队列阻塞浅析_JAVA_编程开发_程序员俱乐部

　 ? 这几天所做的项目中涉及到了队列阻塞机制，通过研究整理如下。在这里和大家分享。

? ? ? ?队列以一种先进先出的方式。如果你向一个已经满了的阻塞队列中添加一个元素，或是从一个空的阻塞队列中移除一个元素，将导致线程阻塞。在多线程进行合作时，阻塞队列是很有用的工具。工作者线程可以定期的把中间结果存到阻塞队列中。而其他工作者线程把中间结果取出并在将来修改它们。队列会自动平衡负载。如果第一个线程集运行的比第二个慢，则第二个线程集在等待结果时就会阻塞。如果第一个线程集运行的快，那么它将等待第二个线程集赶上来。

import java.io.*;
import java.util.*;
import java.util.concurrent.*;

public class BlockingQueueTest
{
   public static void main(String[] args)
   {
      Scanner in = new Scanner(System.in);
      System.out.print("Enter base directory (e.g. /usr/local/jdk1.6.0/src): ");
      String directory = in.nextLine();
      System.out.print("Enter keyword (e.g. volatile): ");
      String keyword = in.nextLine();

      final int FILE_QUEUE_SIZE = 10;
      final int SEARCH_THREADS = 100;

      BlockingQueue<File> queue = new ArrayBlockingQueue<File>(FILE_QUEUE_SIZE);

      FileEnumerationTask enumerator = new FileEnumerationTask(queue, new File(directory));
      new Thread(enumerator).start();
      for (int i = 1; i <= SEARCH_THREADS; i++)
         new Thread(new SearchTask(queue, keyword)).start();
   }
}

/**
 * This task enumerates all files in a directory and its subdirectories.
 */
class FileEnumerationTask implements Runnable
{
   /**
    * Constructs a FileEnumerationTask.
    * @param queue the blocking queue to which the enumerated files are added
    * @param startingDirectory the directory in which to start the enumeration
    */
   public FileEnumerationTask(BlockingQueue<File> queue, File startingDirectory)
   {
      this.queue = queue;
      this.startingDirectory = startingDirectory;
   }

   public void run()
   {
      try
      {
         enumerate(startingDirectory);
         queue.put(DUMMY);
      }
      catch (InterruptedException e)
      {
      }
   }

   /**
    * Recursively enumerates all files in a given directory and its subdirectories
    * @param directory the directory in which to start
    */
   public void enumerate(File directory) throws InterruptedException
   {
      File[] files = directory.listFiles();
      for (File file : files)
      {
         if (file.isDirectory()) enumerate(file);
         else queue.put(file);
      }
   }

   public static File DUMMY = new File("");

   private BlockingQueue<File> queue;
   private File startingDirectory;
}

/**
 * This task searches files for a given keyword.
 */
class SearchTask implements Runnable
{
   /**
    * Constructs a SearchTask.
    * @param queue the queue from which to take files
    * @param keyword the keyword to look for
    */
   public SearchTask(BlockingQueue<File> queue, String keyword)
   {
      this.queue = queue;
      this.keyword = keyword;
   }

   public void run()
   {
      try
      {
         boolean done = false;
         while (!done)
         {
            File file = queue.take();
            if (file == FileEnumerationTask.DUMMY)
            {
               queue.put(file);
               done = true;
            }
            else search(file);            
         }
      }
      catch (IOException e)
      {
         e.printStackTrace();
      }
      catch (InterruptedException e)
      {
      }      
   }

   /**
    * Searches a file for a given keyword and prints all matching lines.
    * @param file the file to search
    */
   public void search(File file) throws IOException
   {
      Scanner in = new Scanner(new FileInputStream(file));
      int lineNumber = 0;
      while (in.hasNextLine())
      {
         lineNumber++;
         String line = in.nextLine().trim();
         if (line.contains(keyword)) System.out.printf("%s:%d    %s%n", file.getPath(), lineNumber, line);
      }
      in.close();
   }

   private BlockingQueue<File> queue;
   private String keyword;
}