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Old September 22nd, 2006, 05:38 AM
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Default Sharing a data structure across multiple threads

Hi, I would like to share a data structure (e.g. HashMap) across multiple threads. Each thread will be adding an entry to the HashMap. A part of the code is:
Code:
HashMap info = new HashMap();
File[] contents = mainDir.listFiles();

for (int i = 0; i < contents.length; i++) {           
                    spawnThread st = new spawnThread(contents[i], info);
                                        Thread t = new Thread(st);
                    t.start();
 }


How can I get it done properly?


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Old October 9th, 2006, 08:32 PM
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Default

It looks like you've already passed the info HashMap to the new thread, so therefore the parent thread and the child thread should both have access to it. However, HashMaps are not thread-safe. Therefore you should have a synchronized() block around any code that does gets or sets on the HashMap.

E.g.:

synchronized(info)
{
    info.put("Foo", new Boolean(true));
}

synchronized(info)
{
    boolean foo = ((Boolean)info.get("Foo")).booleanValue();
}


Jon Emerson
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