并發(fā)編程系列之FutureTask源碼學(xué)習(xí)筆記
1、什么是FutureTask類？
在上一章節(jié)的學(xué)習(xí)中，我們知道了Future類的基本用法，知道了Future其實(shí)就是為了監(jiān)控線程任務(wù)執(zhí)行的，接著本博客繼續(xù)學(xué)習(xí)FutureTask。然后什么是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(chǔ)實(shí)現(xiàn)類。同時(shí)FutureTask還實(shí)現(xiàn)了Runnable接口，所以FutureTask也可以作為一個(gè)獨(dú)立的Runnable任務(wù)。
2、使用FutureTask封裝Callable任務(wù)
線程中是不能直接傳入Callable任務(wù)的，所以需要借助FutureTask，F(xiàn)utureTask可以用來(lái)封裝Callable任務(wù)，下面給出一個(gè)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
* 修改后版本: 修改人： 修改日期: 2021/08/28 18:04 修改內(nèi)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(shí)現(xiàn)了RunnableFuture接口
public class FutureTask<V> implements RunnableFuture<V> {
// ...
}
RunnableFuture接口是怎么樣的？可以看出其實(shí)是繼承了Runnable，F(xiàn)uture
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里畫出FutureTask的uml類圖：
在這里插入圖片描述
所以，可以說(shuō)FutureTask本質(zhì)就是一個(gè)Runnable任務(wù)
4、FutureTask源碼學(xué)習(xí)
- FutureTask類屬性
public class FutureTask<V> implements RunnableFuture<V> {
// 狀態(tài)：存在以下7中狀態(tài)
private volatile int state;
// 新建
private static final int NEW = 0;
// 任務(wù)完成中
private static final int COMPLETING = 1;
// 任務(wù)正常完成
private static final int NORMAL = 2;
// 任務(wù)異常
private static final int EXCEPTIONAL = 3;
// 任務(wù)取消
private static final int CANCELLED = 4;
// 任務(wù)中斷中
private static final int INTERRUPTING = 5;
// 任務(wù)已中斷
private static final int INTERRUPTED = 6;
// 支持結(jié)果返回的Callable任務(wù)
private Callable<V> callable;
// 任務(wù)執(zhí)行結(jié)果：包含正常和異常的結(jié)果，通過(guò)get方法獲取
private Object outcome;
// 任務(wù)執(zhí)行線程
private volatile Thread runner;
// 棧結(jié)構(gòu)的等待隊(duì)列，該節(jié)點(diǎn)是棧中的最頂層節(jié)點(diǎn)
private volatile WaitNode waiters;
}
- 構(gòu)造方法
// 傳入callable任務(wù)
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(wù)、結(jié)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(gè)Runnable任務(wù)，run方法實(shí)現(xiàn)
public void run() {
// 兩種情況直接返回
// 1：狀態(tài)不是NEW，說(shuō)明已經(jīng)執(zhí)行過(guò)，獲取已經(jīng)取消任務(wù)，直接返回
// 2：狀態(tài)是NEW，將當(dāng)前執(zhí)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(zhí)行了給如的Callable任務(wù)
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
// 異常的情況，設(shè)置異常
setException(ex);
}
if (ran)
// 任務(wù)正常執(zhí)行，設(shè)置結(jié)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(wù)被中斷，執(zhí)行中斷處理
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
setException方法：
protected void setException(Throwable t) {
// CAS,將狀態(tài)由NEW改為COMPLETING（中間狀態(tài)）
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
// 返回結(jié)果
outcome = t;
// 將狀態(tài)改為EXCEPTIONAL
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}
- get獲取執(zhí)行結(jié)果
public V get() throws InterruptedException, ExecutionException {
int s = state;
// 任務(wù)還沒(méi)完成，調(diào)用awaitDonw
if (s <= COMPLETING)
s = awaitDone(false, 0L);
// 返回結(jié)果
return report(s);
}
get超時(shí)的方法
public V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
// unit是時(shí)間單位，必須傳
if (unit == null)
throw new NullPointerException();
int s = state;
// 超過(guò)阻塞時(shí)間timeout，拋出TimeoutException
if (s <= COMPLETING &&
(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
throw new TimeoutException();
return report(s);
}
重點(diǎn)看下awaitDone方法：
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
// 計(jì)算截止時(shí)間
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
//
boolean queued = false;
// 無(wú)限循環(huán)，判斷條件是否符合
for (;;) {
// 1、線程是否被中斷，是的情況，移除節(jié)點(diǎn)，同時(shí)拋出InterruptedException
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
// 2、獲取當(dāng)前狀態(tài)，如果狀態(tài)大于COMPLETING
// 說(shuō)明任務(wù)完成了，有可能正常執(zhí)行完成，也有可能是取消了任務(wù)
int s = state;
if (s > COMPLETING) {
if (q != null)
// thread置為null 等待JVM gc
q.thread = null;
//返回結(jié)果
return s;
}
//3、如果狀態(tài)處于中間狀態(tài)COMPLETING
//表示任務(wù)已經(jīng)結(jié)束但是任務(wù)執(zhí)行線程還沒(méi)來(lái)得及給outcome賦值
else if (s == COMPLETING) // cannot time out yet
// 這種情況線程yield讓出執(zhí)行權(quán)，給其它線程先執(zhí)行
Thread.yield();
// 4、如果等待節(jié)點(diǎn)為空，則構(gòu)造一個(gè)等待節(jié)點(diǎn)
else if (q == null)
q = new WaitNode();
// 5、如果還沒(méi)有入隊(duì)列，則把當(dāng)前節(jié)點(diǎn)加入waiters首節(jié)點(diǎn)并替換原來(lái)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(shí)間，則先計(jì)算要等待的時(shí)間
// 如果已經(jīng)超時(shí)，則刪除對(duì)應(yīng)節(jié)點(diǎn)并返回對(duì)應(yīng)的狀態(tài)
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
// 阻塞等待特定時(shí)間
LockSupport.parkNanos(this, nanos);
}
else
// 讓線程等待，阻塞當(dāng)前線程
LockSupport.park(this);
}
}
- cancel取消任務(wù)
public boolean cancel(boolean mayInterruptIfRunning) {
// 如果任務(wù)已經(jīng)結(jié)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(wù)的情況
if (mayInterruptIfRunning) {
try {
Thread t = runner;
// 調(diào)用線程的interrupt來(lái)停止線程
if (t != null)
t.interrupt();
} finally { // final state
// 修改狀態(tài)為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(wú)限循環(huán)，遍歷waiters列表，喚醒節(jié)點(diǎn)中的線程，然后將Callable置為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;
// 置為null，讓JVM gc
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();
callable = null; // to reduce footprint
}
