线程池
假如没有线程池，当存在较多的并发任务的时候，每执行一次任务，系统就要创建一个线程，任务完成后进行销毁，一旦并发任务过多，频繁的创建和销毁线程将会大大降低系统的效率。线程池能够对线程进行统一的分配，通过固定数量的线程来负责处理任务，避免了频繁的创建和销毁对象，使线程能够重复的利用，执行多个任务。
Java线程池的结构
Executor 最顶层接口，仅有execute方法。真正的线程池接口应该是它的子接口ExecutorService
ExecutorService接口，主要对Executor接口补充了一些方法，例如shutdown()、submit()等方法
ThreadPoolExecutor ExecutorService的默认实现，作为自定义线程池的主要类。
ScheduledExecutorService 用来解决任务重复执行的问题
ScheduledThreadPoolExecutor 继承ThreadPoolExecutor的ScheduledExecutorService接口实现，周期性任务调度的类实现。
ThreadPoolExecutor
ThreadPoolExecutor使ExecutorService的默认实现，其功能也最为基础，通过对该类的源码解析来了解整个线程池的工作机制。
ThreadPoolExecutor最完整的构造器
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;
}corePoolSize 线程池中保存的线程个数，包含了空闲线程
maximumPoolSize 为线程池中最多允许线程个数
keepAliveTime 当线程产生的数量多于core时候，空闲线程在这个时间如果没有新任务将被终止
unit keepAliveTime的时间单位
workQueue 线程工作任务队列。任务被执行前保存至工作队列 常用的有ArrayBlockingQueue、LinkedBlockingQuene、priorityBlockingQuene等等
threadFactory 执行程序创建新线程时使用的工厂，默认为DefaultThreadFactory
handler 由于超出线程范围和队列容量而使执行被阻塞时所使用的处理程序，主要有四种AbortPolicy、CallerRunsPolicy、DiscardOldestPolicy、DiscardPolicy
工作流程图如图
源码解析
首先，来看一下几个变量定义
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
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
private static int runStateOf(int c) { return c & ~CAPACITY; }
private static int workerCountOf(int c) { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }线程池使用一个整型变量的高三位作为线程池的状态。
高位111为RUNNING 注：RUNNING为负数，最小
高位000为SHUTDOWN
高位001表示STOP
高位010表示TIDYING
高位100表示TERMINATED
workerCountOf方法用来取低29位的数值，返回线程池的线程数。
runStateOf方法则用来取高3位的数值，返回当前线程池的状态。
然后，来看用于提交任务的submit方法。
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}该方法将Runnable进行了封装，我们这里先不考虑，先看看执行的关键方法execute();
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) { // workerCountOF()方法获取当前线程数量，若线程数量小于核心线程数的时候 直接进入addWorker 启动运行
if (addWorker(command, true))
return;
c = ctl.get();
}
// 上面是当线程数量小于core的时候，下面是针对线程数量大于core
if (isRunning(c) && workQueue.offer(command)) { //若线程池处于运行状态，则添加到阻塞队列中
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command)) //recheck 若线程池已经关闭，就remove任务
reject(command); //执行拒绝策略
else if (workerCountOf(recheck) == 0) //线程池处于running状态，但是没有线程，则创建线程
addWorker(null, false);
}
else if (!addWorker(command, false)) // 线程池关闭或者往workQueue提交任务失败，则reject任务
reject(command);
}execute方法中可以看到，addWork作为一个创建线程并执行任务的重要方法
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 说明线程池不是running; 当线程池关闭，任务为null，workQueue不为空的时候才可以往下进行
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize)) // core为true 则超过core添加失败
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
}
}
//到这里说明能够创建线程执行任务
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
w = new Worker(firstTask);
final Thread t = w.thread; //获取 worker 中的线程对象，Worker的构造方法会调用 ThreadFactory 来创建一个新的线程
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock(); //上锁
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)) { // 线程池运行或者 线程池关闭 任务为null
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
workers.add(w); //添加进HashSet
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true; //添加成功
}
} finally {
mainLock.unlock();
}
if (workerAdded) { //如果添加成功则启动线程
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted) //启动失败则把w从works中移除
addWorkerFailed(w);
}
return workerStarted;
}接下来看看Work类中的run方法，中的runWorker方法
public void run() {
runWorker(this);
}
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // 释放锁，允许中断
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null) { //getTask方法不断从阻塞队列中获取任务
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(); //执行任务的run方法
} 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 {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}
三种常见线程池
Executors类中为我们提供了集中常见的线程池，分别有FixedThreadPool、SingleThreadExecutor、CachedThreadPool三种
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}FixedThreadPool顾名思义 可以设置固定数量的线程池。但是LinkedBlockingQueue是*阻塞队列，队列可存放的大小为Integer.MAX_VALUE，因此maximumPoolSize和keepAliveTime将会是个无用参数，拒绝策略也没用
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}同上，不过线程池大小仅为1
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}线程池的线程数可达到Integer.MAX_VALUE。使用SynchronousQueue作为阻塞队列。newCachedThreadPool在没有任务执行时，当线程的空闲时间超过keepAliveTime，会自动释放线程资源，当提交新任务时，如果没有空闲线程，则创建新线程执行任务，会导致一定的系统开销；