java.utils.concurrent 新類庫中的構件（四）PriorityBlockingQueue

java.utils.concurrent 新類庫中的構件（四）PriorityBlockingQueue

這是一個很基礎的優先級隊列，它具有可阻塞的讀取操作。下面是一個實例，其中在優先級隊列中的對象是按照優先級順序從隊列中出現的任務。PrioritizedTask被賦予了一個優先級數字，以此來提供這種順序

package concurrency;//: concurrency/PriorityBlockingQueueDemo.java
import java.util.concurrent.*;
import java.util.*;
import static net.mindview.util.Print.*;

class PrioritizedTask implements
Runnable, Comparable<PrioritizedTask>  {
  private Random rand = new Random(47);
  private static int counter = 0;
  private final int id = counter++;
  private final int priority;
  protected static List<PrioritizedTask> sequence = new ArrayList<PrioritizedTask>();
  public PrioritizedTask(int priority) {
    this.priority = priority;
    sequence.add(this);
  }
  public int compareTo(PrioritizedTask arg) {
    return priority < arg.priority ? 1 :
      (priority > arg.priority ? -1 : 0);
  }
  public void run() {
    try {
      TimeUnit.MILLISECONDS.sleep(rand.nextInt(250));
    } catch(InterruptedException e) {
      // Acceptable way to exit
    }
    print(this);
  }
  public String toString() {
    return String.format("[%1$-3d]", priority) +
      " Task " + id;
  }
  public String summary() {
    return "(" + id + ":" + priority + ")";
  }
  public static class EndSentinel extends PrioritizedTask {
    private ExecutorService exec;
    public EndSentinel(ExecutorService e) {
      super(-1); // Lowest priority in this program
      exec = e;
    }
    public void run() {
      int count = 0;
      for(PrioritizedTask pt : sequence) {
        printnb(pt.summary());
        if(++count % 5 == 0)
          print();
      }
      print();
      print(this + " Calling shutdownNow()");
      exec.shutdownNow();
    }
  }
}

class PrioritizedTaskProducer implements Runnable {
  private Random rand = new Random(47);
  private Queue<Runnable> queue;
  private ExecutorService exec;
  public PrioritizedTaskProducer(Queue<Runnable> q, ExecutorService e) {
    queue = q;
    exec = e; // Used for EndSentinel
  }
  public void run() {
    // Unbounded queue; never blocks.
    // Fill it up fast with random priorities:
    for(int i = 0; i < 20; i++) {
      queue.add(new PrioritizedTask(rand.nextInt(10)));
      Thread.yield();
    }
    // Trickle in highest-priority jobs:
    try {
      for(int i = 0; i < 10; i++) {
        TimeUnit.MILLISECONDS.sleep(250);
        queue.add(new PrioritizedTask(10));
      }
      // Add jobs, lowest priority first:
      for(int i = 0; i < 10; i++)
        queue.add(new PrioritizedTask(i));
      // A sentinel to stop all the tasks:
      queue.add(new PrioritizedTask.EndSentinel(exec));
    } catch(InterruptedException e) {
      // Acceptable way to exit
    }
    print("Finished PrioritizedTaskProducer");
  }
}

class PrioritizedTaskConsumer implements Runnable {
  private PriorityBlockingQueue<Runnable> q;
  public PrioritizedTaskConsumer(
    PriorityBlockingQueue<Runnable> q) {
    this.q = q;
  }
  public void run() {
    try {
      while(!Thread.interrupted())
        // Use current thread to run the task:
        q.take().run();
    } catch(InterruptedException e) {
      // Acceptable way to exit
    }
    print("Finished PrioritizedTaskConsumer");
  }
}

public class PriorityBlockingQueueDemo {
  public static void main(String[] args) throws Exception {
    Random rand = new Random(47);
    ExecutorService exec = Executors.newCachedThreadPool();
    PriorityBlockingQueue<Runnable> queue = new PriorityBlockingQueue<Runnable>();
    exec.execute(new PrioritizedTaskProducer(queue, exec));
    exec.execute(new PrioritizedTaskConsumer(queue));
  }
} /* (Execute to see output) *///:~

與前一個示例相同，PrioritizedTask對象的創建序列被記錄在sequence List中，用戶和實際的執行順序比較。run()方法將休眠一小段隨機時間，然後打印對象信息，而EndSentinel提供了和前面相同的功能，要確保它是隊列中的最後一個對象。

PrioritzedTaskProducer和PriorittizedTaskComsumer通過PriorityBlockinQueue彼此連接。因爲這種隊列的阻塞特性提供了所必須的同步，所以你應該注意到了，這裏不需要任何顯示的同步—不必考慮當你從這種隊列讀取時，其中是否有元素，因爲這個隊列在沒有任何元素時將直接阻塞讀取者。