本篇内容介绍了“jdk线程池怎么实现”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！
jdk线程池ThreadPoolExecutor的7个参数
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
核心线程个数 ,int类型
maximunPoolSize
最大线程数 ,int类型
keepAliveTime存活时间
传long类型的值,
当线程池中的线程数大于corePoolSize核心线程个数,且线程是闲置状态,则这些空闲线程的最大存活时间是KeepAliveTime
TimeUnit
存活时间的单位, 有时/分/秒/毫秒等可选配置
workQueue
存放待执行任务的阻塞队列, 可传入
arrayBlockingQueue 基于数组的有界阻塞队列;
linkedBlockingQueue基于链表的无界阻塞队列;
synchronousQueue最多只有1个元素的同步队列, 队列容量是1;
priorityBlockingQueue带优先级的无界阻塞队列,出队元素是优先级最高或最低的元素;
DelayQueue 带延迟功能的无界阻塞队列, 过期元素才会出队,队头元素是快要过期的元素.
以上几个Queue都是BlockingQueue的实现类
threadFactory
创建线程的工厂,
jdk提供了DefaultThreadFactory默认工厂,
用Executors.defaultThreadFactory()就行.
RejectedExecutionHandler拒绝策略
当队列满且线程数达到maximunPoolSize最大线程数后采取的策略, 可传入
AbortPolicy 抛出异常,这个是默认策略.
CallersRunPolicy 由调用者所在的线程执行任务
DiscardOldestPolicy 丢弃最老的任务
DiscardPolicy 丢弃新任务,不抛出异常
jdk提供的Executors快速创建线程池的用法
jdk封装了一个Executors类可以直接创建各种线程池,
用法形如
ExecutorService pool = Executors.newXXXXXPool()可以用Executors类创建业务常用的3种线程池
固定线程池
public static ExecutorService newFixedThreadPool(int nThreads) { return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>()); }创建一个核心线程数和最大线程数相同的线程池,都为nThreads,
且线程池的阻塞队列长度是Integer.MAX_VALUE,
且keepAliveTime=0,说明只要线程个数比核心线程个数多并且当前空闲则回收.
单线程线程池
public static ExecutorService newSingleThreadExecutor() { return new FinalizableDelegatedExecutorService (new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>())); }创建一个核心线程数和最大线程数都是1的线程池,
且线程池的阻塞队列长度是Integer.MAX_VALUE,
且keepAliveTime=0,说明只要线程个数比核心线程个数多并且当前空闲则回收.
已缓存的线程池
public static ExecutorService newCachedThreadPool() { return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>()); }创建一个按需创建线程的线程池,初始线程个数为0,最多线程个数为
Integer.MAX_VALUE,并且阻塞队列为同步队列.
keepAliveTime=60,说明当前线程在60s内空闲则回收.
CachedThreadPool的特殊之处在于,加入同步队列的任务会被马上执行,同步队列里边最多只有1个任务.
使用创建好的ExecutorService 线程池执行异步任务
submit操作
提交一个任务, 任务参数可以是 Runnable实现类 或 Callable 实现类.
返回的类型是Future 表示异步计算的结果, 可以用future.get()方法拿到数据.
shutdown操作
调用shutdown方法后,线程池就不会再接受新的任务了,但是工作队列里边的任务还是要执行的, 该方法会立刻返回,不等待队列任务完成再返回.
使用线程池的情况下当程序结束时记得调用shutdown关闭线程池, 如果不关闭线程池,则会导致 线程池资源一直不被释放.
shutdownNow操作
调用shutdownNow方法后,线程池就不会再接受新的任务了,并且会丢弃工作队列里边的任务,正在执行的任务会被中断,该方法会立刻返回,并不等待激活的任务执行完成. 返回值为这时候队列里面被丢弃的任务列表.
awaitTermination操作
当线程调用awaitTermination方法后,当前线程会被阻塞, 直到线程池状态变为TERMINATED 才返回,或者等待时间超时才返回.
案例1-测试FixedThreadPool执行CallableTask任务
package cn.demo;import cn.hutool.core.util.RandomUtil;import java.util.concurrent.ExecutionException;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Future;public class ExecutorTestsForCallableTask { public static void main(String[] args) throws ExecutionException, InterruptedException { String res1 = ""; String res2 = ""; String res3 = ""; String res4 = ""; ExecutorService fixedThreadPool = Executors.newFixedThreadPool(3); //submit 提交4个任务, 实际执行时,任务是并发执行的,执行顺序不固定 Future<String> submit1 = fixedThreadPool.submit( new TestCallableTask(RandomUtil.randomInt(30,1000),"t1")); Future<String> submit2 = fixedThreadPool.submit( new TestCallableTask(RandomUtil.randomInt(100,400),"t2")); Future<String> submit3 = fixedThreadPool.submit( new TestCallableTask(RandomUtil.randomInt(30,350),"t3")); Future<String> submit4 = fixedThreadPool.submit( new TestCallableTask(RandomUtil.randomInt(310,500),"t4")); res1 = submit1.get(); System.out.println(res1); res2 = submit2.get(); System.out.println(res2); res3 = submit3.get(); System.out.println(res3); res4 = submit4.get(); System.out.println(res4); fixedThreadPool.shutdown(); }}package cn.demo;import cn.hutool.core.util.RandomUtil;import java.time.LocalDateTime;import java.util.concurrent.Callable;public class TestCallableTask implements Callable<String> { private int testIntVal; private String taskSeq; public TestCallableTask(int testIntVal, String taskSeq) { this.testIntVal = testIntVal; this.taskSeq = taskSeq; } @Override public String call() throws Exception { String s = LocalDateTime.now().toString(); System.out.println(s+"->"+taskSeq+" run ...."); int i = testIntVal; System.out.println(i); try { Thread.sleep(RandomUtil.randomInt(100,300)); } catch (InterruptedException e) { e.printStackTrace(); } if (i>300){ return "300more"; }else { return "300less"; } }}案例2-测试FixedThreadPool执行RunnableTask任务
package cn.demo;import java.util.concurrent.*;public class ExecutorTestsForRunnableTask { public static void main(String[] args) throws ExecutionException, InterruptedException { String res1 = ""; String res2 = ""; String res3 = ""; String res4 = ""; ExecutorService fixedThreadPool = Executors.newFixedThreadPool(3); //submit 提交4个任务, 实际执行时,任务是并发执行的,执行顺序不固定 Task1Param task1Param = new Task1Param(); task1Param.setUrl("f23r3r"); task1Param.setName("1heg43t34t34t"); Future<String> stringFuture = fixedThreadPool.submit( new TestTask1Runnable(task1Param), "success1 ok"); Task1Param t2 = new Task1Param(); t2.setUrl("gnsg2323"); t2.setName("2wwswer2r1asdaaws"); Future<String> f2 = fixedThreadPool.submit(new TestTask1Runnable(t2), "success2 ok"); Task1Param t3 = new Task1Param(); t3.setUrl("thwasr23r"); t3.setName("3erzawfe23rawsf"); Future<String> f3 = fixedThreadPool.submit(new TestTask1Runnable(t3), "success3 ok"); Task1Param t4 = new Task1Param(); t4.setUrl("mjkdsragt"); t4.setName("4tbertydraewrsfk"); Future<String> f4 = fixedThreadPool.submit(new TestTask1Runnable(t4), "success4 ok"); res1 = stringFuture.get(); System.out.println(res1); res2 = f2.get(); System.out.println(res2); res3 = f3.get(); System.out.println(res3); res4 = f4.get(); System.out.println(res4); fixedThreadPool.shutdown(); }}package cn.demo;import cn.hutool.core.util.RandomUtil;import java.time.LocalDateTime;public class TestTask1Runnable implements Runnable{ private Task1Param task1Param; public TestTask1Runnable(Task1Param task1Param) { this.task1Param = task1Param; } @Override public void run() { try { Thread.sleep(RandomUtil.randomInt(200,600)); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(task1Param.getName()); System.out.println(task1Param.getUrl()); String s = LocalDateTime.now().toString(); System.out.println(s+" TestTask1Runnable run ...."); }}使用自定义的ThreadPoolExecutor来执行异步任务
package cn.demo;import cn.hutool.core.util.RandomUtil;import java.util.concurrent.*;public class TpeTest { private final static ThreadPoolExecutor pool = new ThreadPoolExecutor( 1,1, 1L, TimeUnit.MINUTES, new ArrayBlockingQueue<Runnable>(1), new ThreadPoolExecutor.CallerRunsPolicy()); public static void main(String[] args) throws ExecutionException, InterruptedException { Future<String> submit1 = pool.submit( new TestCallableTask(RandomUtil.randomInt(30,1000),"t1")); Future<String> submit2 = pool.submit( new TestCallableTask(RandomUtil.randomInt(100,400),"t2")); Future<String> submit3 = pool.submit( new TestCallableTask(RandomUtil.randomInt(30,350),"t3")); Future<String> submit4 = pool.submit( new TestCallableTask(RandomUtil.randomInt(310,500),"t4")); System.out.println("task1-"+submit1.get()); System.out.println("task2-"+submit2.get()); System.out.println("task3-"+submit3.get()); System.out.println("task4-"+submit4.get()); pool.shutdown(); }}线程池使用FutureTask时需要注意的事情
线程池使用FutureTask时,如果把拒绝策略设置为 DiscardPolicy 和 DiscardOldestPolicy,并且在被拒绝的任务的Future对象上调用了无参get方法,那么调用线程会一直被阻塞.
如上面的代码,如果把CallerRunsPolicy替换成 DiscardPolicy 或 DiscardOldestPolicy ,就会导致任务一直被阻塞,一直无法取到future.get()的值.
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