FutureTask ialah pengiraan tak segerak yang boleh dibatalkan.
FutureTask menyediakan pelaksanaan asas Masa Depan Anda boleh memanggil kaedah untuk memulakan dan membatalkan pengiraan, bertanya sama ada pengiraan telah selesai dan mendapatkan hasil pengiraan.
FutureTask hanya boleh mendapatkan hasil pengiraan selepas pengiraan selesai Setelah pengiraan selesai, ia tidak boleh dimulakan semula atau dibatalkan melainkan kaedah runAndReset dipanggil.
Selain melaksanakan antara muka Masa Depan, FutureTask juga melaksanakan antara muka Runnable, jadi FutureTask boleh dilaksanakan oleh Pelaksana kumpulan benang, atau ia boleh dilaksanakan terus menggunakan panggilan benang tak segerak (futureTask.run ()).
Pertama, mari kita lihat struktur warisan kelas FutureTask, seperti yang ditunjukkan di bawah ini melaksanakan antara muka RunnableFuture dan RunnableFuture mewarisi daripada Masa Depan dan antara muka berfungsi Runnable, jadi intipati FutureTask Ia adalah Masa Depan yang boleh dijalankan.
Antara muka Masa Depan menetapkan beberapa fungsi yang mesti dilaksanakan oleh kelas pengkomputeran tak segerak Kod sumber adalah seperti berikut:
package java.util.concurrent;
public interface Future<V> {
/**
* 尝试取消任务的执行，并返回取消结果。
* 参数mayInterruptIfRunning：是否中断线程。
*/
boolean cancel(boolean mayInterruptIfRunning);
/**
* 判断任务是否被取消（正常结束之前被被取消返回true）
*/
boolean isCancelled();
/**
* 判断当前任务是否执行完毕，包括正常执行完毕、执行异常或者任务取消。
*/
boolean isDone();
/**
* 获取任务执行结果，任务结束之前会阻塞。
*/
V get() throws InterruptedException, ExecutionException;
/**
* 在指定时间内尝试获取执行结果。若超时则抛出超时异常TimeoutException
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}. Runnable Kita semua biasa dengan antara muka Ia adalah antara muka yang berfungsi, yang sering kita gunakan untuk mencipta benang.
package java.lang;
?
@FunctionalInterface
public interface Runnable {
? ?
? ?public abstract void run();
}FutureTask ialah tugasan yang akan dilaksanakan. Ia mengandungi pelaksanaan khusus antara muka di atas secara dalaman mentakrifkan keadaan tugasan dan beberapa pemalar keadaan boleh dipanggil lulus dalam boleh dipanggil atau boleh dijalankan melalui pembina, dan ia akhirnya akan ditukar kepada boleh dipanggil secara dalaman, kerana kita perlu mendapatkan hasil pelaksanaan tugas tak segerak, dan hanya utas yang dibuat melalui Callable akan mengembalikan hasilnya.
Kita boleh menilai keputusan pulangan isCancelled() dan isDone() pada Masa Depan melalui keadaan semasa.
Berikut ialah kod sumber FutureTask, termasuk komen analisis kod sumber teras
package java.util.concurrent;
import java.util.concurrent.locks.LockSupport;
public class FutureTask<V> implements RunnableFuture<V> {
/**
* 任务的运行状态
*/
private volatile int state;
private static final int NEW = 0; // 新建
private static final int COMPLETING = 1; // 完成
private static final int NORMAL = 2; // 正常
private static final int EXCEPTIONAL = 3; // 异常
private static final int CANCELLED = 4; // 取消
private static final int INTERRUPTING = 5; // 中断中
private static final int INTERRUPTED = 6; // 中断的
private Callable<V> callable;
/**
* 返回结果
*/
private Object outcome;
private volatile Thread runner;
private volatile WaitNode waiters;
...
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW;
}
public FutureTask(Runnable runnable, V result) {
this.callable = Executors.callable(runnable, result);
this.state = NEW;
}
public boolean isCancelled() {
return state >= CANCELLED;
}
public boolean isDone() {
return state != NEW;
}
/*
* 取消任务实现
* 如果任务还没有启动就调用了cancel(true)，任务将永远不会被执行。
* 如果任务已经启动，参数mayInterruptIfRunning将决定任务是否应该中断执行该任务的线程，以尝试中断该任务。
* 如果任务任务已经取消、已经完成或者其他原因不能取消，尝试将失败。
*/
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;
}
/*
* 等待获取结果
* 获取当前状态，判断是否执行完成。并且判断时间是否超时
* 如果任务没有执行完成，就阻塞等待完成，若超时抛出超时等待异常。
*/
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
/*
* 等待获取结果
* 获取当前状态，判断是否执行完成。
* 如果任务没有执行完成，就阻塞等待完成。
*/
public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
if (unit == null)
throw new NullPointerException();
int s = state;
if (s <= COMPLETING &&
(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
throw new TimeoutException();
return report(s);
}
/**
* 根据状态判断返回结果还是异常
*/
private V report(int s) throws ExecutionException {
Object x = outcome;
if (s == NORMAL)
return (V)x;
if (s >= CANCELLED)
throw new CancellationException();
throw new ExecutionException((Throwable)x);
}
protected void done() { }
/**
* 设置结果借助CAS确认状态是否完成状态
*/
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
/**
* 设置异常，当运行完成出现异常，设置异常状态
*/
protected void setException(Throwable t) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}
/*
* 执行callable获取结果，或者异常
* 判断状态是不是启动过的，如果是新建才可以执行run方法
*/
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 = null;
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
/**
* 重新执行
*/
protected boolean runAndReset() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return false;
boolean ran = false;
int s = state;
try {
Callable<V> c = callable;
if (c != null && s == NEW) {
try {
c.call(); // don't set result
ran = true;
} catch (Throwable ex) {
setException(ex);
}
}
} finally {
runner = null;
s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
return ran && s == NEW;
}
/*
* 处理可能取消的中断
*/
private void handlePossibleCancellationInterrupt(int s) {
if (s == INTERRUPTING)
while (state == INTERRUPTING)
Thread.yield();
}
static final class WaitNode {
volatile Thread thread;
volatile WaitNode next;
WaitNode() { thread = Thread.currentThread(); }
}
/**
* 移除并唤醒所有等待线程,执行done,置空callable
*/
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);
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();
callable = null; // to reduce footprint
}
/**
* 等待完成
* 首先判断是否超时
* 处理中断的，然后处理异常状态的，处理完成的...
*/
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);
}
}
/**
* 去除等待
*/
private void removeWaiter(WaitNode node) {
if (node != null) {
node.thread = null;
retry:
for (;;) { // restart on removeWaiter race
for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
s = q.next;
if (q.thread != null)
pred = q;
else if (pred != null) {
pred.next = s;
if (pred.thread == null) // check for race
continue retry;
}
else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
q, s))
continue retry;
}
break;
}
}
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long stateOffset;
private static final long runnerOffset;
private static final long waitersOffset;
static {
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class<?> k = FutureTask.class;
stateOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("state"));
runnerOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("runner"));
waitersOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("waiters"));
} catch (Exception e) {
throw new Error(e);
}
}
}Secara umumnya, kita boleh Adalah dianggap bahawa FutureTask mempunyai tiga keadaan berikut:
Belum dimulakan: FutureTask yang baru dibuat tidak dimulakan sebelum run() dilaksanakan.
private static final int NEW = 0; // 新建
Dimulakan: Apabila kaedah jalankan objek FutureTask dimulakan dan dilaksanakan, FutureTask berada dalam keadaan mula.
Selesai: FutureTask berakhir seperti biasa, atau FutureTaskpelaksanaan dibatalkan (kaedah pembatalan objek FutureTask), atau kaedah larian objek FutureTask membuang pengecualian dan berakhir dengan gangguan, FutureTask Semua telah selesai status.
private static final int COMPLETING = 1; // 完成 private static final int NORMAL = 2; // 完成后正常设置结果 private static final int EXCEPTIONAL = 3; // 完成后异常设置异常 private static final int CANCELLED = 4; // 执行取消 private static final int INTERRUPTING = 5; // 中断中 private static final int INTERRUPTED = 6; // 中断的
Gunakan satu (terus buat urutan baharu untuk memanggil):
FutureTask<Integer> task = new FutureTask<>(new Callable() {
@Override
public Integer call() throws Exception {
return sum();
}
});
new Thread(task).stat();
Integer result = task.get();Gunakan dua (gabungkan penggunaan kolam benang)
FutureTask<Integer> task = new FutureTask<>(new Callable() {
@Override
public Integer call() throws Exception {
return sum();
}
});
Executors.newCachedThreadPool().submit(task);
Integer result = task.get();Atas ialah kandungan terperinci Analisis kod sumber Java FutureTask dan butiran penggunaan. Untuk maklumat lanjut, sila ikut artikel berkaitan lain di laman web China PHP!
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