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多线程(Multithreading)是Java的一个特性，它可以允许一个程序的多个部分(也就是线程)并发地执行，以达到最大程度利用CPU的目的。

关于多线程编程(Multithread Programming)，下面介绍一下Future的特性和基本用法。

About Future

Future(java.util.concurrent Interface Future<V>)表示异步计算的结果。Future接口提供了检查计算是否完成、检查计算是否被取消、等待计算完成并获取计算结果等方法。

A Future 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 get when the computation has completed, blocking if necessary until it is ready. Cancellation is performed by the cancel 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 Future for the sake of cancellability but not provide a usable result, you can declare types of the form Future<?> and return null as a result of the underlying task.

https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/Future.html

Future Methods

下面对Future的几个方法做一下简要介绍并提供代码演示。

private static ExecutorService executor = Executors.newSingleThreadExecutor();

public static Future<Integer> calculate(Integer input) {
    return executor.submit(() -> {
        Thread.sleep(3000);
         return input * input;
    });

}

get()

V get()

Waits if necessary for the computation to complete, and then retrieves its result.

get()方法用户返回计算结果，如果计算还没有完成，则在get的时候会进行阻塞，直到获取到结果为止。

get() Sample Example

Future<Integer> calculateFuture = calculate(100);

System.out.println("calculate result: "+calculateFuture.get());

在阻塞3s以后，可以得到执行结果：

calculate result: 10000

get(long timeout, TimeUnit unit)

V    get(long timeout, TimeUnit unit)

Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.

get(long timeout, TimeUnit unit)方法的耐心是有限的，如果在指定时间内没有完成计算，则会抛出TimeoutException.

get(long timeout, TimeUnit unit) Sample Example

Future<Integer> calculateFuture = calculate(100);

System.out.println(calculateFuture.get(2, TimeUnit.SECONDS));

等待2s以后，抛出异常：

Exception in thread "main" java.util.concurrent.TimeoutException

isDone()

boolean    isDone()

Returns true if this task completed.

isDone()方法用于判断当前Future是否执行完成。

isDone() Sample Example

Future<Integer> calculateFuture = calculate(100);

System.out.println(calculateFuture.isDone());
while (!calculateFuture.isDone()){ }
System.out.println(calculateFuture.isDone());

首先输出false，循环3秒以后输出true。

cancel(boolean mayInterruptIfRunning)

boolean    cancel(boolean mayInterruptIfRunning)

Attempts to cancel execution of this task.

取消当前线程的执行。参数表示是否在线程执行的过程中阻断。

isCancelled()

boolean    isCancelled()

Returns true if this task was cancelled before it completed normally.

判断当前task是否被取消。

isCancelled() Sample Example

Future<Integer> calculateFuture = calculate(100);

System.out.println("isCancelled = "+calculateFuture.isCancelled());
System.out.println("cancel success = "+calculateFuture.cancel(true));
System.out.println("isCancelled = "+calculateFuture.isCancelled());

Future ExecutorService

下面提供一个通过线程池来异步获取执行结果并汇总的代码示例。

Callable<Long> callable = new Callable<Long>() {
    @Override
    public Long call() throws Exception {

        long start = System.currentTimeMillis();
        Thread.sleep(100);
        long end = System.currentTimeMillis();

        long seed = end - start;
        System.out.println("seed=" + seed);

        return seed;
    }
};

List<Callable<Long>> tasks = new ArrayList<>();
tasks.add(callable);
tasks.add(callable);
tasks.add(callable);
tasks.add(callable);
tasks.add(callable);
tasks.add(callable);
tasks.add(callable);
tasks.add(callable);
tasks.add(callable);
tasks.add(callable);

int poolSize = Runtime.getRuntime().availableProcessors();
System.out.println("poolSize=" + poolSize);
ExecutorService executorService = Executors.newFixedThreadPool(poolSize);
List<Future<Long>> futures = executorService.invokeAll(tasks);

long result = 0;
for (Future<Long> future : futures) {
    result += future.get();
}
System.out.println("result=" + result);
executorService.shutdown();

运行程序的主机是8核的，因此启动了8个线程，每次会同时执行8个task。

下面是运行结果：

poolSize=8
seed=109
seed=109
seed=109
seed=109
seed=109
seed=109
seed=109
seed=109
seed=110
seed=110
result=1092

links

SourceCode-Github

Future-JSE8

Future-baeldung.com

Future-geeksforgeeks.org

Future-dzone.com