How to share variables between two threads in Java?

Question

I am a beginner learning Java. Now I have a question, could someone help me solve it? :)

I have a class named Server which maintains a hashmap variable, and I it has two methods named send() and receive(). I want the two methods to run concurrently. The send() method will use the hashmap variable and the receive() method will change the hashmap variable.

Please help work it out. Thanks a lot!

prasanth · Answer 1 · 2013-05-03T06:52:59.113

1

You can use Collections.synchronizedMap(new HashMap());

Edit 1: code template

public class Server
{
    private static Map<String,String> map = Collections.synchronizedMap(new HashMap<String, String>());

    public void send()
    {
        //
        // Do some operation using map variable
    }

    public void receive()
    {
        // change the map variable as you want
        //
    }
}

edited May 03 '13 at 06:52

answered May 03 '13 at 05:28

prasanth

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could u give me a code template for my question? I really don't know how to write the code. Thx – user2193727 May 03 '13 at 06:46
thanks for your codes! And how can I let the two methods run at the same time? – user2193727 May 03 '13 at 07:03
You run your program in multithreaded environment?? – prasanth May 03 '13 at 07:21
yes, I am using this way to solve it: I create two thread classes and put send and receive methods in each of the run() method of the two thread classes. The execute the thread.start() method. is it right? – user2193727 May 03 '13 at 07:34

score 1 · Answer 2 · answered May 03 '13 at 05:29

1

Define the HashMap in the main thread that spawns the send and receive threads. Make sure you use a SynchronizedMap so that you don't run into race condition.

answered May 03 '13 at 05:29

Harshal Pandya

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could u give me a code template for my question? I really don't know how to write the code. Thx – user2193727 May 03 '13 at 06:42
this question has been asked before. You can checkout the questions listed at the top. If you're still having trouble, we can help you out. – Harshal Pandya May 03 '13 at 12:17

score 0 · Answer 3 · answered May 03 '13 at 05:28

0

Items are shared by default. What you'll need is some form of lock or mutex to control sharing and prevent the two threads from trying to modify it at the same time.

Or, you can just use ConcurrentHashMap

answered May 03 '13 at 05:28

Jean-Bernard Pellerin

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could u give me a code template for my question? I really don't know how to write the code. Thx – user2193727 May 03 '13 at 06:48

score 0 · Answer 4 · answered May 03 '13 at 05:28

0

There are two solutions, simplest one is to use thread safe hashmap i.e. ConcurrentHashMap. You need not to worry about the synchronization issues as JVM will take care of it. It will be good to go through the Javadoc for the same:

http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ConcurrentHashMap.html

Other way is to synchronize the send() and receive()methods or the portion of the methods where you update the hashmap.

answered May 03 '13 at 05:28

Juned Ahsan

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could u give me a code template for my question? I really don't know how to write the code. Thx – user2193727 May 03 '13 at 06:53

score 0 · Answer 5 · answered May 03 '13 at 05:31

0

Use COncurrentHashMap

It allows concurrent modification of the Map from several threads without the need to block them.

answered May 03 '13 at 05:31

Suresh Atta

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How to share variables between two threads in Java?

5 Answers5