java執行緒池原始碼解析
阿新 • • 發佈：2020-08-22
主要介紹執行緒池相關知識，關於執行緒池，首先我們思考下為什麼要用執行緒池。如果單純的使用執行緒，執行緒的建立和銷燬都是自己來完成，如果併發請求過多，可能造成資源耗盡。執行緒池可以對執行緒進行統一分配，調優和監控。本篇文章為《圖靈學院》課程筆記
- 降低資源消耗（執行緒無限制地建立，然後使用完畢後銷燬）
- 提高響應速度（無須建立執行緒）
- 提高執行緒的可管理性
java是如何實現和管理執行緒池的，jdk5開始把工作單元和任務執行分離，工作單元包括callable、runnable，而執行機制由Executor提供，Executor的實現還提供了對執行緒生命週期的管理
相關介面
介面介紹
-
java.util.concurrent.Executor （執行器，執行方法）
-
java.util.concurrent.ExecutorService （執行服務） 包含服務的生命週期
-
java.util.concurrent.ScheduledExecutorService （排程相關的服務）
核心介面實現
- java.util.concurrent.ThreadPoolExecutor （普通的的執行緒池實現類）
- java.util.concurrent.ScheduledThreadPoolExecutor （排程的核心實現類）
| 名稱 | 方法 | 說明 | 型別 |
|---|---|---|---|
| java.util.concurrent. Executor |
execute | 執行介面 | 介面 |
| java.util.concurrent. ExecutorService |
submit(java.util.concurrent.Callable) | 提交介面 | 介面 |
| java.util.concurrent. AbstractExecutorService |
submit(Callable |
把執行和提交介面 進行合併區別:有 返回值和無返回值 |
抽象類 |
| java.util.concurrent. ThreadPoolExecutor |
execute(Runnable command) | 調 用 runwork 方 法 getTask（從佇列 拿資料） |
實現類 |
| java.util.concurrent. |
scheduleAtFixedRate、scheduleWithFixedDelay | 定義方法 | 介面 |
| java.util.concurrent. ScheduledThreadPoolExecutor |
delayedExecute | 具體實現 add>task>addWo rk |
實現類 |
內部類分為兩種
- policy 策略
- worker 工作
內部工作原理（構造方法賦值）
- corePool：核心執行緒池大小
- maximumPool：最大執行緒池大小
- BlockingQueue：任務工作佇列
- keepAliveTime：執行緒活躍時間，如果執行緒數量大於核心執行緒數量，多餘執行緒空閒時間超時候被銷燬
- RejectedExecutionHandler：當ThreadPoolExecutor關閉或最大執行緒池已經滿了，executor將呼叫的handler
- ThreadFactory：使用ThreadFactory建立執行緒，預設使用defaultThreadFactory
執行緒池的執行思路
- 如果當前執行緒池中的執行緒數目小於corePoolSize，則每來一個任務，就會建立一個執行緒去執行這個任務；
- 如果當前執行緒池中的執行緒數目>=corePoolSize，則每來一個任務，會嘗試將其新增到任務快取隊列當中，若新增成功，則該任務會等待空閒執行緒將其取出去執行；若新增失敗（一般來說是任務快取佇列已滿），則會嘗試建立新的執行緒去執行這個任務；
- 如果當前執行緒池中的執行緒數目達到maximumPoolSize，則會採取任務拒絕策略進行處理；
- 如果執行緒池中的執行緒數量大於 corePoolSize時，如果某執行緒空閒時間超過keepAliveTime，執行緒將被終止，直至執行緒池中的執行緒數目不大於corePoolSize；如果允許為核心池中的執行緒設定存活時間，那麼核心池中的執行緒空閒時間超過keepAliveTime，執行緒也會被終止
拒接策略
ThreadPoolExecutor.AbortPolicy:丟棄任務並丟擲RejectedExecutionException異常(預設)
ThreadPoolExecutor.DiscardPolicy：也是丟棄任務，但是不丟擲異常
ThreadPoolExecutor.DiscardOldestPolicy：丟棄佇列最前面的任務，然後重新嘗試執行任務（重複此過程）
ThreadPoolExecutor.CallerRunsPolicy：由呼叫執行緒處理該任務
原始碼解析
執行緒池的執行原理
初始化構造器
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();
//判斷是否小於核心數量，是直接新增work成功後直接退出
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
// 增加失敗後繼續獲取標記
c = ctl.get();
}
//判斷是執行狀態並且扔到workQueue裡成功後
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
//再次check判斷執行狀態如果是非執行狀態就移除出去&reject掉
if (! isRunning(recheck) && remove(command))
reject(command);
//否則發現可能執行執行緒數是0那麼增加一個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.如果非執行狀態
//2.不是這種情況（停止狀態並且是null物件並且workQueue不等於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work數量 然後跳出去
break retry;
c = ctl.get(); // Re-read ctl 增加work失敗後繼續遞迴
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. 鎖定後並重新檢查下 是否存線上程工廠的失敗或者鎖定前的關閉
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成功就呼叫start執行
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
ThreadPoolExecutor#runWorker
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();//1.取到當前執行緒
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 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();// 確保執行緒是能中斷的
try {
beforeExecute(wt, task); //開始任務前的鉤子
Throwable thrown = null;
try {
task.run();//執行任務
} 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); //任務後的鉤子
}
} 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
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);
}
}
執行緒池排程原理
排程核心構造器
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
if (isShutdown() &&
!canRunInCurrentRunState(task.isPeriodic()) &&
remove(task))
task.cancel(false);
else
ensurePrestart();
}
}
通過DelayedWorkQueue 延遲佇列實現 offer獲取物件的延遲
ScheduledThreadPoolExecutor.DelayedWorkQueue#offer
public boolean offer(Runnable x) {
if (x == null)
throw new NullPointerException();
RunnableScheduledFuture<?> e = (RunnableScheduledFuture<?>)x; //當前物件
final ReentrantLock lock = this.lock;
lock.lock();
try {
int i = size;
if (i >= queue.length) //擴容
grow();
size = i + 1;
if (i == 0) {
queue[0] = e;
setIndex(e, 0); //第一個直接設定索引和下標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執行緒
}
} 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執行的時候呼叫queue的take方法
public RunnableScheduledFuture<?> take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
for (;;) {
RunnableScheduledFuture<?> first = queue[0];//獲取第一個物件
if (first == null)
available.await();
else {
long delay = first.getDelay(NANOSECONDS);//延遲時間
if (delay <= 0)//到時間了
return finishPoll(first);
first = null; // don't retain ref while waiting
if (leader != null)
available.await();//因為沒有執行執行緒初始化，所以等等什麼時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]; //重排序佇列
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的要執行成功設定下次執行時間和增加額外任務
setNextRunTime();
reExecutePeriodic(outerTask);
}
}
非同步結果原始碼分析
怎麼拿到的非同步任務結果？
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執行緒中斷
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waint物件
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);//連結串列的物件下一個置成當前的waitNode
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos); //等待時間阻塞
}
else
LockSupport.park(this); //一直阻塞
}
}
什麼時候回填的結果
FutureTask#run
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread())) //如果狀態不是new 或者 runner狀態置不成功直接退出
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();//執行ok 的時候返回result
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)//正常成功set result物件
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) { //如果執行緒執行中，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;
}
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