併發編程系列之FutureTask源碼學習筆記
1、什麼是FutureTask類?
在上一章節的學習中,我們知道了Future類的基本用法,知道了Future其實就是爲了監控線程任務執行的,接着本博客繼續學習FutureTask。然後什麼是FutureTask類?
Future是1.5版本引入的異步編程的頂層抽象接口,FutureTask則是Future的基礎實現類。同時FutureTask還實現了Runnable接口,所以FutureTask也可以作爲一個獨立的Runnable任務。
2、使用FutureTask封裝Callable任務
線程中是不能直接傳入Callable任務的,所以需要藉助FutureTask,FutureTask可以用來封裝Callable任務,下面給出一個例子:
package com.example.concurrent.future;
import java.util.Random;
import java.util.concurrent.*;
/**
* <pre>
* FutureTask例子
* </pre>
* <p>
* <pre>
* @author nicky.ma
* 修改記錄
* 修改後版本: 修改人: 修改日期: 2021/08/28 18:04 修改內容:
* </pre>
*/
public class FutureTaskExample {
public static void main(String[] args) throws ExecutionException, InterruptedException {
FutureTask futureTask = new FutureTask(new CallableTask());
Thread t = new Thread(futureTask);
t.start();
System.out.println(futureTask.get());
}
static class CallableTask implements Callable<Integer> {
@Override
public Integer call() throws Exception{
Thread.sleep(1000L);
return new Random().nextInt();
}
}
}
3、FutureTask UML類圖
翻下FutureTask的源碼,可以看出實現了RunnableFuture接口
public class FutureTask<V> implements RunnableFuture<V> {
// ...
}
RunnableFuture接口是怎麼樣的?可以看出其實是繼承了Runnable,Future
public interface RunnableFuture<V> extends Runnable, Future<V> {
/**
* Sets this Future to the result of its computation
* unless it has been cancelled.
*/
void run();
}
在idea裏畫出FutureTask的uml類圖:
所以,可以說FutureTask本質就是一個Runnable任務
4、FutureTask源碼學習
- FutureTask類屬性
public class FutureTask<V> implements RunnableFuture<V> {
// 狀態:存在以下7中狀態
private volatile int state;
// 新建
private static final int NEW = 0;
// 任務完成中
private static final int COMPLETING = 1;
// 任務正常完成
private static final int NORMAL = 2;
// 任務異常
private static final int EXCEPTIONAL = 3;
// 任務取消
private static final int CANCELLED = 4;
// 任務中斷中
private static final int INTERRUPTING = 5;
// 任務已中斷
private static final int INTERRUPTED = 6;
// 支持結果返回的Callable任務
private Callable<V> callable;
// 任務執行結果:包含正常和異常的結果,通過get方法獲取
private Object outcome;
// 任務執行線程
private volatile Thread runner;
// 棧結構的等待隊列,該節點是棧中的最頂層節點
private volatile WaitNode waiters;
}
- 構造方法
// 傳入callable任務
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
// 傳入runnable任務、結果變量result
public FutureTask(Runnable runnable, V result) {
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}
- 是一個Runnable任務,run方法實現
public void run() {
// 兩種情況直接返回
// 1:狀態不是NEW,說明已經執行過,獲取已經取消任務,直接返回
// 2:狀態是NEW,將當前執行線程保存在runner字段(runnerOffset)中,如果賦值失敗,直接返回
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 {
// 執行了給如的Callable任務
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
// 異常的情況,設置異常
setException(ex);
}
if (ran)
// 任務正常執行,設置結果
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;
// 任務被中斷,執行中斷處理
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
setException方法:
protected void setException(Throwable t) {
// CAS,將狀態由NEW改爲COMPLETING(中間狀態)
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
// 返回結果
outcome = t;
// 將狀態改爲EXCEPTIONAL
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}
- get獲取執行結果
public V get() throws InterruptedException, ExecutionException {
int s = state;
// 任務還沒完成,調用awaitDonw
if (s <= COMPLETING)
s = awaitDone(false, 0L);
// 返回結果
return report(s);
}
get超時的方法
public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
// unit是時間單位,必須傳
if (unit == null)
throw new NullPointerException();
int s = state;
// 超過阻塞時間timeout,拋出TimeoutException
if (s <= COMPLETING &&
(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
throw new TimeoutException();
return report(s);
}
重點看下awaitDone方法:
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
// 計算截止時間
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
//
boolean queued = false;
// 無限循環,判斷條件是否符合
for (;;) {
// 1、線程是否被中斷,是的情況,移除節點,同時拋出InterruptedException
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
// 2、獲取當前狀態,如果狀態大於COMPLETING
// 說明任務完成了,有可能正常執行完成,也有可能是取消了任務
int s = state;
if (s > COMPLETING) {
if (q != null)
// thread置爲null 等待JVM gc
q.thread = null;
//返回結果
return s;
}
//3、如果狀態處於中間狀態COMPLETING
//表示任務已經結束但是任務執行線程還沒來得及給outcome賦值
else if (s == COMPLETING) // cannot time out yet
// 這種情況線程yield讓出執行權,給其它線程先執行
Thread.yield();
// 4、如果等待節點爲空,則構造一個等待節點
else if (q == null)
q = new WaitNode();
// 5、如果還沒有入隊列,則把當前節點加入waiters首節點並替換原來waiters
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
nanos = deadline - System.nanoTime();
//如果需要等待特定時間,則先計算要等待的時間
// 如果已經超時,則刪除對應節點並返回對應的狀態
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
// 阻塞等待特定時間
LockSupport.parkNanos(this, nanos);
}
else
// 讓線程等待,阻塞當前線程
LockSupport.park(this);
}
}
- cancel取消任務
public boolean cancel(boolean mayInterruptIfRunning) {
// 如果任務已經結束,則直接返回false
if (!(state == NEW &&
UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
return false;
try { // in case call to interrupt throws exception
// 需要中斷任務的情況
if (mayInterruptIfRunning) {
try {
Thread t = runner;
// 調用線程的interrupt來停止線程
if (t != null)
t.interrupt();
} finally { // final state
// 修改狀態爲INTERRUPTED
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
}
}
} finally {
finishCompletion();
}
return true;
}
finishCompletion方法:
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
// 無限循環,遍歷waiters列表,喚醒節點中的線程,然後將Callable置爲null
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
// 喚醒線程
LockSupport.unpark(t);
}
WaitNode next = q.next;
if (next == null)
break;
// 置爲null,讓JVM gc
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();
callable = null; // to reduce footprint
}