1：使用
package com.huiguan.facade.mobile;
import java.util.concurrent.*;
/**
* @author chengwei
* @since $$Revision:1.0.0, $$Date: 2018/4/26 11:39 $$
*/
public class FutureDemo {
public static void main(String[] args) {
try {
//创建线程池，此线程池创建为非推荐方式
ExecutorService threadPool = Executors.newCachedThreadPool();
//task是线程
Task task = new Task();
FutureTask<String> futureTask = new FutureTask<String>(task);
//submit运行有返回值的线程，execute运行无返回值的线程
threadPool.submit(futureTask);
//get方法会阻塞，直到task线程运行结束，才会获取返回值
String s = futureTask.get();
System.out.println(s);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
}
package com.huiguan.facade.mobile;
import java.util.concurrent.Callable;
/**
* @author chengwei
* @since $$Revision:1.0.0, $$Date: 2018/4/26 11:39 $$
*/
public class Task implements Callable {
@Override
public String call() throws Exception {
//线程休眠1s，方便观察FutureTask.get的阻塞现象
Thread.sleep(1000);
return "qqqqqqq";
}
}2. 解析FutureTask.get()源码
2.1 原理
当futuretask任务线程未运行结束时，调用get方法时会把futuretask放入链表中，使用LockSupport.park将当前主线程置为休眠状态；当futuretask任务线程运行结束，会遍历链表，使用LockSupport.unpark唤醒主线程，主线程获取futuretask线程返回的值。
2.2 源码
Futureta各种状态
* Possible state transitions:
//Futuretask运行时各种状态转化
* NEW -> COMPLETING -> NORMAL
* NEW -> COMPLETING -> EXCEPTIONAL
* NEW -> CANCELLED
* NEW -> INTERRUPTING -> INTERRUPTED
*/
private volatile int state;
//创建状态
private static final int NEW = 0;
//运行中
private static final int COMPLETING = 1;
//正常结束
private static final int NORMAL = 2;
//异常结束
private static final int EXCEPTIONAL = 3;
//取消
private static final int CANCELLED = 4;
//中断
private static final int INTERRUPTING = 5;
//中断结束
private static final int INTERRUPTED = 6;主线程运行逻辑
java.util.concurrent.FutureTask#get()
public V get() throws InterruptedException, ExecutionException {
int s = state;
//当是创建或者运行中状态时，休眠，等待task运行结束
if (s <= COMPLETING)
s = awaitDone(false, 0L);
//运行结束，获取返回值
return report(s);
}java.util.concurrent.FutureTask#awaitDone
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
//线程中断，抛出异常
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
//非创建和运行中状态，直接返回状态值
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
//当是运行中状态时，让出cpu，由自己和其他线程重新竞争
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
//新建一个等待链表，等待链表是任务线程处于运行中，把主线程放入等待链表，等待任务线程运行结束
else if (q == null)
q = new WaitNode();
//用CAS算法，将当前主线程放入等待链表waiters中，CAS算法：compareAndSwapObject（this，vOffset，A，B）内存值V，旧的预期值A，要修改的新值B。当且仅当预期值A和内存值V相同时，将内存值V修改为B，否则什么都不做。可以保证线程安全。
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
//LockSupport.park类似于wait，使当前线程等待
else
LockSupport.park(this);
}
}
java.util.concurrent.FutureTask#report 获取返回值
/** The result to return or exception to throw from get() */
private Object outcome; // non-volatile, protected by state reads/writes
private V report(int s) throws ExecutionException {
Object x = outcome;
if (s == NORMAL)
//正常结束，获取返回值
return (V)x;
if (s >= CANCELLED)
throw new CancellationException();
throw new ExecutionException((Throwable)x);
}任务线程运行逻辑
java.util.concurrent.FutureTask#run
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
//运行结束，返回返回值，唤醒等待的线程
set(result);
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}java.util.concurrent.FutureTask#set
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
//设置返回值
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
//唤醒等待线程
finishCompletion();
}
}java.util.concurrent.FutureTask#finishCompletion
private void finishCompletion() {
// assert state > COMPLETING;
//遍历waiters链表
for (WaitNode q; (q = waiters) != null;) {
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
//唤醒等待线程
LockSupport.unpark(t);
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();
callable = null; // to reduce footprint
}