如何为Java中的CompletableFuture执行资源清理?

How to perform resource cleanup for CompletableFuture in Java?

我在 CompletableFuture 中有一段代码,如果有异常则执行重试,否则完成任务。我已将资源传递给 SupplierConsumer 以执行任务,并希望在所有任务完成后关闭这些资源(重试 3 次后 success/exception)。

这是一段代码:

Supplier mySupplier = new MySupplier(localContext);
CompletableFuture<String> future = CompletableFuture.supplyAsync(mySupplier);
for(int j = 0; j < (retryCount - 1); j++) {
    LOGGER.debug("MySupplier accept() Retry count: "+j);
    future = future.handleAsync((value, throwable) -> throwable == null? CompletableFuture.completedFuture(value): CompletableFuture.supplyAsync(mySupplier)).thenComposeAsync(Function.identity());
}

我打算将它放在供应商的 finally 块下,但如果发生第一个异常,资源将被关闭,我需要它们进行接下来的两次重试。

1) 如何让它发挥作用?

2)还有没有办法只在异常情况下打印重试次数?

由于您似乎并不关心中间结果,最简单的解决方案是将您的 Supplier 简单地包装在另一个处理重试的结果中:

class SupplierRetrier<T> implements Supplier<T> {
    private static final Logger LOGGER = LoggerFactory.getLogger(SupplierRetrier.class);
    final Supplier<T> wrappee;
    final int maxRetries;

    SupplierRetrier(Supplier<T> wrappee, int maxRetries) {
        Objects.requireNonNull(wrappee);
        if (maxRetries <= 0) {
            throw new IllegalArgumentException("maxRetries must be more than 0: " + maxRetries);
        }
        this.wrappee = wrappee;
        this.maxRetries = maxRetries;
    }

    @Override
    public T get() {
        RuntimeException lastException = null;
        for (int i = 0; i < maxRetries; i++) {
            try {
                LOGGER.info("MySupplier accept() Retry count: "+i);
                return wrappee.get();
            } catch (RuntimeException e) {
                lastException = e;
            }
        }
        throw lastException;
    }
}

然后您可以简单地使用它:

CompletableFuture<String> future = CompletableFuture.supplyAsync(
        new SupplierRetrier<>(mySupplier, retryCount));

为了清理上下文,只需对生成的未来添加一个 whenComplete() 调用。无论未来的结果如何,这将被执行。

future.whenComplete((r, e) -> {
    try {
        localContext.close();
    } catch (Exception e2) {
        throw new RuntimeException("Failed to close context", e2);
    }
});

1) 对于资源清理,使用 whenCompletewhenCompleteAsync

2) 对于重试计数,使用长度 1AtomicIntegerint[]。 (无论是否抛出 Exception,此值都可用)

int[] retryCounter = { 0 };
// AtomicInteger retryCounter = new AtomicInteger();

for (int i = 0; i < noOfRetries; i++)
{
  CompletableFuture<CompletableFuture<String>> handleAsync = cf.handleAsync((result, throwable) ->
    {
      if (throwable == null)
        return CompletableFuture.completedFuture(result);

      retryCounter[0]++;
      // retryCounter.incrementAndGet();

      return CompletableFuture.supplyAsync(supplier);
    });
  cf = handleAsync.thenCompose(Function.identity());
}

cf = cf.whenCompleteAsync((result, throwable) ->
  {
    System.out.println("Clean up");

    System.out.println("Retry count: " + retryCounter[0]);
    // System.out.println("Retry count: " + retryCounter.get());
  });

System.out.println("Wating for result...");
System.out.println("Result: " + cf.get());