一個線程運行過程中 如果發生不可恢復的錯誤 會導致線程退出.在java的世界里面 提供了UncaughtExceptionHandler 讓應用開發人員捕捉到這種異常情況.jdk注釋如下.

/**
     * Interface for handlers invoked when a <tt>Thread</tt> abruptly
     * terminates due to an uncaught exception.
     * <p>When a thread is about to terminate due to an uncaught exception
     * the Java Virtual Machine will query the thread for its
     * <tt>UncaughtExceptionHandler</tt> using
     * {@link #getUncaughtExceptionHandler} and will invoke the handler's
     * <tt>uncaughtException</tt> method, passing the thread and the
     * exception as arguments.
     * If a thread has not had its <tt>UncaughtExceptionHandler</tt>
     * explicitly set, then its <tt>ThreadGroup</tt> object acts as its
     * <tt>UncaughtExceptionHandler</tt>. If the <tt>ThreadGroup</tt> object
     * has no
     * special requirements for dealing with the exception, it can forward
     * the invocation to the {@linkplain #getDefaultUncaughtExceptionHandler
     * default uncaught exception handler}.
     *
     * @see #setDefaultUncaughtExceptionHandler
     * @see #setUncaughtExceptionHandler
     * @see ThreadGroup#uncaughtException
     * @since 1.5
     */

學習zookeeper 源碼過程中 發現zookeeper 提供了Listener這種更友好的方法 代碼如下
ZookeeperThread

/**
 * This is the main class for catching all the uncaught exceptions thrown by the
 * threads.
 */
public class ZooKeeperThread extends Thread {

    private static final Logger LOG = LoggerFactory
            .getLogger(ZooKeeperThread.class);

    private UncaughtExceptionHandler uncaughtExceptionalHandler = new UncaughtExceptionHandler() {

        @Override
        public void uncaughtException(Thread t, Throwable e) {
            handleException(t.getName(), e);
        }
    };

    public ZooKeeperThread(Runnable thread, String threadName) {
        super(thread, threadName);
        setUncaughtExceptionHandler(uncaughtExceptionalHandler);
    }

    public ZooKeeperThread(String threadName) {
        super(threadName);
        setUncaughtExceptionHandler(uncaughtExceptionalHandler);
    }

    /**
     * This will be used by the uncaught exception handler and just log a
     * warning message and return.
     *
     * @param thName
     *            - thread name
     * @param e
     *            - exception object
     */
    protected void handleException(String thName, Throwable e) {
        LOG.warn("Exception occured from thread {}", thName, e);
    }
}

ZooKeeperCriticalThread

/**
 * Represents critical thread. When there is an uncaught exception thrown by the
 * thread this will exit the system.
 */
public class ZooKeeperCriticalThread extends ZooKeeperThread {
    private static final Logger LOG = LoggerFactory
            .getLogger(ZooKeeperCriticalThread.class);
    private final ZooKeeperServerListener listener;

    public ZooKeeperCriticalThread(String threadName,
            ZooKeeperServerListener listener) {
        super(threadName);
        this.listener = listener;
    }

    /**
     * This will be used by the uncaught exception handler and make the system
     * exit.
     *
     * @param threadName
     *            - thread name
     * @param e
     *            - exception object
     */
    @Override
    protected void handleException(String threadName, Throwable e) {
        LOG.error("Severe unrecoverable error, from thread : {}", threadName, e);
        listener.notifyStopping(threadName, ExitCode.UNEXPECTED_ERROR);
    }
}

**ZooKeeperServerListener **

/**
 * Listener for the critical resource events.
 */
public interface ZooKeeperServerListener {

    /**
     * This will notify the server that some critical thread has stopped. It
     * usually takes place when fatal error occurred.
     * 
     * @param threadName
     *            - name of the thread
     * @param errorCode
     *            - error code
     */
    void notifyStopping(String threadName, int errorCode);
}