CODEWITHSUNDEEP
javatomcatclassloadertomcat8embedded-tomcat-8
Oren
Oren

Reputation: 21

shared classloader with embedded Tomcat 8

I have upgraded tomcat from version 7.0.34 to version 8.0.33, and since then I have been facing a problem to share the web application context and Junit context.

I have a web application with singleton class that gathers statistic data about the web application. I also have Junit that runs the web application in embedded tomcat. the Junit queries the web application and then checks the statistic data.

I try to make a simple example:

the singleton:

  public class Counter {

  private static Counter instance;
  private AtomicLong counter;

  private Counter(){}

  public static Counter getInstance(){
    if(instance == null){
      synchronized (Counter.class) {
        if(instance == null){
          instance = new Counter();
        }
      }
    }

    return instance;
  }

  public long incrementAndGet(){
    return counter.incrementAndGet();
  }

  public long getValue(){
    return counter.get();
  }

}

the servlet:

@WebServlet(name="servlet",loadOnStartup=1, urlPatterns="/servletTest")
public class Servlet extends HttpServlet{

  @Override
  protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
      resp.getWriter().write("Hi, you are the #" + Counter.getInstance().incrementAndGet() + " visitor");
  }
}

contextListener:

public class MyContextListener implements ServletContextListener{
  @Override
  public void contextDestroyed(ServletContextEvent arg0) {}

  @Override
  public void contextInitialized(ServletContextEvent arg0) {
    Counter.getInstance().incrementAndGet(); 
  }
}

Test unit:

  public void mainTest() throws ServletException, LifecycleException{
    Tomcat tomcat = new Tomcat();

   tomcat.setPort(50000);
   StandardContext ctx = (StandardContext) tomcat.addWebapp("/fe", System.getProperty("FEBaseDir")); //The FEBaseDir property is supposed to be taken from Maven build using 'test' profile

   tomcat.start();

   Counter.getInstance().getValue();

  }

when I used Tomcat 7, everything worked fine. but since I upgraded tomcat to tomcat 8.0.33, It hasn't been working. the singleton class with the static data loads twice. first by the tomcat and then by the Junit itself.

I have tried to pass tomcat a classloader but it doesn't work.

 public void mainTest() throws ServletException, LifecycleException{
    Tomcat tomcat = new Tomcat();

   tomcat.setPort(50000);
   StandardContext ctx = (StandardContext) tomcat.addWebapp("/fe", System.getProperty("FEBaseDir")); //The FEBaseDir property is supposed to be taken from Maven build using 'test' profile

   ctx.setCrossContext(true);
   ctx.setLoader((Loader) new WebappLoader(Thread.currentThread().getContextClassLoader()));

   ctx.setParentClassLoader(Thread.currentThread().getContextClassLoader());

   tomcat.getEngine().setParentClassLoader(Thread.currentThread().getContextClassLoader());
   tomcat.getHost().setParentClassLoader(Thread.currentThread().getContextClassLoader());
   tomcat.getService().setParentClassLoader(Thread.currentThread().getContextClassLoader());
   tomcat.getServer().setParentClassLoader(Thread.currentThread().getContextClassLoader());
   tomcat.start();

   Counter.getInstance().getValue();

  }

What am I doing wrong?

Upvotes: 1

Views: 1627

Answers (1)

vanOekel
vanOekel

Reputation: 6538

You could try using the setDelegate method in StandardContext to prevent the web-app classloader from reloading the Counter class, but this impacts security in a bad manner so I advice against that.
The usual way to expose statistics is to use JMX (MBeans). You enable this by calling the setUseNaming method in StandardContext with value true.

You can register a mbean like this (copied from here):

MBeanServer mBeanServer = ManagementFactory.getPlatformMBeanServer();
ObjectName beanPoolName = new ObjectName("com.zaxxer.hikari:type=Pool (" + poolName + ")");
mBeanServer.registerMBean(hikariPool, beanPoolName);

And you can retrieve a value like this (copied from here):

MBeanServer mBeanServer = ManagementFactory.getPlatformMBeanServer();
ObjectName poolName = new ObjectName("com.zaxxer.hikari:type=Pool (foo)");
HikariPoolMXBean poolProxy = JMX.newMXBeanProxy(mBeanServer, poolName, HikariPoolMXBean.class);

int idleConnections = poolProxy.getIdleConnections();

See also this SO question and you'll probably have to read some more documentation (in my experience, it takes a while to understand the whole JMX thing and get it to work). I have not tried this in combination with unit-tests though, so YMMV.

Upvotes: 2

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