什么是堆栈跟踪,如何使用它来调试应用程序错误?

有时,当我运行我的应用程序时,它给我一个错误,看起来像:

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)

人们将其称为“堆栈跟踪”。什么是堆栈跟踪?关于程序中发生的错误,它能告诉我什么?

关于这个问题-我经常看到一个问题,这个问题是新手程序员“遇到错误”的地方,他们只是粘贴了堆栈跟踪和一些随机的代码块,却不了解堆栈跟踪是什么或如何使用它。该问题旨在为可能需要帮助来了解堆栈跟踪值的新手程序员提供参考。

回答:

简而言之,堆栈跟踪是在抛出Exception时应用程序处于中间的方法调用的列表。

简单的例子

使用问题中给出的示例,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title)某物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

这与“ Caused by”不同。有时,例外会有多个“由…引起”部分。对于这些,您通常希望找到“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 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,我们将向上移至其上方的“ 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)。

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