一、常見線程池：
1. 固定線程數量的線程池
i. 通過Executors.newFixedThreadPool 來創建
ii. 核心線程數和最大線程數一樣
iii. 達到核心線程數後，空閒線程不會超時被終止或釋放。
iiii. 每添加一個任務後，會將任務添加到工作任務列隊列，線程池創建一個線程，線程數等於核心線程數時，就不會再創建線程。
2. 單線程的線程池
i 通過 Executors.newSingelThreadExecutor 來創建。
ii. 核心線程數和最大線程數一樣，且均爲 1 ，也就是隻有一個線程在執行隊列的工作任務。
iii. 使用了代理模式來創建線程池。
3. 可緩存的線程池
i 通過 Executors.newCachedThreadPool 來創建
ii. 核心線程數爲 0 ，最大線程數爲無限大（實際是 int 最大值），空閒線程可以緩存 60s，空閒超過60s的線程會被回收。
iii. 使用了 SynchronousQueue 同步隊列，添加任務的同時，需有工作線程來取任務纔可完成任務的添加和執行。
4. 定時執行的線程池
i. 通過 Executors.newScheduledThreadPool 來創建
ii. 可指定核心線程數，最大線程數爲無限大（實際是 int 最大值），核心線程數空閒不會超過被回收。
iii. 使用了 DelayedWorkQueue 延時隊列，通過延時隊列來控制時間來執行
二、 線程執行源碼解釋：
/**
* Executes the given task sometime in the future. The task
* may execute in a new thread or in an existing pooled thread.
*
* If the task cannot be submitted for execution, either because this
* executor has been shutdown or because its capacity has been reached,
* the task is handled by the current {@code RejectedExecutionHandler}.
*
* @param command the task to execute
* @throws RejectedExecutionException at discretion of
* {@code RejectedExecutionHandler}, if the task
* cannot be accepted for execution
* @throws NullPointerException if {@code command} is null
*/
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();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}
execute 底層執行過程：
1. 檢查線程池的運行狀態和工作線程數量，如果工作線程數 < 核心線程數，則會創建一個新的線程來執行給定任務（調用addWorker來執行任務）
2. 如果超過了核心線程數，把任務放到工作隊列中，若工作隊列沒有滿，則添加任務後，仍檢查一下線程池狀態，如果沒有繼續運行（不是RUNNING狀態）就把任務移除，使用拒絕策略來處理當前任務；否則將創建或喚醒工作線程來執行任務
3. 線程池非RUNNING狀態或添加任務失敗後，使用拒絕策略來處理當前任務
addWorker 源碼分析：
/**
* Checks if a new worker can be added with respect to current
* pool state and the given bound (either core or maximum). If so,
* the worker count is adjusted accordingly, and, if possible, a
* new worker is created and started, running firstTask as its
* first task. This method returns false if the pool is stopped or
* eligible to shut down. It also returns false if the thread
* factory fails to create a thread when asked. If the thread
* creation fails, either due to the thread factory returning
* null, or due to an exception (typically OutOfMemoryError in
* Thread.start()), we roll back cleanly.
*
* @param firstTask the task the new thread should run first (or
* null if none). Workers are created with an initial first task
* (in method execute()) to bypass queuing when there are fewer
* than corePoolSize threads (in which case we always start one),
* or when the queue is full (in which case we must bypass queue).
* Initially idle threads are usually created via
* prestartCoreThread or to replace other dying workers.
*
* @param core if true use corePoolSize as bound, else
* maximumPoolSize. (A boolean indicator is used here rather than a
* value to ensure reads of fresh values after checking other pool
* state).
* @return true if successful
*/
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
// 1. 檢查狀態
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
// 檢查工作隊列
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize)) // 是否超過核心線程數/最大線程數
return false;
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
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 {
// 創建 worker（AQS）
w = new Worker(firstTask);
final Thread t = w.thread;
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.
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);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}