简单来说，stack trace是一个方法调用的列表，该应用程序在抛出异常时处于中间状态.
Simple Example
通过问题中给出的示例，我们可以准确地确定在应用程序中引发异常的位置.让我们看看堆栈跟踪:
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)
这是一个非常简单的堆栈跟踪.如果我们从"at…"列表的开头开始，我们可以知道我们的错误发生在哪里.我们要找的是应用程序中的topmost方法调用.在这种情况下，它是:
at com.example.myproject.Book.getTitle(Book.java:16)
要调试它，我们可以打开Book.java，看看第16行，它是:
15 public String getTitle() {
16 System.out.println(title.toString());
17 return title;
18 }
在上面的代码中，这将表示某个值(可能是title)是null.
Example with a chain of exceptions
有时，应用程序会捕获异常并将其作为另一个异常的原因重新抛出.这通常如下所示:
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 }
这可能会给您一个堆栈跟踪，如下所示:
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
这一次的不同之处在于"原因".有时异常会有多个"原因"部分.对于这些问题，您通常希望找到"根本原因"，它将是堆栈跟踪中"起因"最低的部分之一.在我们的 case 中，它是:
Caused by: java.lang.NullPointerException <-- root cause
at com.example.myproject.Book.getId(Book.java:22) <-- important line
再一次，除了这个例外，我们想看看Book.java的第22行，看看是什么导致了这里的NullPointerException.
More daunting example with library code个
通常，堆栈跟踪比上述两个示例要复杂得多.下面是一个示例(这是一个很长的示例，但演示了链接异常的几个级别):
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) <-- 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
在这个例子中，还有很多.我们最关心的是寻找来自our code的方法，也就是com.example.myproject包中的任何方法.从上面的第二个例子中，我们首先要寻找根本原因，即:
Caused by: java.sql.SQLException
但是，该下的所有方法调用都是库代码.所以我们将从move up到"Caused by" aboveit，在"由"挡路引起的那个中，查找first method call originating from our code，它是:
at com.example.myproject.MyEntityService.save(MyEntityService.java:59)
与前面的示例一样，我们应该查看第59行的MyEntityService.java，因为这是这个错误的起因(这个错误有点明显，因为SQLException声明了错误，但是调试过程是我们想要的).