<s1 title="Servlet">

<p>Servlets are Java classes which service HTTP requests.  The only
requirement for writing a servlet is that it implements the
javax.servlet.Servlet interface.</p>

<p>Servlets are loaded from the classpath like all Java classes.
Normally, users put servlets in <var/WEB-INF/classes/> so Resin will
automatically reload them when they change.</p>

<p><a href='jsp.xtp'>JSP</a> pages are implemented as
Servlets, and tend to be more efficient for pages with lots of text.</p>

<s2 title='Examples'>

<s3 title='Configuring the web.xml'>

<p>The following is a complete working web.xml to run this example.</p>

<p>The <var/servlet-mapping/> tells Resin that the URL
<var//hello/> should invoke the <var/hello-world/> servlet.</p>

<p>The <var/servlet/> tells Resin that <var/hello-world/> uses the
<var/test.HelloWorld/> class and that the value of the <var/greeting/>
init parameter is <var/Hello World/>.</p>

<example title='WEB-INF/web.xml'>
&lt;web-app>
  &lt;servlet-mapping url-pattern='/hello'
                   servlet-name='hello-world'/>

  &lt;servlet servlet-name='hello-world'
           servlet-class='test.HelloWorld'>
    &lt;init-param greeting='Hello, World'/>
&lt;/web-app>
</example>

<p>The Java code, <var/HelloWorld.java/> belongs in</p>
<def>
$app-dir/WEB-INF/classes/test/HelloWorld.java
</def>

<p>Or, if you're compiling the servlet yourself, the class file belongs in</p>
<def>
$app-dir/WEB-INF/classes/test/HelloWorld.class
</def>

<p>Following is the actual servlet code.  It just prints a trivial
HTML page filled with the greeting specified in the web.xml.</p>

<p><var/init()/> and <var/destroy()/> are included mostly for
illustration.  Resin will call <var/init()/> when it starts the servlet
and <var/destroy/> before Resin destroys it.</p>

<example>
package test;

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class HelloWorld extends HttpServlet {
  private String greeting;

  public void init()
    throws ServletException
  {
    greeting = getInitParameter("greeting");
  }

  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
    throws ServletException, IOException
  {
    PrintWriter out = response.getWriter();

    out.println("&lt;title>" + greeting + "&lt;/title>");
    out.println("&lt;h1>" + greeting + "&lt;/h1>");
  }
  
  public void destroy()
  {
    // nothing to do
  }
}
</example>

</s3>

<s3 title='Servlet Example for JSP Programmers'>

<p>Because Resin compiles JSP pages into servlets, programmers familiar
with JSP can start writing servlets fairly easily.  The following template
can be used to see how to write a servlet for someone familiar with JSP.
</p>

<example>
package test;

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class HelloWorld extends HttpServlet {
  public void service(HttpServletRequest request,
  		      HttpServletResponse response)
    throws ServletException, IOException
  {
    PrintWriter out = response.getWriter();
    ServletContext application = getServletContext();
    HttpSession session = request.getSession();

    try {
      // <var/code goes here/>

      // The equivalent of jsp:include:
      // request.getRequestDispatcher("/inc.jsp").include(request, response);
    } catch (ServletException e) {
      throw e;
    } catch (Exception e) {
      throw new ServletException(e);
    }
  }
}
</example>

</s3>

<s3 title='Using Databases from a Servlet'>

<p>The following is a sample design pattern for getting new database
connections.  The <var/try ... finally/> block is very important.  Without
the close in the finally block, Resin's database pool can loose connections.</p>

<p>Configuring the database is described in the
<a href="db-config.xtp">database configuration</a> page.</p>

<example title="TestDatabase.java">
package test;

import java.io.*;

import java.sql.*;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.naming.*;
import javax.sql.*;

public class TestDatabase extends HttpServlet {
  DataSource pool;

  public void init()
    throws ServletException
  {
    try {
      Context env = (Context) new InitialContext().lookup("java:comp/env");

      pool = (DataSource) env.lookup("jdbc/test");

      if (pool == null)
        throw new ServletException("`jdbc/test' is an unknown DataSource");
    } catch (NamingException e) {
      throw new ServletException(e);
    }
  }

  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
    throws IOException, ServletException
  {
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();

    Connection conn = null;
    try {
      conn = pool.getConnection();

      // <var/code for the servlet using the database goes here/>

      rs.close();
      stmt.close();
    } catch (SQLException e) {
      throw new ServletException(e);
    } finally {
      try {
        if (conn != null)
          conn.close();
      } catch (SQLException e) {
      }
    }
  }
}
</example>

</s3>

</s2>

<s2 title="Servlet Configuration">

<defun title='init-param'>
<p>Initializes servlet variables. <code/servlet-param/>
defines initial values for <code/getServletConfig().getInitParameter("foo")/>.
</p>

<p>The full servlet 2.2 syntax is supported and allows a simple shortcut.</p>
<example>
&lt;web-app id='/'&gt;

&lt;servlet servlet-name='test.HelloWorld'&gt;
  &lt;init-param foo='bar'/&gt;

  &lt;init-param&gt;
    &lt;param-name&gt;baz&lt;/param-name&gt;
    &lt;param-value&gt;value&lt;/param-value&gt;
  &lt;/init-param&gt;
&lt;/servlet&gt;

&lt;/web-app>
</example>
</defun>

<defun title="load-on-startup">
<p>If present, starts the servlet when the server starts.</p>
<example>
&lt;web-app id='/'>

&lt;servlet servlet-name='hello'
         servlet-class='test.HelloWorld'&gt;
  &lt;load-on-startup/&gt;
&lt;/servlet&gt;

&lt;/web-app&gt;
</example>
</defun>

<defun title='run-at' version='Resin 1.1'>
<p>If present, calls the servlet's <var/service()/> method
at the specified times.
&lt;run-at> lets servlet writers execute periodic tasks without worrying
about creating a new Thread.</p>

<p>The value is a list of 24-hour times when the
servlet should be automatically executed.  To run the servlet every 6
hours, you could use:</p>

<example>
&lt;servlet servlet-name='test.HelloWorld'&gt;
  &lt;run-at&gt;0:00, 6:00, 12:00, 18:00&lt;/run-at&gt;
&lt;/servlet&gt;
</example>

<p>If the hour is omitted, the servlet runs every hour at the
specified minute.  To run the server every 15 minutes, you could use:
</p>

<example>
&lt;servlet servlet-name='test.HelloWorld'&gt;
  &lt;run-at&gt;:00, :15, :30, :45&lt;/run-at&gt;
&lt;/servlet&gt;
</example>
</defun>

<defun title='servlet'>
<p>Defines a servlet alias for later mapping.</p>

<deftable>
<tr><th>Attribute<th>Description
<tr><td>servlet-name<td>The servlet's name (alias)
<tr><td>servlet-class<td>The servlet's class (defaults to servlet-name)
<tr><td>init-param<td>Initialization parameters
<tr><td>load-on-startup<td>Initializes the servlet when the server starts.
<tr><td>run-at<td>Times to execute the servlet automatically
</deftable>

<p>The following example defines a servlet alias 'hello'</p>
<example>
&lt;web-app id='/'&gt;

&lt;servlet-mapping url-pattern='/hello.html'
                 servlet-name='hello'/&gt;

&lt;servlet servlet-name='hello'
         servlet-class='test.HelloWorld'&gt;
  &lt;init-param title='Hello, World'/&gt;
&lt;/servlet&gt;

&lt;servlet servlet-name='cron'
         servlet-class='test.DailyChores'&gt;
  &lt;run-at&gt;3:00&lt;/run-at&gt;
&lt;/servlet&gt;

&lt;/web-app&gt;
</example>
</defun>

<defun title='servlet-class'>
<p>Class of the servlet.  The CLASSPATH for servlets includes
the WEB-INF/classes directory and all jars in the WEB-INF/lib directory.
</p>
</defun>

<defun title='servlet-name'>
<p>Alias of the servlet.</p>
</defun>

<defun title="servlet-mapping">
<p>Maps from a URL to the servlet to execute.  The servlet-mapping
has a url-pattern to match the URL and a servlet-name to match the
configured servlet.</p>

<example title="typical servlet-mapping">
&lt;servlet>
  &lt;servlet-name>hello&lt;/servlet-name>
  &lt;servlet-class>test.HelloServlet&lt;/servlet-class>
&lt;/servlet>

&lt;servlet-mapping>
  &lt;url-pattern>/hello/*&lt;/url-pattern>
  &lt;servlet-name>hello&lt;/servlet-name>
&lt;/servlet-mapping>
</example>

<p>Resin allows for a shortcut combining the servlet and
the servlet mapping:</p>

<example title="shortcut servlet-mapping">
&lt;servlet-mapping url-pattern="/hello/*"
                 servlet-class="test.HelloServlet"/>
</example>
</defun>

<defun title="url-pattern" version="Servlet 2.2">
<p>Matches a set of URLs for servlet-mapping.</p>

<deftable>
<tr><th width='25%'>Pattern<th>Description
<tr><td>/foo/bar.html<td>Matches exactly the /foo/bar.html URL.
<tr><td>/foo/*<td>Matches /foo and any children
<tr><td>*.foo<td>Matches any URL with a .foo extension
<tr><td>/<td>Replaces the default servlet.
</deftable>

<p><var///> defines a default handler and <var//*/> defines a prefix handler.
<var//*/> will override extension handlers like <var/*.foo/>. <var///>
will only be used if no other pattern matches.</p>

<p>No default.  Either url-pattern or url-regexp is required.</p>
</defun>

</s2>
</s1>

