I am creating a ScheduledExecutorService through Dropwizard's LifecycleEnvironment.scheduledExecutorService()
. I schedule four tasks on it, which throw an exception after 3 seconds. The problem is that no stack trace is printed for exceptions and hence I cannot trace why it happened. The task which throws exception one is never restarted.
I tried setting a default uncaught exception handler but it didn't help either:
Thread.setDefaultUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
@Override
public void uncaughtException(Thread thread, Throwable throwable) {
System.err.println("An exception occurred as below:");
throwable.printStackTrace(System.err);
}
});
Following is the complete code:
This is the main driving class which extends Application:
App.java
import io.dropwizard.Application;
import io.dropwizard.setup.Environment;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
public class App extends Application<AppConfiguration> {
public void run(AppConfiguration appConfiguration, Environment environment) throws Exception {
final ScheduledExecutorService scheduledExecutorService = environment.lifecycle()
.scheduledExecutorService("throwing-exception-threads").threads(4)
.build();
scheduledExecutorService.scheduleAtFixedRate(new ThreadToDie(), 0,5, TimeUnit.SECONDS);
scheduledExecutorService.scheduleAtFixedRate(new ThreadToDie(), 0,5, TimeUnit.SECONDS);
scheduledExecutorService.scheduleAtFixedRate(new ThreadToDie(), 0,5, TimeUnit.SECONDS);
scheduledExecutorService.scheduleAtFixedRate(new ThreadToDie(), 0,5, TimeUnit.SECONDS);
final HelloWorldResource resource = new HelloWorldResource();
environment.jersey().register(resource);
}
public static void main(String[] args) throws Exception {
new App().run(args);
}
}
Code for task being scheduled at fixed interval is below. It prints counter value which is incremented at each second. When the counter is 3 it throws a NullPointerException.
ThreadToDie.java
public class ThreadToDie implements Runnable {
int i = 0;
@Override
public void run() {
i++;
System.out.printf("Value of i: %d\n", i);
if (i % 3 == 0) {
System.out.printf("Throwing NullPointerException\n");
throw new NullPointerException("This should be printed.");
}
}
}
For the sake of completeness following are the Configuration class and A HelloWorld API class. Though in the question asked what they contain is not relevant.
AppConfiguration.java
import io.dropwizard.Configuration;
public class AppConfiguration extends Configuration {
}
HelloWorldResource.java
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.QueryParam;
import javax.ws.rs.core.MediaType;
import java.util.Optional;
@Path("/hello")
@Produces(MediaType.APPLICATION_JSON)
public class HelloWorldResource {
@GET
public String hello(@QueryParam("name") Optional<String> name) {
final String retVal = String.format("Hello %s!", name.orElse("World"));
return retVal;
}
}
See Why is UncaughtExceptionHandler not called by ExecutorService? for an explanation on why the UncaughtExceptionHandler
is never triggered - each task you supply is handled by a worker that catches all exceptions.
When you submit a task to an executor you receive back a Future
and can access the exception in that way:
ScheduledFuture<?> future = scheduledExecutorService.scheduleAtFixedRate(new ThreadToDie(), 0,5, TimeUnit.SECONDS);
try {
future.get();
} catch (ExecutionException ex) {
ex.getCause().printStackTrace();
}
Future.get
will wait until the task has completed or errored.
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