[Guava Retrying] Guava Retrying

This is a small extension to Google's Guava library to allow for the creation of configurable retrying strategies for an arbitrary function call...

Posted by Mr.Vincent on 2020-04-30
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上章节我们学习了 Spring Retry 的重试机制,本章将学习另一种重试机制 Guava Retrying
在学习前,我们先来回顾下重试的使用场景。

重试的使用场景

在很多业务场景中,为了排除系统中的各种不稳定因素,以及逻辑上的错误,并最大概率保证获得预期的结果,重试机制都是必不可少的。尤其是调用远程服务,在高并发场景下,很可能因为服务器响应延迟或者网络原因,造成我们得不到想要的结果,或者根本得不到响应。这个时候,一个优雅的重试调用机制,可以让我们更大概率保证得到预期的响应。

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通常情况下,我们会通过定时任务进行重试。例如某次操作失败,则记录下来,当定时任务再次启动,则将数据放到定时任务的方法中,重新跑一遍。最终直至得到想要的结果为止。无论是基于定时任务的重试机制,还是我们自己写的简单的重试器,缺点都是重试的机制太单一,而且实现起来不优雅。那么问题来了如何优雅地设计重试实现?

一个完备的重试实现,要很好地解决如下问题:

  • 什么条件下重试
  • 什么条件下停止
  • 如何停止重试
  • 停止重试等待多久
  • 如何等待
  • 请求时间限制
  • 如何结束
  • 如何监听整个重试过程

并且,为了更好地封装性,重试的实现一般分为两步:

  • 使用工厂模式构造重试器
  • 执行重试方法并得到结果

一个完整的重试流程可以简单示意为:

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接下来就让我们看看 guava-retrying 是怎么实现的重试机制的。

基础用法

guava-retrying 是基于谷歌的核心类库 guava 的重试机制实现,可以说是一个重试利器。下面就快速看一下它的用法。

pom.xml

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<!-- guava-retrying -->
<dependency>
<groupId>com.github.rholder</groupId>
<artifactId>guava-retrying</artifactId>
<version>2.0.0</version>
</dependency>

需要注意的是,此版本依赖的是 27.0.1 版本的 guava。如果你项目中的 guava 低几个版本没问题,但是低太多就不兼容了。这个时候你需要升级你项目的guava版本,或者直接去掉你自己的
guava 依赖,使用 guava-retrying 传递过来的 guava 依赖。

实现 Callable

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Callable<Boolean> callable = new Callable<Boolean>() {
public Boolean call() throws Exception {
return true; // do something useful here
}
};

Callablecall 方法中是你自己实际的业务调用。

通过 RetryerBuilder 构造 Retryer

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Retryer<Boolean> retryer = RetryerBuilder.<Boolean>newBuilder()
.retryIfResult(Predicates.<Boolean>isNull())
.retryIfExceptionOfType(IOException.class)
.retryIfRuntimeException()
.withStopStrategy(StopStrategies.stopAfterAttempt(3))
.build();

使用重试器执行你的业务

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retryer.call(callable);

eg:

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public Boolean test() throws Exception {
//定义重试机制
Retryer<Boolean> retryer = RetryerBuilder.<Boolean>newBuilder()
//retryIf 重试条件
.retryIfException()
.retryIfRuntimeException()
.retryIfExceptionOfType(Exception.class)
.retryIfException(Predicates.equalTo(new Exception()))
.retryIfResult(Predicates.equalTo(false))

//等待策略:每次请求间隔1s
.withWaitStrategy(WaitStrategies.fixedWait(1, TimeUnit.SECONDS))

//停止策略 : 尝试请求6次
.withStopStrategy(StopStrategies.stopAfterAttempt(6))

//时间限制 : 某次请求不得超过2s , 类似: TimeLimiter timeLimiter = new SimpleTimeLimiter();
.withAttemptTimeLimiter(AttemptTimeLimiters.fixedTimeLimit(2, TimeUnit.SECONDS))

.build();

//定义请求实现
Callable<Boolean> callable = new Callable<Boolean>() {
int times = 1;

@Override
public Boolean call() throws Exception {
log.info("call times={}", times);
times++;

if (times == 2) {
throw new NullPointerException();
} else if (times == 3) {
throw new Exception();
} else if (times == 4) {
throw new RuntimeException();
} else if (times == 5) {
return false;
} else {
return true;
}

}
};
//利用重试器调用请求
return retryer.call(callable);
}

实现原理

guava-retrying 的核心是 Attempt 类、Retryer 类以及一些 Strategy(策略) 相关的类。

Attempt

Attempt 既是一次重试请求(call),也是请求的结果,并记录了当前请求的次数、是否包含异常和请求的返回值。

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/**
* An attempt of a call, which resulted either in a result returned by the call,
* or in a Throwable thrown by the call.
*
* @param <V> The type returned by the wrapped callable.
* @author JB
*/
public interface Attempt<V>

Retryer

Retryer 通过 RetryerBuilder 这个工厂类进行构造。RetryerBuilder 负责将定义的重试策略赋值到 Retryer 对象中。在 Retryer 执行 call 方法的时候,会将这些重试策略一一使用。下面就看一下Retryer的call方法的具体实现。

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/**
* Executes the given callable. If the rejection predicate
* accepts the attempt, the stop strategy is used to decide if a new attempt
* must be made. Then the wait strategy is used to decide how much time to sleep
* and a new attempt is made.
*
* @param callable the callable task to be executed
* @return the computed result of the given callable
* @throws ExecutionException if the given callable throws an exception, and the
* rejection predicate considers the attempt as successful. The original exception
* is wrapped into an ExecutionException.
* @throws RetryException if all the attempts failed before the stop strategy decided
* to abort, or the thread was interrupted. Note that if the thread is interrupted,
* this exception is thrown and the thread's interrupt status is set.
*/
public V call(Callable<V> callable) throws ExecutionException, RetryException {
long startTime = System.nanoTime();
//说明:根据 attemptNumber 进行循环 —— 也就是重试多少次
for (int attemptNumber = 1; ; attemptNumber++) {
//说明:进入方法不等待,立即执行一次
Attempt<V> attempt;
try {
//说明:执行 callable 中的具体业务
//attemptTimeLimiter 限制了每次尝试等待的时常
V result = attemptTimeLimiter.call(callable);
//利用调用结果构造新的 attempt
attempt = new ResultAttempt<V>(result, attemptNumber, TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime));
} catch (Throwable t) {
attempt = new ExceptionAttempt<V>(t, attemptNumber, TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime));
}

//说明:遍历自定义的监听器
for (RetryListener listener : listeners) {
listener.onRetry(attempt);
}

//说明:判断是否满足重试条件,来决定是否继续等待并进行重试
if (!rejectionPredicate.apply(attempt)) {
return attempt.get();
}

//说明:此时满足停止策略,因为还没有得到想要的结果,因此抛出异常
if (stopStrategy.shouldStop(attempt)) {
throw new RetryException(attemptNumber, attempt);
} else {
//说明:执行默认的停止策略 —— 线程休眠
long sleepTime = waitStrategy.computeSleepTime(attempt);
try {
//说明:也可以执行定义的停止策略
blockStrategy.block(sleepTime);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
throw new RetryException(attemptNumber, attempt);
}
}
}
}

Retryer执行过程如下:

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eg:

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/**
* @author vincent
*/
@Slf4j
public class GuavaRetrying {
@Data
class WeatherInfo {
/**
* city : 上海
* cityid : 101190408
* temp : 23.5
* WD : 东北风
* WS : 小于3级
* SD : 80%
* AP : 1006.4hPa
* njd : 2903
* WSE : <3
* time : 17:00
* sm : 1.1
* isRadar : 1
* Radar : JC_RADAR_AZ9210_JB)!!!
*/
private String city;
private String cityid;
private String temp;
private String WD;
private String WS;
private String SD;
private String AP;
private String njd;
private String WSE;
private String time;
private String sm;
private String isRadar;
private String Radar;
}

/**
* Guava retryer: 重试机制
* .retryIfException() -> 抛出异常会进行重试
* .retryIfResult(Predicates.equalTo(false)) -> 如果接口返回的结果不符合预期,也需要重试
* .withWaitStrategy(WaitStrategies.fixedWait(1, TimeUnit.SECONDS)) -> 重试策略, 此处设置的是重试间隔时间
* .withStopStrategy(StopStrategies.stopAfterAttempt(3)) -> 重试次数
* .withAttemptTimeLimiter(AttemptTimeLimiters.fixedTimeLimit(2, TimeUnit.SECONDS)) -> 某次请求不得超过2s,
* 使用这个属性会报 "java.lang.NoSuchMethodError: com.google.common.util.concurrent.SimpleTimeLimiter: method <init>()V not found" 个人感觉是 jar 暂时没找到原因
*/
private Retryer<WeatherInfo> retryer = RetryerBuilder.<WeatherInfo>newBuilder()
.retryIfException()
.withWaitStrategy(WaitStrategies.fixedWait(1, TimeUnit.SECONDS))
.withStopStrategy(StopStrategies.stopAfterAttempt(3))
// .withAttemptTimeLimiter(AttemptTimeLimiters.fixedTimeLimit(3, TimeUnit.SECONDS))
.withRetryListener(new RetryListener() {
@Override
public <WeatherInfo> void onRetry(Attempt<WeatherInfo> attempt) {
// attempt.getAttemptNumber() -> 第几次重试,(注意:第一次重试其实是第一次调用)
log.debug("第 retryTime = {}次重试......", attempt.getAttemptNumber());
// attempt.getDelaySinceFirstAttempt() -> 距离第一次重试的延迟
log.debug("距离上一次重试的 delay = {}......", attempt.getDelaySinceFirstAttempt());
if (attempt.hasException()) {
// attempt.hasException() -> 是异常终止
log.debug("异常终止 causeBy = {}......", attempt.getExceptionCause().toString());
} else {
// attempt.hasResult() -> 是正常返回
log.debug("成功获取结果为 {}......", attempt.getResult());
}
}
})
.build();

/**
* 调取上海当前天气的情况
*
* @return
*/
public WeatherInfo getWeather() {
//上海区域编码
String AreaCode = "101020100";
String url = "http://www.weather.com.cn/data/sk/" + AreaCode + ".html";
//这里用的是 hutool 工具类,进行 HttpClient 请求和 Json 转化
String json = HttpUtil.get(url, CharsetUtil.CHARSET_UTF_8);
JSON parse = JSONUtil.parse(json);
WeatherInfo weatherinfo = parse.getByPath("weatherinfo", WeatherInfo.class);

//模拟调取外部接口时发生异常
int a = 1 / 0;
return weatherinfo;
}

@Test
public void t2() throws Throwable {
log.debug("开始调取外部接口......");
WeatherInfo weatherInfo = retryer.call(() -> getWeather());
log.debug("调取外部接口结束......");
log.info("调用返回的天气情况 {}!!!", weatherInfo);
log.info("go on with something......");
}
}

案例源码:https://github.com/V-Vincen/retry-mechanism
参考:https://juejin.im/post/5c77e3bcf265da2d914da410#heading-3


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