The first request to a JSP typically has a processing delay as the JSP container compiles the JSP. Specifically, the container generates Java source code for a servlet and compiles the source code to a class file. You can prevent your users from experiencing this delay by compiling the JSPs before they are requested.

The Tomcat manual describes how to compile JSPs at build time. An Ant target runs the Japser JSP Engine to convert JSPs to servlet code. Another Ant target compiles the servlet code to class files. Servlet declarations and mappings for the generated servlets must be merged into the web.xml file. I’m going to present another solution that compiles JSPs when the application starts.

The JSP specification says a container must accept the request parameter jsp_precompile on a request to a JSP. The parameter can have values true or false, or no value at all. If the parameter has value true or no value, then it is a suggestion to the container to compile the JSP. Even if container chooses to ignore this suggestion, it must not let the page process the request.

A custom servlet context listener compiles all JSPs in the Web application when the application starts. To use it, define a listener in the web.xml file:

<listener>
  <listener-class>com.github.pukkaone.jsp.JspCompileListener</listener-class>
</listener>

Details

The JspCompileListener class implements the ServletContextListener interface so it receives a notification when the Web application starts.

public class JspCompileListener implements ServletContextListener {
    private ServletContext servletContext;
    private HttpServletRequest request = createHttpServletRequest();
    private HttpServletResponse response = createHttpServletResponse();

The contextInitialized method receives a notification when the application starts.

    public void contextInitialized(ServletContextEvent event) {
        servletContext = event.getServletContext();

        compileJspsInDirectory("/");
    }

The compileJspsInDirectory method finds and compiles all JSPs in a directory. It recursively descends into subdirectories and processes JSPs found there.

    @SuppressWarnings("unchecked")
    private void compileJspsInDirectory(String dirPath) {
        Set<String> paths = servletContext.getResourcePaths(dirPath);
        for (String path : paths) {
            if (path.endsWith(".jsp")) {
                RequestDispatcher requestDispatcher =
                        servletContext.getRequestDispatcher(path);
                if (requestDispatcher == null) {
                    // Should have gotten a RequestDispatcher for the path
                    // because the path came from the getResourcePaths() method.
                    throw new Error(path + " not found");
                }

                try {
                    servletContext.log("Compiling " + path);
                    requestDispatcher.include(request, response);
                } catch (Exception e) {
                    servletContext.log("include", e);
                }
            } else if (path.endsWith("/")) {
                // Recursively process subdirectories.
                compileJspsInDirectory(path);
            }
        }
    }

The call to the RequestDispatcher.include method sends a request to a JSP. One of the arguments must be a HttpServletRequest implementation that returns jsp_precompile for the query string. I could have written a class providing stub implementations for the HttpServletRequest methods, but the interface has so many methods that the code to create a JDK dynamic proxy implementing the interface is more concise. Another JDK dynamic proxy implements the HttpServletResponse interface in a similar fashion.

    private HttpServletRequest createHttpServletRequest() {
        InvocationHandler handler = new InvocationHandler() {
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                if (method.getName().equals("getQueryString")) {
                    return "jsp_precompile";
                }
                return null;
            }
        };

        return (HttpServletRequest) Proxy.newProxyInstance(
                getClass().getClassLoader(),
                new Class<?>[] { HttpServletRequest.class },
                handler);
    }