一、FutureTask对象的使用
当需要创建一个线程时,通常会有两种方式，实现Runnable 接口或者继承Thread 类，但不管使用这两种的那一个，线程执行后都不会有返回值，因为这俩的run方法都没有返回值。FutureTask对象就用来让一个线程执行完任务后可以有返回值,想获取这个返回值的线程在此线程没结束前会被阻塞住。
@Slf4j
public class Test1 {
public static void main(String[] args) throws ExecutionException, InterruptedException {
FutureTask<String> task = new FutureTask<>(new Callable<String>() {
@Override
public String call() throws Exception {
Thread.sleep(3000);
log.info("task end ");
return "success";
}
});
Thread t1 = new Thread(task);
t1.start();
//主线程调用get方法后如何t1还没有返回结果主线程就会被阻塞住
String res = task.get();
log.info("res:{}",res);
}
}
所以用FutureTask对象创建线程实现了让一个线程t1等待另一个线程t2的返回值的效果。
二、FutureTask源码解读
// 实现了Runnable接口所以可以用来创建线程
public class FutureTask<V> implements RunnableFuture<V> {
// FutureTask内部维护的状态变量，表示当前对象的状态
private volatile int state;
private static final int NEW = 0;
private static final int COMPLETING = 1;
private static final int NORMAL = 2;
private static final int EXCEPTIONAL = 3;
private static final int CANCELLED = 4;
private static final int INTERRUPTING = 5;
private static final int INTERRUPTED = 6;
// 当前对象要执行的任务
private Callable<V> callable;
// 任务执行完后的返回值
private Object outcome; // non-volatile, protected by state reads/writes
//存储当前对象会被那个线程执行
private volatile Thread runner;
//等待队列，任务未执行完前获取值的线程都会在这里阻塞(调用park方法)
private volatile WaitNode waiters;
//线程启动时执行的方法
public void run() {
//先校验当前对象的状态是不是New,不是就直接返回，线程就会结束运行;
//使用cas操作把当前对象的runner属性设置成当前线程本身，如果失败了就直接返回
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
//callable封装了要执行的任务
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
//执行任务
result = c.call();
ran = true;
} catch (Throwable ex) {
//执行任务过程中抛异常的情况
result = null;
ran = false;
//异常情况的处理
setException(ex);
}
if (ran)
//走到这里表示任务正常执行完成
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)
//走到这里表示执行这个任务的线程被打断了
handlePossibleCancellationInterrupt(s);
}
}
//处理任务正常结束的情况
protected void set(V v) {
//cas操作设置state状态为COMPLETING
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;//返回值赋值
//操作内存的方式设置状态为完成
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
//这个方法中会唤醒当前正在等待此线程返回值的其他线程
private void finishCompletion() {
// assert state > COMPLETING;
//遍历等待链表
for (WaitNode q; (q = waiters) != null;) {
//修改链表头为null,防止其他线程也来遍历
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
for (;;) {
//以下从链表中取出每个线程用unpark方法唤醒
Thread t = q.thread;
if (t != null) {
q.thread = null;
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
}
//异常情况的处理
protected void setException(Throwable t) {
//cas操作设置状态为COMPLETING
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
//返回值设置为异常对象
outcome = t;
//设置状态为EXCEPTIONAL
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
//唤醒等待者
finishCompletion();
}
}
//以上这些就是线程执行FutureTask对象的过程,执行完后会把结果放到outcome属性中,
//接下来看下另一个线程是如何获取此线程的返回值的
//需要等待此FutureTask对象执行结果的线程调用此方法
public V get() throws InterruptedException, ExecutionException {
//获取当前状态
int s = state;
//状态值小于等于COMPLETING表示还未执行完，当前线程就需要阻塞等待
if (s <= COMPLETING)
//实现等待的效果
s = awaitDone(false, 0L);
//返回结果
return report(s);
}
//实现等待的效果
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;//定义q,值是null
boolean queued = false;
//死循环中处理各种情况
for (;;) {
//处理等待线程被打断的情况
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
//获取状态
int s = state;
//能进入这个循环表示任务已经执行完了
if (s > COMPLETING) {
if (q != null)
q.thread = null;
//返回状态,后续report方法会用到这个值
return s;
}
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
else if (q == null)
//创建一个链表节点赋值给q,下次循环会把q加入链表
q = new WaitNode();
else if (!queued)
//把q加入等待队列
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
//带时限的等待处理
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
//阻塞等待的线程
LockSupport.parkNanos(this, nanos);
}
else
//阻塞等待的线程,线程被唤醒后会继续进行下一次循环
LockSupport.park(this);
}
}
}
