Guava - Detailed Explanation of Parallel Programming Futures

Guava provides many useful extensions for Java parallel programming Future, mainly through the ListenableFuture interface, and with the help of Futures static extensions.

ListenableFuture, inheriting from Future, allows us to add callback functions that return values or methods immediately when the thread operation is completed or when the method execution is completed.

Add callback functions to ListenableFuture:

Futures.addCallback(ListenableFuture<V>, FutureCallback<V>, Executor)

where FutureCallback is an interface that includes onSuccess(V), onFailure(Throwable).

Use, for example:

Futures.addCallback(ListenableFuture, new FutureCallback<Object>() {
  public void onSuccess(Object result) {
    System.out.printf("onSuccess with: %s%n", result);
  }
  public void onFailure(Throwable thrown) {
    System.out.printf("onFailure %s%n", thrown.getMessage());
  }
});

At the same time, Guava provides many useful extensions for Future extensions, mainly through the ListenableFuture interface, and with the help of Futures static extensions.

1. transform: Transforms the return value of ListenableFuture.
2. allAsList: Merges multiple ListenableFutures, returning a List object composed of multiple Future return values when all Futures are successful. Note: When one of the Futures fails or is cancelled, it will enter failure or cancellation.
3. successfulAsList: Similar to allAsList, the only difference is that for failed or cancelled Future return values, null is used instead. It will not enter the failure or cancellation process.
4. immediateFuture/immediateCancelledFuture: Immediately returns a ListenableFuture with a pending value.
5. makeChecked: Converts ListenableFuture to CheckedFuture. CheckedFuture is a ListenableFuture that includes multiple versions of the get method, with method declarations throwing checked exceptions. This makes it easier to create a Future that can throw exceptions within the execution logic.
6. JdkFutureAdapters.listenInPoolThread(future): guava provides an interface function to convert JDK Future to ListenableFuture.
   

The following is a test demo for Future:

@Test
public void should_test_furture() throws Exception {
  ListeningExecutorService service = MoreExecutors.listeningDecorator(Executors.newFixedThreadPool(10));
  ListenableFuture future1 = service.submit(new Callable<Integer>() {
    public Integer call() throws InterruptedException {
      Thread.sleep(1000);
      System.out.println("call future 1.");
      return 1;
    }
  });
  ListenableFuture future2 = service.submit(new Callable<Integer>() {
    public Integer call() throws InterruptedException {
      Thread.sleep(1000);
      System.out.println("call future 2.");
  //    throw new RuntimeException("----call future 2.");
      return 2;
    }
  });
  final ListenableFuture allFutures = Futures.allAsList(future1, future2);
  final ListenableFuture transform = Futures.transform(allFutures, new AsyncFunction<List<Integer>, Boolean>() {
    @Override
    public ListenableFuture apply(List<Integer> results) throws Exception {
      return Futures.immediateFuture(String.format("success future:%d", results.size()));
    }
  });
  Futures.addCallback(transform, new FutureCallback<Object>() {
    public void onSuccess(Object result) {
      System.out.println(result.getClass());
      System.out.printf("success with: %s%n", result);
    }
    public void onFailure(Throwable thrown) {
      System.out.printf("onFailure%s%n", thrown.getMessage());
    }
  });
  System.out.println(transform.get());
}

Official material homepage:https://awk.so/@code.google.com!/p/guava-libraries/wiki/ListenableFutureExplained

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