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    <p>In simple terms, a <strong>stack trace</strong> is a list of the method calls that the application was in the middle of when an Exception was thrown.</p> <p><strong>Simple Example</strong></p> <p>With the example given in the question, we can determine exactly where the exception was thrown in the application. Let's have a look at the stack trace:</p> <pre><code>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) </code></pre> <p>This is a very simple stack trace. If we start at the beginning of the list of "at ...", we can tell where our error happened. What we're looking for is the <strong>topmost</strong> method call that is part of our application. In this case, it's:</p> <pre><code>at com.example.myproject.Book.getTitle(Book.java:16) </code></pre> <p>To debug this, we can open up <code>Book.java</code> and look at line <code>16</code>, which is:</p> <pre><code>15 public String getTitle() { 16 System.out.println(title.toString()); 17 return title; 18 } </code></pre> <p>This would indicate that something (probably <code>title</code>) is <code>null</code> in the above code.</p> <p><strong>Example with a chain of exceptions</strong></p> <p>Sometimes applications will catch an Exception and re-throw it as the cause of another Exception. This typically looks like:</p> <pre><code>34 public void getBookIds(int id) { 35 try { 36 book.getId(id); // this method it throws a NullPointerException on line 22 37 } catch (NullPointerException e) { 38 throw new IllegalStateException("A book has a null property", e) 39 } 40 } </code></pre> <p>This might give you a stack trace that looks like:</p> <pre><code>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:36) ... 1 more </code></pre> <p>What's different about this one is the "Caused by". Sometimes exceptions will have multiple "Caused by" sections. For these, you typically want to find the "root cause", which will be one of the lowest "Caused by" sections in the stack trace. In our case, it's:</p> <pre><code>Caused by: java.lang.NullPointerException &lt;-- root cause at com.example.myproject.Book.getId(Book.java:22) &lt;-- important line </code></pre> <p>Again, with this exception we'd want to look at line <code>22</code> of <code>Book.java</code> to see what might cause the <code>NullPointerException</code> here.</p> <p><strong>More daunting example with library code</strong></p> <p>Usually stack traces are much more complex than the two examples above. Here's an example (it's a long one, but demonstrates several levels of chained exceptions):</p> <pre><code>javax.servlet.ServletException: Something bad happened 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) &lt;-- 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 </code></pre> <p>In this example, there's a lot more. What we're mostly concerned about is looking for methods that are from <em>our code</em>, which would be anything in the <code>com.example.myproject</code> package. From the second example (above), we'd first want to look down for the root cause, which is:</p> <pre><code>Caused by: java.sql.SQLException </code></pre> <p>However, all the method calls under that are library code. So we'll move up to the "Caused by" above it, and look for the first method call originating from our code, which is:</p> <pre><code>at com.example.myproject.MyEntityService.save(MyEntityService.java:59) </code></pre> <p>Like in previous examples, we should look at <code>MyEntityService.java</code> on line <code>59</code>, because that's where this error originated (this one's a bit obvious what went wrong, since the SQLException states the error, but the debugging procedure is what we're after).</p>
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    1. POWhat is a stack trace, and how can I use it to debug my application errors?
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    1. USRob Hruska
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    1. POWhat is a stack trace, and how can I use it to debug my application errors?
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    1. COYou've not explained the possibility of having multiple root causes in the stacktrace.
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      1. USBuhake Sindi
    2. CO@The Elite Gentleman - Well, there's ultimately only one root cause, no? The bottom-most "Caused by" will be the root. I could probably get a trace that has multiple "cause" chaining. I'll see what I can do.
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    3. CO@Rob Hruska, sorry, I meant that you can potentially have more than 1 "Caused By" in a stacktrace.
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