Java并发编程之美——第六章 Java并发包中锁原理剖析
Java并发编程之美——第五章 Java并发包中并发List（CopyOnWriteArrayList）源码剖析
Java并发编程之美——第四章 Java并发包中原子 *** 作类原理剖析
Java并发编程之美——第三章 Java并发包中ThreadLocalRandom类原理剖析
Java并发编程之美——第二章 并发编程的其他知识
Java并发编程之美——第一章 Java并发编程基础
文章目录- Time 2022-01-02——Hireek
- 介绍
- 为什么需要线程池？（线程池解决了什么问题？）
- ThreadPoolExecutor
- 类图
- field
- Worker——private final class
- public void execute(Runnable command)
- private boolean addWorker(Runnable firstTask, boolean core)
- final void runWorker(Worker w)
- 总结
-
当执行大量异步任务时线程池能够提供较好的性能。在不使用线程池时，每当需要执行异步任务时直接new一个线程来运行，而线程的创建和销毁是需要开销的。线程池里面的线程是可复用的，不需要每次执行异步任务时都重新创建和销毁线程。
-
线程池也提供了一种资源限制和管理的手段，比如可以限制线程的个数，动态新增线程等。
读者也可以自行思考下如何实现线程池？
- 池化线程
- 如何创建线程
- 调度
- 复用线程
- 销毁
直接看最核心线程池的实现 ThreadPoolExecutor
ThreadPoolExecutor 类图ThreadPool-related class description：
The Executor implementations provided in this package implement ExecutorService, which is a more extensive interface. The ThreadPoolExecutor class provides an extensible thread pool implementation. The Executors class provides convenient factory methods for these Executors.
fieldpublic class ThreadPoolExecutor extends AbstractExecutorService {
// 原子int，封装了线程数量和状态，高3位表示状态，低29位表示线程池数量 最大(2^29)-1
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
// 00011111111111111111111111111111
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
// (高3位) : 11100000000000000000000000000000
private static final int RUNNING = -1 << COUNT_BITS;
// (高三位) : 00000000000000000000000000000000
private static final int SHUTDOWN = 0 << COUNT_BITS;
// (高三位) : 00100000000000000000000000000000
private static final int STOP = 1 << COUNT_BITS;
// (高三位) : 01000000000000000000000000000000
private static final int TIDYING = 2 << COUNT_BITS;
// (高三位) : 01100000000000000000000000000000
private static final int TERMINATED = 3 << COUNT_BITS;
// 保存任务的阻塞队列
private final BlockingQueue workQueue;
// 锁
private final ReentrantLock mainLock = new ReentrantLock();
private final HashSet workers = new HashSet();
private final Condition termination = mainLock.newCondition();
private int largestPoolSize; // tracks 跟踪当前线程池的线程（已经存在的）的最大值。
private long completedTaskCount;
// Factory for new threads. All threads are created using this factory (via method addWorker).
// 默认实现DefaultThreadFactory
private volatile ThreadFactory threadFactory;
private volatile RejectedExecutionHandler handler;
// 存活时间
private volatile long keepAliveTime;
private volatile boolean allowCoreThreadTimeOut;
private volatile int corePoolSize;
private volatile int maximumPoolSize; // 线程池允许创建线程的最大值
}
Worker——private final class
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable {
private static final long serialVersionUID = 6138294804551838833L;
final Thread thread;
Runnable firstTask;
volatile long completedTasks;
Worker(Runnable firstTask) {
// 在构造函数内首先设置Worker的状态为-1，这是为了避免当前Worker在调用rnnWorker方法前被中断
// (当其他线程调用了线程池的shutdownNow时，如果Worker状态>=O则会中断该线程)。
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
public void run() {
runWorker(this);
}
}
public void execute(Runnable command)
注释已经很明白了，就不讲了。
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
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(); // recheck
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}
主要分析下addWorker
private boolean addWorker(Runnable firstTask, boolean core)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 && // !SHUTDOWN
firstTask == null && // SHUTDOWN && firstTask!=null
! workQueue.isEmpty())) // SHUTDOWN && workQueue.isEmpty()
return false;
// for(;;) compareAndIncrementWorkerCount
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
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 {
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
// 加独占锁，为了实现workers同步，因为可能多个线程调用了线程池的execute方法
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
// 重新检查线程池状态，以避免在获取锁前调用了shutdown
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) { // shutdown状态还可能会添加新的线程，继续执行队列中任务
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 {
// 释放锁
mainLock.unlock();
}
if (workerAdded) {
t.start(); // 启动线程！
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}
final void runWorker(Worker w)
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null) { // getTask
// 加锁是为了避免在任务运行期间，其他线程调用了shutdown后正在执行的任务被中断(shutdown只会中断当前被阻塞挂起的线程）
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();
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run();
} catch (RuntimeException x) { // catch e 防止线程死亡
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 {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
当然还有getTask()、shutdown()等源码待研究。具体的拒绝策略…
总结线程池巧妙地使用一个Integer类型的原子变量来记录线程池状态和线程池中的线程个数。通过线程池状态来控制任务的执行，每个Worker线程可以处理多个任务。线程池通过线程的复用减少了线程创建和销毁的开销。还是比较巧妙的。
参考：
-
java并发编程之美
-
https://www.cnblogs.com/liuzhihu/p/8177371.html
加油，温润自己的心！
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