When i execute the below code,I am seeing the output as:
Finally
Exception in thread "main" java.lang.NullPointerException
at ClientTestConcepts.main(ClientTestConcepts.java:9)
Who prints the bold faced statements.
public class ClientTestConcepts {
public static void main(String []args){
try{
throw new NullPointerException();
}
finally{
System.out.println("Finally");
}
}
}
The Java runtime.
It catches all exceptions not handled in user code, and prints them on the error output (by default).
Each thread has a default uncaught exception handler that runs when an exception makes it to the top of the stack. The one you are observing is provided by ThreadGroup.uncaughtException:
Called by the Java Virtual Machine when a thread in this thread group stops because of an uncaught exception, and the thread does not have a specific Thread.UncaughtExceptionHandler installed.
The uncaughtException method of ThreadGroup does the following: (...)
... a message containing the thread's name, as returned from the thread's getName method, and a stack backtrace, using the Throwable's printStackTrace method, is printed to the standard error stream.
If you want some other behaviour register an uncaught exception handler for the thread.
Related
Will an unhandled runtime exception stop the whole server (i.e. Spring Boot application) or just the specific HTTP request ?
It is true that an unhandled runtime exception will instantly shut down normal Java application, right ?
Will an unhandled runtime exception stop the whole server (i.e. Spring Boot application) or just the specific HTTP request ?
Just the current HTTP request ... assuming that the exception is thrown by the request thread. (If it is thrown by a child thread, it won't even terminate the HTTP request.)
It is true that an unhandled runtime exception will instantly shut down normal Java application, right?
Actually, not right.
An uncaught exception causes the thread on which it was thrown to terminate. In a simple Java application where there is only one (non-daemon) "main" thread, and an uncaught exception occurs on that thread, the JVM then exit because the last non-daemon thread has terminated.
But if there are other threads ... the uncaught exception doesn't stop the JVM.
To illustrate, run this program:
public class Test {
public static void main(String[] args) {
new Thread(() -> {
try {
Thread.sleep(10000);
} catch (Exception e) {
System.err.println("Caught " + e);
}
}).start();
throw new RuntimeException("Goodbye cruel world");
}
}
When you run this, you will observe that there is a 10 second delay between the stacktrace printing (from the uncaught exception in main) and the program actually ending. The JVM is waiting for the child thread to terminate.
This could also indirectly answer your main question ... except:
We don't know for sure if the request threads are daemon threads or not.
We don't know for sure that the request threads are not actually catching / handling the exceptions thrown by your request processing code.
Suffice it to say that a framework will typically do something sensible depending one what the thrown exception is. For example, a framework might handle Error exceptions on a worker thread by attempting a shutdown ... on the basis that the error may have put the JVM into a potentially unsafe / non-recoverable state.
Just the thread in question
An unhandled RuntimeException will kill the thread, not the application. Usually each HTTP request is handled by its own thread, so nothing else in the server should be affected. I don’t know how the server handles the dead thread. A wise move would probably be creating a new thread for upcoming requests.
Only when all threads (except so-called demon threads) are terminated, will your Spring Boot application stop. This in turn means that in an application that isn’t threaded — one that has everything running in the main thread — an uncaught exception (RuntimeException or some other type) will bring the whole program down.
In most applications it matters which thread is killed. Your application probably has more threads handling HTTP requests and can spawn new ones as needed. In this case a thread death will hardly be noticed except by the user having sent this HTTP request. Other threads are probably more vital to the functioning of the server, and such a thread being killed, while not stopping the server completely, will stop it from working properly. For a different example, if the event dispatching thread in a desktop application is killed by an exception, the application can no longer accept user input.
Try it out for yourself
It doesn’t take a lot to try it out. The following program spawns a thread that throws an unhandled exception.
public class DemoUnhandledException {
public static void main(String[] args) throws InterruptedException {
new Thread(() -> { throw new RuntimeException("Demo"); }).start();
TimeUnit.MINUTES.sleep(1);
System.out.println("A minute later the main thread is still alive and well");
}
}
Let the program run for a full minute to get the full output, which is:
Exception in thread "Thread-0" java.lang.RuntimeException: Demo
at ovv.so.exception.DemoUnhandledException.lambda$0(DemoUnhandledException.java:8)
at java.base/java.lang.Thread.run(Thread.java:834)
A minute later the main thread is still alive and well
Link
How uncaught exceptions are handled in Java on Javamex
Will an unhandled runtime exception stop the whole server (i.e. Spring Boot application) or just the specific HTTP request ?
There is usually a thread pool in web server to handle all requests(e.g tomcat). Uncaught exceptions that occur in your controller will eventually be caught somewhere, so it will not cause the worker thread in the thread pool to be killed.
Tomcat#AbstractEndpoint#processSocket:
Executor executor = getExecutor();
if (dispatch && executor != null) {
// handle request in thread poll
executor.execute(sc);
}
I did a simple debug. Assuming that an unhandle exception occurred in your controller, then the exception was actually caught by FrameworkServlet#processRequest. After the capture, it will be wrapped into a NestedServletException and continue to be thrown to the upper layer. Eventually, it will be caught and printed in StandardWrapperValve#invoke.
I have some method with render() name which includes very difficult logic and I call it so in my code
Future<File> fileFuture = null;
try {
fileFuture = executor.getThreadPoolExecutor()
.submit(() -> render());
return fileFuture.get(10,TimeUnit.SECONDS);
} catch (TimeoutException e) {
fileFuture.cancel(true);
throw new MyTimeOutException(e);
}
}
render() method opens some I/O resources and when job doesn't finish
till timeout I get such error when timeout happens
java.nio.channels.ClosedByInterruptException
I investigated this problem and found that this happens because
I/O resources stay still open and Thread cannot be terminated
till these resources won't be closed.
But isn't any way for avoiding this exception
I just need to stop my async method when timeout happens.
and I also want add that I use Spring boot if there is any solution with
Spring please tell me
This exception ClosedByInterruptException will be thrown when a thread performing a blocking read or write on an InterruptibleChannel, like a FileChannel, it's interrupted.
When you call fileFuture.cancel(true) the boolean flag indicates that if the task is blocked, the thread may be interrupted. So, as the thread is blocked in a read or write operation it throws a ClosedByInterruptException.
If you want to get rid of the ClosedByInterruptException, then you render() method to tackle with ClosedByInterruptException, to end the process gracefully if it's thrown.
I'm trying to understand concurrent execution in Java, but given this code :
class Inter extends Thread {
public void run() {
System.out.println("Starting...");
try {
sleep(10000);
} catch (InterruptedException e) {
System.out.println("Interrupted."); }
System.out.println("Finished.");
}
public static void main(String[] args) {
Inter hi = new Inter();
hi.start();
System.out.println("Sending interruption...");
hi.interrupt();
System.out.println("Sent.");
}
}
I don't know why always give me this trace :
Sending interruption...
Sent.
Starting...
Interrupted.
Finished.
No matter how many times I run :
$ java Inter
As fars as I know in Java, when we execute the start() method in a new thread, the execution of this thread starts.
So , since the main thread and the Inter thread are concurrently executed, why can't be this a possible trace, ?
Starting..
Sending interruption..
Sent
Interrupted
Finished
So, since the main thread and the Inter thread are concurrently executed, why can't be this a possible trace?
Yes, it can. If you run your program a thousand times, most probably you will have that output at least once.
It's up to the operating system thread scheduler to arrange the threads execution in order to give that possible output, but we have no control over the scheduler. Hence, the importance of properly designing your code to prevent race conditions.
Is it possible to recover java thread by doing the next?
Thread.setDefaultExceptionHandler(new UncaughtExceptionHandler() {
public void unchaughtException(Thread t, Throwable e) {
t.start();
}
});
Yes it is possible to run a Thread in a Thread.UncaughtExceptionHandler.uncaughtException ... provided that the Thread hasn't been started previously.
But it is NOT possible to start the Thread that was pass as the t argument. That will (always) be a Thread that has already been started and has terminated.
You can start a given Thread at most once. If you try to start one a second time you will get an InvalidStateException. Always.
No, you cannot run the thread that threw the exception, as shown in your code. It has already run. That's how it threw the exception. A thread cannot be started more than once.
I have an application that runs headless and needs to be able to notify administrators of problems. I built an email notification framework for it to use, and basically if an exception is thrown and caught, depending on the area in code and the criticality of the exception, it may be passed into the alert notification framework which triggers an email to go out to the listed admins with the stack trace and other debug info.
This works, pretty well.
Before deploying, I was putting it through it's paces. One of my tests is to throw an unhandled exception from a random spot in code - simulating a potentially serious runtime problem where exceptions are being thrown that we did not expect.
For example, here is a method I inserted the test unhandled exception:
/**
* Closes connection.
*/
public void closeConnection() {
if (true)
throw new NullPointerException("Test unexpected exception NPE");
LOG.info("Closing SFTP connection");
getSftpChannel().exit();
getSession().disconnect();
LOG.debug("SFTP Connection closed");
}
The code runs, and when it gets to this unhandled exception, the program hardlocks (because exception is thrown and the sftp connection thread is never closed, so it holds the jvm open until the connection timeout).
I had thought it would crash the JVM, or pass it up to it's caller which eventually would bubble up to the alert system.
In this scenario, I had assumed this NPE would throw out of this exception, isn't handled by it's caller or the caller's caller, etc, so it should bubble to the main() and then crash the JVM since even main does not catch Exception or NPE's.
Question: What is going on here and how can I ensure a scenario like this won't hang in production? Do I just had a huge catch-all catch clause to my main() and have it catch all Exception so that every exception gets handled?
EDIT FOR CLARITY: The question is more-or-less -- Why does an unhandled exception that is not explicitly thrown in a method's signature, nor handled by a caller, not crash the JVM?
To answer your question: Why does an unhandled exception that is not explicitly thrown in a method's signature, nor handled by a caller, not crash the JVM?
Assuming this code is running in a Thread, the reason is, that unless you've set an UnhandledExceptionHandler on either the Thread class or the Thread instance, then the 'main' thread group is the default UnhandledExceptionHandler. By default, the thread group handles unhandled exceptions by logging the stack trace to system.out, the thread 'dies' and the JVM does not crash.
You may want to consider implementing an UnhandledExceptionHandler that uses your email framework to notify you of these failures.
As other posters have suggested, the code should clean up resources like Channels and Session in a finally block.
use a finally block to ensure the connection is closed.
public void closeConnection() {
try {
if (true)
throw new NullPointerException("Test unexpected exception NPE");
} finally {
LOG.info("Closing SFTP connection");
getSftpChannel().exit();
getSession().disconnect();
LOG.debug("SFTP Connection closed");
}
}