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Learn how to resolve the `Failed to start component` error in your Spring Boot and Tomcat 9 application by addressing common configuration mistakes.
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This video is based on the question https://stackoverflow.com/q/66180575/ asked by the user 'Luciana Oliveira' ( https://stackoverflow.com/u/15200705/ ) and on the answer https://stackoverflow.com/a/66184078/ provided by the user 'Andy Wilkinson' ( https://stackoverflow.com/u/1384297/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Fixing the Failed to Start Component Error in Tomcat 9 with Spring Boot Applications

As a developer, few things can be more frustrating than encountering an application deployment failure, especially with a detailed stack trace. If you've found yourself puzzled by the Failed to start component [StandardEngine[Catalina].StandardHost[localhost].StandardContext] error while trying to deploy a Spring Boot application on Tomcat 9, you’re not alone. In this post, we are going to walk through the probable causes and solutions for this issue, allowing you to get your application up and running smoothly.

Understanding the Problem

When deploying a Spring Boot application packaged as a .war file in Tomcat, you may encounter a NullPointerException caused by the org.springframework.boot.cli.app.SpringApplicationWebApplicationInitializer. This typically indicates a misconfiguration in your application, particularly when it comes to your pom.xml file and other setup files that define your project setup.

Common Symptoms

Error Logs: You’ll usually see a stack trace in your logs that contains the message about the IllegalStateException alongside a NullPointerException.

Deploy Failure: Your web application fails to start, and Tomcat throws the above error in the logs.

Analyzing the Solution

To address the root causes of this issue, there are a few key steps you can follow. They mainly revolve around misconfigured dependencies in your pom.xml file. Here’s how to properly configure your Spring Boot application for deployment to Tomcat.

Step 1: Remove Unnecessary Dependencies

Upon reviewing your pom.xml, it appears that you have included the spring-boot-cli dependency. This is not recommended for applications meant to be run in a servlet container like Tomcat:

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Solution:

Remove the spring-boot-cli dependency from your pom.xml. This will prevent the CLI features from interfering with your application, thus eliminating the NullPointerException you’re experiencing.

Step 2: Validate Version Compatibility

Another important aspect to consider is ensuring that your dependencies are compatible. In your pom.xml, you are using spring-boot-starter-parent version 2.4.2 but have overridden the spring-boot-maven-plugin to 2.2.6.RELEASE.

Solution:

It is best practice to allow the spring-boot-starter-parent to manage the versions of its dependencies. Therefore, you should remove the specific version for the spring-boot-maven-plugin to align it with the parent version:

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Example of a Clean pom.xml

Here’s how your revised pom.xml could look after making the necessary changes:

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Conclusion

Deploying a Spring Boot application to Tomcat can sometimes lead to configuration challenges. However, by carefully analyzing and adjusting your pom.xml, you can resolve the Failed to start component error effectively. Remember to check for unnecessary dependencies and ensure compatible versions across your project.

For further assistance, you can always consult the Spring Boot documentation or community forums to get help from fellow developers facing similar issues.

Thanks for reading, and best of luck with your Spring Boot applications!