Tomcat 8.0.36: WebappClassLoaderBase elusive nullpointerexception

Question

Issue:

The issue is that I am getting a NullPointerException in WebappClassLoaderBase in Tomcat without too much explanation of why that happens. It starts to happen from version 8.0.35 as far as I can tell.....

The exception trace:

java.lang.NullPointerException
    org.apache.catalina.loader.WebappClassLoaderBase.binaryNameToPath(WebappClassLoaderBase.java:2562)
    org.apache.catalina.loader.WebappClassLoaderBase.findLoadedClass0(WebappClassLoaderBase.java:2730)
    org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1208)
    org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1166)
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:490)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:616)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:528)
    org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1099)
    org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:670)
    org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2508)
    org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2497)
    java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    java.lang.Thread.run(Thread.java:745)

The web.xml which generates the issue:

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://java.sun.com/xml/ns/javaee"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    version="3.0">

<servlet>
    <servlet-name>FirstServlet</servlet-name>
    <async-supported>true</async-supported>
</servlet>

<servlet>
    <servlet-name>SecondServlet</servlet-name>
    <enabled>false</enabled>
</servlet>

<session-config>
    <session-timeout>7</session-timeout>
    <cookie-config>
        <http-only>true</http-only>
        <secure>true</secure>
    </cookie-config>
</session-config>

<security-constraint>
    <display-name>HTTPS</display-name>
    <web-resource-collection>
        <web-resource-name>HTTPS Constraint</web-resource-name>
        <url-pattern>/*</url-pattern>
    </web-resource-collection>
    <user-data-constraint>
        <transport-guarantee>CONFIDENTIAL</transport-guarantee>
    </user-data-constraint>
</security-constraint>
<!-- IMPORTED web.xml MODULES END -->

</web-app>

Answer 1

I solved it by adding <servlet-class> tags in the web.xml file.

AFAICT I should not need to do this, as in my code I have @WebServlet annotations which should already have registered the class names with Tomcat.

Answer 2

I saw this issue when I developed on Debian Wheezy Linux, Java 8, Jersey 2.24, Tomcat 8.0.38, and tried deploying the war file to a Tomcat 8.0 in my developer files that defaulted to using Java 7 from the operating system. I have multiple versions of Java installed for development. To fix, I created a short script to export the JAVA_HOME and JRE_HOME variables as the corresponding Java 8 locations and then run the Tomcat bin/shutdown.sh and bin/startup.sh in order. This solved the problem.

If the application is being deployed to an operational Tomcat, then the /etc/default/tomcat8 file can be edited to set JAVA_HOME because the default on the OS may be the wrong version.

The bottom line is be careful assuming that Tomcat will find the correct Java version on the system.