Java线程池(3)- JUC Executors 源码分析

4、JUC Executors 使用&源码分析

未完待续，写作中…

1、JUC里有几种线程池？各自的使用场景？

FixedThreadPool

public static ExecutorService newFixedThreadPool(int nThreads) {return new ThreadPoolExecutor(nThreads, nThreads,0L, TimeUnit.MILLISECONDS,new LinkedBlockingQueue<Runnable>());
}
public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {return new ThreadPoolExecutor(nThreads, nThreads,0L, TimeUnit.MILLISECONDS,new LinkedBlockingQueue<Runnable>(),threadFactory);
}

CachedThreadPool

public static ExecutorService newCachedThreadPool() {return new ThreadPoolExecutor(0, Integer.MAX_VALUE,60L, TimeUnit.SECONDS,new SynchronousQueue<Runnable>());
}
public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {return new ThreadPoolExecutor(0, Integer.MAX_VALUE,60L, TimeUnit.SECONDS,new SynchronousQueue<Runnable>(),threadFactory);
}

SingleThreadExecutor

public static ExecutorService newSingleThreadExecutor() {return new FinalizableDelegatedExecutorService(new ThreadPoolExecutor(1, 1,0L, TimeUnit.MILLISECONDS,new LinkedBlockingQueue<Runnable>()));
}
static class FinalizableDelegatedExecutorServiceextends DelegatedExecutorService {FinalizableDelegatedExecutorService(ExecutorService executor) {super(executor);}protected void finalize() {super.shutdown();}
}

4.1、简单使用线程池

public class FixedThreadPoolTest {public static void main(String[] args) {ExecutorService executorService = Executors.newFixedThreadPool(3);for (int i = 0; i<6; i++) {executorService.execute(new Runnable() {public void run() {System.out.println(Thread.currentThread().getName());ThreadUtils.sleep(1000);}});}}
}

ExecutorService executorService = Executors.newFixedThreadPool(3),创建一个拥有 固定数量线程 的线程池。

4.1.1、Executors、ExecutorService命名

我初学JUC 线程池的时候，很不能理解，为什么要用 Executors 这个命名的工具类来创建线程池？线程池为什么还是ExecutorService 接口的实现类？

不知道大家有没有这个疑惑？

线程池的作用是干嘛的？单纯的线程池没什么意义，执行任务才是线程池存在的意义。所以从执行任务这个角度来看，使用 Executors 和 ExectutorService 两个命名就显得合情合理了！！

ThreadPoolExecutor 源码

NO.	项目	二进制
A	CAPACITY	0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
B	~CAPACITY	1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1	RUNNING(-1)	1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
2	SHUTDOWN(0)	0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
3	STOP(1)	0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
4	TIDYING(2)	0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
5	TERMINATED(3)	1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

 private static int runStateOf(int c)     { return c & ~CAPACITY; }private static int workerCountOf(int c)  { return c & CAPACITY; }

int的前3位表示状态，通过与~CAPACITY进行与运算可以取得（~CAPACITY 的前三位为全1）
int的后29位表示 线程数量，通过与CAPACITY进行与运算可以取得（CAPACITY 的后29位为全1）

点击参考：反码 & 为什么使用反码

java.util.concurrent.ThreadPoolExecutor#execute源码：

public void execute(Runnable command) {if (command == null)throw new NullPointerException();int c = ctl.get();if (workerCountOf(c) < corePoolSize) {if (addWorker(command, true))// 新增工作线程return;c = ctl.get();}// 当前 "线程池处于RUNNING状态",则 "添加当前任务到任务队列"if (isRunning(c) && workQueue.offer(commxand)) {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);}

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()))return false;for (;;) {int wc = workerCountOf(c);if (wc >= CAPACITY ||wc >= (core ? corePoolSize : maximumPoolSize))return false;if (compareAndIncrementWorkerCount(c))break retry;c = ctl.get();  // Re-read ctlif (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;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 startablethrow 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;
}

⭐️ addWorker(command, true) 上面时候返回 false ?

1、JUC里有几种线程池？各自的使用场景？

1、fixed（固定数量的线程）；
使用 固定数量的线程 & 无界队列 来处理你提交的任务，只有指定数量的线程来处理任务。
这些线程都处于运行中时提交任务，任务需要放入无界队列中等待处理。

2 、cached（线程数量不固定，无论来多少任务都是不停的创建线程来执行，如果线程空闲了一定的时间，就会释放掉这个线程）；

3 、single（线程池里就只有一个线程）

4、scheduled（提交进去的线程，会在指定的时间过后才去执行）

最最常见的线程池就是fixed线程池，线程数量是固定的，不会超过这个数量的线程，就执行你不断提交的任务