guava-retrying重试工具库: 隔多长时间重试

guava-retrying提供了WaitStrategy接口，用来控制2次重试的时间间隔，这个接口与StopStrategy有的类似。内置的等待策略在WaitStrategies中定义。

import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.Callable;

public class AlwaysExceptionTask implements Callable {

    private static final SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss,SSS");
    private int times = 1;

    @Override
    public Boolean call() throws Exception {
        System.out.println(df.format(new Date()));
        int thisTimes = times;
        times++;

        if (thisTimes == 1) {
            throw new NullPointerException();
        } else if (thisTimes == 2) {
            throw new IOException();
        } else if (thisTimes == 3) {
            throw new ArithmeticException();
        } else {
            throw new Exception();
        }
    }
}

WaitStrategies.noWait()失败后立刻重试，没有等待时间。

Retryer retryer = RetryerBuilder.newBuilder()
		.retryIfException()
		.withWaitStrategy(WaitStrategies.noWait())
		.build();

System.out.println("begin..." + df.format(new Date()));

try {
	retryer.call(new AlwaysExceptionTask());
} catch (Exception e) {
	System.err.println("still failed after retry." + e.getCause().toString());
}

System.out.println("end..." + df.format(new Date()));

WaitStrategies.fixedWait(1, TimeUnit.SECONDS)间隔固定时间之后重试，比如每隔1s重试一次。

Retryer retryer = RetryerBuilder.newBuilder()
		.retryIfException()
		.withWaitStrategy(WaitStrategies.fixedWait(1, TimeUnit.SECONDS))
		.build();

System.out.println("begin..." + df.format(new Date()));

try {
	retryer.call(new AlwaysExceptionTask());
} catch (Exception e) {
	System.err.println("still failed after retry." + e.getCause().toString());
}

WaitStrategies.randomWait(3, TimeUnit.SECONDS)间隔随机时间后重试，比如间隔0~3中随机时间后重试。

Retryer retryer = RetryerBuilder.newBuilder()
		.retryIfException()
		.withWaitStrategy(WaitStrategies.randomWait(3, TimeUnit.SECONDS))
		.build();

System.out.println("begin..." + df.format(new Date()));

try {
	retryer.call(new AlwaysExceptionTask());
} catch (Exception e) {
	System.err.println("still failed after retry." + e.getCause().toString());
}

System.out.println("end..." + df.format(new Date()));

WaitStrategies.randomWait(2, TimeUnit.SECONDS, 5, TimeUnit.SECONDS)最小值，最大值之间的随机时间。

Retryer retryer = RetryerBuilder.newBuilder()
		.retryIfException()
		.withWaitStrategy(WaitStrategies.randomWait(2, TimeUnit.SECONDS, 5, TimeUnit.SECONDS))
		.build();

System.out.println("begin..." + df.format(new Date()));

try {
	retryer.call(new AlwaysExceptionTask());
} catch (Exception e) {
	System.err.println("still failed after retry." + e.getCause().toString());
}

System.out.println("end..." + df.format(new Date()));

WaitStrategies.incrementingWait增量重试，重试的次数越多，等待时间间隔越长。incrementingWait需要传递2个参数，一个是initialSleepTime(第一次到第二次尝试的间隔)，一个是increment(每增加一次尝试，需要增加的时间间隔)。第一次尝试是不需要等待的，因为guava-retrying中的第一次尝试，对应正常的第一次调用。从第二次重试开始，第n-1次到n次间隔是：initialSleepTime + (n-2)*increment。

1、WaitStrategies.incrementingWait(0, TimeUnit.SECONDS, 0, TimeUnit.SECONDS)等价于     WaitStrategies.noWait()。

2、 WaitStrategies.incrementingWait(1, TimeUnit.SECONDS, 0, TimeUnit.SECONDS)等价于WaitStrategies.fixedWait(1, TimeUnit.SECONDS)

3、 WaitStrategies.incrementingWait(0, TimeUnit.SECONDS, 1, TimeUnit.SECONDS)等价于WaitStrategies.fixedWait(1, TimeUnit.SECONDS)。

4、  WaitStrategies.incrementingWait(1, TimeUnit.SECONDS, 1, TimeUnit.SECONDS)

WaitStrategies.fibonacciWait()按照斐波那契数列等待。fibonacciWait(long multiplier,long maximumTime,TimeUnit maximumTimeUnit)，multiplier单位固定是ms，maximumTime最大等待时间。n=1的时候，是无需等待的。当n>=2的时候，开始符合斐波那契数列。n=2的时候，等待1 * multiplier毫秒；n=3的时候，等待1 * multiplier毫秒；n=4的时候，等待2 * multiplier毫秒；n=5的时候，等待3 * multiplier毫秒；n=6的时候，等待5 * multiplier毫秒；n=7的时候，等待8 * multiplier毫秒；n=8的时候，等待15 * multiplier毫秒；......但是等待时间最长不超过maximumTime。

Retryer retryer = RetryerBuilder.newBuilder()
		.retryIfException()
		.withWaitStrategy(WaitStrategies.fibonacciWait(100, 10, TimeUnit.SECONDS))
		.build();

System.out.println("begin..." + df.format(new Date()));

try {
	retryer.call(new AlwaysExceptionTask());
} catch (Exception e) {
	System.err.println("still failed after retry." + e.getCause().toString());
}

WaitStrategies.exponentialWait按照指数递增(2的n次方)来等待，各个参数含义与fibonacciWait相同。

Retryer retryer = RetryerBuilder.newBuilder()
		.retryIfException()
		.withWaitStrategy(WaitStrategies.exponentialWait(100, 10, TimeUnit.SECONDS))
		.build();

System.out.println("begin..." + df.format(new Date()));

try {
	retryer.call(new AlwaysExceptionTask());
} catch (Exception e) {
	System.err.println("still failed after retry." + e.getCause().toString());
}

WaitStrategies.exceptionWait根据抛出的异常来决定等待的时间长短，没有什么实际用处，不过作为学习还是可以了解下，说不定将来能排上用场呢。下面我们定义1个任务，随机抛出不同异常。

class RandomExceptionTask implements Callable {
	@Override
	public Boolean call() throws Exception {
		int round = (int) Math.floor(Math.random() * 8);
		System.out.println("round=" + round + ",time=" + df.format(new Date()));
		if (round == 1) {
			throw new NullPointerException();
		} else if (round == 2) {
			throw new IOException();
		} else if (round == 3) {
			throw new ArithmeticException();
		} else if (round == 4) {
			throw new IllegalStateException();
		} else if (round == 5) {
			throw new IndexOutOfBoundsException();
		} else {
			return true;
		}
	}
}

如果我 们希望实现这种效果：如果出现了 NullPointerException，那么等待1s后再重试；如果抛出IOException，那等待2s后再重试；如果出现了ArithmeticException，那么等待3s后再重试；如果出现了IllegalStateException，那么等待4s后再重试；如果出现了IndexOutOfBoundsException，那么等待5s后再重试；否则不等待，立刻重试。

// 根据不同异常,等待不同时间后重试
private static  Function itsFunction(Class exceptionClass) {
	Function result = new Function() {
		@Nullable
		@Override
		public Long apply(@Nullable T input) {
			if (input instanceof NullPointerException) {
				return 1 * 1000L;
			} else if (input instanceof IOException) {
				return 2 * 1000L;
			} else if (input instanceof ArithmeticException) {
				return 3 * 1000L;
			} else if (input instanceof IllegalStateException) {
				return 4 * 1000L;
			} else if (input instanceof IndexOutOfBoundsException) {
				return 5 * 1000L;
			} else {
				return 0L;
			}
		}
	};
	return result;
}

下面是测试代码，可以看出不同的异常确实会导致不同的重试间隔。

WaitStrategy exceptionJoin = WaitStrategies.join(
		WaitStrategies.exceptionWait(NullPointerException.class, itsFunction(NullPointerException.class)),
		WaitStrategies.exceptionWait(IOException.class, itsFunction(IOException.class)),
		WaitStrategies.exceptionWait(ArithmeticException.class, itsFunction(ArithmeticException.class)),
		WaitStrategies.exceptionWait(IllegalStateException.class, itsFunction(IllegalStateException.class)),
		WaitStrategies.exceptionWait(IndexOutOfBoundsException.class, itsFunction(IndexOutOfBoundsException.class))
		);

Retryer retryer = RetryerBuilder.newBuilder()
		.retryIfException()
		.withWaitStrategy(exceptionJoin)
		.build();

System.out.println("begin..." + df.format(new Date()));

try {
	retryer.call(new RandomExceptionTask());
} catch (Exception e) {
	System.err.println("still failed after retry." + e.getCause().toString());
}

System.out.println("end..." + df.format(new Date()));