Can I create a custom classpath on a per application basis in Tomcat

Question

For some applications I use ZK, others Hibernate, other Apache Commons, etc.

I don't want to deploy a 75MB war file, just because it uses lots of libraries.

I don't want to add the libraries to my tomcat lib folder, or nor the classpath to it's configuration as I may have an old application using library x.1 and another application using library x.2

For this reason, it would be great to have something in the web.xml or context.xml where I say something like:

<classpath>/usr/local/tomcat/custom-libs/zk-5.0.4</classpath>

Note: The above is pseudo-code

Answer 1

From Tomcat 7 there is no mention of not being able to use the VirtualWebappLoader in production. I tried it and it works like a dream. Simply add the following to META-INF/context.xml:

<?xml version="1.0" encoding="UTF-8"?>

<Context antiJARLocking="true" path="/websandbox">
    <Loader className="org.apache.catalina.loader.VirtualWebappLoader"
          virtualClasspath="/usr/.../*.jar;/usr/.../*.jar"/>
</Context>

In Netbeans, under packaging, I just untick all the packages, taking the .war size down to nothing, make sure the dependencies are in the correct folders on the server and upload. Yey! No more 100 MB WAR file.

Answer 2

Addition @Spider answer.

Tomcat Context hold Loader element. According to docs deployment descriptor (what in <Context> tag) can be placed in:

• $CATALINA_BASE/conf/server.xml - bad - require server restarts in order to reread config
• $CATALINA_BASE/conf/context.xml - bad - shared across all applications
• $CATALINA_BASE/work/$APP.war:/META-INF/context.xml - bad - require repackaging in order to change config
• $CATALINA_BASE/work/[enginename]/[hostname]/$APP/META-INF/context.xml - nice , but see last option!!
• $CATALINA_BASE/webapps/$APP/META-INF/context.xml - nice , but see last option!!
• $CATALINA_BASE/conf/[enginename]/[hostname]/$APP.xml - best - completely out of application and automatically scanned for changes!!!

Here my config which demonstrate how to use development version of project files out of $CATALINA_BASE hierarchy (note that I place this file into src/test/resources dir and intruct Maven to preprocess ${basedir} placeholders through pom.xml <filtering>true</filtering> so after build in new environment I copy it to $CATALINA_BASE/conf/Catalina/localhost/$APP.xml ):

<Context docBase="${basedir}/src/main/webapp"
         reloadable="true">
    <!-- http://tomcat.apache.org/tomcat-7.0-doc/config/context.html -->
    <Resources className="org.apache.naming.resources.VirtualDirContext"
               extraResourcePaths="/WEB-INF/classes=${basedir}/target/classes,/WEB-INF/lib=${basedir}/target/${project.build.finalName}/WEB-INF/lib"/>
    <Loader className="org.apache.catalina.loader.VirtualWebappLoader"
            virtualClasspath="${basedir}/target/classes;${basedir}/target/${project.build.finalName}/WEB-INF/lib"/>
    <JarScanner scanAllDirectories="true"/>

    <!-- Use development version of JS/CSS files. -->
    <Parameter name="min" value="dev"/>
    <Environment name="app.devel.ldap" value="USER" type="java.lang.String" override="true"/>
    <Environment name="app.devel.permitAll" value="true" type="java.lang.String" override="true"/>
</Context>

UPDATE Tomcat 8 change syntax for <Resources> and <Loader> elements, corresponding part now look like:

<Resources>
    <PostResources className="org.apache.catalina.webresources.DirResourceSet"
                   webAppMount="/WEB-INF/classes" base="${basedir}/target/classes" />
    <PostResources className="org.apache.catalina.webresources.DirResourceSet"
                   webAppMount="/WEB-INF/lib" base="${basedir}/target/${project.build.finalName}/WEB-INF/lib" />
</Resources>

Answer 3

This is what the META-INF/context.xml file can be used for. You defined your own WebappLoader, which loads classes for your particular webapp. This is the reference I used: http://tomcat.apache.org/tomcat-5.5-doc/config/loader.html (Edit: for Tomcat 6: http://tomcat.apache.org/tomcat-6.0-doc/config/loader.html , for Tomcat 7: http://tomcat.apache.org/tomcat-7.0-doc/config/loader.html )

Also this fellow here seems to post a solution to your exact problem (example included): http://java.dzone.com/articles/extending-tomcat-webapploader

Answer 4

Another a bit hacky alternative.

You can write a 5-6 line custom class loader which derives from urlclassloader , and simply adds your classpath jars using addUrl() method.

Then set it as the context class loader of the thread in your application code.

Thread.setContextClassLoader(new CustomClassloader(path, parentClassLoader)

where parent class loader typically is

Thread.getContextClassloader()