What is stack trace, and how to use it to find errors in application development?

In simple terms, the stack trace is a list of methods that were called before the exception occurred in the application.

Simple case

In this example, we can determine exactly when the exception occurred. Consider the stack trace:

Exception in thread "main" java.lang.NullPointerException
       at com.example.myproject.Book.getTitle(Book.java:16)
       at com.example.myproject.Author.getBookTitles(Author.java:25)
       at com.example.myproject.Bootstrap.main(Bootstrap.java:14)

This is an example of a very simple trace. If we go through the list of lines like "at..." from the very beginning, we can understand where the error occurred. We are looking at upper function call. In our case, this is:

at com.example.myproject.Book.getTitle(Book.java:16)

To debug this fragment, open Book.java and see what is on the line 16:

public String getTitle() {
    System.out.println(title.toString()); <-- line 16
    return title;
}

This means that in the above code snippet, some variable (probably title) has the value null.

Example of an exception chain

Sometimes applications catch an exception and throw it as another exception. It usually looks like so:

try {
    ....
} catch (NullPointerException e) {
    throw new IllegalStateException("A book has a null property", e)
}

The trace in this case can look like this:

Exception in thread "main" java.lang.IllegalStateException: A book has a null property
       at com.example.myproject.Author.getBookIds(Author.java:38)
       at com.example.myproject.Bootstrap.main(Bootstrap.java:14)
Caused by: java.lang.NullPointerException
       at com.example.myproject.Book.getId(Book.java:22)
       at com.example.myproject.Author.getBookIds(Author.java:35)
       ... 1 more

In this case, the difference is in the "Caused by" attribute." ("What caused it"). Sometimes exceptions can have multiple "Caused by" sections. Usually, you need to find the original cause, which turns out to be in the very last (bottom) "Caused by" section of the trace. In our case, this is:

Caused by: java.lang.NullPointerException <-- root cause
       at com.example.myproject.Book.getId(Book.java:22) <-- important line

Similarly, with such an exception, you need to refer to the line 22 of the book Book.java to find out that caused this exception– NullPointerException.

Another scary example with the library code

As a rule, the trace has a much more complex appearance than in the cases discussed above. Here is an example (a long trace showing several levels of exception chains):

javax.servlet.ServletException: Произошло что–то ужасное
   at com.example.myproject.OpenSessionInViewFilter.doFilter(OpenSessionInViewFilter.java:60)
   at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
   at com.example.myproject.ExceptionHandlerFilter.doFilter(ExceptionHandlerFilter.java:28)
   at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
   at com.example.myproject.OutputBufferFilter.doFilter(OutputBufferFilter.java:33)
   at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
   at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
   at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
   at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
   at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
   at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
   at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
   at org.mortbay.jetty.Server.handle(Server.java:326)
   at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
   at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:943)
   at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:756)
   at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:218)
   at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
   at org.mortbay.jetty.bio.SocketConnector$Connection.run(SocketConnector.java:228)
   at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)
Caused by: com.example.myproject.MyProjectServletException
   at com.example.myproject.MyServlet.doPost(MyServlet.java:169)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
   at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
   at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
   at com.example.myproject.OpenSessionInViewFilter.doFilter(OpenSessionInViewFilter.java:30)
   ... 27 more
Caused by: org.hibernate.exception.ConstraintViolationException: could not insert: [com.example.myproject.MyEntity]
   at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:96)
   at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:66)
   at org.hibernate.id.insert.AbstractSelectingDelegate.performInsert(AbstractSelectingDelegate.java:64)
   at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:2329)
   at org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:2822)
   at org.hibernate.action.EntityIdentityInsertAction.execute(EntityIdentityInsertAction.java:71)
   at org.hibernate.engine.ActionQueue.execute(ActionQueue.java:268)
   at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:321)
   at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:204)
   at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:130)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:210)
   at org.hibernate.event.def.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:56)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:195)
   at org.hibernate.event.def.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:50)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:93)
   at org.hibernate.impl.SessionImpl.fireSave(SessionImpl.java:705)
   at org.hibernate.impl.SessionImpl.save(SessionImpl.java:693)
   at org.hibernate.impl.SessionImpl.save(SessionImpl.java:689)
   at sun.reflect.GeneratedMethodAccessor5.invoke(Unknown Source)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
   at java.lang.reflect.Method.invoke(Method.java:597)
   at org.hibernate.context.ThreadLocalSessionContext$TransactionProtectionWrapper.invoke(ThreadLocalSessionContext.java:344)
   at $Proxy19.save(Unknown Source)
   at com.example.myproject.MyEntityService.save(MyEntityService.java:59) <-- relevant call (see notes below)
   at com.example.myproject.MyServlet.doPost(MyServlet.java:164)
   ... 32 more
Caused by: java.sql.SQLException: Violation of unique constraint MY_ENTITY_UK_1: duplicate value(s) for column(s) MY_COLUMN in statement [...]
   at org.hsqldb.jdbc.Util.throwError(Unknown Source)
   at org.hsqldb.jdbc.jdbcPreparedStatement.executeUpdate(Unknown Source)
   at com.mchange.v2.c3p0.impl.NewProxyPreparedStatement.executeUpdate(NewProxyPreparedStatement.java:105)
   at org.hibernate.id.insert.AbstractSelectingDelegate.performInsert(AbstractSelectingDelegate.java:57)
   ... 54 more

This example shows a far from complete call stack. What is of the greatest interest here is the search for functions from of our code - from the package com.example.myproject. In in the previous example, we first wanted to find the "root cause", namely:

Caused by: java.sql.SQLException

However, all method calls in this case refer to the library code. So we'll go to the previous "Caused by" section and find the first method call from our code, namely:

at com.example.myproject.MyEntityService.save(MyEntityService.java:59)

Similar to the previous examples, you should pay attention to MyEntityService.java, line 59: this is where the error appeared (in this case, the situation is quite obvious, since the error is reported by SQLException, but in in this question, we are considering the debugging procedure using tracing).

_{Translation of the response: "What is a stack trace, and how can I use it to debug my application errors?" @Rob Hruska}