Understanding Classloader Behavior in Tomcat When Running Multiple Applications

What is the classloader behavior in Tomcat when deploying multiple web applications?

// Example classloader hierarchy in Tomcat

// Define Context Loader
web.xml
<Context>
   <Loader className="org.apache.catalina.loader.WebappClassLoader" />
</Context>

Classloaders in Tomcat manage the loading of classes and resources for web applications, allowing each deployed application isolated control over its classes. Understanding how classloaders behave when multiple applications are deployed is crucial in avoiding class conflicts and ensuring correct resource loading.

// Configure the shared classloader in context.xml
<Context>
   <Loader className="org.apache.catalina.loader.WebappClassLoader" />
   <Loader delegate="true"/>
</Context>

// Example of organizing shared resources in `shared/lib`
/shared/lib/library1.jar
/shared/lib/library2.jar

Causes

• Each web application has its own classloader hierarchy in Tomcat, primarily consisting of the Context Classloader, Loader Classloader, and System Classloader.
• The hierarchy follows a parent-first delegation model to avoid conflicts between classes loaded by different applications.
• If two applications contain classes with the same name but are loaded by different classloaders, this can lead to ClassCastExceptions and other runtime issues.

Solutions

• Be aware of the classloader hierarchy: Web applications should ideally not share classes unless explicitly intended via shared libraries.
• Utilize the shared classloader mechanism by placing common libraries in the `shared` directory of Tomcat if multiple applications require the same library.
• Encapsulate application-specific classes within the `WEB-INF/classes` directory to avoid namespace collisions.

Mistake: Assuming that libraries in `WEB-INF/lib` are accessible across applications.

Solution: Each application maintains a separate classloader; libraries should be placed in the `shared` directory for access across apps.

Mistake: Not understanding the effect of `delegate` in ClassLoader configurations.

Solution: Ensure that `delegate=true` for shared classloaders to load classes from the shared space first before looking into the application's classes.