ThreadPoolExecutor源碼解析
今天為了給一個朋友做一份文檔，從源碼層級解析一下ThreadPoolExecutor。然後就直接在源碼上寫備註的形式解析，看這篇文章的朋友，就和看源碼一樣，一步步的跟著向下執行的看就好
1、常用變量的解釋
// 1. ctl，可以看做一個int類型的數字，高3位表示線程池狀態，低29位表示worker數量
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
// 2. COUNT_BITS，Integer.SIZE為32，所以COUNT_BITS為29
private static final int COUNT_BITS = Integer.SIZE - 3;
// 3. CAPACITY，線程池允許的最大線程數。1左移29位，然後減1，即為 2^29 - 1
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
// 4. 線程池有5種狀態，按大小排序如下：RUNNING < SHUTDOWN < STOP < TIDYING < TERMINATED
private static final int RUNNING = -1 << COUNT_BITS;
private static final int SHUTDOWN = 0 << COUNT_BITS;
private static final int STOP = 1 << COUNT_BITS;
private static final int TIDYING = 2 << COUNT_BITS;
private static final int TERMINATED = 3 << COUNT_BITS;
// Packing and unpacking ctl
// 5. runStateOf()，獲取線程池狀態，通過按位與操作，低29位將全部變成0
private static int runStateOf(int c) { return c & ~CAPACITY; }
// 6. workerCountOf()，獲取線程池worker數量，通過按位與操作，高3位將全部變成0
private static int workerCountOf(int c) { return c & CAPACITY; }
// 7. ctlOf()，根據線程池狀態和線程池worker數量，生成ctl值
private static int ctlOf(int rs, int wc) { return rs | wc; }
/*
- Bit field accessors that don't require unpacking ctl.
- These depend on the bit layout and on workerCount being never negative.
*/
// 8. runStateLessThan()，線程池狀態小於xx
private static boolean runStateLessThan(int c, int s) {
return c < s;}
// 9. runStateAtLeast()，線程池狀態大於等於xx
private static boolean runStateAtLeast(int c, int s) {
return c >= s;}
純乾貨，面試重災區--多線程源碼解析，你該這樣回答
2、構造方法
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
// 基本類型參數校驗
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
// 空指針校驗
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
// 根據傳入參數`unit`和`keepAliveTime`，將存活時間轉換為納秒存到變量`keepAliveTime `中
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;}
3、提交執行task的過程
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
/*
* Proceed in 3 steps:
*
* 1. If fewer than corePoolSize threads are running, try to
* start a new thread with the given command as its first
* task. The call to addWorker atomically checks runState and
* workerCount, and so prevents false alarms that would add
* threads when it shouldn't, by returning false.
*
* 2. If a task can be successfully queued, then we still need
* to double-check whether we should have added a thread
* (because existing ones died since last checking) or that
* the pool shut down since entry into this method. So we
* recheck state and if necessary roll back the enqueuing if
* stopped, or start a new thread if there are none.
*
* 3. If we cannot queue task, then we try to add a new
* thread. If it fails, we know we are shut down or saturated
* and so reject the task.
*/
int c = ctl.get();
// worker數量比核心線程數小，直接創建worker執行任務
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
// worker數量超過核心線程數，任務直接進入隊列
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
// 線程池狀態不是RUNNING狀態，說明執行過shutdown命令，需要對新加入的任務執行reject()操作。
// 這兒為什麼需要recheck，是因為任務入隊列前後，線程池的狀態可能會發生變化。
if (! isRunning(recheck) && remove(command))
reject(command);
// 這兒為什麼需要判斷0值，主要是在線程池構造方法中，核心線程數允許為0
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
// 如果線程池不是運行狀態，或者任務進入隊列失敗，則嘗試創建worker執行任務。
// 這兒有3點需要注意：
// 1. 線程池不是運行狀態時，addWorker內部會判斷線程池狀態
// 2. addWorker第2個參數表示是否創建核心線程
// 3. addWorker返回false，則說明任務執行失敗，需要執行reject操作
else if (!addWorker(command, false))
reject(command);}
4、addworker源碼解析
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
// 外層自旋
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// 這個條件寫得比較難懂，我對其進行了調整，和下面的條件等價
// (rs > SHUTDOWN) ||
// (rs == SHUTDOWN && firstTask != null) ||
// (rs == SHUTDOWN && workQueue.isEmpty())
// 1. 線程池狀態大於SHUTDOWN時，直接返回false
// 2. 線程池狀態等於SHUTDOWN，且firstTask不為null，直接返回false
// 3. 線程池狀態等於SHUTDOWN，且隊列為空，直接返回false
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
// 內層自旋
for (;;) {
int wc = workerCountOf(c);
// worker數量超過容量，直接返回false
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
// 使用CAS的方式增加worker數量。
// 若增加成功，則直接跳出外層循環進入到第二部分
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
// 線程池狀態發生變化，對外層循環進行自旋
if (runStateOf(c) != rs)
continue retry;
// 其他情況，直接內層循環進行自旋即可
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
// worker的添加必須是串行的，因此需要加鎖
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
// 這兒需要重新檢查線程池狀態
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
// worker已經調用過了start()方法，則不再創建worker
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
// worker創建並添加到workers成功
workers.add(w);
// 更新`largestPoolSize`變量
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
// 啟動worker線程
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
// worker線程啟動失敗，說明線程池狀態發生了變化（關閉操作被執行），需要進行shutdown相關操作
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;}
5、線程池worker任務單元
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable{
/**
* This class will never be serialized, but we provide a
* serialVersionUID to suppress a javac warning.
*/
private static final long serialVersionUID = 6138294804551838833L;
/** Thread this worker is running in. Null if factory fails. */
final Thread thread;
/** Initial task to run. Possibly null. */
Runnable firstTask;
/** Per-thread task counter */
volatile long completedTasks;
/**
* Creates with given first task and thread from ThreadFactory.
* @param firstTask the first task (null if none)
*/
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
// 這兒是Worker的關鍵所在，使用了線程工廠創建了一個線程。傳入的參數為當前worker
this.thread = getThreadFactory().newThread(this);
}
/** Delegates main run loop to outer runWorker */
public void run() {
runWorker(this);
}
// 省略代碼...}
6、核心線程執行邏輯-runworker
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
// 調用unlock()是為了讓外部可以中斷
w.unlock(); // allow interrupts
// 這個變量用於判斷是否進入過自旋（while循環）
boolean completedAbruptly = true;
try {
// 這兒是自旋
// 1. 如果firstTask不為null，則執行firstTask；
// 2. 如果firstTask為null，則調用getTask()從隊列獲取任務。
// 3. 阻塞隊列的特性就是：當隊列為空時，當前線程會被阻塞等待
while (task != null || (task = getTask()) != null) {
// 這兒對worker進行加鎖，是為了達到下面的目的
// 1. 降低鎖範圍，提升性能
// 2. 保證每個worker執行的任務是串行的
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
// 如果線程池正在停止，則對當前線程進行中斷操作
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
// 執行任務，且在執行前後通過`beforeExecute()`和`afterExecute()`來擴展其功能。
// 這兩個方法在當前類裡面為空實現。
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run();
} 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);
}
} finally {
// 幫助gc
task = null;
// 已完成任務數加一
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
// 自旋操作被退出，說明線程池正在結束
processWorkerExit(w, completedAbruptly);
}}
怎麼樣，不知道大家看明白了沒有，可能對於有一些朋友來說有那麼一點點的困難，但是沒關係啊
既然已經說道thread了，咱也別落下什麼