Java基本线程池带锁的实现.ReentrantLock

Question

我是 Java 新手。我只是在尝试线程，我想创建类似线程池的东西(如果这实际上是我正在做的事情......)。

基本上，我有一个 while 循环，它会触发线程，直到仍然有任务要执行&&，同时最大并发线程不大于 n。每个线程使用 java.util.concurrent.locks.ReentrantLock 围绕任务计数变量提供锁，该变量在每个线程中减少，并且线程计数变量在线程启动时增加，并在线程结束前减少(代码闻起来？):

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class Test { 
   public static void main(String[] args) {

      //overcome limitation of closure not being able to modify outer variable with arr.
      final int[] runningThreads = {0};
      final int[] taskcount = {10};

      final Lock _mutex = new ReentrantLock(true);

      int maxThreadQty = 3;


      while ((taskcount[0] > 0) && (runningThreads[0] < maxThreadQty))  {

         new Thread("T") {
             public void run() {
                  System.out.println("New Thread Started");
                  _mutex.lock();
                  runningThreads[0]++;
                  System.out.println("Running Threads: " + runningThreads[0]);
                  System.out.println("Times to go: " + taskcount[0]);
                  _mutex.unlock();
                  // actually do something;
                  _mutex.lock();
                  taskcount[0]--;
                  runningThreads[0]--;
                  _mutex.unlock();
             }

          }.start();
      }
   }
}

当我运行代码时，新线程不断被激发，任务数量只减少了两到三倍……

输出的最后几行(读取时间作为待完成的任务):

Running Threads: 565
Times to go: 8
Running Threads: 566
Times to go: 8
Running Threads: 567
Times to go: 8
Running Threads: 568
Times to go: 8
Running Threads: 569
Times to go: 8
Running Threads: 570
Times to go: 8
Running Threads: 571
Times to go: 8
Running Threads: 572
Times to go: 8
Running Threads: 573
Times to go: 8
Running Threads: 574
Times to go: 8
Running Threads: 575
Times to go: 8

CTRL-C

我确信我使用线程或锁的方式一定有问题。但是作为一个java新手，我可能会错过很多东西(甚至可能是最基本的)，需要一些帮助和一些让我回到正确的道路上将非常感激......!谢谢。

我用它作为线程的引用:http://tutorials.jenkov.com/java-concurrency/creating-and-starting-threads.html

然后通过这个 stackoverflow 答案来了解如何使用 ReentrantLock:https://stackoverflow.com/a/12510490/988591

这对于闭包无法修改外部变量的解决方法(使用数组值): http://c2.com/cgi/wiki?ClosuresThatWorkAroundFinalLimitation

Answer 1

你不能使用 built in thread pool功能？

如果不是，问题是 runningThreads 在每个线程启动并获取锁之前不会增加。实际上，主线程可能会运行相当长的时间，同时无限制地旋转新线程。

一种解决方案可能是在启动新线程之前增加主线程上的 runningThreads 变量，但在每个工作线程内减少该变量。

我不想暗示您的代码的其他所有内容都是“好的”(创建强大的线程池实现可能是一项相当困难且复杂的任务)，但可以避免该问题的最小更改可能是

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class Test { 
   public static void main(String[] args) {

      //overcome limitation of closure not being able to modify outer variable with arr.
      final int[] runningThreads = {0};
      final int[] taskcount = {10};

      final Lock _mutex = new ReentrantLock(true);

      int maxThreadQty = 3;


      while ((taskcount[0] > 0) && (runningThreads[0] < maxThreadQty))  {
         System.out.println("New Thread Started");
         _mutex.lock();
         runningThreads[0]++;
         System.out.println("Running Threads: " + runningThreads[0]);
         System.out.println("Times to go: " + taskcount[0]);
         _mutex.unlock();
         new Thread("T") {
             public void run() {
                  // actually do something;
                  _mutex.lock();
                  taskcount[0]--;
                  runningThreads[0]--;
                  _mutex.unlock();
             }

          }.start();
      }
   }
}