This tutorial aims to solve the problem of some Tomcat indicators (such as tomcat.cache.access, tomcat.global.error) disappearing in MetricsEndpoint after the Spring Boot Actuator is upgraded. The core solution is to configure server.tomcat.mbeanregistry.enabled=true in application.properties to enable Tomcat's MBean registration mechanism to resume monitoring of these key runtime indicators.

Tomcat indicator missing phenomenon

After upgrading the spring-boot-starter-actuator dependency of a Spring Boot application to a specific version (such as 2.7.0 and higher), developers may find that the Tomcat-related metrics obtained through the /actuator/metrics endpoint are no longer complete. Specifically, some important performance monitoring indicators, such as tomcat.cache.access, tomcat.cache.hit, tomcat.global.error, etc., may disappear from the list of available indicators. These indicators are crucial for a deep understanding of the runtime status, cache efficiency, and error handling of the Tomcat server. Their absence will seriously affect the application monitoring capabilities.

Mbean registration mechanism and Actuator

Spring Boot Actuator collects application indicators in a variety of ways. For some specific indicators of underlying containers such as Tomcat, it usually relies on Java Management Extensions (JMX) MBeans. As a Java application server, Tomcat registers its internal components (such as connectors, thread pools, caches, etc.) as MBeans and exposes its runtime data through the JMX interface. Actuator can discover and use these MBeans to generate corresponding metrics.

In some Spring Boot version updates, Tomcat's MBean registration behavior may have changed or is no longer enabled by default. When Tomcat does not register its MBeans with the JMX MBeanServer, Actuator cannot discover these MBeans and naturally cannot expose the metrics associated with them. Therefore, to restore these missing Tomcat metrics, the key is to ensure that Tomcat's MBean registration mechanism is correctly enabled.

core solution

The core method to solve the problem of missing Tomcat indicators is very straightforward: explicitly enable Tomcat's MBean registration function in the configuration file of the Spring Boot application. This can be achieved by adding a configuration item in application.properties or application.yml.

Configuration example

Add the following configuration in the application.properties file:

 server.tomcat.mbeanregistry.enabled=true

If your project uses application.yml, the configuration is as follows:

 server:
  tomcat:
    mbeanregistry:
      enabled: true

After adding this configuration, when the Spring Boot application starts Tomcat, it will ensure that Tomcat's MBean registration mechanism is activated. Once the MBeans are registered, spring-boot-starter-actuator is able to rediscover and collect these metrics and expose them through the /actuator/metrics endpoint.

Important tips and verification

1. Version compatibility: The above solution mainly addresses the problem of missing Tomcat indicators caused by Spring Boot Actuator version upgrade (especially 2.7.0 and later versions). For other versions or different questions, you may want to consult the official Spring Boot documentation or Tomcat's JMX configuration guide.
2. Metric verification: After the application is restarted, you can verify that Tomcat metrics have been restored by accessing the /actuator/metrics endpoint. You should see familiar metrics such as tomcat.cache.access, tomcat.global.error, etc. For more precise verification, you can access the metric-specific endpoint, such as /actuator/metrics/tomcat.cache.access.
3. Performance considerations: Enabling Tomcat Mbean registration usually does not introduce significant performance overhead, because JMX MBeans are mainly used for monitoring and management, and will only be accessed when requested. But in extremely high-concurrency or resource-constrained environments, performance testing is always recommended to ensure there are no unintended effects.
4. Other monitoring: In addition to Tomcat indicators, Actuator also provides a wealth of JVM, system, and application custom indicators. It is recommended to use various indicators in combination to build a comprehensive application monitoring system.

Summarize

When the Spring Boot Actuator upgrade causes Tomcat indicators to be missing, this problem can be effectively solved by simply setting server.tomcat.mbeanregistry.enabled=true in application.properties. This configuration item ensures that Tomcat's MBean registration mechanism is activated, allowing Actuator to re-collect and expose these key runtime performance indicators. In a production environment, complete monitoring data is critical for troubleshooting, performance optimization, and system health assessment, so ensuring that all necessary metrics are available is fundamental to maintaining high availability and high-performance applications.

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