簡介
Future主要用於有返回值的異步任務。最核心的類是FutureTask,它是Future接口唯一的實現類。
FutureTask
可以看出FutureTask類實現了Runnable和Future接口。
內部屬性有
private volatile int state;
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;
其中
- state表示任務執行狀態,有下面幾個取值
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; - callable是實際要執行的run方法的對象
- outCome是執行的結果
- runner是執行run方法的線程
- waitNodes是一個鏈表,存儲調用get或者別的方法而阻塞的線程
構造函數
有兩週構造函數,分別傳入Runnable對象和Callable對象
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
public FutureTask(Runnable runnable, V result) {
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}
當傳入構造函數的對象是Runnable時,調用Executors.callable(Runnable,Result)方法返回一個callable的適配器,這個適配器的call方法會調用runnable的run方法,並固定返回傳入的result對象。
run方法
在run方法調用callable的call方法,並把返回結果賦值給outcome屬性。
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)
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
用於獲取執行的結果,如果沒有執行完則阻塞等待,可以設置超時時間。
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
awaitDone方法
在get方法中調用awaitDone方法阻塞等待。awaitDone方法會把當前調用get的線程yield掛起並放在一個鏈表中,執行完成之後會一一喚醒
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
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
LockSupport.park(this);
}
}
finishCompletion執行完成調用
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;
LockSupport.unpark(t); // 喚醒線程
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();
callable = null; // to reduce footprint
}
使用方法
可以直接使用callable或者包裝成FutureTask
public static void main(String[] args) throws ExecutionException, InterruptedException {
/**
* 使用FutureTask
*/
// FutureTask<String> futureTask=new FutureTask<String>(new MyTask());
// ExecutorService myExecuterservice = Executors.newFixedThreadPool(2);
// myExecuterservice.submit(futureTask);
// myExecuterservice.shutdown();
// System.out.println(futureTask.get());
/**
* 直接使用callable
*/
ExecutorService myExecutorService = Executors.newFixedThreadPool(2);
Future<String> future = myExecutorService.submit(new MyTask());
System.out.println(future.get());
myExecutorService.shutdown();
}
}
class MyTask implements Callable<String>{
public String call(){
System.out.println("call begins...");
try {
Thread.sleep(1000l);
} catch (InterruptedException e) {
e.printStackTrace();
}
return "Hello from mytask!";
}