Liferay 6.1+Tomcat7.0环境下javax.servlet-api-3.0.1.jar无法加载问题排查

Hey, I’ve run into this exact issue with Liferay portlets before—let’s break down what’s causing the error and how to fix it quickly.

为什么会出现这个报错？ #

The error message points to a key rule from the Servlet Spec 2.3, Section 9.7.2: your web application is trying to include a jar file that contains core Servlet API classes, which are already provided by the Tomcat server bundled with Liferay.

Here’s the breakdown:

• Liferay 6.1.1 CE GA2 uses Tomcat 7.0.27, which already includes its own copy of the Servlet API (matching Servlet 3.0 spec).
• Your custom SiteSkills_CMS-portlet has packaged javax.servlet-api-3.0.1.jar inside its WEB-INF/lib directory. This jar contains the javax.servlet.Servlet.class—a class that Tomcat already manages.
• To avoid class loading conflicts, Tomcat refuses to load your duplicate jar, which prevents the entire web app from starting up.

怎么解决这个问题？ #

There are two straightforward fixes, depending on how you build your portlet:

1. Use "provided" scope for Servlet API dependency (Recommended) #

If you’re using a build tool like Maven or Gradle, mark the Servlet API dependency as provided. This tells the build tool not to package the jar into your portlet’s WEB-INF/lib folder, since the server (Tomcat/Liferay) will provide it at runtime.

Maven example:

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>3.0.1</version>
    <scope>provided</scope>
</dependency>

Gradle example:

providedCompile 'javax.servlet:javax.servlet-api:3.0.1'

2. Manually remove the conflicting jar #

If you’re not using a build tool, simply:

• Locate your portlet’s WEB-INF/lib directory (in the temp folder mentioned, or your packaged WAR file)
• Delete the javax.servlet-api-3.0.1.jar file
• Redeploy the portlet to Liferay

After making either change, restart your Liferay server (or just redeploy the portlet) and the error should disappear.

内容的提问来源于stack exchange，提问作者rahul