引子：
上图是两个系统交互的情况，现在我想将对外系统的调用做成异步实现，那么就需要考虑两个问题：
主线程可以得到异步线程的结果，在得到结果之后再进行operation-4
?主线程如何得到异步线程结果？
?主线程在得到异步线程的结果之前是否可以不等待？
可以使用Future模式来实现。
Future模式在请求发生时返回一个Future对象给发起请求的客户端，然后由一个新的线程执行真正的异步业务处理，当客户端需要异步处理的结果时，通过返回给客户端的Future对象的get()方法获取异步处理结果！
JDK的Future模式：Java.util.concurrent.future.Future接口
/**
* A <tt>Future</tt> represents the result of an asynchronous
* computation. Methods are provided to check if the computation is
* complete, to wait for its completion, and to retrieve the result of
* the computation. The result can only be retrieved using method
* <tt>get</tt> when the computation has completed, blocking if
* necessary until it is ready. Cancellation is performed by the
* <tt>cancel</tt> method. Additional methods are provided to
* determine if the task completed normally or was cancelled. Once a
* computation has completed, the computation cannot be cancelled.
* If you would like to use a <tt>Future</tt> for the sake
* of cancellability but not provide a usable result, you can
* declare types of the form {@code Future<?>} and
* return <tt>null</tt> as a result of the underlying task.
*
* <p>
* <b>Sample Usage</b> (Note that the following classes are all
* made-up.) <p>
* <pre> {@code
* interface ArchiveSearcher { String search(String target); }
* class App {
* ExecutorService executor = ...
* ArchiveSearcher searcher = ...
* void showSearch(final String target)
* throws InterruptedException {
* Future<String> future
* = executor.submit(new Callable<String>() {
* public String call() {
* return searcher.search(target);
* }});
* displayOtherThings(); // do other things while searching
* try {
* displayText(future.get()); // use future
* } catch (ExecutionException ex) { cleanup(); return; }
* }
* }}</pre>
*
* The {@link FutureTask} class is an implementation of <tt>Future</tt> that
* implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>.
* For example, the above construction with <tt>submit</tt> could be replaced by:
* <pre> {@code
* FutureTask<String> future =
* new FutureTask<String>(new Callable<String>() {
* public String call() {
* return searcher.search(target);
* }});
* executor.execute(future);}</pre>
*
* <p>Memory consistency effects: Actions taken by the asynchronous computation
* <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
* actions following the corresponding {@code Future.get()} in another thread.
*
* @see FutureTask
* @see Executor
* @since 1.5
* @author Doug Lea
* @param <V> The result type returned by this Future‘s <tt>get</tt> method
*/
public interface Future<V> {
/**
* Attempts to cancel execution of this task. This attempt will
* fail if the task has already completed, has already been cancelled,
* or could not be cancelled for some other reason. If successful,
* and this task has not started when <tt>cancel</tt> is called,
* this task should never run. If the task has already started,
* then the <tt>mayInterruptIfRunning</tt> parameter determines
* whether the thread executing this task should be interrupted in
* an attempt to stop the task.
*
* <p>After this method returns, subsequent calls to {@link #isDone} will
* always return <tt>true</tt>. Subsequent calls to {@link #isCancelled}
* will always return <tt>true</tt> if this method returned <tt>true</tt>.
*
* @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
* task should be interrupted; otherwise, in-progress tasks are allowed
* to complete
* @return <tt>false</tt> if the task could not be cancelled,
* typically because it has already completed normally;
* <tt>true</tt> otherwise
*/
boolean cancel(boolean mayInterruptIfRunning);
/**
* Returns <tt>true</tt> if this task was cancelled before it completed
* normally.
*
* @return <tt>true</tt> if this task was cancelled before it completed
*/
boolean isCancelled();
/**
* Returns <tt>true</tt> if this task completed.
*
* Completion may be due to normal termination, an exception, or
* cancellation -- in all of these cases, this method will return
* <tt>true</tt>.
*
* @return <tt>true</tt> if this task completed
*/
boolean isDone();
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
*/
V get() throws InterruptedException, ExecutionException;
/**
* Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result, if available.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
* @throws TimeoutException if the wait timed out
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
对应的任务接口是：java.util.concurrent.Callable
/**
* A task that returns a result and may throw an exception.
* Implementors define a single method with no arguments called
* <tt>call</tt>.
*
* <p>The <tt>Callable</tt> interface is similar to {@link
* java.lang.Runnable}, in that both are designed for classes whose
* instances are potentially executed by another thread. A
* <tt>Runnable</tt>, however, does not return a result and cannot
* throw a checked exception.
*
* <p> The {@link Executors} class contains utility methods to
* convert from other common forms to <tt>Callable</tt> classes.
*
* @see Executor
* @since 1.5
* @author Doug Lea
* @param <V> the result type of method <tt>call</tt>
*/
public interface Callable<V> {
/**
* Computes a result, or throws an exception if unable to do so.
*
* @return computed result
* @throws Exception if unable to compute a result
*/
V call() throws Exception;
}
把javaDoc上的代码摘抄出来看下~
public interface ArchiveSearcher {
String search(String target);
}
public class App {
ExecutorService executor = ;
ArchiveSearcher searcher =
void showSearch(final String target)
throws InterruptedException {
Future<String> future = executor.submit(new Callable<String>() {
public String call() {
return searcher.search(target);
}
});
displayOtherThings(); // do other things while searching
try {
displayText(future.get()); // use future
} catch (ExecutionException ex) {
cleanup();
return;
}
}
}
简单实用介绍：
/**
* 异步任务
*/
public class AsynchronousTask implements Callable<String> {
private String data;
public AsynchronousTask(String data) {
this.data = data;
}
public String call() throws Exception {
try {
System.out.println(Thread.currentThread().getName() + "AsynchronousTask start...");
//模拟异步任务的处理
Thread.sleep(1000);
System.out.println((Thread.currentThread().getName() + "AsynchronousTask end..."));
} catch (Exception ex) {
ex.printStackTrace();
}
//返回异步任务的处理结果
return "hello future";
}
}
public class FutureTest {
public static void main(String[] args) throws Exception {
ExecutorService executor = Executors.newFixedThreadPool(10);
//进行异步任务处理
Future<String> submit = executor.submit(new AsynchronousTask("futire test"));
System.out.println(Thread.currentThread().getName() + "FutureTest start");
//这里使用sleep方法表示对其他业务逻辑的处理
//与此同时异步任务也在执行，从而充分利用了等待时间
Thread.sleep(100);
System.out.println(Thread.currentThread().getName() + "FutureTest end");
//submit.get()获取异步执行结果
//如果异步任务call没有执行完成，则依然会等待
System.out.println("数据" + submit.get());
}
}
时间： 2024-08-27 19:42:41