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copied!<p>When the Servlet container starts, it:</p> <ol> <li>reads <code>web.xml</code>;</li> <li>finds the declared Servlets in the classpath; and</li> <li>loads and instantiates each Servlet <strong>only once</strong>.</li> </ol> <p>Roughly, like this:</p> <pre><code>String urlPattern = parseWebXmlAndRetrieveServletUrlPattern(); String servletClass = parseWebXmlAndRetrieveServletClass(); HttpServlet servlet = (HttpServlet) Class.forName(servletClass).newInstance(); servlet.init(); servlets.put(urlPattern, servlet); // Similar to a map interface. </code></pre> <p>Those Servlets are stored in memory and reused every time the request URL matches the Servlet's associated <code>url-pattern</code>. The servlet container then executes code similar to:</p> <pre><code>for (Entry<String, HttpServlet> entry : servlets.entrySet()) { String urlPattern = entry.getKey(); HttpServlet servlet = entry.getValue(); if (request.getRequestURL().matches(urlPattern)) { servlet.service(request, response); break; } } </code></pre> <p>The <a href="http://java.sun.com/javaee/5/docs/api/javax/servlet/http/HttpServlet.html#service%28javax.servlet.http.HttpServletRequest,%20javax.servlet.http.HttpServletResponse%29" rel="noreferrer"><code>GenericServlet#service()</code></a> on its turn decides which of the <code>doGet()</code>, <code>doPost()</code>, etc.. to invoke based on <a href="http://java.sun.com/javaee/5/docs/api/javax/servlet/http/HttpServletRequest.html#getMethod%28%29" rel="noreferrer"><code>HttpServletRequest#getMethod()</code></a>.</p> <p>You see, the servletcontainer reuses the <strong>same servlet instance</strong> for every request. In other words: the servlets are shared among <em>every request</em>. That's why it's extremely important to write servlet code the threadsafe manner --which is actually simple: just do <strong>not</strong> assign request or session scoped data as servlet instance variables, but just as method local variables. E.g.</p> <pre><code>public class MyServlet extends HttpServlet { private Object thisIsNOTThreadSafe; protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { Object thisIsThreadSafe; thisIsNOTThreadSafe = request.getParameter("foo"); // BAD!! Shared among all requests! thisIsThreadSafe = request.getParameter("foo"); // OK, this is thread safe. } } </code></pre>

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