Tomcat casting servlets to javax.servlet.Servlet instead of jakarta.servlet.http.HttpServlet

You’re basically physically including Tomcat 10.x specific libraries in WAR and then deploying the WAR to Tomcat 9.x. This is not the correct approach at all. Moreover, Tomcat 10.x was the first version to be Jakartified, not Tomcat 9.x.

For Tomcat 9.x, which is based on Servlet 4.0, JSP 2.3, EL 3.0, WS 1.1 and JASIC 1.0, you should use javax.* imports and the entire <dependencies> section should minimally look like:

<dependencies>
    <dependency>
        <groupId>javax.servlet</groupId>
        <artifactId>javax.servlet-api</artifactId>
        <version>4.0.0</version>
        <scope>provided</scope>
    </dependency>
    <dependency>
        <groupId>javax.servlet.jsp</groupId>
        <artifactId>javax.servlet.jsp-api</artifactId>
        <version>2.3.0</version>
        <scope>provided</scope>
    </dependency>
    <dependency>
        <groupId>javax.el</groupId>
        <artifactId>javax.el-api</artifactId>
        <version>3.0.0</version>
        <scope>provided</scope>
    </dependency>
    <dependency>
        <groupId>javax.websocket</groupId>
        <artifactId>javax.websocket-api</artifactId>
        <version>1.1</version>
        <scope>provided</scope>
    </dependency>
    <dependency>
        <groupId>javax.security.enterprise</groupId>
        <artifactId>javax.security.enterprise-api</artifactId>
        <version>1.0</version>
        <scope>provided</scope>
    </dependency>
</dependencies>

For Tomcat 10.x, which is based on Servlet 5.0, JSP 3.0, EL 4.0, WS 2.0 and JASIC 2.0, you should use jakarta.* imports and the entire <dependencies> section should minimally look like:

<dependencies>
    <dependency>
        <groupId>jakarta.servlet</groupId>
        <artifactId>jakarta.servlet-api</artifactId>
        <version>5.0.0</version>
        <scope>provided</scope>
    </dependency>
    <dependency>
        <groupId>jakarta.servlet.jsp</groupId>
        <artifactId>jakarta.servlet.jsp-api</artifactId>
        <version>3.0.0</version>
        <scope>provided</scope>
    </dependency>
    <dependency>
        <groupId>jakarta.el</groupId>
        <artifactId>jakarta.el-api</artifactId>
        <version>4.0.0</version>
        <scope>provided</scope>
    </dependency>
    <dependency>
        <groupId>jakarta.websocket</groupId>
        <artifactId>jakarta.websocket-api</artifactId>
        <version>2.0.0</version>
        <scope>provided</scope>
    </dependency>
    <dependency>
        <groupId>jakarta.security.enterprise</groupId>
        <artifactId>jakarta.security.enterprise-api</artifactId>
        <version>2.0.0</version>
        <scope>provided</scope>
    </dependency>
</dependencies>

Please note that the <scope> is explicitly set to provided for them, which means that Maven should not include the physical JAR files in /WEB-INF/lib of the generated WAR file (because it’s already provided by Tomcat itself!). Otherwise you only end up in runtime conflicts caused by duplicate classes in the runtime classpath.

Please also note that Tomcat is not a JEE server and thus importing either javax:javaee-api for Tomcat 9.x or jakarta.platform:jakarta.jakartaee-api for Tomcat 10.x is per definition wrong. Because it will allow you to compile your code against other JEE components such as JSF, JSTL, CDI, BV, EJB, JPA, JAX-RS, JSONB, etc etc while Tomcat actually doesn’t offer them out the box. Tomcat only offers Servlet, JSP, EL, WS and JASIC out the box, so you should only declare them in pom.xml.

For example, JSTL needs to be installed separately as instructed in How to install JSTL? The absolute uri: http://java.sun.com/jstl/core cannot be resolved and JSF needs to be installed separately as instructed in How to properly install and configure JSF libraries via Maven? and CDI needs to be installed separately as instructed in How to install and use CDI on Tomcat?

If you’re however very well aware of this limitation during developing code for Tomcat (i.e. make sure yourself that you don’t accidentally use e.g. JSTL, CDI, BV, JPA, etc without actually installing them in Tomcat first), and merely want to minimize pom.xml boilerplate, then you can also get away with this minimalist dependencies configuration for Tomcat 9.x:

<dependencies>
    <dependency>
        <groupId>javax</groupId>
        <artifactId>javaee-web-api</artifactId>
        <version>8.0</version>
        <scope>provided</scope>
    </dependency>
</dependencies>

Or this for Tomcat 10.x:

<dependencies>
    <dependency>
        <groupId>jakarta.platform</groupId>
        <artifactId>jakarta.jakartaee-web-api</artifactId>
        <version>9.0.0</version>
        <scope>provided</scope>
    </dependency>
</dependencies>

See also:

• Tomcat versions
• What exactly is Java EE?