简单来说，堆栈跟踪是应用程序在引发
Exception时处于中间的方法调用的列表。
简单的例子
通过问题中给出的示例，我们可以准确确定在应用程序中引发异常的位置。让我们看一下堆栈跟踪：
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，我们可以判断出错误发生在哪里。我们正在寻找的是应用程序中最顶层的方法调用。在这种情况下，它是：
at com.example.myproject.Book.getTitle(Book.java:16)
为了调试它，我们可以打开Book.java并查看line 16，它是：
15 public String getTitle() {16 System.out.println(title.toString());17 return title;18 }这将表明上面的代码中有（可能是title）某些内容null。
带有一系列异常的示例
有时，应用程序会捕获异常并将其重新引发为另一个异常的原因。通常看起来像：
34 public void getBookIds(int id) {35 try {36 book.getId(id); // this method it throws a NullPointerException on line 2237 } catch (NullPointerException e) {38 throw new IllegalStateException("A book has a null property", e)39 }40 }这可能会给你一个堆栈跟踪，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
与此不同的是
Caused by。有时，例外会有多个
Caused by部分。对于这些，你通常希望找到“根本原因”，这将是堆栈跟踪中最低的“Cause by”部分之一。在我们的例子中是：
Caused by: java.lang.NullPointerException <-- root cause at com.example.myproject.Book.getId(Book.java:22) <-- important line
同样，对于此例外，我们会想看看行22的Book.java，看看有什么可能导致
NullPointerException这里。
库代码更令人生畏的示例
通常，堆栈跟踪要比上面的两个示例复杂得多。这是一个示例（虽然很长，但是展示了多个级别的链接异常）：
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 moreCaused 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 moreCaused 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，还有更多。我们最关心的是从代码中寻找方法，这些方法可能是com.example.myproject程序包中的任何方法。在上面的第二个示例中，我们首先要查找根本原因，即：
Caused by: java.sql.SQLException
但是，该方法下的所有方法调用都是库代码。因此，我们将移至其上方的“ Caused by”，并寻找源于我们代码的第一个方法调用，即：
at com.example.myproject.MyEntityService.save(MyEntityService.java:59)
就像在前面的例子中，我们应该看看MyEntityService.java就行59，因为这是此错误的起源（这个有点明显出了什么问题，因为SQLException中发生的错误，但调试程序是什么，我们以后是）。
