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copied!<p>The DB-independent approach would be to let the <code>User</code> have a <code>static Map<User, HttpSession></code> variable and implement <a href="https://docs.oracle.com/javaee/7/api/javax/servlet/http/HttpSessionBindingListener.html" rel="noreferrer"><code>HttpSessionBindingListener</code></a> (and <a href="http://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#equals-java.lang.Object-" rel="noreferrer"><code>Object#equals()</code></a> and <a href="http://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#hashCode--" rel="noreferrer"><code>Object#hashCode()</code></a>). This way your webapp will still function after an unforeseen crash which may cause that the DB values don't get updated (you can of course create a <code>ServletContextListener</code> which resets the DB on webapp startup, but that's only more and more work).</p> <p>Here's how the <code>User</code> should look like:</p> <pre><code>public class User implements HttpSessionBindingListener { // All logins. private static Map<User, HttpSession> logins = new ConcurrentHashMap<>(); // Normal properties. private Long id; private String username; // Etc.. Of course with public getters+setters. @Override public boolean equals(Object other) { return (other instanceof User) && (id != null) ? id.equals(((User) other).id) : (other == this); } @Override public int hashCode() { return (id != null) ? (this.getClass().hashCode() + id.hashCode()) : super.hashCode(); } @Override public void valueBound(HttpSessionBindingEvent event) { HttpSession session = logins.remove(this); if (session != null) { session.invalidate(); } logins.put(this, event.getSession()); } @Override public void valueUnbound(HttpSessionBindingEvent event) { logins.remove(this); } } </code></pre> <p>When you login the <code>User</code> as follows:</p> <pre><code>User user = userDAO.find(username, password); if (user != null) { sessionMap.put("user", user); } else { // Show error. } </code></pre> <p>then it will invoke the <code>valueBound()</code> which will remove any previously logged in user from the <code>logins</code> map and invalidate the session.</p> <p>When you logout the <code>User</code> as follows:</p> <pre><code>sessionMap.remove("user"); </code></pre> <p>or when the session is timed out, then the <code>valueUnbound()</code> will be invoked which removes the user from the <code>logins</code> map.</p>

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