retrying重试工具库: RetryListener

当发生重试之后，假如我们需要做一些额外的处理动作，比如发个告警邮件啥的，那么可以使用RetryListener。每次重试之后，guava-retrying会自动回调我们注册的监听。可以注册多个RetryListener，会按照注册顺序依次调用。

import com.github.rholder.retry.Attempt;
import com.github.rholder.retry.RetryListener;

import java.util.concurrent.ExecutionException;

public class MyRetryListener implements RetryListener {

    @Override
    public  void onRetry(Attempt attempt) {

        // 第几次重试,(注意:第一次重试其实是第一次调用)
        System.out.print("[retry]time=" + attempt.getAttemptNumber());

        // 距离第一次重试的延迟
        System.out.print(",delay=" + attempt.getDelaySinceFirstAttempt());

        // 重试结果: 是异常终止, 还是正常返回
        System.out.print(",hasException=" + attempt.hasException());
        System.out.print(",hasResult=" + attempt.hasResult());

        // 是什么原因导致异常
        if (attempt.hasException()) {
            System.out.print(",causeBy=" + attempt.getExceptionCause().toString());
        } else {
            // 正常返回时的结果
            System.out.print(",result=" + attempt.getResult());
        }

        // bad practice: 增加了额外的异常处理代码
        try {
            Boolean result = attempt.get();
            System.out.print(",rude get=" + result);
        } catch (ExecutionException e) {
            System.err.println("this attempt produce exception." + e.getCause().toString());
        }

        System.out.println();
    }
}

public class TestRetryListener {

    private static Callable maySuccessTask = new Callable() {
        private int times = 0;
        @Override
        public Boolean call() throws Exception {
            System.out.println("called");
            times++;

            if (times == 1) {
                throw new NullPointerException();
            } else if (times == 2) {
                return false;
            } else {
                return true;
            }
        }
    };


    public static void main(String[] args) throws Exception {
        // 异常或者返回false都继续重试
        // 每秒重试一次
        // 最多重试5次
        // 允许添加多个RetryListener
        Retryer retryer = RetryerBuilder.newBuilder()
                .retryIfResult(Predicates.equalTo(false))
                .retryIfException()
                .withWaitStrategy(WaitStrategies.fixedWait(1, TimeUnit.SECONDS))
                .withStopStrategy(StopStrategies.stopAfterAttempt(5))
                .withRetryListener(new MyRetryListener<>())
                .withRetryListener(new MyRetryListener<>())
                .build();

        try {
            retryer.call(maySuccessTask);
        } catch (Exception e) {
            System.err.println("still failed after retry.");
            e.printStackTrace();
        }
    }
}