1 問題

我們知道hashMap線程是不安全的，一般而言，我們怎么創建線程安全的HashMap呢？

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2 解決辦法

我們可以使用Collections.synchronizedMap來創建HashMap,如下

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

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3?Collections.synchronizedMap源碼部分實現

我們先看synchronizedMap如果創建

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

我們看下具體的SynchronizedMap類的部分

private static class SynchronizedMap<K,V>implements Map<K,V>, Serializable {// use serialVersionUID from JDK 1.2.2 for interoperabilityprivate static final long serialVersionUID = 1978198479659022715L;private final Map<K,V> m;     // Backing Mapfinal Object      mutex;    // Object on which to synchronizeSynchronizedMap(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 實現了Map接口的代理類，該類中對Map接口中的方法還是使用synchronized 同步關鍵字來保證對Map的操作是線程安全的。

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

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