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    <p>If you want the <code>ArrayList</code>s to store per-client info and access it by client ID, I suggest just using a <a href="http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ConcurrentHashMap.html" rel="nofollow"><code>java.util.concurrent.ConcurrentHashMap</code></a> with the client ID as key, as that is very straightforward to implement.</p> <p>If you really need an <code>ArrayList</code>, I would hide it as an implementation detail in the server code. Then you would call some server methods from your <code>Runnable</code>s to update the <code>ArrayList</code>. In this way you can start with something very simple like:</p> <pre><code>public class Server { ... private ArrayList&lt;Info&gt; infos = new ArrayList&lt;Info&gt;(); ... // Create one method for each *abstract* operation // that you need to perform on the ArrayList // (Do *NOT* implement read / write primitives // unless that is exactly what you need. The idea is // that all synchronization that you might need on // the ArrayList happens inside those methods, so // the Runnables are unaware of the implementation // you haven chosen.) // Here I have used example operations that are // a bit more complex than read / write primitives // so the intention is more clear. public void clearInfo(int pos) { synchronized (infos) { infos.set(pos, null); } } public Info updateInfo(int pos, Info info) { Info oldInfo = null; synchronized (infos) { oldInfo = infos.get(pos); infos.set(pos, info); } return oldInfo; } ... public Info readInfo(int pos) { Info info = null; synchronized (infos) { infos.get(pos); info.incrementReadCount(); } return null; } ... } ... public class ClientRunnable implements Runnable { ... private Server server; ... @Override public void run() { ... Info info = // getInfo(); int writePos = // getWritePos(); Info old = server.updateInfo(writePos, info); ... int readPos = // getReadPos(); Info newInfo = server.readInfo(readPos); ... } ... } </code></pre> <p>Then, when you profile your application, if you find contention in accesses to your list you can fine tune locking or even change your implementation to a lock-free one while minimizing impact in your code. Locking is quite fast in Java nowadays so in many cases this simple solution might be good enough.</p>
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