线程池ThreadPoolExecutor的使用
标签（空格分隔）： ThreadPoolExecutor
一 创建ThreadPoolExecutor
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue<Runnable> workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler){}
参数说明：
- corePoolSize，核心线程数，池中所保存的最少线程个数。
- maximumPoolSize，池中允许的最大线程数，核心线程数+非核心线程数。
- keepAliveTime，当池中总线程数大于核心时，此为终止多余的空闲线程 等待新任务的最长时间。
- unit，keepAliveTime参数的时间单位，有DAYS、HOURS、 MINUTES、SECONDS、MILLISECONDS、MICROSECONDS、NANOSECONDS。常用MILLISECONDS。
- workQueue，执行前用于保持任务的队列。
- threadFactory，执行程序创建新线程时使用的工厂，默认使用Executors.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（策略），详情见ThreadPoolExecutor的内部静态类（一共有4个，都实现了RejectedExecutionHandler接口），默认使用AbortPolicy（用于被拒绝任务的处理程序，它将抛出 RejectedExecutionException）。
二 提交任务
public Future<?> submit(Runnable task){}
public <T> Future<T> submit(Callable<T> task){}
public void execute(Runnable command){}
说明：
- 常见有3种提交任务方式，其中submit是继承是基类AbstractExecutorService（类如其名，是抽样类），execute方法是其自己实现，查看源码可知submit底层还是调用execute方法。
- execute和submit方法的区别，二者的入参类型明显不同，而且execute没有返回值，submit是带有返回值的；
三 基本知识点
- 使用完线程池后，corePoolSize会保持不变，在核心线程创建之后的非核心线程会在keepAliveTime之后失效。非核心线程就像任务比较多的时候临时招进来的临时工，当活干完就不再需要。
@Test
public void corePoolSizeIsKeep(){
ThreadPoolExecutor executor =
new ThreadPoolExecutor(6, 10, 5L, TimeUnit.SECONDS, new SynchronousQueue<>());
submit9(executor);
watch(executor);
}
运行结果见下图：
corePoolSize不会变化.png
- 当使用无界的任务队列创建线程池时，即便提交的任务数超过了核心线程数，也不会再开辟新的非核心线程。
@Test
public void meaninglessMaximunPoolSize(){
ThreadPoolExecutor executor =
new ThreadPoolExecutor(3, 6, 5L, TimeUnit.SECONDS, new LinkedBlockingQueue<>());
submit9(executor);
watch(executor);
}
运行结果见下图：
无意义的maximumPoolSize.png
- 当使用了有界的任务队列创建线程，并且提交的任务数超过核心线程数加上任务队列，但不超过最大线程数+任务队列大小时，会开辟新的非核心线程。
@Test
public void maximunPoolSize(){
ThreadPoolExecutor executor =
new ThreadPoolExecutor(3, 4, 5L, TimeUnit.SECONDS, new LinkedBlockingQueue<>(2));
submit6(executor);
watch(executor);
}
运行结果见下图：
maximumPoolSize.png
- 当使用了有界的任务队列创建线程，并且提交的任务数超过最大线程数+任务队列大小时，会启用拒绝策略AbortPolicy并抛出异常。
@Test
public void abortPolicyReject(){
ThreadPoolExecutor executor =
new ThreadPoolExecutor(3, 5, 5L, TimeUnit.SECONDS, new LinkedBlockingQueue<>(1));
submit6(executor);
executor.submit(new SleepChild());
watch(executor);
}
----------
java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.FutureTask@3f8f9dd6 rejected from java.util.concurrent.ThreadPoolExecutor@aec6354[Running, pool size = 5, active threads = 5, queued tasks = 1, completed tasks = 0]
at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2047)
at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:823)
at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1369)
at java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:112)
at com.taobao.juc.ExecutorsTest.abortPolicyReject(ExecutorsTest.java:123)
- ThreadPoolExecutor.CallerRunsPolicy策略的解决方法是提交任务的线程不再提交任务，而是帮助执行任务。
@Test
public void CallerRunsPolicy(){
ThreadPoolExecutor executor =
new ThreadPoolExecutor(3, 5, 5L, TimeUnit.SECONDS, new LinkedBlockingQueue<>(1), new CallerRunsPolicy());
submit6(executor);
executor.submit(new SleepChild());
watch(executor);
}
运行结果如下：
CallerRunsPolicy策略.png
四 附录
- SleepChild的源码
public class SleepChild implements Runnable {
@Override
public void run() {
try {
Thread.sleep(2000);
System.out.println(Thread.currentThread().getName() + " run.");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
- 其他源码
private void submit6(ThreadPoolExecutor executor){
executor.submit(new SleepChild());
executor.submit(new SleepChild());
executor.submit(new SleepChild());
System.out.println("===================first====================");
System.out.println("core pool size: " + executor.getCorePoolSize() + ", pool size: " + executor.getPoolSize() +
", work queue size: " + executor.getQueue().size());
executor.submit(new SleepChild());
executor.submit(new SleepChild());
executor.submit(new SleepChild());
System.out.println("===================second====================");
System.out.println("core pool size: " + executor.getCorePoolSize() + ", pool size: " + executor.getPoolSize() +
", work queue size: " + executor.getQueue().size());
}
----------
private void submit9(ThreadPoolExecutor executor){
executor.submit(new SleepChild());
executor.submit(new SleepChild());
executor.submit(new SleepChild());
System.out.println("===================first====================");
System.out.println("core pool size: " + executor.getCorePoolSize() + ", pool size: " + executor.getPoolSize() +
", work queue size: " + executor.getQueue().size());
executor.submit(new SleepChild());
executor.submit(new SleepChild());
executor.submit(new SleepChild());
System.out.println("===================second====================");
System.out.println("core pool size: " + executor.getCorePoolSize() + ", pool size: " + executor.getPoolSize() +
", work queue size: " + executor.getQueue().size());
executor.submit(new SleepChild());
executor.submit(new SleepChild());
executor.submit(new SleepChild());
System.out.println("===================third====================");
System.out.println("core pool size: " + executor.getCorePoolSize() + ", pool size: " + executor.getPoolSize() +
", work queue size: " + executor.getQueue().size());
}
-------------
private void watch(ThreadPoolExecutor executor){
//主线程检测线程池的状态
while(true){
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("watch, core pool size: " + executor.getCorePoolSize() + ", pool size: " + executor.getPoolSize() +
", work queue size: " + executor.getQueue().size());
}
}
