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Asynchronous Programming

You can use either synchronous or asynchronous methods to call operations on AWS services. Synchronous methods block your thread's execution until the client receives a response from the service. Asynchronous methods return immediately, giving control back to the calling thread without waiting for a response.

Because an asynchronous method returns before a response is available, you need a way to get the response when it's ready. The |sdk-java| provides two ways: Future objects and callback methods.

Java Futures

Asynchronous methods in the |sdk-java| return a :javase-ref:`Future <java/util/concurrent/Future>` object that contains the results of the asynchronous operation in the future.

Call the :code-java:`Future` :methodname:`isDone()` method to see if the service has provided a response object yet. When the response is ready, you can get the response object by calling the :code-java:`Future` :methodname:`get()` method. You can use this mechanism to periodically poll for the asynchronous operation's results while your application continues to work on other things.

Here is an example of an asynchronous operation that calls a |LAM| function, receiving a :code-java:`Future` that can hold an :aws-java-class:`InvokeResult <services/lambda/model/InvokeResult>` object. The :classname:`InvokeResult` object is retrieved only after :methodname:`isDone()` is true.

.. literalinclude:: snippets/lambda_invoke_example/src/main/java/example/lambda/InvokeLambdaFunctionAsync.java
   :language: java
   :lines: 18-

Asynchronous Callbacks

In addition to using the Java :code-java:`Future` object to monitor the status of asynchronous requests, the SDK also enables you to implement a class that uses the :aws-java-class:`AsyncHandler <handlers/AsyncHandler>` interface. :code-java:`AsyncHandler` provides two methods that are called depending on how the request completed: :methodname:`onSuccess` and :methodname:`onError`.

The major advantage of the callback interface approach is that it frees you from having to poll the :code-java:`Future` object to find out when the request has completed. Instead, your code can immediately start its next activity, and rely on the SDK to call your handler at the right time.

.. literalinclude:: snippets/lambda_invoke_example/src/main/java/example/lambda/InvokeLambdaFunctionCallback.java
   :language: java
   :lines: 18-


Best Practices

Callback Execution

Your implementation of :classname:`AsyncHandler` is executed inside the thread pool owned by the asynchronous client. Short, quickly executed code is most appropriate inside your :classname:`AsyncHandler` implementation. Long-running or blocking code inside your handler methods can cause contention for the thread pool used by the asynchronous client, and can prevent the client from executing requests. If you have a long-running task that needs to begin from a callback, have the callback run its task in a new thread or in a thread pool managed by your application.

Thread Pool Configuration

The asynchronous clients in the |sdk-java| provide a default thread pool that should work for most applications. You can implement a custom :javase-ref:`ExecutorService <java/util/concurrent/ExecutorService>` and pass it to |sdk-java| asynchronous clients for more control over how the thread pools are managed.

For example, you could provide an :classname:`ExecutorService` implementation that uses a custom :javase-ref:`ThreadFactory <java/util/concurrent/ThreadFactory>` to control how threads in the pool are named, or to log additional information about thread usage.

|S3| Asynchronous Access

The :aws-java-class:`TransferManager <services/s3/transfer/TransferManager>` class in the SDK offers asynchronous support for working with the |S3|. :classname:`TransferManager` manages asynchronous uploads and downloads, provides detailed progress reporting on transfers, and supports callbacks into different events.

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