什麼是Stacktrace?

Stacktrace（堆棧跟蹤）是一個非常有用的調試工具. 在未捕獲的異常被拋出時（或者手動製造堆棧跟蹤的時候）它讓你看到你調到的堆（意思是，在某一點調用方法的堆). 不僅顯示出出現錯誤的地方, 也顯出程序在那個地方是如何結束的. 所以還要知道如下問題:

什麼是 Exception?

Exception（異常）是運行環境告訴你出現的錯誤. 比較常見的有 NullPointerException, IndexOutOfBoundsException 和ArithmeticException. 這些都是在不正確做事情時發生的錯誤.比如但試圖指向一個空對象時會拋出一個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.

怎麼使用堆棧跟蹤處理異常?

每當應用程序崩潰, Stack trace 會被寫進控制檯 (這種情況下, 在logcat裏). 該 stack trace 解決問題的重要信息.

Android Studio

I在窗口底部欄, 點擊 Android Monitor 按鈕. 還可以按 alt+6. Devices 面板選中設備或模擬器. 然後, 找出紅色顯示的 stack trace. logcat可能有很多信息，最快的方式找出stack trace是clear一下 (使用左側回收站圖標按鈕),使應用再崩潰一次.

Eclipse

略

簡單示例

在堆棧跟蹤中，我們可以看到在應用程序中異常被拋出的位置 :

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)

這是一個簡單的stack trace.從 "at ..."開頭的列表, 我們可以看到哪裏發生了錯誤. 先看最上面的一行:

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

調試它,我們可以雙擊(Book.java:16)打開 Book.java 並且遊標會指向第 16行:

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

這表明上面代碼的某個對象(或許是 title) 是 null .

異常鏈的示例：

有時候應用程序會捕獲一個異常並把它作爲另一個異常的原因拋出. 經典模式如下:

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

這可能會給你的stack trace 像這樣:

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

與上個stack trace例子區別的是多了 "Caused by". 有時候，異常會有多個 "Caused by"節段. 這種情況下, 你通常需要找到 "root cause"（根本原因）, 通常是在stack trace最下面 "Caused by" 節段. 在這裏:

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

同樣，在此例，我們要看的是Book.java 的第22行，是什麼導致出現 NullPointerException 的.

含有庫的示例：

通常 stack traces 比上面兩個例子複雜的多. 如下這個例子 (雖然長, 但有着 chained exceptions的幾個層次):

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

這個例子中有很多信息. 我們最關心的是要找到自己代碼中的方法, 從第二個例子就知道，首先要找出 root cause（根本原因）:

Caused by: java.sql.SQLException

然而，所有的方法都實在庫代碼下調用的. 所以向上面的 "Caused by" 移動, 尋找是自己代碼調用的方法:

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

象在前面的例子,我們應該看向 MyEntityService.java 的第59行，這是錯誤出現的地方（這裏明顯不同，因爲即使 SQLException 出現了error,但調試是從我們的代碼開始的).

代碼控制stack trace

Throwable family 提供的很多個stack trace特性-可以操縱 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)

操縱 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，在內部類中:

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會出現:

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)