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本文小編為大家詳細(xì)介紹“Java異步任務(wù)計(jì)算FutureTask源碼分析”，內(nèi)容詳細(xì)，步驟清晰，細(xì)節(jié)處理妥當(dāng)，希望這篇“Java異步任務(wù)計(jì)算FutureTask源碼分析”文章能幫助大家解決疑惑，下面跟著小編的思路慢慢深入，一起來學(xué)習(xí)新知識(shí)吧。
FutureTask 是一個(gè)可取消的異步計(jì)算。
FutureTask提供了對(duì)Future的基本實(shí)現(xiàn)，可以調(diào)用方法去開始和取消一個(gè)計(jì)算，可以查詢計(jì)算是否完成，并且獲取計(jì)算結(jié)果。
FutureTask只能在計(jì)算完成后獲取到計(jì)算結(jié)果，一旦計(jì)算完成，將不能重啟或者取消，除非調(diào)用runAndReset方法。
FutureTask除了實(shí)現(xiàn)了Future接口以外，還實(shí)現(xiàn)了Runnable接口，因此FutureTask是可以交由線程池的Executor執(zhí)行，也可以直接使用一個(gè)異步線程調(diào)用執(zhí)行（futureTask.run()）。
首先，我們看一下FutureTask類的繼承結(jié)構(gòu)，如下圖，它實(shí)現(xiàn)的是RunnableFuture接口，而RunnableFuture繼承自Future和函數(shù)式接口Runnable，所以說FutureTask本質(zhì)就是一個(gè)可運(yùn)行的Future。
Future 接口約定了一些異步計(jì)算類必須要實(shí)現(xiàn)的功能，源碼如下：
package java.util.concurrent;
public interface Future<V> {
/**
* 嘗試取消任務(wù)的執(zhí)行，并返回取消結(jié)果。
* 參數(shù)mayInterruptIfRunning：是否中斷線程。
*/
boolean cancel(boolean mayInterruptIfRunning);
/**
* 判斷任務(wù)是否被取消（正常結(jié)束之前被被取消返回true）
*/
boolean isCancelled();
/**
* 判斷當(dāng)前任務(wù)是否執(zhí)行完畢，包括正常執(zhí)行完畢、執(zhí)行異?；蛘呷蝿?wù)取消。
*/
boolean isDone();
/**
* 獲取任務(wù)執(zhí)行結(jié)果，任務(wù)結(jié)束之前會(huì)阻塞。
*/
V get() throws InterruptedException, ExecutionException;
/**
* 在指定時(shí)間內(nèi)嘗試獲取執(zhí)行結(jié)果。若超時(shí)則拋出超時(shí)異常TimeoutException
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}Runnable 接口我們都很熟悉，他就是一個(gè)函數(shù)式接口，我們常用其創(chuàng)建一個(gè)線程。
package java.lang;
?
@FunctionalInterface
public interface Runnable {
? ?
? ?public abstract void run();
}FutureTask就是一個(gè)將要被執(zhí)行的任務(wù)，它包含了以上接口具體的實(shí)現(xiàn)，F(xiàn)utureTask內(nèi)部定義了任務(wù)的狀態(tài)state和一些狀態(tài)的常量，它的內(nèi)部核心是一個(gè)Callable callable，我們通過構(gòu)造函數(shù)可以傳入callable或者是runnable，最后都會(huì)內(nèi)部轉(zhuǎn)為callable，因?yàn)槲覀冃枰@取異步任務(wù)的執(zhí)行結(jié)果，只有通過Callable創(chuàng)建的線程才會(huì)返回結(jié)果。
我們可以通過此時(shí)的狀態(tài)判斷Future中isCancelled()，isDone()的返回結(jié)果。
以下為FutureTask源碼，內(nèi)含核心源碼分析注釋
package java.util.concurrent;
import java.util.concurrent.locks.LockSupport;
public class FutureTask<V> implements RunnableFuture<V> {
/**
* 任務(wù)的運(yùn)行狀態(tài)
*/
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; // 異常
private static final int CANCELLED = 4; // 取消
private static final int INTERRUPTING = 5; // 中斷中
private static final int INTERRUPTED = 6; // 中斷的
private Callable<V> callable;
/**
* 返回結(jié)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;
}
/*
* 取消任務(wù)實(shí)現(xiàn)
* 如果任務(wù)還沒有啟動(dòng)就調(diào)用了cancel(true)，任務(wù)將永遠(yuǎn)不會(huì)被執(zhí)行。
* 如果任務(wù)已經(jīng)啟動(dòng)，參數(shù)mayInterruptIfRunning將決定任務(wù)是否應(yīng)該中斷執(zhí)行該任務(wù)的線程，以嘗試中斷該任務(wù)。
* 如果任務(wù)任務(wù)已經(jīng)取消、已經(jīng)完成或者其他原因不能取消，嘗試將失敗。
*/
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;
}
/*
* 等待獲取結(jié)果
* 獲取當(dāng)前狀態(tài)，判斷是否執(zhí)行完成。并且判斷時(shí)間是否超時(shí)
* 如果任務(wù)沒有執(zhí)行完成，就阻塞等待完成，若超時(shí)拋出超時(shí)等待異常。
*/
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
/*
* 等待獲取結(jié)果
* 獲取當(dāng)前狀態(tài)，判斷是否執(zhí)行完成。
* 如果任務(wù)沒有執(zhí)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);
}
/**
* 根據(jù)狀態(tài)判斷返回結(jié)果還是異常
*/
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() { }
/**
* 設(shè)置結(jié)果借助CAS確認(rèn)狀態(tài)是否完成狀態(tài)
*/
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
/**
* 設(shè)置異常，當(dāng)運(yùn)行完成出現(xiàn)異常，設(shè)置異常狀態(tài)
*/
protected void setException(Throwable t) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}
/*
* 執(zhí)行callable獲取結(jié)果，或者異常
* 判斷狀態(tài)是不是啟動(dòng)過的，如果是新建才可以執(zhí)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);
}
}
/**
* 重新執(zhí)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;
}
/*
* 處理可能取消的中斷
*/
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(); }
}
/**
* 移除并喚醒所有等待線程,執(zhí)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
* 首先判斷是否超時(shí)
* 處理中斷的，然后處理異常狀態(tài)的，處理完成的...
*/
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);
}
}
}一般來說，我們可以認(rèn)為FutureTask具有以下三種狀態(tài)：
未啟動(dòng)：新建的FutureTask，在run()沒執(zhí)行之前，F(xiàn)utureTask處于未啟動(dòng)狀態(tài)。
private static final int NEW = 0; // 新建
已啟動(dòng)：FutureTask對(duì)象的run方法啟動(dòng)并執(zhí)行的過程中，F(xiàn)utureTask處于已啟動(dòng)狀態(tài)。
已完成：FutureTask正常執(zhí)行結(jié)束，或者FutureTask執(zhí)行被取消(FutureTask對(duì)象cancel方法)，或者FutureTask對(duì)象run方法執(zhí)行拋出異常而導(dǎo)致中斷而結(jié)束，F(xiàn)utureTask都處于已完成狀態(tài)。
private static final int COMPLETING = 1; // 完成 private static final int NORMAL = 2; // 完成后正常設(shè)置結(jié)果 private static final int EXCEPTIONAL = 3; // 完成后異常設(shè)置異常 private static final int CANCELLED = 4; // 執(zhí)行取消 private static final int INTERRUPTING = 5; // 中斷中 private static final int INTERRUPTED = 6; // 中斷的
使用一（直接新建一個(gè)線程調(diào)用）：
FutureTask<Integer> task = new FutureTask<>(new Callable() {
@Override
public Integer call() throws Exception {
return sum();
}
});
new Thread(task).stat();
Integer result = task.get();使用二（結(jié)合線程池使用）
FutureTask<Integer> task = new FutureTask<>(new Callable() {
@Override
public Integer call() throws Exception {
return sum();
}
});
Executors.newCachedThreadPool().submit(task);
Integer result = task.get();讀到這里，這篇“Java異步任務(wù)計(jì)算FutureTask源碼分析”文章已經(jīng)介紹完畢，想要掌握這篇文章的知識(shí)點(diǎn)還需要大家自己動(dòng)手實(shí)踐使用過才能領(lǐng)會(huì)，如果想了解更多相關(guān)內(nèi)容的文章，歡迎關(guān)注億速云行業(yè)資訊頻道。
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