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) {/*****/}


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