When deploying applications on Apache Tomcat, a popular servlet container, developers may encounter an issue known as a "memory leak". Memory leaks occur when the garbage collector in the Java Virtual Machine (JVM) is unable to reclaim memory that is no longer in use. Over time, memory leaks can lead to increased memory usage, performance degradation, and application crashes.

Understanding Memory Leaks in Tomcat

What causes memory leaks during redeployment?

Memory leaks during redeployment primarily occur when old instances of classes or objects continue to be referenced, even after an application has been undeployed. These lingering references prevent the JVM from freeing the associated memory.

Common Sources of Memory Leaks

1. ClassLoader Leaks: Each deployed application in Tomcat is loaded using a separate classloader. If static references are left behind after undeployment, these can keep the classloader in memory, consequently retaining all classes, static variables, and resources loaded by it.
2. Unreleased Threads: Thread pools or scheduled tasks that continue running post-undeployment can hold references to objects, inhibiting garbage collection.
3. Listeners, Filters, or Servlet Contexts: These elements may establish static references or be registered but not deregistered upon undeployment, causing memory retention.
4. Database Connections: JDBC resources like unclosed connections or improperly managed JNDI resources can lead to memory issues.
5. Third-party Libraries: Libraries such as logging frameworks (e.g., Log4j) may maintain static references.

Detecting Memory Leaks

Diagnosing memory leaks requires monitoring tools and practices:

• Verbose GC Logging: Enable garbage collection logging to monitor memory usage patterns over time.
• Profiling Tools: Utilize tools like VisualVM or YourKit to inspect heap dumps and analyze object retention.
• Leak Detection Utilities: Libraries like Apache Tomcat provides `org.apache.catalina.loader.WebappClassLoaderBase` with facilities to detect potential memory leaks.

Resolving Memory Leaks

To mitigate and resolve memory leaks, consider the following approaches:

• Proper Resource Management: Ensure that all resources such as database connections, input streams, and file handles are explicitly closed.
• Thread Management: Use `ServletContextListener` to manage thread lifecycles, ensuring threads are ceased during context destruction.
• Weak References: Replace static references with `WeakReference` or `SoftReference` alternatives to allow garbage collection.
• Classloader Storage Usage: Avoid storing objects in static fields when they are loaded by the web application’s classloader.
• Periodic Full GC: Schedule periodic full garbage collections in lower environments to check for retained memory.

Example: Memory Leak due to Unreleased Thread

Consider the use of a scheduled executor in a web application: