java - Tomcat8 memory leak

Question

When I try to stop tomcat8 on Java 8, I get a few memory leaks errors:

org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesThreads The web application [ROOT] appears to have started a thread named [Abandoned connection cleanup thread] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread:
 java.lang.Object.wait(Native Method)
 java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:142)
 com.mysql.jdbc.AbandonedConnectionCleanupThread.run(AbandonedConnectionCleanupThread.java:40)
23-Jan-2015 08:18:10.202 WARNING [localhost-startStop-2] org.apache.catalina.loader.WebappClassLoaderBase.clearReferencesThreads The web application [ROOT] appears to have started a thread named [pool-5-thread-1] but has failed to stop it. This is very likely to create a memory leak. Stack trace of thread


23-Jan-2015 08:18:10.205 SEVERE [localhost-startStop-2] org.apache.catalina.loader.WebappClassLoaderBase.checkThreadLocalMapForLeaks The web application [ROOT] created a ThreadLocal with key of type [com.util.ThreadLocalProperties$1] (value [com.util.ThreadLocalProperties$1@2fafda6e]) and a value of type [java.util.Properties] (value [{}]) but failed to remove it when the web application was stopped. Threads are going to be renewed over time to try and avoid a probable memory leak.

The class ThreadLocalProperties is:

public class ThreadLocalProperties extends Properties {

    private static final long serialVersionUID = -4260632266508618756L;
    private final ThreadLocal<Properties> localProperties = new ThreadLocal<Properties>() {
        @Override
        protected Properties initialValue() {
            return new Properties();
        }
    };

    public ThreadLocalProperties(Properties properties) {
        super(properties);
    }

    @Override
    public String getProperty(String key) {
        String localValue = localProperties.get().getProperty(key);
        return localValue == null ? super.getProperty(key) : localValue;
    }

    @Override
    public Object setProperty(String key, String value) {
        return localProperties.get().setProperty(key, value);
    }

    public ThreadLocal<Properties> getThreadLocal() {
        return localProperties;
    }
}

and I start and stop it like this:

@WebListener()
public class GeneralListener implements ServletContextListener {

    ThreadLocalProperties threadLocalProperties = new ThreadLocalProperties(System.getProperties());

    @Override
    public void contextDestroyed(ServletContextEvent arg0) {
        threadLocalProperties.getThreadLocal().remove();
    }

    @Override
    public void contextInitialized(ServletContextEvent arg0) {
        System.setProperties(threadLocalProperties);
    }
}

Why would i get all this memory leaks errors? as well, when I run shutdown, it doesn't shut it down, I have to manually kill the process.

What is wrong?

Answer 1

There is nothing to worry about. This is a standard message tomcat outputs when it detects the application has started it's own Thread or created a ThreadLocal. If you terminate the thread on shutdown and remove the threadlocals when they are no longer needed, then there will be no problem.

Why would I get all this memory leaks errors? as well, when I run shutdown, it doesn't shut it down, I have to manually kill the process.

I've seen this behavior, when an application has started ScheduledExecutor (but this will happen with any other Thread/TheadPool) and didn't shut it down on contextDestroyed. So check if you are shutting down your threads on application/server stop.

Now on your concrete problem why the server doesn't stop: JDBC drivers are registered in the JVM as singletons, and are shared with all webapps. More info here. The best solution to your problem is to move the MySQL driver in Tomcat's /lib folder. If you cannot do that you can try this but it's more like a hack than a real solution.