Exceptions between threads....

Question

Hi all,
Was wondering if anyone could help. I am trying to catch an exception in a thread and throw it all the way "back into" the calling thread.
However, I don't seem to be able to do this (which I'm not actually surprised about), and I get the following exception:
Exception in thread "Thread-0" java.lang.NullPointerException
     at testproject.TestClass$1.run(TestClass.java:20)
From the following example code:
package testproject;
public class TestClass {
  public TestClass() {
    try {
      executeThread();
    } catch(NullPointerException exp) {
      System.out.println("Inside last try/catch");
      exp.printStackTrace();
  private void executeThread() throws NullPointerException {
    try {
      Thread t = new Thread(){
        public void run() {
          System.out.println("Starting execute");
          String testString = null;
          System.out.println(Thread.currentThread().getName());
          System.out.println(testString.substring(3));
      t.start();
    } catch(NullPointerException exp) {
      System.out.println("Inside thread try/catch");
      throw exp;
  public static void main(String args[]) {
    TestClass c = new TestClass();
}Any ideas how I can go about doing this, or the next best thing?
Thanks.

Answer 1

If you want the exception to be visible by another thread, then the run() method will have to NOT throw any exceptions, as far as I know. You can catch() any exceptions in the run() method itself, and set a property (such as the exception object itself) which can be retrieved by normal accessor methods by the other thread(s).
So the executeThread method could be rewritten like this:
MyThread t = new MyThread();
t.start();
t.join();
Exception e = t.getException();
if (e != null)
  throw e;
private class MyThread extends Thread {
// NOTE: You could (and should) implement Runnable instead
  private Exception ex;
  public Exception getException() { return ex; }
  public void run() {
    try {
      // implementation which may throw an exception...
    catch (Exception e) {
      ex = e;
}Message was edited by:
warnerja