主要介紹線程池相關(guān)知識(shí)，關(guān)于線程池虚婿，首先我們思考下為什么要用線程池旋奢。如果單純的使用線程，線程的創(chuàng)建和銷(xiāo)毀都是自己來(lái)完成然痊，如果并發(fā)請(qǐng)求過(guò)多至朗，可能造成資源耗盡。線程池可以對(duì)線程進(jìn)行統(tǒng)一分配剧浸，調(diào)優(yōu)和監(jiān)控爽丹。本篇文章為《圖靈學(xué)院》課程筆記
- 降低資源消耗（線程無(wú)限制地創(chuàng)建筑煮，然后使用完畢后銷(xiāo)毀）
- 提高響應(yīng)速度（無(wú)須創(chuàng)建線程）
- 提高線程的可管理性
java是如何實(shí)現(xiàn)和管理線程池的，jdk5開(kāi)始把工作單元和任務(wù)執(zhí)行分離粤蝎，工作單元包括callable真仲、runnable，而執(zhí)行機(jī)制由Executor提供初澎，Executor的實(shí)現(xiàn)還提供了對(duì)線程生命周期的管理
相關(guān)接口
image
接口介紹
java.util.concurrent.Executor （執(zhí)行器秸应，執(zhí)行方法）
java.util.concurrent.ExecutorService （執(zhí)行服務(wù)） 包含服務(wù)的生命周期
java.util.concurrent.ScheduledExecutorService （調(diào)度相關(guān)的服務(wù)）
核心接口實(shí)現(xiàn)
- java.util.concurrent.ThreadPoolExecutor （普通的的線程池實(shí)現(xiàn)類）
- java.util.concurrent.ScheduledThreadPoolExecutor （調(diào)度的核心實(shí)現(xiàn)類）
| 名稱 | 方法 | 說(shuō)明 | 類型 |
|---|---|---|---|
| java.util.concurrent.<br />Executor | execute | 執(zhí)行接口 | 接口 |
| java.util.concurrent.<br />ExecutorService | submit(java.util.concurrent.Callable) | 提交接口 | 接口 |
| java.util.concurrent.<br />AbstractExecutorService | submit(Callable<T> task) | 把執(zhí)行和提交接口<br />進(jìn)行合并區(qū)別:有<br />返回值和無(wú)返回值 | 抽象類 |
| java.util.concurrent.<br />ThreadPoolExecutor | execute(Runnable command) | 調(diào) 用 runwork 方 法 getTask（從隊(duì)列 拿數(shù)據(jù)） |
實(shí)現(xiàn)類 |
| java.util.concurrent.<br />ScheduledExecutorService | scheduleAtFixedRate、scheduleWithFixedDelay | 定義方法 | 接口 |
| java.util.concurrent.<br />ScheduledThreadPoolExecutor | delayedExecute | 具體實(shí)現(xiàn) add>task>addWo rk |
實(shí)現(xiàn)類 |
image
內(nèi)部類分為兩種
- policy 策略
- worker 工作
內(nèi)部工作原理（構(gòu)造方法賦值）
- corePool：核心線程池大小
- maximumPool：最大線程池大小
- BlockingQueue：任務(wù)工作隊(duì)列
- keepAliveTime：線程活躍時(shí)間碑宴，如果線程數(shù)量大于核心線程數(shù)量软啼，多余線程空閑時(shí)間超時(shí)候被銷(xiāo)毀
- RejectedExecutionHandler：當(dāng)ThreadPoolExecutor關(guān)閉或最大線程池已經(jīng)滿了，executor將調(diào)用的handler
- ThreadFactory：使用ThreadFactory創(chuàng)建線程延柠，默認(rèn)使用defaultThreadFactory
線程池的運(yùn)行思路
- 如果當(dāng)前線程池中的線程數(shù)目小于corePoolSize祸挪，則每來(lái)一個(gè)任務(wù)，就會(huì)創(chuàng)建一個(gè)線程去執(zhí)行這個(gè)任務(wù)贞间；
- 如果當(dāng)前線程池中的線程數(shù)目>=corePoolSize贿条，則每來(lái)一個(gè)任務(wù)，會(huì)嘗試將其添加到任務(wù)緩存隊(duì)列當(dāng)中增热，若添加成功整以，則該任務(wù)會(huì)等待空閑線程將其取出去執(zhí)行；若添加失斁稹（一般來(lái)說(shuō)是任務(wù)緩存隊(duì)列已滿）公黑，則會(huì)嘗試創(chuàng)建新的線程去執(zhí)行這個(gè)任務(wù)；
- 如果當(dāng)前線程池中的線程數(shù)目達(dá)到maximumPoolSize摄咆，則會(huì)采取任務(wù)拒絕策略進(jìn)行處理凡蚜；
- 如果線程池中的線程數(shù)量大于 corePoolSize時(shí)，如果某線程空閑時(shí)間超過(guò)keepAliveTime吭从，線程將被終止番刊，直至線程池中的線程數(shù)目不大于corePoolSize；如果允許為核心池中的線程設(shè)置存活時(shí)間影锈，那么核心池中的線程空閑時(shí)間超過(guò)keepAliveTime芹务，線程也會(huì)被終止
[圖片上傳失敗...(image-c13745-1598087752516)]
拒接策略
ThreadPoolExecutor.AbortPolicy:丟棄任務(wù)并拋出RejectedExecutionException異常(默認(rèn))
ThreadPoolExecutor.DiscardPolicy：也是丟棄任務(wù)，但是不拋出異常
ThreadPoolExecutor.DiscardOldestPolicy：丟棄隊(duì)列最前面的任務(wù)鸭廷，然后重新嘗試執(zhí)行任務(wù)（重復(fù)此過(guò)程）
ThreadPoolExecutor.CallerRunsPolicy：由調(diào)用線程處理該任務(wù)
源碼解析
線程池的執(zhí)行原理
初始化構(gòu)造器
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.acc = System.getSecurityManager() == null ?
null :
AccessController.getContext();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
ThreadPoolExecutor#execute
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
//判斷是否小于核心數(shù)量枣抱，是直接新增work成功后直接退出
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
// 增加失敗后繼續(xù)獲取標(biāo)記
c = ctl.get();
}
//判斷是運(yùn)行狀態(tài)并且扔到workQueue里成功后
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
//再次check判斷運(yùn)行狀態(tài)如果是非運(yùn)行狀態(tài)就移除出去&reject掉
if (! isRunning(recheck) && remove(command))
reject(command);
//否則發(fā)現(xiàn)可能運(yùn)行線程數(shù)是0那么增加一個(gè)null的worker
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
//直接增加worker如果不成功直接reject
else if (!addWorker(command, false))
reject(command);
}
ThreadPoolExecutor#addWorker
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
// 兩種情況
//1.如果非運(yùn)行狀態(tài)
//2.不是這種情況（停止?fàn)顟B(tài)并且是null對(duì)象并且workQueue不等于null）
return false;
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;// 判斷是否飽和容量了
if (compareAndIncrementWorkerCount(c)) //增加一個(gè)work數(shù)量 然后跳出去
break retry;
c = ctl.get(); // Re-read ctl 增加work失敗后繼續(xù)遞歸
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
//增加一個(gè)worker
w = new Worker(firstTask);
final Thread t = w.thread;
//判斷是否 為null
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired. 鎖定后并重新檢查下 是否存在線程工廠的失敗或者鎖定前的關(guān)閉
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
workers.add(w); //增加work
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) { //本次要是新增加work成功就調(diào)用start運(yùn)行
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
ThreadPoolExecutor#runWorker
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();//1.取到當(dāng)前線程
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null) { //獲取任務(wù) 看看是否能拿到
w.lock();
// If pool is stopping, ensure thread is interrupted;
// if not, ensure thread is not interrupted. This
// requires a recheck in second case to deal with
// shutdownNow race while clearing interrupt
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();// 確保線程是能中斷的
try {
beforeExecute(wt, task); //開(kāi)始任務(wù)前的鉤子
Throwable thrown = null;
try {
task.run();//執(zhí)行任務(wù)
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
afterExecute(task, thrown); //任務(wù)后的鉤子
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
ThreadPoolExecutor#processWorkerExit
private void processWorkerExit(Worker w, boolean completedAbruptly) {
if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
decrementWorkerCount();
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
completedTaskCount += w.completedTasks;
workers.remove(w); //移除work
} finally {
mainLock.unlock();
}
tryTerminate();
int c = ctl.get();
if (runStateLessThan(c, STOP)) { //判斷是否還有任務(wù)
if (!completedAbruptly) {
int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
if (min == 0 && ! workQueue.isEmpty())
min = 1;
if (workerCountOf(c) >= min)
return; // replacement not needed
}
addWorker(null, false);
}
}
image
線程池調(diào)度原理
調(diào)度核心構(gòu)造器
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
new DelayedWorkQueue());
}
ScheduledThreadPoolExecutor#delayedExecute
private void delayedExecute(RunnableScheduledFuture<?> task) {
if (isShutdown())
reject(task);
else {
super.getQueue().add(task);//增加任務(wù)
if (isShutdown() &&
!canRunInCurrentRunState(task.isPeriodic()) &&
remove(task))
task.cancel(false);
else
ensurePrestart();
}
}
通過(guò)DelayedWorkQueue 延遲隊(duì)列實(shí)現(xiàn) offer獲取對(duì)象的延遲
ScheduledThreadPoolExecutor.DelayedWorkQueue#offer
public boolean offer(Runnable x) {
if (x == null)
throw new NullPointerException();
RunnableScheduledFuture<?> e = (RunnableScheduledFuture<?>)x; //當(dāng)前對(duì)象
final ReentrantLock lock = this.lock;
lock.lock();
try {
int i = size;
if (i >= queue.length) //擴(kuò)容
grow();
size = i + 1;
if (i == 0) {
queue[0] = e;
setIndex(e, 0); //第一個(gè)直接設(shè)置索引和下標(biāo)0
} else {
siftUp(i, e); //篩選到上邊
}
if (queue[0] == e) {
leader = null;
available.signal(); //喚醒所有的被擠壓的wait線程
}
} finally {
lock.unlock();
}
return true;
}
ScheduledThreadPoolExecutor.DelayedWorkQueue#siftUp
private void siftUp(int k, RunnableScheduledFuture<?> key) {
while (k > 0) {
int parent = (k - 1) >>> 1;
RunnableScheduledFuture<?> e = queue[parent];
if (key.compareTo(e) >= 0)
break;
queue[k] = e;
setIndex(e, k);
k = parent;
}
queue[k] = key;
setIndex(key, k);
}
ScheduledThreadPoolExecutor.ScheduledFutureTask#compareTo
public int compareTo(Delayed other) {
if (other == this) // compare zero if same object
return 0;
if (other instanceof ScheduledFutureTask) {
ScheduledFutureTask<?> x = (ScheduledFutureTask<?>)other;
long diff = time - x.time; //判斷time
if (diff < 0)
return -1;
else if (diff > 0)
return 1;
else if (sequenceNumber < x.sequenceNumber)
return -1;
else
return 1;
}
long diff = getDelay(NANOSECONDS) - other.getDelay(NANOSECONDS);
return (diff < 0) ? -1 : (diff > 0) ? 1 : 0;
}
ThreadPoolExecutor#ensurePrestart
確保有work執(zhí)行
void ensurePrestart() {
int wc = workerCountOf(ctl.get());
if (wc < corePoolSize)
addWorker(null, true);
else if (wc == 0)
addWorker(null, false);
}
ScheduledThreadPoolExecutor.DelayedWorkQueue#take
work運(yùn)行的時(shí)候調(diào)用queue的take方法
public RunnableScheduledFuture<?> take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
for (;;) {
RunnableScheduledFuture<?> first = queue[0];//獲取第一個(gè)對(duì)象
if (first == null)
available.await();
else {
long delay = first.getDelay(NANOSECONDS);//延遲時(shí)間
if (delay <= 0)//到時(shí)間了
return finishPoll(first);
first = null; // don't retain ref while waiting
if (leader != null)
available.await();//因?yàn)闆](méi)有執(zhí)行線程初始化，所以等等什么時(shí)候有了自己被他人喚醒
else {
Thread thisThread = Thread.currentThread();
leader = thisThread;
try {
available.awaitNanos(delay); //各種condition的awaitNanos
} finally {
if (leader == thisThread)
leader = null;
}
}
}
}
} finally {
if (leader == null && queue[0] != null)
available.signal();
lock.unlock();
}
}
ScheduledThreadPoolExecutor.DelayedWorkQueue#finishPoll
private RunnableScheduledFuture<?> finishPoll(RunnableScheduledFuture<?> f) {
int s = --size;
RunnableScheduledFuture<?> x = queue[s]; //重排序隊(duì)列
queue[s] = null;
if (s != 0)
siftDown(0, x);
setIndex(f, -1);
return f
}
ScheduledThreadPoolExecutor.ScheduledFutureTask#run
public void run() {
boolean periodic = isPeriodic();
if (!canRunInCurrentRunState(periodic))
cancel(false);
else if (!periodic)
ScheduledFutureTask.super.run();
else if (ScheduledFutureTask.super.runAndReset()) {//有period的要執(zhí)行成功設(shè)置下次執(zhí)行時(shí)間和增加額外任務(wù)
setNextRunTime();
reExecutePeriodic(outerTask);
}
}
異步結(jié)果源碼分析
怎么拿到的異步任務(wù)結(jié)果辆床？
FutureTask#awaitDone
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()) { //check線程中斷
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
if (s > COMPLETING) { //判斷是否完成
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(); //生成一個(gè)waint對(duì)象
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);//鏈表的對(duì)象下一個(gè)置成當(dāng)前的waitNode
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos); //等待時(shí)間阻塞
}
else
LockSupport.park(this); //一直阻塞
}
}
什么時(shí)候回填的結(jié)果
FutureTask#run
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread())) //如果狀態(tài)不是new 或者 runner狀態(tài)置不成功直接退出
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();//運(yùn)行ok 的時(shí)候返回result
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)//正常成功set result對(duì)象
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);
}
}
FutureTask#cancel
public boolean cancel(boolean mayInterruptIfRunning) {
if (!(state == NEW &&
UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
mayInterruptIfRunning ? INTERRUPTING : CANCELLED))) //CAS 置成stateOffset 的中斷或者取消
return false;
try { // in case call to interrupt throws exception
if (mayInterruptIfRunning) { //如果線程運(yùn)行中佳晶，可能中斷
try {
Thread t = runner;
if (t != null)
t.interrupt();
} finally { // final state
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
}
}
} finally {
finishCompletion();
}
return true;
}
原文地址
