java11-concurrency-thread-uncaught-exception

Reference: advanced-exception-handling
Reference: https://www.amazon.com/Java-Concurrency-Practice-Brian-Goetz/dp/0321349601

preface

Thread.UncaughtExceptionHandler

• interface for handlers invoked when a Thread abruptly terminates due to an uncaught exception.

@FunctionalInterface
public interface UncaughtExceptionHandler {
    /**
     * Method invoked when the given thread terminates due to the
     * given uncaught exception.
     * <p>Any exception thrown by this method will be ignored by the
     * Java Virtual Machine.
     * @param t the thread
     * @param e the exception
     */
    void uncaughtException(Thread t, Throwable e);
}

• per-thread basis
  • thread.setUncaughtExceptionHandler(...)
  • thread.getUncaughtExceptionHandler()
• default
  • Thread.setDefaultUncaughtExceptionHandler(...)
  • Thread.getDefaultUncaughtExceptionHandler()
• from Thread class

  // null unless explicitly set
  private volatile UncaughtExceptionHandler uncaughtExceptionHandler;
  
  // null unless explicitly set
  private static volatile UncaughtExceptionHandler defaultUncaughtExceptionHandler;
  

thread group

• is represented by ThreadGroup class
• thread.getThreadGroup()
• a thread is always a member of a thread group
• by default, the thread group is inherited from creator thread
• the JVM creates a thread group called main and a thread in this group called main, which is responsible for running the main() at startup
• thread groups are arranged in a tree-like structure
• thread group main does not have a parent
• ThreadGroup implements Thread.UncaughtExceptionHandler

unhandled exceptions

When thread terminates due to an uncaught exception:

1. (first per-thread basis): if thread.getUncaughtExceptionHandler() is not null, its method uncaughtException(Thread t, Throwable e) is called
2. otherwise, (second per-ThreadGroup basis): thread group method uncaughtException(Thread t, Throwable e) is called - if uncaughtException method is not @Override, uncaughtException is called recursively in the consecutive parents (note that main parent is null)
3. (bubbles up to the top-level): top-level thread group handler delegates to the default system handler Thread.getDefaultUncaughtExceptionHandler() (if one exists; the default is none) and otherwise prints the stack trace to the console System.err

project description

We will provide simple examples of how to handle uncaught exceptions thrown in threads.

1. handler for a specific thread

   final class SpecificUncaughtExceptionsHandler implements Thread.UncaughtExceptionHandler {
       @Override
       public void uncaughtException(Thread t, Throwable e) {
           System.out.println("Caught specific: " + e
                   + " in: " + t.getName());
       }
   }
   

   and test:

   var thread = new Thread(endsExceptionally);
   thread.setUncaughtExceptionHandler(new SpecificUncaughtExceptionsHandler());
   
   thread.start();
   thread.join();
   

   will print:

   Caught specific: java.lang.RuntimeException: exception! in: Thread-0
   

2. global handler

   final class GlobalUncaughtExceptionsHandler implements Thread.UncaughtExceptionHandler {
       @Override
       public void uncaughtException(Thread t, Throwable e) {
           System.out.println("Caught global: " + e
                   + " in: " + t.getName());
       }
   }
   

   and test:

   Thread.setDefaultUncaughtExceptionHandler(new GlobalUncaughtExceptionsHandler());
   
   var thread = new Thread(endsExceptionally);
   
   thread.start();
   thread.join();
   

   will print:

   Caught global: java.lang.RuntimeException: exception! in: Thread-0
   

3. group handler

   final class MyThreadGroup extends ThreadGroup {
   
       MyThreadGroup() {
           super("My Thread Group");
       }
   
       @Override
       public void uncaughtException(Thread t, Throwable e) {
           System.out.println("Caught in My Thread Group: " + e
                   + " in: " + t.getName());
       }
   }
   

   and tests:

   var t = new Thread(new MyThreadGroup(), endsExceptionally);
   
   t.start();
   t.join();
   

   will print:

   Caught in My Thread Group: java.lang.RuntimeException: exception! in: Thread-0
   

4. unhandled at all

   var thread = new Thread(endsExceptionally);
   
   thread.start();
   thread.join();
   

   will print:

   Exception in thread "Thread-0" java.lang.RuntimeException: exception!
   	at ThreadExceptionTest.lambda$new$0(ThreadExceptionTest.java:8)
   	at java.base/java.lang.Thread.run(Thread.java:834)