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mbf . position ( 0 );
mbf . put ( b ); // Zeros the file
Even with buffers, there are limitations of what can be done in Java for large (e.g.,
transferring 10G between filesystems) I/O operations that perform synchronously
on a single thread. Before Java 7, these types of operations would typically be done
by writing custom multithreaded code, and managing a separate thread for per‐
forming a background copy. Let's move on to look at the new asynchronous I/O fea‐
tures that were added with JDK 7.
d O
Async I/O
The key to the new asynchronous functionality are some new subclasses of Channel
that can deal with I/O operations that need to be handed off to a background
thread. The same functionality can be applied to large, long-running operations,
and to several other use cases.
In this section, we'll deal exclusively with AsynchronousFileChannel for file I/O,
but there are a couple of other asynchronous channels to be aware of. We'll deal
with asynchronous sockets at the end of the chapter. We'll look at:
AsynchronousFileChannel for file I/O
AsynchronousSocketChannel for client socket I/O
AsynchronousServerSocketChannel for asynchronous sockets that accept
incoming connections
There are two different ways to interact with an asynchronous channel— Future
style, and callback style.
Future-Based Style
We'll meet the Future interface in detail in Chapter 11 , but for the purpose of this
chapter, it can be thought of as an ongoing task that may or may not have completed
yet. It has two key methods:
Returns a Boolean indicating whether the task has finished.
get( Returns the result. If finished, returns immediately. If not finished, blocks until
Let's look at an example of a program that reads a large file (possibly as large as 100
Mb) asynchronously:
try ( AsynchronousFileChannel channel =
AsynchronousFileChannel . open ( Paths . get ( "input.txt" ))) {
ByteBuffer buffer = ByteBuffer . allocateDirect ( 1024 * 1024 * 100 );
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