Java之通过Collections.synchronizedMap创建线程安全的HashMap

1 问题

我们知道hashMap线程是不安全的，一般而言，我们怎么创建线程安全的HashMap呢？

2 解决办法

我们可以使用Collections.synchronizedMap来创建HashMap,如下

static Map<String, String> results = Collections.synchronizedMap(new HashMap<String, String>());
 

3 Collections.synchronizedMap源码部分实现

我们先看synchronizedMap如果创建

      public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) {
      return new SynchronizedMap<K,V>(m);
      }
  
 

我们看下具体的SynchronizedMap类的部分

      private static class SynchronizedMap<K,V>
      implements Map<K,V>, Serializable {
      // use serialVersionUID from JDK 1.2.2 for interoperability
      private static final long serialVersionUID = 1978198479659022715L;
      private final Map<K,V> m; // Backing Map
      final Object mutex; // Object on which to synchronize
       SynchronizedMap(Map<K,V> m) {
      if (m==null)
      throw new NullPointerException();
      this.m = m;
       mutex = this;
       }
       SynchronizedMap(Map<K,V> m, Object mutex) {
      this.m = m;
      this.mutex = mutex;
       }
      public int size() {
      synchronized(mutex) {return m.size();}
       }
      public boolean isEmpty(){
      synchronized(mutex) {return m.isEmpty();}
       }
      public boolean containsKey(Object key) {
      synchronized(mutex) {return m.containsKey(key);}
       }
      public boolean containsValue(Object value){
      synchronized(mutex) {return m.containsValue(value);}
       }
      public V get(Object key) {
      synchronized(mutex) {return m.get(key);}
       }
      public V put(K key, V value) {
      synchronized(mutex) {return m.put(key, value);}
       }
      public V remove(Object key) {
      synchronized(mutex) {return m.remove(key);}
       }
      public void putAll(Map<? extends K, ? extends V> map) {
      synchronized(mutex) {m.putAll(map);}
       }
      public void clear() {
      synchronized(mutex) {m.clear();}
       }
      private transient Set<K> keySet = null;
      private transient Set<Map.Entry<K,V>> entrySet = null;
      private transient Collection<V> values = null;
      public Set<K> keySet() {
      synchronized(mutex) {
      if (keySet==null)
       keySet = new SynchronizedSet<K>(m.keySet(), mutex);
      return keySet;
       }
       }
      public Set<Map.Entry<K,V>> entrySet() {
      synchronized(mutex) {
      if (entrySet==null)
       entrySet = new SynchronizedSet<Map.Entry<K,V>>(m.entrySet(), mutex);
      return entrySet;
       }
       }
      public Collection<V> values() {
      synchronized(mutex) {
      if (values==null)
       values = new SynchronizedCollection<V>(m.values(), mutex);
      return values;
       }
       }
      public boolean equals(Object o) {
      if (this == o)
      return true;
      synchronized(mutex) {return m.equals(o);}
       }
      public int hashCode() {
      synchronized(mutex) {return m.hashCode();}
       }
      public String toString() {
      synchronized(mutex) {return m.toString();}
       }
      private void writeObject(ObjectOutputStream s) throws IOException {
      synchronized(mutex) {s.defaultWriteObject();}
       }
       }
  
 

SynchronizedMap 实现了Map接口的代理类，该类中对Map接口中的方法还是使用synchronized 同步关键字来保证对Map的操作是线程安全的。

synchronized(mutex) {/*****/}
 

文章来源: chenyu.blog.csdn.net，作者：chen.yu，版权归原作者所有，如需转载，请联系作者。

原文链接：chenyu.blog.csdn.net/article/details/103640286