本文详细介绍了Java线程池的原理与应用，包括ExecutorService接口、四种常见线程池的创建，以及线程池核心参数如corePoolSize、maximumPoolSize、keepAliveTime等的解析，探讨了线程池如何提高系统效率并避免资源浪费。同时，文章还涉及线程创建工厂、任务执行方法及Worker对象的源码分析。
一.介绍：
- java 里面线程池的顶级接口是 Executor，但是严格意义上讲 Executor 并不是一个线程池，而 只是一个执行线程的工具。真正的线程池接口是ExecutorService。
- 线程池：虽然线程的创建销毁的开销相对较小，但是频繁得创建和销毁也会消耗有限的资源，从而带来性能上的浪费，也不够高效。因此线程池的出，现就是为了解决这一问题，即在初始状态创建并维护一定数量的空闲线程，当有需要执行的任务，就交付给线程中的一个线程，任务执行结束后，该线程也不会死亡，而是回到线程池中重新变为空闲状态。
- 线程池的好处：减少线程频繁创建销毁的资源开销，同时能够有效控制系统中并发线程的数量，防止系统性能的剧烈下降。
二.常用线程池
- newCachedThreadPool():创建一个具有缓存功能的线程池，系统根据需要创建线程
- newFixedThreadPool(int nThreads):创建一个可重用的，具有固定线程数的线程池
- newSingleThreadExecutor():创建一个单线程线程池
- newScheduledThreadPool(int corePoolSize):创建具有指定线程数的线程池，它将在指定延迟后执行线程任务
- newSingleThreadScheduledExecutor():创建一个延迟执行的单线程线程池
- newWorkingStealingPool(int parallelism):创建持有足够的线程的线程池来支持给定的并行级别，以充分支持多CPU并行能力。
- newWorkingStealingPool():根据CPU个数设置并行级别。
三.源码分析
public interface Executor {
void execute(Runnable command);
}public interface ExecutorService extends Executor {
/*
* 不再接受新任务
* 当所有已提交任务执行完后，就关闭
* 如果已经关闭，则调用没有其他作用
*/
void shutdown();
/*
* 试图停止所有正在执行的活动任务，暂停处理正在等待的任务
* 并返回等待执行的任务列表
*/
List<Runnable> shutdownNow();
boolean isShutdown();
boolean isTerminated();
//所有任务完成后shutdown，或者超时interrupted后会shutdown
boolean awaitTermination(long timeout, TimeUnit unit)
throws InterruptedException;
// 提交任务
<T> Future<T> submit(Callable<T> task);
<T> Future<T> submit(Runnable task, T result);
Future<?> submit(Runnable task);
// 批量提交任务
<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
throws InterruptedException;
<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
long timeout, TimeUnit unit)
throws InterruptedException;
<T> T invokeAny(Collection<? extends Callable<T>> tasks)
throws InterruptedException, ExecutionException;
<T> T invokeAny(Collection<? extends Callable<T>> tasks,
long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
3.1.线程池参数
Executor
ExecutorService
AbstractExecutorService
ThreadPoolExecutor
线程池通过Executors类创建出不同线程池，默认四种线程池创建方法都是默认给开发者配置好了相应参数，通过不同参数使用构造方法最终构造出我们看到的不同线程池。
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
return new DelegatedScheduledExecutorService
(new ScheduledThreadPoolExecutor(1));
}不同参数构造方法，上面线程池都是调用以下方法构建的 ，最终也是调用到下面构造方法
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.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}（1）corePoolSize与maximumPoolSize
- 线程池中的线程数，即使这部分线程空闲，也不会被销毁
- 如果一个新任务提交，且创建的线程数少于corePoolSize，则新的线程被创建，即使已经创建的线程当中有空闲线程
- 如果已经创建的线程大于corePoolSize，且小于maximumPoolSize，会先把任务提交到阻塞队列，在阻塞队列满的情况下，才会新建线程
- 线程池中的线程数不可能大于maximumPoolSize
（2）keepAliveTime与unit
- keepAliveTime代表线程空闲时的存活时间，即当线程没有任务执行时，继续存活的时间；默认情况下，该参数只在线程数大于corePoolSize时才有用
- unit则为keepAliveTime的时间单位
（3）workQueue
- 添加策略
-
正在执行的线程小于corePoolSize，创建新线程
-
正在执行的线程大于等于corePoolSize，把任务添加到阻塞队列
-
阻塞队列满了，且正在执行的线程小于maximumPoolSize，创建新线程
-
否则拒绝任务
-
阻塞队列类型
- ArrayBlockingQueue：基于数组结构的有界阻塞队列，按FIFO排序任务；
- LinkedBlockingQuene：基于链表结构的阻塞队列，按FIFO排序任务，吞吐量通常要高于ArrayBlockingQuene；
- SynchronousQuene：一个不存储元素的阻塞队列，每个插入操作必须等到另一个线程调用移除操作，否则插入操作一直处于阻塞状态，吞吐量通常要高于LinkedBlockingQuene；
- priorityBlockingQuene：具有优先级的无界阻塞队列；
（4）threadFactory
- 创建线程的工厂，通过自定义的线程工厂可以给每个新建的线程设置一个具有识别度的线程名。
（5）handler
- 线程池的饱和策略，当阻塞队列满了，且没有空闲的工作线程，如果继续提交任务，采取的处理策略。
3.2.线程创建工厂
public interface ThreadFactory {
/**
* 创建一个线程接口
*/
Thread newThread(Runnable r);
}默认线程创建工厂
private static class DefaultThreadFactory implements ThreadFactory {
private static final AtomicInteger poolNumber = new AtomicInteger(1);
private final ThreadGroup group;
private final AtomicInteger threadNumber = new AtomicInteger(1);
private final String namePrefix;
DefaultThreadFactory() {
SecurityManager s = System.getSecurityManager();
group = (s != null) ? s.getThreadGroup() :
Thread.currentThread().getThreadGroup();
namePrefix = "pool-" +
poolNumber.getAndIncrement() +
"-thread-";
}
public Thread newThread(Runnable r) {
/**
* 创建线程组，线程名，和stackSize
*/
Thread t = new Thread(group, r,
namePrefix + threadNumber.getAndIncrement(),
0);
/**
* 默认不是后台线程
*/
if (t.isDaemon())
t.setDaemon(false);
/**
* 如果不是normal优先级则设置normal优先级
*/
if (t.getPriority() != Thread.NORM_PRIORITY)
t.setPriority(Thread.NORM_PRIORITY);
return t;
}
}
private static class PrivilegedThreadFactory extends DefaultThreadFactory {
final AccessControlContext acc;
final ClassLoader ccl;
PrivilegedThreadFactory() {
super();
/**
* 获取类加载器
*/
this.acc = AccessController.getContext();
this.ccl = Thread.currentThread().getContextClassLoader();
}
public Thread newThread(final Runnable r) {
return super.newThread(new Runnable() {
/**
* 创建一个线程以后在runable里面调用submit过来的runnable执行体
*/
public void run() {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
Thread.currentThread().setContextClassLoader(ccl);
r.run();
return null;
}
}, acc);
}
});
}
}3.2.Execute任务执行方法
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
//如果当前任务数小于核心线程数
if (workerCountOf(c) < corePoolSize) {
//开启新线程执行任务
if (addWorker(command, true))
return;
c = ctl.get();
}
// 线程数大于corePoolSize，或者新建线程不成功，条件任务队列
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
// 如果此时ThreadPoolExecutor已经关闭了
if (! isRunning(recheck) && remove(command))
reject(command);
// 或者线程均被释放了，新建一个线程
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
// 阻塞队列满了，则尝试新建非core线程
else if (!addWorker(command, false))
reject(command);
}
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// 如果线程池是不是关闭，当前添加任务为空，任务队列为空，则说明没有要添加的新任务
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 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 {
//创建任务对象
w = new Worker(firstTask);
//获取任务线程
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();
//将任务添加到正在执行任务列表中
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
//启动线程执行任务
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}3.3.Worker对象实现源码
private final class Worker
extends AbstractQueuedSynchronizer
implements Runnable
{
....
Worker(Runnable firstTask) {
setState(-1); // inhibit interrupts until runWorker
this.firstTask = firstTask;
//通过ThreadFactory创建一个新线程，键
this.thread = getThreadFactory().newThread(this);
}
//线程执行体
public void run() {
runWorker(this);
}
......
}3.3.线程执行任务实体
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) {
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) {
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);
}
}
private Runnable getTask() {
boolean timedOut = false; // Did the last poll() time out?
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
decrementWorkerCount();
return null;
}
int wc = workerCountOf(c);
// Are workers subject to culling?
boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
if ((wc > maximumPoolSize || (timed && timedOut))
&& (wc > 1 || workQueue.isEmpty())) {
if (compareAndDecrementWorkerCount(c))
return null;
continue;
}
try {
Runnable r = timed ?
workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
workQueue.take();
if (r != null)
return r;
timedOut = true;
} catch (InterruptedException retry) {
timedOut = false;
}
}
}
3.4 submit方法
// Callable包装成一个FutureTask，然后提交给execute函数执行
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
// 把Callable包装成一个FutureTask
RunnableFuture<T> ftask = newTaskFor(task);
// 通过execute调用
execute(ftask);
return ftask;
}
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