How does Tomcat set the context class loader of threads handling HTTP request?

Question

A war is deployed to Tomcat 8.5.27. During this deployment, entity classes are dynamically added to a persistence.xml file on the classpath.

Then Hibernate 5.1.2 is asked to load the persistence unit containing those entity classes. They get added to a java.net.FactoryURLClassLoader instance. Here is an excerpt from the deployment log showing a dump of this class loader:

[INFO] [talledLocalContainer] 15-Feb-2019 15:03:46.722 INFOS [localhost-startStop-1] org.apache.catalina.core.ApplicationContext.log ** SEARCHING: generated.com.company.Baz
[INFO] [talledLocalContainer] 15-Feb-2019 15:03:46.727 INFOS [localhost-startStop-1] org.apache.catalina.core.ApplicationContext.log ClassLoader: java.net.FactoryURLClassLoader@701f7082
[INFO] [talledLocalContainer]   class generated.com.company.Foo
[INFO] [talledLocalContainer]   class generated.com.company.Bar
(...)
[INFO] [talledLocalContainer]   class generated.com.company.GooClassLoader: ParallelWebappClassLoader
[INFO] [talledLocalContainer]   context: webappContext
[INFO] [talledLocalContainer]   delegate: false
[INFO] [talledLocalContainer] ----------> Parent Classloader:
[INFO] [talledLocalContainer] java.net.URLClassLoader@45d6cf48
[INFO] [talledLocalContainer] 
[INFO] [talledLocalContainer]   class com.sun.jersey.server.impl.container.servlet.JerseyServletContainerInitializer
[INFO] [talledLocalContainer]   class javax.ws.rs.core.Application
(...)

Please note that all the above actions seem performed by the localhost-startStop-1 thread.

Later, when a request hits the webapp, the thread handling that request (http-nio-8080-exec-6 see in log below) is unable to find the classes under generated.com.company. Here is an excerpt of the dump of this thread class loader:

[INFO] [talledLocalContainer] 15-Feb-2019 15:04:18.705 INFOS [http-nio-8080-exec-6] org.apache.catalina.core.ApplicationContext.log  ** SEARCHING: generated.com.company.Foo
[INFO] [talledLocalContainer] 15-Feb-2019 15:04:18.724 INFOS [http-nio-8080-exec-6] org.apache.catalina.core.ApplicationContext.log ClassLoader: ParallelWebappClassLoader
[INFO] [talledLocalContainer]   context: webappContext
[INFO] [talledLocalContainer]   delegate: false
[INFO] [talledLocalContainer] ----------> Parent Classloader:
[INFO] [talledLocalContainer] java.net.URLClassLoader@45d6cf48
[INFO] [talledLocalContainer] 
[INFO] [talledLocalContainer]   class com.sun.jersey.server.impl.container.servlet.JerseyServletContainerInitializer
[INFO] [talledLocalContainer]   class javax.ws.rs.core.Application
(...)

Here is the code used to generate the dump:

private static void dumpLoadedClasses() {
        try {
            ClassLoader myCL = Thread.currentThread().getContextClassLoader();
            StringBuilder sb = new StringBuilder();
            while (myCL != null) {
                sb.append("ClassLoader: " + myCL);
                for (Iterator<Class<?>> iter = list(myCL); iter.hasNext();) {
                    sb.append("\n\t" + iter.next());
                }
                myCL = myCL.getParent();
            }

            log.info(sb.toString());
        } catch (Exception e) {
            log.warn("", e);
        }
}

Why http-nio-8080-exec-6 doesn't have java.net.FactoryURLClassLoader@701f7082 as his context class loader ?

jnr jnr · Accepted Answer · 2019-02-16T09:09:23

0

votes

Consider using ClassLoader.getParent(), maybe they have common ancestor

How does Tomcat set the context class loader of threads handling HTTP request?

1 Answers