14.9 执行器

大量生命周期很短的线程，应该使用线程池。另一方面，线程池限定了线程的总数，避免由于产生大量线程而导致性能降低。

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1 线程池

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

/**
 * @version 1.02 2015-06-21
 * @author Cay Horstmann
 */
public class ThreadPoolTest
{
   public static void main(String[] args) throws Exception
   {
      try (Scanner in = new Scanner(System.in))
      {
         System.out.print("Enter base directory (e.g. /usr/local/jdk5.0/src): ");
         String directory = in.nextLine();
         System.out.print("Enter keyword (e.g. volatile): ");
         String keyword = in.nextLine();

         ExecutorService pool = Executors.newCachedThreadPool();

         MatchCounter counter = new MatchCounter(new File(directory), keyword, pool);
         Future<Integer> result = pool.submit(counter);

         try
         {
            System.out.println(result.get() + " matching files.");
         }
         catch (ExecutionException e)
         {
            e.printStackTrace();
         }
         catch (InterruptedException e)
         {
         }
         pool.shutdown();

         int largestPoolSize = ((ThreadPoolExecutor) pool).getLargestPoolSize();
         System.out.println("largest pool size=" + largestPoolSize);
      }
   }
}

/**
 * This task counts the files in a directory and its subdirectories that contain a given keyword.
 */
class MatchCounter implements Callable<Integer>
{
   private File directory;
   private String keyword;
   private ExecutorService pool;
   private int count;

   /**
    * Constructs a MatchCounter.
    * @param directory the directory in which to start the search
    * @param keyword the keyword to look for
    * @param pool the thread pool for submitting subtasks
    */
   public MatchCounter(File directory, String keyword, ExecutorService pool)
   {
      this.directory = directory;
      this.keyword = keyword;
      this.pool = pool;
   }

   public Integer call()
   {
      count = 0;
      try
      {
         File[] files = directory.listFiles();
         List<Future<Integer>> results = new ArrayList<>();

         for (File file : files)
            if (file.isDirectory())
            {
               MatchCounter counter = new MatchCounter(file, keyword, pool);
               Future<Integer> result = pool.submit(counter);
               results.add(result);
            }
            else
            {
               if (search(file)) count++;
            }

         for (Future<Integer> result : results)
            try
            {
               count += result.get();
            }
            catch (ExecutionException e)
            {
               e.printStackTrace();
            }
      }
      catch (InterruptedException e)
      {
      }
      return count;
   }

   /**
    * Searches a file for a given keyword.
    * @param file the file to search
    * @return true if the keyword is contained in the file
    */
   public boolean search(File file)
   {
      try
      {
         try (Scanner in = new Scanner(file, "UTF-8"))
         {
            boolean found = false;
            while (!found && in.hasNextLine())
            {
               String line = in.nextLine();
               if (line.contains(keyword)) found = true;
            }         
            return found;
         }
      }
      catch (IOException e)
      {
         return false;
      }
   }
}

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2 预定执行

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3 控制任务组

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4 Fork-Join框架

import java.util.concurrent.*;
import java.util.function.*;

/**
 * This program demonstrates the fork-join framework.
 * @version 1.01 2015-06-21
 * @author Cay Horstmann
 */
public class ForkJoinTest
{
   public static void main(String[] args)
   {
      final int SIZE = 10000000;
      double[] numbers = new double[SIZE];
      for (int i = 0; i < SIZE; i++) numbers[i] = Math.random();
      Counter counter = new Counter(numbers, 0, numbers.length, x -> x > 0.5);
      ForkJoinPool pool = new ForkJoinPool();
      pool.invoke(counter);
      System.out.println(counter.join());
   }
}

class Counter extends RecursiveTask<Integer>
{
   public static final int THRESHOLD = 1000;
   private double[] values;
   private int from;
   private int to;
   private DoublePredicate filter;

   public Counter(double[] values, int from, int to, DoublePredicate filter)
   {
      this.values = values;
      this.from = from;
      this.to = to;
      this.filter = filter;
   }

   protected Integer compute()
   {
      if (to - from < THRESHOLD)
      {
         int count = 0;
         for (int i = from; i < to; i++)
         {
            if (filter.test(values[i])) count++;
         }
         return count;
      }
      else
      {
         int mid = (from + to) / 2;
         Counter first = new Counter(values, from, mid, filter);
         Counter second = new Counter(values, mid, to, filter);
         invokeAll(first, second);
         return first.join() + second.join();
      }
   }
}

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5 Completable Futures