线程池
记忆场景图
三大方法创建线程池（不用）
阿里巴巴开发手册 不允许使用Executors去创建线程池。因为，FixedThreadPool和SingleThreadPool允许的请求队列长度为Interger.Max_Value，可能导致oom。CacheThreadPool和ScheduledThreadPool允许创建的线程数量为Interger.Max_Value，可能导致oom。
故必须使用ThreadPoolExecutor创建线程池
- // ExecutorService executorService = Executors.newSingleThreadExecutor();// 单个线程java.util.concurrent.Executors$FinalizableDelegatedExecutorService
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
- // ExecutorService executorService = Executors.newFixedThreadPool(5);// 创建固定线程池大小java.util.concurrent.ThreadPoolExecutor
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
- // ExecutorService executorService = Executors.newCachedThreadPool();// 可伸缩的线程池，遇强则强。
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
七大参数（自定义ThreadPoolExecutor指定）
public ThreadPoolExecutor(int corePoolSize, // 1.核心线程池大小
int maximumPoolSize, // 2.最大核心线程池大小
long keepAliveTime, // 3.超时了无人调用就释放
TimeUnit unit, // 4.超时单位
BlockingQueue<Runnable> workQueue, //5. 阻塞队列
ThreadFactory threadFactory, // 6.创建线程的工厂
RejectedExecutionHandler handler) {
// 7.拒绝策略
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;
}
调优！CPU密集型&IO密集型
调优。最大线程池如何定义？maximumPoolSize
- CPU密集型 12核CPU同时运行，保证cpu效率最高
// CPU密集型。防止运维打死我，获取此电脑的cpu处理器核数
int cpuProcessor = Runtime.getRuntime().availableProcessors();
- IO密集型 > 判断程序中十分耗io的线程。15个大型任务，io十分占用资源。定义多余15个线程。
四种拒绝策略（自定义ThreadPoolExecutor指定）
- new ThreadPoolExecutor.AbortPolicy() ); // 流产堕胎拒绝策略，>2+5个线程后抛异常报RejectedExecutionException错
- // new ThreadPoolExecutor.CallerRunsPolicy() ); // 哪来的滚自己公司办卡，打来电话滚跑策略。main线程去打发执行
- // new ThreadPoolExecutor.DiscardPolicy() ); // 队列满不抛异常，一走了之抛弃策略
- // new ThreadPoolExecutor.DiscardOldestPolicy() ); // 队列满不抛异常，尝试和最早执行的线程竞争，不当备胎，努力尝试后不行一走了之抛弃策略、
/**
* 三大方法
* 七大参数
* 四种拒绝策略
* @Author Weton Li
* @Date 2021/2/11 09:09
*/
public class MyThreadPoolExecutor {
public static void main(String[] args) {
// ExecutorService executorService = Executors.newSingleThreadExecutor();// 单个线程java.util.concurrent.Executors$FinalizableDelegatedExecutorService
// ExecutorService executorService = Executors.newFixedThreadPool(5);// 创建固定线程池大小java.util.concurrent.ThreadPoolExecutor
// ExecutorService executorService = Executors.newCachedThreadPool();// 可伸缩的线程池，遇强则强。
// CPU密集型。防止运维打死我，获取此电脑的cpu处理器核数
int cpuProcessor = Runtime.getRuntime().availableProcessors();
/*自定义线程池，工作只使用此方法*/
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
2,4,3, TimeUnit.SECONDS,
new LinkedBlockingDeque<>(3),
Executors.defaultThreadFactory(),
new ThreadPoolExecutor.AbortPolicy() ); // 流产堕胎拒绝策略，>2+5个线程后抛异常报RejectedExecutionException错
// new ThreadPoolExecutor.CallerRunsPolicy() ); // 哪来的滚自己公司办卡，打来电话滚跑策略。main线程去打发执行
// new ThreadPoolExecutor.DiscardPolicy() ); // 队列满不抛异常，一走了之抛弃策略
// new ThreadPoolExecutor.DiscardOldestPolicy() ); // 队列满不抛异常，尝试和最早执行的线程竞争，不当备胎，努力尝试后不行一走了之抛弃策略、
try {
for (int i = 0; i < 8; i++) {
// new Thread().start();
// executorService.execute(()->{ // 使用线程池创建线程
threadPoolExecutor.execute(()->{
System.out.println(Thread.currentThread().getName()+"执行ok");
});
}
// System.out.println(executorService.getClass());
System.out.println(threadPoolExecutor.getClass());
} catch (Exception e) {
e.printStackTrace();
} finally {
// 关闭线程池
// executorService.shutdown();
threadPoolExecutor.shutdown();
}
}
}