Java中的size(),put(),remove(),get()原子同步HashMap? [英] Are size(), put(), remove(), get() atomic in Java synchronized HashMap?
问题描述
我将Java Map声明为
Map< String,String> map = Collections.synchronizedMap(new HashMap< String,String>());
来处理并发问题,并在地图上同步所有操作。但是,当操作是原子操作时,我读取 synchronizedMap
时不需要同步。我检查了Java API,并且HashMap的文档似乎没有提及哪些是原子的,所以我不确定哪些是原子的。
我正在同步以下是对地图的调用:
map.size()
map.put()
map.remove()
map.get()
但是如果有些是原子的,似乎同步并不是必需的。哪个是原子的?
同步映射顾名思义是同步的。它的每一个操作都是原子的。
你可以认为你的同步映射的每个方法都是用<$ c
请注意,虽然单个操作是原子操作,但如果将它们组合起来,它们不再是原子操作,例如:
字符串值= map.get(key);
map.put(key,value +2);
不等同于您的自定义同步代码:
synchronized(map){
String value = map.get(key);
map.put(key,value +2);
}
而是:
synchronized(map){
String value = map.get(key);
}
同步(地图){
map.put(key,value +2);
}
I am declaring a Java Map as
Map<String, String> map = Collections.synchronizedMap(new HashMap<String, String>());
to deal with the concurrency issues, and synchronizing on the map for all the operations on it. However, I read that synchronization isn't necessary on a synchronizedMap
when the operations are atomic. I checked the Java API and the documentation of HashMap doesn't seem to mention which are atomic, so I'm not sure which are.
I'm synchronizing on the following calls to the map:
map.size()
map.put()
map.remove()
map.get()
But if some are atomic, it seems synchronization isn't necessary for these. Which are atomic?
A synchronized map as the name suggests is synchronized. Every operation on it is atomic in respect to any other operation on it.
You can think of it as if every method of your synchronized map is declared with a synchronized
keyword.
Please bear in mind that although individual operations are atomic, if you combine them they're no longer atomic, for instance:
String value = map.get("key");
map.put("key", value+"2");
is not equivalent to your custom synchronized code:
synchronized (map) {
String value = map.get("key");
map.put("key", value+"2");
}
but rather:
synchronized (map) {
String value = map.get("key");
}
synchronized (map) {
map.put("key", value+"2");
}
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