EJB 3 Timer问题

Question

I am using JBoss 4.2.3 with JDK 1.5. I have created a stateless EJB whose purpose is to delete a file after a specified period of time (in milliseconds).

The EJB code is:

import java.io.File;

import javax.annotation.Resource;
import javax.ejb.Stateless;
import javax.ejb.Timeout;
import javax.ejb.Timer;
import javax.ejb.TimerService;

import org.jboss.annotation.ejb.LocalBinding;

@Stateless
@LocalBinding(jndiBinding = "TimedFileDeletion")
public class TimedFileDeletionBean implements TimedFileDeletionBeanLocal {

    @Resource
    TimerService timerService;

    File fileToDelete;

    public void setRequiredInfo(long intervalDuration, File fileToDelete) {
        timerService.createTimer(intervalDuration, "Created new timer");
        this.fileToDelete = fileToDelete;
    }

    @Timeout
    public void timeout(Timer timer) {
        System.out.println("Timeout occurred");

        if(fileToDelete.exists()) {
            fileToDelete.delete();
        }
    }
}

The local interface is:

import java.io.File;

public interface TimedFileDeletionBeanLocal {

    public void setRequiredInfo(long intervalDuration, File fileToDelete);
}

When I call the bean through the web container (I use the Stripes framework) the timeout method is called after the specified time but it only prints "Timeout occurred", it does not delete the file and it throws an exception. This is the console output:

INFO  [STDOUT] Timeout occurred
ERROR [TimerImpl] Error invoking ejbTimeout: javax.ejb.EJBException: java.lang.NullPointerException

Any advice would be appreciated.

Answer 1

One thing that appears that might be a problem is that you're passing into the setRequiredInfo method a reference to a File. That reference is then stored locally, using a debugger I would verify that the reference value is the same when the timer fires. I suspect that it may no longer be the same file referenced or that the File object may be transient.

Also, just a little warning with EJBTimers and JBoss. This version of JBoss spins a thread for every EJB with a timer. So, if you have 500 files to delete with these EJBs, JBoss will spin up 500 threads. This behavior, while undesirable, does comply with the EJB spec (which is ambiguous on implementation). These threads will be recreated if the container restarts and the timers are still waiting to fire.

Answer 2

In stateless session bean, conversational state is not maintained. Stateless bean instance variables are shared between invocations, so they may overlap.

Therefore even if you set file using setRequiredInfo(), on timeout it gets the fileToDelete null.

Try checking null before doing operation. Below is some code snippet might help you.

class FileUtility {

// Make singleton class to store list of files to delete

public static List<File> files;

//-- get/set accessing methods

}

//---------------------

public void setRequiredInfo(long intervalDuration, File fileToDelete) {
        timerService.createTimer(intervalDuration, fileToDelete.getName()+Math.random());
        FileUtility.files.add(fileToDelete);
}

//---------------------

@Timeout
    public void timeout(Timer timer) {
        System.out.println("Timeout occurred");

    for(File fileToDelete : Fileutility.files){

        if(fileToDelete.exists()) {
            fileToDelete.delete();
        }
    }
}