使用Stacktrace处理异常

什么是Stacktrace?

 Stacktrace（堆栈跟踪）是一个非常有用的调试工具. 在未捕获的异常被抛出时（或者手动制造堆栈跟踪的时候）它让你看到你调到的堆（意思是，在某一点调用方法的堆). 不仅显示出出现错误的地方, 也显出程序在那个地方是如何结束的. 所以还要知道如下问题:

什么是 Exception?

Exception（异常）是运行环境告诉你出现的错误. 比较常见的有 NullPointerException, IndexOutOfBoundsException 和ArithmeticException. 这些都是在不正确做事情时发生的错误.比如但试图指向一个空对象时会抛出一个NullPointerException :

Object a = null;
a.toString();    //this line throws a NullPointerException

Object[] b = new Object[5];
System.out.println(b[10]);    //this line throws an IndexOutOfBoundsException,
                              //because b is only 5 elements long
int ia = 5;
int ib = 0;
ia = ia/b;                      //this line throws an  ArithmeticException with the 
                              //message "/ by 0", because you are trying to
                              //divide by 0, which is not possible.

怎么使用堆栈跟踪处理异常?

每当应用程序崩溃, Stack trace 会被写进控制台 (这种情况下, 在logcat里). 该 stack trace 解决问题的重要信息.

Android Studio

I在窗口底部栏, 点击 Android Monitor 按钮. 还可以按 alt+6.  Devices 面板选中设备或模拟器. 然后, 找出红色显示的 stack trace. logcat可能有很多信息，最快的方式找出stack trace是clear一下 (使用左侧回收站图标按钮),使应用再崩溃一次.

Eclipse

略

简单示例

在堆栈跟踪中，我们可以看到在应用程序中异常被抛出的位置 :

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)

这是一个简单的stack trace.从 "at ..."开头的列表, 我们可以看到哪里发生了错误. 先看最上面的一行:

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

调试它,我们可以双击(Book.java:16)打开 Book.java 并且游标会指向第 16行:

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

这表明上面代码的某个对象(或许是 title) 是 null .

异常链的示例：

有时候应用程序会捕获一个异常并把它作为另一个异常的原因抛出. 经典模式如下:

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

这可能会给你的stack trace 像这样:

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

与上个stack trace例子区别的是多了 "Caused by". 有时候，异常会有多个 "Caused by"节段. 这种情况下, 你通常需要找到 "root cause"（根本原因）,  通常是在stack trace最下面 "Caused by" 节段. 在这里:

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

同样，在此例，我们要看的是Book.java 的第22行，是什么导致出现 NullPointerException 的.

含有库的示例：

通常 stack traces 比上面两个例子复杂的多. 如下这个例子 (虽然长, 但有着 chained exceptions的几个层次):

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

这个例子中有很多信息. 我们最关心的是要找到自己代码中的方法, 从第二个例子就知道，首先要找出 root cause（根本原因）:

Caused by: java.sql.SQLException

然而，所有的方法都实在库代码下调用的. 所以向上面的 "Caused by" 移动, 寻找是自己代码调用的方法:

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

象在前面的例子,我们应该看向 MyEntityService.java 的第59行，这是错误出现的地方 （这里明显不同，因为即使 SQLException 出现了error,但调试是从我们的代码开始的).

代码控制stack trace

 Throwable family 提供的很多个stack trace特性-可以操纵 stack trace 信息.如：

1，普通程序:

package test.stack.trace;

public class SomeClass {

    public void methodA() {
        methodB();
    }

    public void methodB() {
        methodC();
    }

    public void methodC() {
        throw new RuntimeException();
    }

    public static void main(String[] args) {
        new SomeClass().methodA();
    }
}

Stack trace:

Exception in thread "main" java.lang.RuntimeException
    at test.stack.trace.SomeClass.methodC(SomeClass.java:18)
    at test.stack.trace.SomeClass.methodB(SomeClass.java:13)
    at test.stack.trace.SomeClass.methodA(SomeClass.java:9)
    at test.stack.trace.SomeClass.main(SomeClass.java:27)

操纵 stack trace:

package test.stack.trace;

public class SomeClass {

    ...

    public void methodC() {
        RuntimeException e = new RuntimeException();
        e.setStackTrace(new StackTraceElement[]{
                new StackTraceElement("OtherClass", "methodX", "String.java", 99),
                new StackTraceElement("OtherClass", "methodY", "String.java", 55)
        });
        throw e;
    }

    public static void main(String[] args) {
        new SomeClass().methodA();
    }
}

Stack trace:

Exception in thread "main" java.lang.RuntimeException
    at OtherClass.methodX(String.java:99)
    at OtherClass.methodY(String.java:55)

2，在内部类中:

public class Test {

    private static void privateMethod() {
        throw new RuntimeException();
    }

    public static void main(String[] args) throws Exception {
        Runnable runnable = new Runnable() {
            @Override public void run() {
                privateMethod();
            }
        };
        runnable.run();
    }
}

stack trace会出现:

Exception in thread "main" java.lang.RuntimeException
        at Test.privateMethod(Test.java:4)
        at Test.access$000(Test.java:1)
        at Test$1.run(Test.java:10)
        at Test.main(Test.java:13)

另外，还可以使用断点控制堆栈跟踪的进程。

持续完善中。。。

参考http://stackoverflow.com/questions/3988788/what-is-a-stack-trace-and-how-can-i-use-it-to-debug-my-application-errors

参考http://stackoverflow.com/questions/23353173/unfortunately-myapp-has-stopped-how-can-i-solve-this

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