代码中运用线程池有两个步骤:创建线程池和线程池中运行线程
1.创建线程池
创建线程池的方法以及实现原码:
Executors.newSingleThreadExecutor();
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}Executors.newCachedThreadPool();
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}Executors.newFixedThreadPool(10)
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}Executors.newScheduledThreadPool(10);
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);
}而new ScheduledThreadPoolExecutor(corePoolSize);代码为:
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
new DelayedWorkQueue());
}继承的父类为ThreadPoolExecutor,对应的构造函数为:
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue) {
this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
Executors.defaultThreadFactory(), defaultHandler);
}通过以上代码可知,新建线程核心类是ThreadPoolExecutor,构造函数主要的属性有corePoolSize,maximumPoolSize,keepAliveTime,unit,workQueue
corePoolSize:核心线程数，可以大致理解为长期驻留的线程数目。
- 核心线程会一直存活，及时没有任务需要执行
- 当线程数小于核心线程数时，即使有线程空闲，线程池也会优先创建新线程处理
- 设置allowCoreThreadTimeout=true（默认false）时，核心线程会超时关闭
maximumPoolSize:线程不够时能够创建的最大线程数
- 当线程数>=corePoolSize，且任务队列已满时。线程池会创建新线程来处理任务
- 当线程数=maxPoolSize，且任务队列已满时，线程池会拒绝处理任务而抛出异常
keepAliveTime:当线程数大于核心线程数时，多余的空闲线程在终止前等待新任务的最长时间。unit:keepAliveTime参数的时间单位
workQueue:工作队列，必须是BlockingQueue。
- 当核心线程数达到最大时，新任务会放在队列中排队等待执行
ThreadFactory:可以定义新建线程newThread方法,定义线程的线程组,名称,是否为守护线程,给线程分配优先级等等
RejectedExecutionHandler:设置处理异常类,实现rejectedExecution方法
设置这些属性必须要清楚当执行线程时线程池新建线程的规则:
- 当线程数小于核心线程数时，创建线程。
- 当线程数大于等于核心线程数，且任务队列未满时，将任务放入任务队列。
- 当线程数大于等于核心线程数，且任务队列已满
- 若线程数小于最大线程数，创建线程
- 若线程数等于最大线程数，抛出异常，拒绝任务
2.线程池中运行线程
阅读本段介绍后,建议查看源码,可以更好的帮助自己详细的理解线程池运行线程的原理.
运行线程有两种方法:execute和submit
execute(Runnable command):
无返回值的线程执行方法,这里只展示一些核心代码,
方法执行后会将要执行的线程放入Worker类组成的hashSet中,然后新起线程执行Worker中的run方法:
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);
}
}其中先获取当前需要执行的线程,当前执行的线程执行完后用getTask获取阻塞队列中的线程来执行
执行时beforeExecute(wt, task);afterExecute(task, thrown);根据需要重写两个方法可以对线程执行前和线程执行后做逻辑处理或者每个线程的异常处理
submit(Callable<T> task):
有返回值的线程执行方法,运用方法代码如下:
public static void main(String[] args) throws ExecutionException, InterruptedException {
ExecutorService executor2 = Executors.newFixedThreadPool(10);
call a=new call();
Future<String> future = executor2.submit(a);
System.out.println(future.get());
}
static class call implements Callable<String>{
@Override
public String call(){
return "result of call";
}
}会打印结果:result of call
原码中,Callable<T> task放在了FutureTask(实现了runable接口)中,然后execute了该FutureTask,下面,看看FutureTask中run方法是怎么写的:
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}可见run方法中执行了Callable<T> task中的call方法,用执行结果执行了set()方法,然后我们用get()方法获取这个返回值,由此可见,这个流程的核心以及和execute方法的最大区别是get和set方法如何传递数据,看下原码:
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}set方法执行了finishCompletion方法(赋值给outcome后的处理方法),get方法执行了awaitDone等待方法,下面看下这两个方法:
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
LockSupport.unpark(t);
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();
callable = null; // to reduce footprint
}
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
else if (q == null)
q = new WaitNode();
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
else
LockSupport.park(this);
}
}awaitDone方法执行了LockSupport.park(this);阻塞了该线程也就是get()方法所在线程
finishCompletion方法执行了LockSupport.unpark(t);重新启动了阻塞的线程,然后get()方法通过标志位来判断获取结果是否完成,是否可以返回结果
3.线程池的状态
复制一段:https://blog.csdn.net/l_kanglin/article/details/57411851?utm_source=copy
1、RUNNING
(1) 状态说明：线程池处在RUNNING状态时，能够接收新任务，以及对已添加的任务进行处理。
(2) 状态切换：线程池的初始化状态是RUNNING。换句话说，线程池被一旦被创建，就处于RUNNING状态，并且线程池中的任务数为0
2、 SHUTDOWN
(1) 状态说明：线程池处在SHUTDOWN状态时，不接收新任务，但能处理已添加的任务。
(2) 状态切换：调用线程池的shutdown()接口时，线程池由RUNNING -> SHUTDOWN。
3、STOP
(1) 状态说明：线程池处在STOP状态时，不接收新任务，不处理已添加的任务，并且会中断正在处理的任务。
(2) 状态切换：调用线程池的shutdownNow()接口时，线程池由(RUNNING or SHUTDOWN ) -> STOP。
4、TIDYING
(1) 状态说明：当所有的任务已终止，ctl记录的”任务数量”为0，线程池会变为TIDYING状态。当线程池变为TIDYING状态时，会执行钩子函数terminated()。terminated()在ThreadPoolExecutor类中是空的，若用户想在线程池变为TIDYING时，进行相应的处理；可以通过重载terminated()函数来实现。
(2) 状态切换：当线程池在SHUTDOWN状态下，阻塞队列为空并且线程池中执行的任务也为空时，就会由 SHUTDOWN -> TIDYING。
当线程池在STOP状态下，线程池中执行的任务为空时，就会由STOP -> TIDYING。
5、 TERMINATED
(1) 状态说明：线程池彻底终止，就变成TERMINATED状态。
(2) 状态切换：线程池处在TIDYING状态时，执行完terminated()之后，就会由 TIDYING -> TERMINATED。