使用do-while设计重试块。

boolean successful = false;
int maxTries = 3;
do{
  try {
    something();
    success = true;
  } catch(Me ifUCan) {
    maxTries--;
  }
} while (!successful || maxTries > 0)

这是一个老问题，但解决方案仍然重要。以下是我在Java 8中不使用任何第三方库的通用解决方案:

public interface RetryConsumer<T> {
    T evaluate() throws Throwable;
}
public interface RetryPredicate<T> {
    boolean shouldRetry(T t);
}
public class RetryOperation<T> {
    private RetryConsumer<T> retryConsumer;
    private int noOfRetry;
    private int delayInterval;
    private TimeUnit timeUnit;
    private RetryPredicate<T> retryPredicate;
    private List<Class<? extends Throwable>> exceptionList;

    public static class OperationBuilder<T> {
        private RetryConsumer<T> iRetryConsumer;
        private int iNoOfRetry;
        private int iDelayInterval;
        private TimeUnit iTimeUnit;
        private RetryPredicate<T> iRetryPredicate;
        private Class<? extends Throwable>[] exceptionClasses;

        private OperationBuilder() {
        }

        public OperationBuilder<T> retryConsumer(final RetryConsumer<T> retryConsumer) {
            this.iRetryConsumer = retryConsumer;
            return this;
        }

        public OperationBuilder<T> noOfRetry(final int noOfRetry) {
            this.iNoOfRetry = noOfRetry;
            return this;
        }

        public OperationBuilder<T> delayInterval(final int delayInterval, final TimeUnit timeUnit) {
            this.iDelayInterval = delayInterval;
            this.iTimeUnit = timeUnit;
            return this;
        }

        public OperationBuilder<T> retryPredicate(final RetryPredicate<T> retryPredicate) {
            this.iRetryPredicate = retryPredicate;
            return this;
        }

        @SafeVarargs
        public final OperationBuilder<T> retryOn(final Class<? extends Throwable>... exceptionClasses) {
            this.exceptionClasses = exceptionClasses;
            return this;
        }

        public RetryOperation<T> build() {
            if (Objects.isNull(iRetryConsumer)) {
                throw new RuntimeException("'#retryConsumer:RetryConsumer<T>' not set");
            }

            List<Class<? extends Throwable>> exceptionList = new ArrayList<>();
            if (Objects.nonNull(exceptionClasses) && exceptionClasses.length > 0) {
                exceptionList = Arrays.asList(exceptionClasses);
            }
            iNoOfRetry = iNoOfRetry == 0 ? 1 : 0;
            iTimeUnit = Objects.isNull(iTimeUnit) ? TimeUnit.MILLISECONDS : iTimeUnit;
            return new RetryOperation<>(iRetryConsumer, iNoOfRetry, iDelayInterval, iTimeUnit, iRetryPredicate, exceptionList);
        }
    }

    public static <T> OperationBuilder<T> newBuilder() {
        return new OperationBuilder<>();
    }

    private RetryOperation(RetryConsumer<T> retryConsumer, int noOfRetry, int delayInterval, TimeUnit timeUnit,
                           RetryPredicate<T> retryPredicate, List<Class<? extends Throwable>> exceptionList) {
        this.retryConsumer = retryConsumer;
        this.noOfRetry = noOfRetry;
        this.delayInterval = delayInterval;
        this.timeUnit = timeUnit;
        this.retryPredicate = retryPredicate;
        this.exceptionList = exceptionList;
    }

    public T retry() throws Throwable {
        T result = null;
        int retries = 0;
        while (retries < noOfRetry) {
            try {
                result = retryConsumer.evaluate();
                if (Objects.nonNull(retryPredicate)) {
                    boolean shouldItRetry = retryPredicate.shouldRetry(result);
                    if (shouldItRetry) {
                        retries = increaseRetryCountAndSleep(retries);
                    } else {
                        return result;
                    }
                } else {
                    // no retry condition defined, no exception thrown. This is the desired result.
                    return result;
                }
            } catch (Throwable e) {
                retries = handleException(retries, e);
            }
        }
        return result;
    }

    private int handleException(int retries, Throwable e) throws Throwable {
        if (exceptionList.contains(e.getClass()) || (exceptionList.isEmpty())) {
            // exception is excepted, continue retry.
            retries = increaseRetryCountAndSleep(retries);
            if (retries == noOfRetry) {
                // evaluation is throwing exception, no more retry left. Throw it.
                throw e;
            }
        } else {
            // unexpected exception, no retry required. Throw it.
            throw e;
        }
        return retries;
    }

    private int increaseRetryCountAndSleep(int retries) {
        retries++;
        if (retries < noOfRetry && delayInterval > 0) {
            try {
                timeUnit.sleep(delayInterval);
            } catch (InterruptedException ignore) {
                Thread.currentThread().interrupt();
            }
        }
        return retries;
    }
}

让我们有一个这样的测试用例:

@Test
public void withPredicateAndException() {
    AtomicInteger integer = new AtomicInteger();
    try {
        Integer result = RetryOperation.<Integer>newBuilder()
                .retryConsumer(() -> {
                    int i = integer.incrementAndGet();
                    if (i % 2 == 1) {
                        throw new NumberFormatException("Very odd exception");
                    } else {
                        return i;
                    }
                })
                .noOfRetry(10)
                .delayInterval(10, TimeUnit.MILLISECONDS)
                .retryPredicate(value -> value <= 6)
                .retryOn(NumberFormatException.class, EOFException.class)
                .build()
                .retry();
        Assert.assertEquals(8, result.intValue());
    } catch (Throwable throwable) {
        Assert.fail();
    }
}

你需要像这样在while循环中封装try-catch: -

int count = 0;
int maxTries = 3;
while(true) {
    try {
        // Some Code
        // break out of loop, or return, on success
    } catch (SomeException e) {
        // handle exception
        if (++count == maxTries) throw e;
    }
}

我已经采取了count和maxTries，以避免运行到一个无限循环，以防异常继续发生在你的try块。

简单的

int MAX = 3;

int count = 0;
while (true) {
    try {
        ...

        break;
    } catch (Exception e) {
        if (count++ < MAX) {
            continue;
        }

        ...
        
        break;
    }
}

这里是我的解决方案类似于其他人可以包装一个函数，但允许您获得函数返回值，如果它成功。

    /**
     * Wraps a function with retry logic allowing exceptions to be caught and retires made.
     *
     * @param function the function to retry
     * @param maxRetries maximum number of retires before failing
     * @param delay time to wait between each retry
     * @param allowedExceptionTypes exception types where if caught a retry will be performed
     * @param <V> return type of the function
     * @return the value returned by the function if successful
     * @throws Exception Either an unexpected exception from the function or a {@link RuntimeException} if maxRetries is exceeded
     */
    @SafeVarargs
    public static <V> V runWithRetriesAndDelay(Callable<V> function, int maxRetries, Duration delay, Class<? extends Exception>... allowedExceptionTypes) throws Exception {
        final Set<Class<? extends Exception>> exceptions = new HashSet<>(Arrays.asList(allowedExceptionTypes));
        for(int i = 1; i <= maxRetries; i++) {
            try {
                return function.call();
            } catch (Exception e) {
                if(exceptions.contains(e.getClass())){
                    // An exception of an expected type
                    System.out.println("Attempt [" + i + "/" + maxRetries + "] Caught exception [" + e.getClass() + "]");
                    // Pause for the delay time
                    Thread.sleep(delay.toMillis());
                }else {
                    // An unexpected exception type
                    throw e;
                }
            }
        }
        throw new RuntimeException(maxRetries + " retries exceeded");
    }

使用带有本地状态标志的while循环。初始化标志为false，并在操作成功时将其设置为true，如下所示:

  boolean success  = false;
  while(!success){
     try{ 
         some_instruction(); 
         success = true;
     } catch (NearlyUnexpectedException e){
       fix_the_problem();
     }
  }

这将不断重试，直到成功。

如果你只想重试一定次数，那么也可以使用计数器:

  boolean success  = false;
  int count = 0, MAX_TRIES = 10;
  while(!success && count++ < MAX_TRIES){
     try{ 
         some_instruction(); 
         success = true;
     } catch (NearlyUnexpectedException e){
       fix_the_problem();
     }
  }
  if(!success){
    //It wasn't successful after 10 retries
  }

这将尝试最多10次，如果不成功，直到那时将退出，如果它成功之前。