Monday, November 27, 2006

Catching all Errors and Exceptions

All errors and exceptions extend from Throwable. By catching Throwable, it is possible to handle all unexpected conditions.

There are several scenarios where it is good practice to catch Throwable. For example, in a server application, the threads that handle requests should catch Throwable and relay any errors or exceptions to the client. Another scenario is a long-running thread that performs some background activity. Such threads should catch Throwable, log any errors or exceptions, and then continue functioning.

It is rarely good practice for a method in a library to catch Throwable. In general, errors and exceptions should not be masked from the caller.

This example demonstrates a long-running thread that catches Throwable and logs the exception.

    class BgThread extends Thread {
// Create a logger. For more information on the logging api's,
Logger logger = Logger.getLogger("com.mycompany.mypackage");

BgThread() {
// As a daemon thread, this thread won't prevent the application from exiting

// Set to true to shut down this thread
boolean stop = false;

public void run() {
while (!stop) {
try {
// Perform work here
} catch (Throwable t) {
// Log the exception and continue
logger.log(Level.SEVERE, "Unexception exception", t);

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