Java Concurrency 简明教程

Java Concurrency - ConcurrentMap Interface

java.util.concurrent.ConcurrentMap 接口是 Map 接口的子接口，支持对基础映射变量执行原子操作。它具有类似于易失变量上的读写操作的 get 和 set 方法。也就是说，某项设置与对同一变量执行的任何后续 get 操作之间存在发生在前关系。此接口确保线程安全和原子性保证。

A java.util.concurrent.ConcurrentMap interface is a subinterface of Map interface, supports atomic operations on underlying map variable. It have get and set methods that work like reads and writes on volatile variables. That is, a set has a happens-before relationship with any subsequent get on the same variable. This interface ensures thread safety and atomicity guarantees.

ConcurrentMap Methods

Sr.No.	Method & Description
1	default V compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction) Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).
2	default V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction) If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.
3	default V computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction) If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.
4	default void forEach(BiConsumer<? super K,? super V> action) Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.
5	default V getOrDefault(Object key, V defaultValue) Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.
6	default V merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction) If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.
7	V putIfAbsent(K key, V value) If the specified key is not already associated with a value, associate it with the given value.
8	boolean remove(Object key, Object value) Removes the entry for a key only if currently mapped to a given value.
9	V replace(K key, V value) Replaces the entry for a key only if currently mapped to some value.
10	boolean replace(K key, V oldValue, V newValue) Replaces the entry for a key only if currently mapped to a given value.
11	default void replaceAll(BiFunction<? super K,? super V,? extends V> function) Replaces each entry’s value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.

Example

以下 TestThread 程序展示了 ConcurrentMap 与 HasMap 的用法。

The following TestThread program shows usage of ConcurrentMap vs HashMap.

import java.util.ConcurrentModificationException;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class TestThread {

   public static void main(final String[] arguments) {
      Map<String,String> map = new ConcurrentHashMap<String, String>();

      map.put("1", "One");
      map.put("2", "Two");
      map.put("3", "Three");
      map.put("5", "Five");
      map.put("6", "Six");

      System.out.println("Initial ConcurrentHashMap: " + map);
      Iterator<String> iterator = map.keySet().iterator();

      try {

         while(iterator.hasNext()) {
            String key = iterator.next();

            if(key.equals("3")) {
               map.put("4", "Four");
            }
         }
      } catch(ConcurrentModificationException cme) {
         cme.printStackTrace();
      }
      System.out.println("ConcurrentHashMap after modification: " + map);

      map = new HashMap<String, String>();

      map.put("1", "One");
      map.put("2", "Two");
      map.put("3", "Three");
      map.put("5", "Five");
      map.put("6", "Six");

      System.out.println("Initial HashMap: " + map);
      iterator = map.keySet().iterator();

      try {

         while(iterator.hasNext()) {
            String key = iterator.next();

            if(key.equals("3")) {
               map.put("4", "Four");
            }
         }
         System.out.println("HashMap after modification: " + map);
      } catch(ConcurrentModificationException cme) {
         cme.printStackTrace();
      }
   }
}

这将产生以下结果。

This will produce the following result.

Output

Initial ConcurrentHashMap: {1 = One, 2 = Two, 3 = Three, 5 = Five, 6 = Six}
ConcurrentHashMap after modification: {1 = One, 2 = Two, 3 = Three, 4 = Four, 5 = Five, 6 = Six}
Initial HashMap: {1 = One, 2 = Two, 3 = Three, 5 = Five, 6 = Six}
java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(Unknown Source)
	at java.util.HashMap$KeyIterator.next(Unknown Source)
	at TestThread.main(TestThread.java:48)