FutureTask
源码分析
(1)FutureTask属性集
* Possible state transitions:
* NEW -> COMPLETING -> NORMAL
* NEW -> COMPLETING -> EXCEPTIONAL
* NEW -> CANCELLED
* NEW -> INTERRUPTING -> INTERRUPTED
*/
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;
/** The underlying callable; nulled out after running */
private Callable callable;
/** The result to return or exception to throw from get() */
private Object outcome; // non-volatile, protected by state reads/writes
/** The thread running the callable; CASed during run() */
private volatile Thread runner;
/** Treiber stack of waiting threads */
private volatile WaitNode waiters;
FutureTask属性:
- state属性记录当前线程的执行状态,用int表示0~6
- callable属性在run方法中调用执行真正的业务逻辑
- outcome属性该FutureTask返回值:异常/get()值
- runner属性表示用来执行callable的线程,在run()/runAndReset()方法中通过CAS设置
- waiters属性等待获取该FutureTask值的线程(们)
(2) FutureTask方法集
run方法:一般放在线程池中的线程中运行,运行过程中通过CAS重置该FutureTask的状态.
public void run() {
//如果当前状态为NEW则设置runner为当前线程
if (state != NEW ||!UNSAFE.compareAndSwapObject(this,runnerOffset,null, Thread.currentThread()))
return;
try {
//在runner中调用callable.call()方法获取结果
Callable 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)
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);
}
}
get方法:用于获取线程执行callable后的结果,当线程还没有执行完成时会调用awaitDone()使当前线程阻塞.
public V get() throws InterruptedException, ExecutionException {
int s = state;
//如果当前状态为=0|1(未完成),则调用get()方法的线程进行阻塞.
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
set方法:通过CAS设置FutureTask的state及outcome属性
//执行完毕将结果设置为outcome,并更改当前FutureTask状态
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
cancle方法:取消线程的执行,首先重置FutureTask的状态然后中断该线程.
public boolean cancel(boolean mayInterruptIfRunning) {
if (!(state == NEW &&
UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
return false;
try { // in case call to interrupt throws exception
if (mayInterruptIfRunning) {
try {
Thread t = runner;
if (t != null)
t.interrupt();
} finally { // final state
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
}
}
} finally {
finishCompletion();
}
return true;
}
finishCompletion方法:将所有的等待线程从队列中移除并唤醒,并设置callable为空.
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
//唤醒因调用get()方法导致等待的所有线程
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
LockSupport.unpark(t);
}
WaitNode next = q.next;
//退出循环条件
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();
callable = null; // to reduce footprint
}
awaitDone方法:等待runnable运行完成或中断,会调用LockSupport.park()方法使当线程中断.
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
//用户当前线程中断
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
如果状态非normal,返回当前状态
int s = state;
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
else if (q == null)
q = new WaitNode();
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);
}
else
//当前线程阻塞状态
LockSupport.park(this);
}
}
执行流程
- 第一步首先编写Callable实现类,重写V call()方法,submit(callable).
- 创建线程池,更新runner变量
- 客户端线程调用get()方法获取结果
- 添加到等待队列中,单链表连接
- 线程池任务执行成功,置状态
- 唤醒客户端线程并返回结果
- 获取submit()方法的返回值FutureTask对象
-
调用FutureTask对象,使当前线程阻塞直至有返回值.
Wechat.png
同步技巧
volatile变量
1. int state 变量在多线程之间(1.客户端线程:调用get()/cancle等()方法的线程 2.线程池中运行run()方法的runner线程)保证可见行
2. Thread runner 当在线程池中运行run()方法时设置池中线程
3. WaitNode waiters 在客户端线程中将waiter node添加到队列中,在线程池线程run()方法中将waiter node唤醒.
CAS
不用过于介绍主要是基于CAS从内存层面直接更改变量值
LockSupport
1. 底层调用UNSAFE的park() or unpark()方法阻塞/唤醒线程.
1. 调用该类的park()方法使线程进入阻塞状态.
2. 通过unpark()方法将线程重新唤醒.
demo
public class FutureCode {
public static void main(String[] args) {
ArrayList> list = new ArrayList<>();
ExecutorService pool = Executors.newFixedThreadPool(5);
for (int i = 0; i < 5; i++) {
FutureTask ft = new FutureTask<>(new Task(i, "" + i));
pool.submit(ft);
list.add(ft);
}
System.out.println("计算完毕");
Integer totalResult = 0;
for (FutureTask ft : list) {
try {
totalResult = totalResult.intValue() + ft.get();
} catch (Exception e) {
e.printStackTrace();
}
}
pool.shutdown();
System.out.println("多任务计算完毕:totalResult=" + totalResult);
}
static class Task implements Callable {
private Integer result;
private String name;
public Task(Integer result, String name) {
this.result = result;
this.name = name;
}
@Override
public Integer call() throws Exception {
for (int i = 0; i < 100; i++) {
result = +i;
}
Thread.sleep(5000);
System.out.println("子线程计算任务:" + name + "执行完成!");
return result;
}
}
}