Here are some valid (as of the time of this writing) reasons for doing build-time precompilation of JSPs:
You need all the performance you can squeeze out of your webapp, and build-time compiled JSPs run faster than JSPs that are compiled inside Tomcat after the webapp is deployed. First, the Java class bytecodes generated in both situations should really be the same, and if they’re not exactly the same, the difference will be very small—certainly not worth a major deployment change such as is necessary to precompile the JSPs before deployment. Also, the time it takes Tomcat to compile the original JSP is usually small and occurs only on the first request of each JSP page after webapp deployment/redeployment. All other requests to the JSP pages serve from the compiled and loaded JSP servlet class (JSPs are compiled into Java servlets). But since JSPs that were compiled before webapp deployment are mapped to the URI space in the web.xml file, Tomcat is able to route requests to them slightly faster than if the JSP page were compiled at webapp runtime. This is because when JSP pages are compiled during runtime, the resulting servlets must be mapped to the URI space first by the regular URI mapper, which sends the request to the JspServlet, then the request is mapped to the requested JSP page by Tomcat’s JspServlet. Note that the runtime compiled JSPs are mapped via two layers of indirection (two distinct mappers), and precompiled JSPs are mapped via only the first layer of indirection. The performance difference comes down to the performance of the two different URI mapper situations. In the end, precompiled JSPs usually run about 4 percent faster. Precompiling them before webapp deployment would save you the small initial request compile time for each JSP page in your webapp, plus the 4 percent performance improvement on each subsequent request for a JSP page. In Tomcat 4.1.x, the runtime JSP request mapper was noticeably slower than the web.xml servlet mapper and made it worth precompiling JSPs before webapp deployment. That made JSP pages faster by approximately 12 percent or so in our tests. But, for Tomcat version 5.0.x and higher, this margin was reduced to about 4 percent or less.
By precompiling JSPs at webapp build or packaging time, the syntax for the JSPs is checked during the JSP compilation process, which means that you can be confident that the JSPs at least compile with no syntax errors before you deploy your webapp. This is great a way to avoid the situation where you have deployed your webapp to your production server(s) only to find out later that one of the JSPs had a syntax error, and it was found by the first user who requested that page. Also, finding errors in the development phase of the code allows the developer to find and fix the errors more rapidly; it shortens the development cycle. This will not prevent every kind of bug because a compiled JSP may still have runtime logic bugs, but at least you can catch all syntax errors in the development environment.
If you have a large number of JSP files in your webapp, each of which is somewhat long (hopefully you are not copying and pasting lots of content from one JSP page to many other JSP pages; you should instead make use of the JSP include feature), the initial compilation time for all the JSP pages combined could be significantly large. If so, you can save time on the production server by precompiling the JSPs before webapp deployment time. This is especially helpful if your traffic load is high, and your server responses would otherwise slow down quite a bit, while the server is initially compiling many JSP pages at the same time when the webapp is first started.
If you have a low server resource situation, for instance, if the Java VM is configured to use a small amount of RAM or the server does not have very many CPU cycles for Tomcat to use, you may not want to do any JSP compilation at all on the server. Instead, you could do the compilation in your development environment and deploy only compiled servlets, which would lighten the utilization of both memory and CPU time for the first request of each JSP file after each new copy of the webapp is deployed.
You are developing a JSP web application that you will sell to customer(s) whom you do not want to have the JSP source code. If you could give the customer(s) the webapp containing just compiled servlets, you could develop the webapp using the original JSPs, and ship it with the compiled JSP servlets. In this use case, precompiling before release to the customer is used as a source code obfuscation mechanism. Keep in mind, though, that compiled Java class files are relatively easy to decompile into readable Java source code, but (as of this writing) there is no way to decompile it all the way back into JSP source code.
Also, as of Tomcat version 5.5, you no longer need a JDK that has a built-in Java source compiler to serve runtime compiled JSPs. Tomcat versions 5.5 and higher come bundled with the Eclipse JDT compiler, which is a Java compiler that is itself written in pure Java. Because the JDT compiler is bundled as part of Tomcat, Tomcat can always compile JSPs into servlets, even when Tomcat is run on a JRE and not a JDK.
Example 4-4 is an Ant build file that you can use to compile your webapp’s JSP files at build time.
Example 4-4. The precompile-jsps.xml Ant build file
If you put this Ant build xmlcontent into a file named something such as precompile-jsps.xml, you can test it alongside any build.xml file you already have, and if you like it, you can merge it into your build.xml.
This build file will find all of your webapp’s JSP files, compile them into servlet classes, and generate servlet mappings for those JSP servlet classes. The servlet mappings it generates must go into your webapp’s WEB-INF/web.xmlfile, but it would be difficult to write an Ant build file that knows how to insert the servlet mappings into your web.xml file in a repeatable way every time the build file runs. Instead, we used an XML entity include so that the generated servlet mappings go into a new file every time the build file runs and that servlet mappings file can be inserted into your web.xmlfile via the XML entity include mechanism. To use it, your webapp’s WEB-INF/web.xmlmust have a special entity declaration at the top of the file, plus a reference to the entity in the content of the web.xml file where you want the servlet mappings file to be included. Here is how an empty servlet 2.5 webapp’s web.xml file looks with these modifications:
<!DOCTYPE jspc-webxml [ <!ENTITY jspc-webxml SYSTEM "jspc-web.xml"> ]>
<!-- We include the JspC-generated mappings here. --> &jspc-webxml;
<!-- Non-generated web.xml content goes here. -->
</web-app>
Make sure your webapp’s web.xml file has the inline DTD (theDOCTYPEtag) all the way at the top of the file and the servlet 2.5web-appschema declaration below that. Then, wherever you want to insert the generated servlet mappings in your web.xml file, put the entity reference&jspc-webxml;. Remember, the entity reference begins with an ampersand (&), then has the name of the entity, and ends with a semicolon (;).
To use the build file, just edit it and set all of the properties at the top to values that match your setup, and then run it like this:
$ ant -f pre-compile-jsps.xml Buildfile: pre-compile-jsps.xml
compile-jsp-servlets: [javac] Compiling 1 source file to /home/jasonb/myproject/webapp-dir/WEB-INF/ classes
BUILD SUCCESSFUL Total time: 7 seconds
Any JSP files you have in your webapp dir will be compiled into servlets, and when you deploy the webapp, the JSP page requests will be mapped to the compiled servlets. Ignore the “SEVERE: Invalid PUBLIC ID: null” message if you get it; it’s bogus. If you want to clean out the compiled servlets and their generated Java source and mappings, just execute the clean target like this:
$ ant -f pre-compile-jsps.xml clean
One thing that this build file does not do: remove all of the JSP files in your webapp after compiling them. We didn’t want you to accidentally delete your JSP files, so we intentionally left it out. Your own build file should do that before the webapp gets deployed. If you forget and accidentally leave the JSP files in the deployed webapp, none of them should get served by Tomcat because the web.xml file explicitly tells Tomcat to use the compiled servlet classes instead.
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