迭代ConcurrentHashMap值是否安全?

问题

在javadoc forConcurrentHashMap中有以下内容:

检索操作(包括get)一般不会阻塞,因此可能与更新操作重叠(包括put和remove)。检索反映了最近完成的更新操作的结果。对于诸如putAll和clear之类的聚合操作,并发检索可能反映仅插入或删除某些条目。类似地,Iterators和Enumerations在迭代器/枚举的创建时或之后的某个时刻返回反映哈希表状态的元素。它们不会抛出ConcurrentModificationException。但是,迭代器设计为一次只能由一个线程使用。

这是什么意思?如果我尝试同时使用两个线程迭代 Map 会发生什么?如果我在迭代时放置或删除 Map 中的值,会发生什么?


#1 热门回答(156 赞)

这是什么意思?

这意味着你从aConcurrentHashMap获得的每个迭代器都设计为由单个线程使用,不应传递。这包括for-each循环提供的语法糖。

如果我尝试同时使用两个线程迭代 Map 会发生什么?

如果每个线程都使用它自己的迭代器,它将按预期工作。

如果我在迭代时放置或删除 Map 中的值会怎样?

如果你这样做,那么事情就不会破坏(这是"并发"中的一部分)。但是,无法保证一个线程将看到另一个线程执行的映射更改(无需从映射中获取新的迭代器)。迭代器保证在创建时反映 Map 的状态。进一步的变化可能会反映在迭代器中,但它们并非必须如此。

总之,一个声明就像

for (Object o : someConcurrentHashMap.entrySet()) {
    // ...
}

几乎每次看到它都会很好(或至少是安全的)。


#2 热门回答(17 赞)

你可以使用此类来测试两个访问线程,并使用一个变更共享实例ConcurrentHashMap

import java.util.Map;
import java.util.Random;
import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class ConcurrentMapIteration
{
  private final Map<String, String> map = new ConcurrentHashMap<String, String>();

  private final static int MAP_SIZE = 100000;

  public static void main(String[] args)
  {
    new ConcurrentMapIteration().run();
  }

  public ConcurrentMapIteration()
  {
    for (int i = 0; i < MAP_SIZE; i++)
    {
      map.put("key" + i, UUID.randomUUID().toString());
    }
  }

  private final ExecutorService executor = Executors.newCachedThreadPool();

  private final class Accessor implements Runnable
  {
    private final Map<String, String> map;

    public Accessor(Map<String, String> map)
    {
      this.map = map;
    }

    @Override
    public void run()
    {
      for (Map.Entry<String, String> entry : this.map.entrySet())
      {
        System.out.println(
            Thread.currentThread().getName() + " - [" + entry.getKey() + ", " + entry.getValue() + ']'
        );
      }
    }
  }

  private final class Mutator implements Runnable
  {

    private final Map<String, String> map;
    private final Random random = new Random();

    public Mutator(Map<String, String> map)
    {
      this.map = map;
    }

    @Override
    public void run()
    {
      for (int i = 0; i < 100; i++)
      {
        this.map.remove("key" + random.nextInt(MAP_SIZE));
        this.map.put("key" + random.nextInt(MAP_SIZE), UUID.randomUUID().toString());
        System.out.println(Thread.currentThread().getName() + ": " + i);
      }
    }
  }

  private void run()
  {
    Accessor a1 = new Accessor(this.map);
    Accessor a2 = new Accessor(this.map);
    Mutator m = new Mutator(this.map);

    executor.execute(a1);
    executor.execute(m);
    executor.execute(a2);
  }
}

不会抛出异常。

在访问者线程之间共享相同的迭代器可能会导致死锁:

import java.util.Iterator;
import java.util.Map;
import java.util.Random;
import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class ConcurrentMapIteration
{
  private final Map<String, String> map = new ConcurrentHashMap<String, String>();
  private final Iterator<Map.Entry<String, String>> iterator;

  private final static int MAP_SIZE = 100000;

  public static void main(String[] args)
  {
    new ConcurrentMapIteration().run();
  }

  public ConcurrentMapIteration()
  {
    for (int i = 0; i < MAP_SIZE; i++)
    {
      map.put("key" + i, UUID.randomUUID().toString());
    }
    this.iterator = this.map.entrySet().iterator();
  }

  private final ExecutorService executor = Executors.newCachedThreadPool();

  private final class Accessor implements Runnable
  {
    private final Iterator<Map.Entry<String, String>> iterator;

    public Accessor(Iterator<Map.Entry<String, String>> iterator)
    {
      this.iterator = iterator;
    }

    @Override
    public void run()
    {
      while(iterator.hasNext()) {
        Map.Entry<String, String> entry = iterator.next();
        try
        {
          String st = Thread.currentThread().getName() + " - [" + entry.getKey() + ", " + entry.getValue() + ']';
        } catch (Exception e)
        {
          e.printStackTrace();
        }

      }
    }
  }

  private final class Mutator implements Runnable
  {

    private final Map<String, String> map;
    private final Random random = new Random();

    public Mutator(Map<String, String> map)
    {
      this.map = map;
    }

    @Override
    public void run()
    {
      for (int i = 0; i < 100; i++)
      {
        this.map.remove("key" + random.nextInt(MAP_SIZE));
        this.map.put("key" + random.nextInt(MAP_SIZE), UUID.randomUUID().toString());
      }
    }
  }

  private void run()
  {
    Accessor a1 = new Accessor(this.iterator);
    Accessor a2 = new Accessor(this.iterator);
    Mutator m = new Mutator(this.map);

    executor.execute(a1);
    executor.execute(m);
    executor.execute(a2);
  }
}

一旦你开始共享相同的Iterator<Map.Entry<String, String>>mong访问器和mutator线程java.lang.IllegalStateExceptions将开始弹出。

import java.util.Iterator;
import java.util.Map;
import java.util.Random;
import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class ConcurrentMapIteration
{
  private final Map<String, String> map = new ConcurrentHashMap<String, String>();
  private final Iterator<Map.Entry<String, String>> iterator;

  private final static int MAP_SIZE = 100000;

  public static void main(String[] args)
  {
    new ConcurrentMapIteration().run();
  }

  public ConcurrentMapIteration()
  {
    for (int i = 0; i < MAP_SIZE; i++)
    {
      map.put("key" + i, UUID.randomUUID().toString());
    }
    this.iterator = this.map.entrySet().iterator();
  }

  private final ExecutorService executor = Executors.newCachedThreadPool();

  private final class Accessor implements Runnable
  {
    private final Iterator<Map.Entry<String, String>> iterator;

    public Accessor(Iterator<Map.Entry<String, String>> iterator)
    {
      this.iterator = iterator;
    }

    @Override
    public void run()
    {
      while (iterator.hasNext())
      {
        Map.Entry<String, String> entry = iterator.next();
        try
        {
          String st =
              Thread.currentThread().getName() + " - [" + entry.getKey() + ", " + entry.getValue() + ']';
        } catch (Exception e)
        {
          e.printStackTrace();
        }

      }
    }
  }

  private final class Mutator implements Runnable
  {

    private final Random random = new Random();

    private final Iterator<Map.Entry<String, String>> iterator;

    private final Map<String, String> map;

    public Mutator(Map<String, String> map, Iterator<Map.Entry<String, String>> iterator)
    {
      this.map = map;
      this.iterator = iterator;
    }

    @Override
    public void run()
    {
      while (iterator.hasNext())
      {
        try
        {
          iterator.remove();
          this.map.put("key" + random.nextInt(MAP_SIZE), UUID.randomUUID().toString());
        } catch (Exception ex)
        {
          ex.printStackTrace();
        }
      }

    }
  }

  private void run()
  {
    Accessor a1 = new Accessor(this.iterator);
    Accessor a2 = new Accessor(this.iterator);
    Mutator m = new Mutator(map, this.iterator);

    executor.execute(a1);
    executor.execute(m);
    executor.execute(a2);
  }
}

#3 热门回答(8 赞)

这意味着你不应在多个线程之间共享迭代器对象。创建多个迭代器并在不同的线程中同时使用它们很好。