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copied!<p>You can benefit from an <a href="http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/atomic/AtomicReference.html" rel="nofollow">AtomicReference</a> in cases where multiple threads access (common) configuration data that can be refreshed at any given moment. Refreshing usually happens by one thread (after a certain event or a certain period), but if multiple threads can refresh at the same time, you need some extra locking (see code example below).</p> <p>The threads getting the configuration data must re-use the returned instance of the configuration data where necessary to prevent mixing old configuration data with new configuration data. This usually means creating a separate class with the configuration data and methods that use the configuration data. But you'll need that anyway to work with the AtomicReference.</p> <p>The following code example should give you an idea about how to use an AtomicReference:</p> <pre><code>import java.util.HashMap; import java.util.concurrent.atomic.AtomicInteger; import java.util.concurrent.atomic.AtomicReference; public class Q20708889 { private static final Object reloadLock = new Object(); private static final AtomicInteger reloadCount = new AtomicInteger(); private AtomicReference<DbSettings> dbSettingsRef = new AtomicReference<DbSettings>(); public DbSettings getDbSettings() { DbSettings dbs = dbSettingsRef.get(); if (dbs == null) { dbs = reload(); } return dbs; } public DbSettings reload() { DbSettings dbs = null; int rld = reloadCount.get(); synchronized (reloadLock) { if (rld < reloadCount.get()) { // Reload was already done dbs = dbSettingsRef.get(); } else { reloadCount.incrementAndGet(); dbs = new DbSettings(); dbs.load(); dbSettingsRef.set(dbs); } } return dbs; } class DbSettings { private HashMap<String, HashMap<String, String>> theSettings; public void load() { theSettings = new HashMap<String, HashMap<String,String>>(); // getting resources from the database } public String getValueSettings(String groupName, String keyName) { String value = null; HashMap<String, String> group = theSettings.get(groupName); if (group != null && !group.isEmpty()) { value = group.get(keyName); } return value; } } } </code></pre>

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