1 同步包装器
TreeMap不是线程安全性的。如果多线程并发访问一个Map对象，当某些线程对该map对象进行结构上的修改时，该map必须要实现同步（结构上的修改是指添加或删除操作，如果仅仅是修改value值，则不是结构上的修改）。可以使用java同步包装器Collections.synchronizedSortedMap实现同步功能。什么是同步包装器？同步线程包装器是为“原集合对象”的实际操作找一个代理对象，代理在“原集合对象”的一切功能之上又增加了同步功能（只是对这个“代理对象”上的操作同步，“原集合对象”上的操作非同步）。java同步线程包装器：
public static Collection synchronizedCollection(Collection c);
public static Set synchronizedSet(Set s);
public static List synchronizedList(List list);
public static Map synchronizedMap(Map m);
public static SortedSet synchronizedSortedSet(SortedSet s);
public static SortedMap synchronizedSortedMap(SortedMap m);
java的同步线程包装器是有条件的同步，只有对集合的原子粒度的操作才同步。对于有并发情况的迭代操作（如Iterator是非线程安全性的），因为迭代操作是通过对对像集的调用间接操作原对像，所以在迭代时要对迭代的对像实现再同步。
SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
...
Set s = m.keySet(); // Needn't be in synchronized block
...
synchronized (m) { // Synchronizing on m, not s!
Iterator i = s.iterator(); // Must be in synchronized block
while (i.hasNext())
foo(i.next());
}
2 synchronizedMap源码
SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
synchronizedSortedMap源码如下：
/**
* Returns a synchronized (thread-safe) sorted map backed by the specified
* sorted map. In order to guarantee serial access, it is critical that
* <strong>all</strong> access to the backing sorted map is accomplished
* through the returned sorted map (or its views).<p>
*
* It is imperative that the user manually synchronize on the returned
* sorted map when iterating over any of its collection views, or the
* collections views of any of its <tt>subMap</tt>, <tt>headMap</tt> or
* <tt>tailMap</tt> views.
* <pre>
* SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
* ...
* Set s = m.keySet(); // Needn't be in synchronized block
* ...
* synchronized (m) { // Synchronizing on m, not s!
* Iterator i = s.iterator(); // Must be in synchronized block
* while (i.hasNext())
* foo(i.next());
* }
* </pre>
* or:
* <pre>
* SortedMap m = Collections.synchronizedSortedMap(new TreeMap());
* SortedMap m2 = m.subMap(foo, bar);
* ...
* Set s2 = m2.keySet(); // Needn't be in synchronized block
* ...
* synchronized (m) { // Synchronizing on m, not m2 or s2!
* Iterator i = s.iterator(); // Must be in synchronized block
* while (i.hasNext())
* foo(i.next());
* }
* </pre>
* Failure to follow this advice may result in non-deterministic behavior.
*
* <p>The returned sorted map will be serializable if the specified
* sorted map is serializable.
*
* @param m the sorted map to be "wrapped" in a synchronized sorted map.
* @return a synchronized view of the specified sorted map.
*/
public static <K,V> SortedMap<K,V> synchronizedSortedMap(SortedMap<K,V> m) {
return new SynchronizedSortedMap<>(m);
}
看到其调用了SynchronizedSortedMap的构造方法，SynchronizedSortedMap是Collections类中的内部类，SynchronizedSortedMap源码如下：
/**
* @serial include
*/
static class SynchronizedSortedMap<K,V>
extends SynchronizedMap<K,V>
implements SortedMap<K,V>
{
private static final long serialVersionUID = -8798146769416483793L;
private final SortedMap<K,V> sm;
SynchronizedSortedMap(SortedMap<K,V> m) {
super(m);
sm = m;
}
SynchronizedSortedMap(SortedMap<K,V> m, Object mutex) {
super(m, mutex);
sm = m;
}
public Comparator<? super K> comparator() {
synchronized (mutex) {return sm.comparator();}
}
public SortedMap<K,V> subMap(K fromKey, K toKey) {
synchronized (mutex) {
return new SynchronizedSortedMap<>(
sm.subMap(fromKey, toKey), mutex);
}
}
public SortedMap<K,V> headMap(K toKey) {
synchronized (mutex) {
return new SynchronizedSortedMap<>(sm.headMap(toKey), mutex);
}
}
public SortedMap<K,V> tailMap(K fromKey) {
synchronized (mutex) {
return new SynchronizedSortedMap<>(sm.tailMap(fromKey),mutex);
}
}
public K firstKey() {
synchronized (mutex) {return sm.firstKey();}
}
public K lastKey() {
synchronized (mutex) {return sm.lastKey();}
}
}
上述代码中用同步方法synchronized加锁的mutex是父类SynchronizedMap中的属性，代表当前对象，SynchronizedMap的源码如下：
/**
* Returns a synchronized (thread-safe) map backed by the specified
* map. In order to guarantee serial access, it is critical that
* <strong>all</strong> access to the backing map is accomplished
* through the returned map.<p>
*
* It is imperative that the user manually synchronize on the returned
* map when iterating over any of its collection views:
* <pre>
* Map m = Collections.synchronizedMap(new HashMap());
* ...
* Set s = m.keySet(); // Needn't be in synchronized block
* ...
* synchronized (m) { // Synchronizing on m, not s!
* Iterator i = s.iterator(); // Must be in synchronized block
* while (i.hasNext())
* foo(i.next());
* }
* </pre>
* Failure to follow this advice may result in non-deterministic behavior.
*
* <p>The returned map will be serializable if the specified map is
* serializable.
*
* @param m the map to be "wrapped" in a synchronized map.
* @return a synchronized view of the specified map.
*/
public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) {
return new SynchronizedMap<>(m);
}
/**
* @serial include
*/
private static class SynchronizedMap<K,V>
implements Map<K,V>, Serializable {
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<>(m.keySet(), mutex);
return keySet;
}
}
public Set<Map.Entry<K,V>> entrySet() {
synchronized (mutex) {
if (entrySet==null)
entrySet = new SynchronizedSet<>(m.entrySet(), mutex);
return entrySet;
}
}
public Collection<V> values() {
synchronized (mutex) {
if (values==null)
values = new SynchronizedCollection<>(m.values(), mutex);
return values;
}
}
public boolean equals(Object o) {
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();}
}
}