Can I serve JSPs from inside a JAR in lib, or is there a workaround?

Question

I have a web application deployed as a WAR file in Tomcat 7. The application is build as a multi-module project:

• core - packaged as JAR, contains most of the backend code
• core-api - packaged as JAR, contains interfaces toward core
• webapp - packaged as WAR, contains frontend code and depends on core
• customer-extensions - optional module, packaged as JAR

Normally, we can put our JSP files in the webapp project, and reference them relative to the context:

/WEB-INF/jsp/someMagicalPage.jsp

The question is what we do about JSP files that are specific to the customer-extensions project, that should not always be included in the WAR. Unfortunately, I cannot refer to JSPs inside JAR files, it appears. Attempting classpath:jsp/customerMagicalPage.jsp results in a file not found in the JspServlet, since it uses ServletContext.getResource().

Traditionally, we "solved" this having maven unpack the customer-extensions JAR, locate the JSPs, and put them in the WAR when building it. But an ideal situation is where you just drop a JAR in the exploded WAR in Tomcat and the extension is discovered - which works for everything but the JSPs.

Is there anyway to solve this? A standard way, a Tomcat-specific way, a hack, or a workaround? For example, I've been thinking of unpacking the JSPs on application startup...

Answer 1

Servlet 3.0 which Tomcat 7 supports includes the ability to package jsps into a jar.

You need to:

• place your jsps in META-INF/resources directory of your jar
• optionally include a web-fragment.xml in the META-INF directory of your jar
• place the jar in WEB-INF/lib directory of your war

You should then be able to reference your jsps in your context. For example if you have a jsp META-INF/resources/test.jsp you should be able reference this at the root of your context as test.jsp

Answer 2

As a workaround, I created a class that opens up a jar file, finds files matching a certain pattern, and extracts those files to a given location relative to the context path.

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.Enumeration;
import java.util.jar.JarEntry;
import java.util.jar.JarFile;

import javax.annotation.PostConstruct;
import javax.servlet.ServletContext;

import org.springframework.util.AntPathMatcher;
import org.springframework.web.context.ServletContextAware;

/**
 * Allows extraction of contents of a JAR file. All files matching a given Ant path pattern will be extracted into a
 * specified path.
 */
public class JarFileResourcesExtractor implements ServletContextAware {

    private String resourcePathPattern;
    private String jarFile;
    private String destination;
    private ServletContext servletContext;
    private AntPathMatcher pathMatcher = new AntPathMatcher();

    /**
     * Creates a new instance of the JarFileResourcesExtractor
     * 
     * @param resourcePathPattern
     *            The Ant style path pattern (supports wildcards) of the resources files to extract
     * @param jarFile
     *            The jar file (located inside WEB-INF/lib) to search for resources
     * @param destination
     *            Target folder of the extracted resources. Relative to the context.
     */
    private JarFileResourcesExtractor(String resourcePathPattern, String jarFile, String destination) {
        this.resourcePathPattern = resourcePathPattern;
        this.jarFile = jarFile;
        this.destination = destination;
    }

    /** 
     * Extracts the resource files found in the specified jar file into the destination path
     * 
     * @throws IOException
     *             If an IO error occurs when reading the jar file
     * @throws FileNotFoundException
     *             If the jar file cannot be found
     */
    @PostConstruct
    public void extractFiles() throws IOException {
        try {
            String path = servletContext.getRealPath("/WEB-INF/lib/" + jarFile);
            JarFile jarFile = new JarFile(path);

            Enumeration<JarEntry> entries = jarFile.entries();
            while (entries.hasMoreElements()) {
                JarEntry entry = entries.nextElement();
                if (pathMatcher.match(resourcePathPattern, entry.getName())) {
                    String fileName = entry.getName().replaceFirst(".*\\/", "");
                    File destinationFolder = new File(servletContext.getRealPath(destination));
                    InputStream inputStream = jarFile.getInputStream(entry);
                    File materializedJsp = new File(destinationFolder, fileName);
                    FileOutputStream outputStream = new FileOutputStream(materializedJsp);
                    copyAndClose(inputStream, outputStream);
                }
            }

        }
        catch (MalformedURLException e) {
            throw new FileNotFoundException("Cannot find jar file in libs: " + jarFile);
        }
        catch (IOException e) {
            throw new IOException("IOException while moving resources.", e);
        }
    }

    @Override
    public void setServletContext(ServletContext servletContext) {
        this.servletContext = servletContext;
    }

    public static int IO_BUFFER_SIZE = 8192;

    private static void copyAndClose(InputStream in, OutputStream out) throws IOException {
        try {
            byte[] b = new byte[IO_BUFFER_SIZE];
            int read;
            while ((read = in.read(b)) != -1) {
                out.write(b, 0, read);
            }
        } finally {
            in.close();
            out.close();
        }
    }
}

And then I configure it as a bean in my Spring XML:

<bean id="jspSupport" class="se.waxwing.util.JarFileResourcesExtractor">
   <constructor-arg index="0" value="jsp/*.jsp"/>
   <constructor-arg index="1" value="myJarFile-1.1.0.jar"/>
   <constructor-arg index="2" value="WEB-INF/classes/jsp"/>
</bean>

It's not an optimal solution to a really annoying problem. The question now becomes, will the guy who maintains this code come and murder me while I sleep for doing this?