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POJava ConcurrentHashMap not thread safe.. wth?
 

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  1. POJava ConcurrentHashMap not thread safe.. wth?
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    <p>I was using HashMap before like </p> <pre><code> public Map&lt;SocketChannel, UserProfile&gt; clients = new HashMap&lt;SocketChannel, UserProfile&gt;(); </code></pre> <p>now I've switched to ConcurrentHashMap to avoid synchronized blocks and now i'm experiencing problems my server is heavily loaded with 200-400 concurrent clients every second which is expected to grow over time.</p> <p>which now looks like this</p> <pre><code>public ConcurrentHashMap&lt;SocketChannel, UserProfile&gt; clients = new ConcurrentHashMap&lt;SocketChannel, UserProfile&gt;(); </code></pre> <p>My server design works like this. I have a worker thread(s) for processing huge amounts of packets. Each packet is checked with a packetHandler sub-routine (not part of thread) pretty much any client can call it at anytime it's almost like static but it isn't.</p> <p>My whole server is mostly single threaded except for the packet processing portion.</p> <p>Anyways so when someone uses a command like count up all clients online and get some information from them.</p> <p>It is also possible that clients can get disconnected and removed from <strong>ConcurrentHashMap</strong> while the counting is in progress (which causes my problems).</p> <p>Also I'd like to add some code here.</p> <pre><code> int txtGirls=0; int vidGirls=0; int txtBoys=0; int vidBoys=0; Iterator i = clients.values().iterator(); while (i.hasNext()) { UserProfile person = (UserProfile)i.next(); if(person != null) { if(person.getChatType()) { if(person.getGender().equals("m")) vidBoys++; else //&lt;-- crash occurs here. vidGirls++; } else if(!person.getChatType()) { if(person.getGender().equals("m")) txtBoys++; else txtGirls++; } } } </code></pre> <p>I mean of course i'm going to fix it by adding a try-catch Exception inside the Iterator to skip these null clients.</p> <p>But what I don't understand if it checks above if(person != null) shouldn't the code nested automatically works..</p> <p>if it doesn't means it got removed while it was iterating which should be impossible since it's thread safe wtf?</p> <p>What should I do? or is try-catch Exception the best way?</p> <p>Here is the exception</p> <pre><code>java.lang.NullPointerException at Server.processPackets(Server.java:398) at PacketWorker.run(PacketWorker.java:43) at java.lang.Thread.run(Thread.java:636) </code></pre> <p>The processPackets contains the code above. and the comment indicates the line count #</p> <p>Thanks for enlightening me.</p>
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    1. COHave you tried Collections.synchronizedMap(map) ?
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    2. CO"TF" is that ConcurrentHashMap **is** thread-safe, but you are expecting something else on top of thread safety.
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    3. COit would also help if you explained what "crash occurs here" means. What kind of "crash"? What's the exception?
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