官网地址:

https://github.com/rholder/guava-retrying

https://codechina.csdn.net/mirrors/rholder/guava-retrying?utm_source=csdn_github_accelerator

1 guava-retry的简介

在日常的一些场景中, 很多需要进行重试的操作.查询资料Guava-Retry工具对于重试操作效果比较好.

Guava retryer是一个基于Guava，提供重试机制的库,是通过定义重试者角色来包装正常逻辑重试，支持重试次数和重试频度控制，能够兼容支持多个异常或者自定义实体对象的重试源定义，让重试功能有更多的灵活性, 而且也是线程安全的，入口调用逻辑采用的是java.util.concurrent.Callable的call方法.

2 guava-retry的使用

1 导入maven依赖

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

2 添加一个重试方法

@Slf4j
public class RetryDemo {
    

    public static boolean retryMethod(Integer param) {
    
        int i = new Random().nextInt(param);
        log.info("随机生成的数:{}", i);

        if (1 == i) {
    
            log.info("为1,返回true.");
            return true;
        } else if (i < 1) {
    
            log.info("小于1,抛出参数异常.");
            throw new IllegalArgumentException("参数异常");
        } else if (i > 1 && i < 10) {
    
            log.info("大于1,小于10,抛出参数异常.");
            return false;
        } else {
    
            //为其他
            log.info("大于10,抛出自定义异常.");
            throw new RemoteAccessException("大于10,抛出自定义异常");
        }
    }
}

3 添加测试类

@Slf4j
public class GuavaRetryTest {
    

    public static void main(String[] args) {
    
        Boolean result = false;
        // RetryerBuilder 构建重试实例对象
        // 1 可以设置重试源且可以支持多个重试源
        // 2 可以设置根据结果重试
        // 3 可以配置等待时间间隔
        // 4 可以配置重试次数或重试超时时间
        Retryer<Boolean> retryer = RetryerBuilder.<Boolean>newBuilder()
                .retryIfExceptionOfType(RemoteAccessException.class)//设置异常重试源
                .retryIfException()
                .retryIfResult(res -> false) //设置根据结果重试
                .withWaitStrategy(WaitStrategies.fixedWait(3, TimeUnit.SECONDS)) //设置等待间隔时间
                .withStopStrategy(StopStrategies.stopAfterAttempt(3)) //设置最大重试次数
                .withRetryListener(new RetryListener() {
    
                    @Override
                    public <V> void onRetry(Attempt<V> attempt) {
    
                        log.info("第【{}】次调用失败", attempt.getAttemptNumber());
                    }
                })
                .build();

        try {
    
            result = retryer.call(new Callable<Boolean>() {
    
                @Override
                public Boolean call() throws Exception {
    
                    return RetryDemo.retryMethod(110);
                }
            });

            // 上述可以简化为
//            result = retryer.call(() -> RetryDemo.retryMethod(110));

        } catch (Exception e) {
    
            e.printStackTrace();
            log.info("超时三次错误,{}", e.getMessage());
        }

        System.out.println("方法调用返回状态= " + result);
    }
}    

说明:

RetryerBuilder类是一个工厂建造者,可以设置多个重试源,可以设置重试次数和重试超时时间等,创建重试者Retryer对象.

RetryerBuilder部分源码:

public class RetryerBuilder<V> {
    
    // 单次任务执行时间限制
    private AttemptTimeLimiter<V> attemptTimeLimiter;
    // 停止策略
    private StopStrategy stopStrategy;
    // 等待策略
    private WaitStrategy waitStrategy;
    // 阻塞策略
    private BlockStrategy blockStrategy;
    // 重试源 支持Exception异常对象和自定义断言对象
    private Predicate<Attempt<V>> rejectionPredicate = Predicates.alwaysFalse();
    // 重试监听
    private List<RetryListener> listeners = new ArrayList<RetryListener>();

    private RetryerBuilder() {
    
    }

    // new一个RetryerBuilder对象
    /**
     * Constructs a new builder
     *
     * @param <V> result of a {@link Retryer}'s call, the type of the call return value
     * @return the new builder
     */
    public static <V> RetryerBuilder<V> newBuilder() {
    
        return new RetryerBuilder<V>();
    }
    
    
    // 创建一个Retryer对象
    /**
     * Builds the retryer.
     *
     * @return the built retryer.
     */
    public Retryer<V> build() {
    
        AttemptTimeLimiter<V> theAttemptTimeLimiter = attemptTimeLimiter == null ? AttemptTimeLimiters.<V>noTimeLimit() : attemptTimeLimiter;
        StopStrategy theStopStrategy = stopStrategy == null ? StopStrategies.neverStop() : stopStrategy;
        WaitStrategy theWaitStrategy = waitStrategy == null ? WaitStrategies.noWait() : waitStrategy;
        BlockStrategy theBlockStrategy = blockStrategy == null ? BlockStrategies.threadSleepStrategy() : blockStrategy;

        return new Retryer<V>(theAttemptTimeLimiter, theStopStrategy, theWaitStrategy, theBlockStrategy, rejectionPredicate, listeners);
    }
    
    // ...
}    

Retryer部分源码:

public final class Retryer<V> {
    
    // 停止策略
    private final StopStrategy stopStrategy;
    // 等待策略
    private final WaitStrategy waitStrategy;
    // 阻塞策略
    private final BlockStrategy blockStrategy;
    // 单次任务执行时间限制
    private final AttemptTimeLimiter<V> attemptTimeLimiter;
    // 重试源 支持Exception异常对象和自定义断言对象
    private final Predicate<Attempt<V>> rejectionPredicate;
    // 重试监听
    private final Collection<RetryListener> listeners;
    
    
    // RetryerBuilder建造者中build方法被使用
    /**
     * Constructor
     *
     * @param attemptTimeLimiter to prevent from any single attempt from spinning infinitely
     * @param stopStrategy       the strategy used to decide when the retryer must stop retrying
     * @param waitStrategy       the strategy used to decide how much time to sleep between attempts
     * @param blockStrategy      the strategy used to decide how to block between retry attempts; eg, Thread#sleep(), latches, etc.
     * @param rejectionPredicate the predicate used to decide if the attempt must be rejected
     *                           or not. If an attempt is rejected, the retryer will retry the call, unless the stop
     *                           strategy indicates otherwise or the thread is interrupted.
     * @param listeners          collection of retry listeners
     */
    @Beta
    public Retryer(@Nonnull AttemptTimeLimiter<V> attemptTimeLimiter,
                   @Nonnull StopStrategy stopStrategy,
                   @Nonnull WaitStrategy waitStrategy,
                   @Nonnull BlockStrategy blockStrategy,
                   @Nonnull Predicate<Attempt<V>> rejectionPredicate,
                   @Nonnull Collection<RetryListener> listeners) {
    
        Preconditions.checkNotNull(attemptTimeLimiter, "timeLimiter may not be null");
        Preconditions.checkNotNull(stopStrategy, "stopStrategy may not be null");
        Preconditions.checkNotNull(waitStrategy, "waitStrategy may not be null");
        Preconditions.checkNotNull(blockStrategy, "blockStrategy may not be null");
        Preconditions.checkNotNull(rejectionPredicate, "rejectionPredicate may not be null");
        Preconditions.checkNotNull(listeners, "listeners may not null");

        this.attemptTimeLimiter = attemptTimeLimiter;
        this.stopStrategy = stopStrategy;
        this.waitStrategy = waitStrategy;
        this.blockStrategy = blockStrategy;
        this.rejectionPredicate = rejectionPredicate;
        this.listeners = listeners;
    }
    
    // 线程安全调用重试方法
    /**
     * 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();
        for (int attemptNumber = 1; ; attemptNumber++) {
    
            Attempt<V> attempt;
            try {
    
                V result = attemptTimeLimiter.call(callable);
                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));
            }

            for (RetryListener listener : listeners) {
    
                listener.onRetry(attempt);
            }

            if (!rejectionPredicate.apply(attempt)) {
    
                return attempt.get();
            }
            if (stopStrategy.shouldStop(attempt)) {
    
                throw new RetryException(attemptNumber, attempt);
            } else {
    
                long sleepTime = waitStrategy.computeSleepTime(attempt);
                try {
    
                    blockStrategy.block(sleepTime);
                } catch (InterruptedException e) {
    
                    Thread.currentThread().interrupt();
                    throw new RetryException(attemptNumber, attempt);
                }
            }
        }
    }
       
}    

3 总结

Guava-Retry, 是一个非常灵活简单上手的工具. 可以完好的把重试功能和业务逻辑解耦, 不但支持多种设置, 如多异常判断,重试次数,重试时间,以及每次重试方法时的监听.并且是属于线程安全的, 在并发的场景下也能稳定支持.

参考资料:

https://blog.csdn.net/zzzgd_666/article/details/84377962