FutureTask 源码分析
JDK源码学习
深入分析下 java.util.concurrent 包下 FutureTask 类
简单画了个UML图，可以看到FutureTask, CompletableFuture 都有实现 Future接口类
FutureTask类
先来看Future的实现类 --> FutureTask间接实现Runnable，Future, 可作为一个任务被执行，也能获取计算结果
有些场景需要异步执行任务， 或子线程并行执行任务，此时就可用FutureTask来实现，可以阻塞获取返回值，也可轮询获取返回值
下面通过例子来分析源码：
public class FutureTest {
public static class Task implements Callable {
@Override
public String call() {
String tid = String.valueOf(Thread.currentThread().getId());
System.out.printf("Thread#%s : in call\n", tid);
return tid;
}
}
public static void main(String[] args) throws InterruptedException, ExecutionException
{
//新建一个任务放入线程池，获取其执行完的返回值
ExecutorService es = Executors.newFixedThreadPool(3);
Future future = es.submit(new Task());
System.out.println(future.get());
}
} 向线程池提交一个有返回值任务， 返回FutureTask实例
抽象类AbstractExecutorService：
public Future submit(Callable task) {
if (task == null) throw new NullPointerException();
RunnableFuture ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
protected RunnableFuture newTaskFor(Callable callable) {
return new FutureTask(callable);
}
FutureTask类：
private Callable callable;
private volatile int state;
public FutureTask(Callable callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
分析execute(ftask)，此方法将任务放入线程池，在未来某个时间点执行任务
类ThreadPoolExecutor:
//运行状态存储在高三位中
private static final int RUNNING = -1 << COUNT_BITS;
//ctl原子整数,存储着有效线程数和线程池状态，ctlOf方法通过或运算符计算， 初始化时工作线程数为0
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
}
int c = ctl.get();
//计算工作线程数是否小于核心线程数
if (workerCountOf(c) < corePoolSize) {
//创建核心线程执行任务
if (addWorker(command, true))
return;
c = ctl.get();
}
//判断线程池是否是运行状态，并向工作队列添加一个任务
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
//线程池不处于运行状态，移除任务
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
//创建新线程
addWorker(null, false);
}
else if (!addWorker(command, false))
//拒绝策略
reject(command);
}继续分析addWorker方法，此方法创建工作线程执行任务
类ThreadPoolExecutor：
private boolean addWorker(Runnable firstTask, boolean core) {
//标记循环的位置
retry:
for (;;) {
//返回池控制状态
int c = ctl.get();
//获取线程池状态
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
//计算工作线程数
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
//cas操作递增工作线程数，当前数+1
if (compareAndIncrementWorkerCount(c))
//跳出循环标记位
break retry;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
//创建worker实例
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
//放入工作线程池集合
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
//启动失败，移除worker, cas操作递减工作线程数，当前数-1
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
private void addWorkerFailed(Worker w) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
if (w != null)
workers.remove(w);
decrementWorkerCount();
//尝试终止线程池
tryTerminate();
} finally {
mainLock.unlock();
}
}通过上面的分析，可以看到任务放入了Worker对象中，然后启动了Worker里面的线程，那这个线程做了什么呢？
这个线程先执行创建Worker对象时提交的任务，然后取工作队列里的任务执行
类ThreadPoolExecutor：
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
//创建Worker对象时提交的任务
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
//getTask() 从任务池中拿一个任务
while (task != null || (task = getTask()) != null) {
w.lock();
// If pool is stopping, ensur thread is interrupted;
// if not, ensure thread is not interrupted. This
// requires a recheck in second case to deal with
// shutdownNow race while clearing interrupt
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
//子类可重写，前置增强
beforeExecute(wt, task);
Throwable thrown = null;
try {
//运行任务
task.run();
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
//从队列中取任务
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// 判断超时标志位
// 核心线程不允许超时，allowCoreThreadTimeOut 默认false
// wc > corePoolSize 判断当前线程数是否大于核心线程数
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
//当前线程数-1， 线程退出
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
//timed = true, 等待keepAliveTime 纳秒取任务
//timed = false, 阻塞直到有可用任务
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
//已超时，作为上面的if条件，用于判断此线程是否退出
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}由于在 ThreadPoolExecutor 提交的是一个FutureTask 任务实现类，所以runWorker 里是调用 FutureTask.run() 方法来执行
类FutureTask：
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
//执行call函数，也就是执行测试例子中的Task.call()方法
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
//设置执行结果
set(result);
}
} finally {
runner = null;
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
protected void set(V v) {
//cas操作变更此任务状态为 COMPLETING
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
//任务结果赋值给outcome, 后续future.get获取的就是outcome值
outcome = v;
//设置最终状态为 NORMAL
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
//移除所有等待的线程并发出信号唤醒
finishCompletion();
}
} 上段代码中 set() 方法设置此任务最终的状态为 NORMAL, 表示此任务已正常执行完成。
接着 finishCompletion() 方法 遍历所有等待的节点，并发送信号唤醒线程
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
//唤醒等待的线程，此处唤醒awaitDone()方法中LockSupport.park阻塞的线程
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
}最终我们来看future.get() 是怎么执行的
类FutureTask：
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
//任务未执行完成，进行阻塞
s = awaitDone(false, 0L);
return report(s);
}
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，则从链表中移除节点，并抛出中断异常
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
//任务完成、取消、异常等，则返回当前任务状态
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
else if (s == COMPLETING) // cannot time out yet
//让出cpu，使当前线程从运行状态变成就绪状态，和其它线程一同竞争cpu
Thread.yield();
else if (q == null)
//创建等待节点
q = new WaitNode();
else if (!queued)
//cas操作更新waiters的头节点为q, 完成入队
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
//到了设置的超时时间，则移除等待队列
removeWaiter(q);
return state;
}
//阻塞nacos纳秒数
LockSupport.parkNanos(this, nanos);
}
else
//阻塞，等待唤醒
LockSupport.park(this);
}
}
private V report(int s) throws ExecutionException {
//此处x的值就是前面set方法赋值的outcome值
Object x = outcome;
if (s == NORMAL)
return (V)x;
if (s >= CANCELLED)
throw new CancellationException();
throw new ExecutionException((Throwable)x);
}总结：
到这里Future类分析完了，基本通过这个例子可看到整个的执行流程，比如创建了FutureTask对像、创建了线程执行任务、控制状态的存储、怎么完成的回调等等。