How to read and understand the java stack trace? [duplicate]

For example, I got a stack trace like this:

java.lang.NullPointerException
abc.investxa.presentation.controllers.UnixServerJobController.handleRequest(UnixServerJobController.java:66)
org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter.handle(SimpleControllerHandlerAdapter.java:48)
org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:875)
org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:807)
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:571)
org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:501)
javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:96)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)

So what is the root cause of this exception? From the stack trace, I found out that there is a problem with doFilter function in the OncePerRequestFilter class! However, when I put a break point there and the program never stop at that break point.

Could anyone give explanation about this!? And in general case, how should I use that stack case for debugging (read from bottom to top or from top to bottom)!

Solution 1:

You should generally read from the top - so in this case, there's a NullPointerException at line 66 of UnixServerJobController, in the handleRequest method. That method was called by SimpleControllerHandlerAdapter.handle, which was called by DispatcherServlet.doDispatch etc.

However, in this particular case it's likely that the first frame of the stack trace is all you need. Look at line 66 of UnixServerJobController, work out what might be null, and act accordingly.

Note that sometimes one exception is wrapped in another (which may in turn be wrapped in another, etc). In this case you should look at each of the stack traces - often it's the "most nested" exception which gives the most useful information, as that's the root cause.

Solution 2:

Generally the exact reason for the Exception is at the first line of your Stack Trace, and for more information about the cause of that exception, you need to gradually move down, and the root cause can often be found somewhere near the bottom of the stack trace.

But in most cases, you can even get the cause of the exception from the first few lines..

So, in this case, your exception is at handleRequest method, and when you move downwards, those are the methods, which invoked your previous method (The one at above the current method in stack trace)

Solution 3:

This tutorial might shed some light on your problem and help you understand things better.

As per your problem, you seem to have a Null Pointer Exception at line 66 of the Unix Server Job Controller class.

Solution 4:

I found this tutorial is highly useful to me https://forums.bukkit.org/threads/how-to-read-stack-traces-and-troubleshoot-your-own-plugins-by-yourself.32457/