FutureTask
源碼分析
(1)FutureTask屬性集
* 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<V> 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屬性:
- state屬性記錄當(dāng)前線程的執(zhí)行狀態(tài),用int表示0~6
- callable屬性在run方法中調(diào)用執(zhí)行真正的業(yè)務(wù)邏輯
- outcome屬性該FutureTask返回值:異常/get()值
- runner屬性表示用來(lái)執(zhí)行callable的線程,在run()/runAndReset()方法中通過(guò)CAS設(shè)置
- waiters屬性等待獲取該FutureTask值的線程(們)
(2) FutureTask方法集
run方法:一般放在線程池中的線程中運(yùn)行,運(yùn)行過(guò)程中通過(guò)CAS重置該FutureTask的狀態(tài).
public void run() {
//如果當(dāng)前狀態(tài)為NEW則設(shè)置runner為當(dāng)前線程
if (state != NEW ||!UNSAFE.compareAndSwapObject(this,runnerOffset,null, Thread.currentThread()))
return;
try {
//在runner中調(diào)用callable.call()方法獲取結(jié)果
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)
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:用于獲取線程執(zhí)行callable后的結(jié)果,當(dāng)線程還沒(méi)有執(zhí)行完成時(shí)會(huì)調(diào)用awaitDone()使當(dāng)前線程阻塞.
public V get() throws InterruptedException, ExecutionException {
int s = state;
//如果當(dāng)前狀態(tài)為=0|1(未完成),則調(diào)用get()方法的線程進(jìn)行阻塞.
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
set方法:通過(guò)CAS設(shè)置FutureTask的state及outcome屬性
//執(zhí)行完畢將結(jié)果設(shè)置為outcome,并更改當(dāng)前FutureTask狀態(tài)
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:取消線程的執(zhí)行,首先重置FutureTask的狀態(tài)然后中斷該線程.
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:將所有的等待線程從隊(duì)列中移除并喚醒,并設(shè)置callable為空.
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
//喚醒因調(diào)用get()方法導(dǎo)致等待的所有線程
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
LockSupport.unpark(t);
}
WaitNode next = q.next;
//退出循環(huán)條件
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運(yùn)行完成或中斷,會(huì)調(diào)用LockSupport.park()方法使當(dāng)線程中斷.
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
//用戶當(dāng)前線程中斷
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
如果狀態(tài)非normal,返回當(dāng)前狀態(tài)
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
//當(dāng)前線程阻塞狀態(tài)
LockSupport.park(this);
}
}
執(zhí)行流程
- 第一步首先編寫(xiě)Callable實(shí)現(xiàn)類(lèi),重寫(xiě)V call()方法,submit(callable).
- 創(chuàng)建線程池,更新runner變量
- 客戶端線程調(diào)用get()方法獲取結(jié)果
- 添加到等待隊(duì)列中,單鏈表連接
- 線程池任務(wù)執(zhí)行成功,置狀態(tài)
- 喚醒客戶端線程并返回結(jié)果
- 獲取submit()方法的返回值FutureTask對(duì)象
-
調(diào)用FutureTask對(duì)象,使當(dāng)前線程阻塞直至有返回值.
Wechat.png
同步技巧
volatile變量
1. int state 變量在多線程之間(1.客戶端線程:調(diào)用get()/cancle等()方法的線程 2.線程池中運(yùn)行run()方法的runner線程)保證可見(jiàn)行
2. Thread runner 當(dāng)在線程池中運(yùn)行run()方法時(shí)設(shè)置池中線程
3. WaitNode waiters 在客戶端線程中將waiter node添加到隊(duì)列中,在線程池線程run()方法中將waiter node喚醒.
CAS
不用過(guò)于介紹主要是基于CAS從內(nèi)存層面直接更改變量值
LockSupport
1. 底層調(diào)用UNSAFE的park() or unpark()方法阻塞/喚醒線程.
1. 調(diào)用該類(lèi)的park()方法使線程進(jìn)入阻塞狀態(tài).
2. 通過(guò)unpark()方法將線程重新喚醒.
demo
public class FutureCode {
public static void main(String[] args) {
ArrayList<FutureTask<Integer>> list = new ArrayList<>();
ExecutorService pool = Executors.newFixedThreadPool(5);
for (int i = 0; i < 5; i++) {
FutureTask<Integer> ft = new FutureTask<>(new Task(i, "" + i));
pool.submit(ft);
list.add(ft);
}
System.out.println("計(jì)算完畢");
Integer totalResult = 0;
for (FutureTask<Integer> ft : list) {
try {
totalResult = totalResult.intValue() + ft.get();
} catch (Exception e) {
e.printStackTrace();
}
}
pool.shutdown();
System.out.println("多任務(wù)計(jì)算完畢:totalResult=" + totalResult);
}
static class Task implements Callable<Integer> {
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("子線程計(jì)算任務(wù):" + name + "執(zhí)行完成!");
return result;
}
}
}