线程安全LinkedHashMap没有Collections.synchronized [英] thread safe LinkedHashMap without Collections.synchronized

问题描述

我正在使用LinkedHashMap，并且环境是多线程的，因此这个结构需要是线程安全的。在特定事件期间，我需要读取整个地图推送到数据库并清除所有。

I am using a LinkedHashMap and the environment is multi threaded so this structure needs to be thread safe. During specific events I need to read the entire map push to db and clear all.

大部分时间只写入此地图。此地图有50个条目限制。

Most of time only writes happen to this map. This map has a limit 50 entries.

我使用的是Oracle MAF，它没有Collections.syncronizedMap可用。那么，我需要在synchronized块中放置什么东西以确保写入和读取不会让我遇到concurrentModificationException等。

I am using Oracle MAF and it does not have Collections.syncronizedMap available. So, what are things I need to put in synchronized blocks to make sure writing and reading doesn't hit me concurrentModificationException etc

几个要求：

1. 我需要像循环队列那样表现它，以便重写LinkedHashMap的removeEldestEntry方法。


2. 我需要保留订单

推荐答案

那么，我需要在同步块中放置什么东西以确保写入和读取不会遇到我的concurrentModificationException等

So, what are things I need to put in synchronized blocks to make sure writing and reading doesn't hit me concurrentModificationException etc

所有方法调用都应该在同步块中。

Everything method call should be in a synchronized block.

棘手的是使用迭代器，因为你必须在迭代器的生命周期中持有锁。例如

The tricky one being the use of an Iterator, as you have to hold the lock for the life of the Iterator. e.g.

// pre Java 5.0 code
synchronized(map) { // the lock has to be held for the whole loop.
    for(Iterator iter = map.entrySet().iterator(); iter.hashNext(); ) {
         Map.Entry entry = iter.next();
         String key = (String) entry.getKey();
         MyType value = (MyType) entry.getValue();
         // do something with key and value.
    }
}

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