简单聊一聊Throwable

Posted by Gityuan on June 20, 2016

一.概述

Android有一套异常处理机制, 分析Crash时最常见的便是先查看其调用栈stackTrace. 对于调用栈, 是从下往上调用的. 其中经常会遇到”Caused by”, 以及”… 8 more”等信息, 具体是什么含义呢, 本文就是解答这个问题.

1.1 初始化Throwable对象

[-> Throwable.java]

public Throwable(String detailMessage, Throwable cause) {
    this.detailMessage = detailMessage;
    this.cause = cause;
    this.stackTrace = EmptyArray.STACK_TRACE_ELEMENT;
    fillInStackTrace(); // [见小节1.2]
}

1.2 fillInStackTrace

[-> Throwable.java]

public Throwable fillInStackTrace() {
    if (stackTrace == null) {
        return this; 
    }
    stackState = nativeFillInStackTrace();
    stackTrace = EmptyArray.STACK_TRACE_ELEMENT;
    return this;
}

二. 打印调用栈

2.1 printStackTrace

[-> Throwable.java]

public void printStackTrace() {
    printStackTrace(System.err);
}

public void printStackTrace(PrintStream err) {
    try {
        printStackTrace(err, "", null); //[见小节2.2]
    } catch (IOException e) {
        throw new AssertionError();
    }
}

2.2 printStackTrace

[-> Throwable.java]

private void printStackTrace(Appendable err, String indent, StackTraceElement[] parentStack)
    throws IOException {
    err.append(toString()); //[见小节2.2.1]
    err.append("\n");

    StackTraceElement[] stack = getInternalStackTrace(); //[见小节2.2.2]
    if (stack != null) {
        //parentStack为null, 则duplicates=0;
        int duplicates = parentStack != null ? countDuplicates(stack, parentStack) : 0;
        for (int i = 0; i < stack.length - duplicates; i++) {
            err.append(indent);
            err.append("\tat ");
            err.append(stack[i].toString());
            err.append("\n");
        }

        if (duplicates > 0) {
            ...
        }
    }

    //打印suppressed异常
    if (suppressedExceptions != null) {
        for (Throwable throwable : suppressedExceptions) {
            err.append(indent);
            err.append("\tSuppressed: ");
            throwable.printStackTrace(err, indent + "\t", stack);
        }
    }

    Throwable cause = getCause();  //[见小节2.2.3]
    if (cause != null) {
        err.append(indent);
        err.append("Caused by: ");
        cause.printStackTrace(err, indent, stack); //[见小节2.2.4]
    }
}

其中indent为空.

2.2.1 toString

[-> Throwable.java]

public String toString() {
    //获取detailMessage
    String msg = getLocalizedMessage();
    //获取exception类名
    String name = getClass().getName();
    if (msg == null) {
        return name;
    }
    return name + ": " + msg;
}

这是输出的第一行.例如:

  1. detailMessage为”Error receiving broadcast Intent”;
  2. name 为java.lang.RuntimeException;

则组合为: java.lang.RuntimeException: Error receiving broadcast Intent

2.2.2 getInternalStackTrace

private StackTraceElement[] getInternalStackTrace() {
    if (stackTrace == EmptyArray.STACK_TRACE_ELEMENT) {
        // 当stackTrace为空, 则获取native调用栈
        stackTrace = nativeGetStackTrace(stackState); 
        stackState = null;
        return stackTrace;
    } else if (stackTrace == null) {
        return EmptyArray.STACK_TRACE_ELEMENT;
    } else {
      return stackTrace;
    }
}

2.2.3 getCause

public Throwable getCause() {
    if (cause == this) {
        return null;
    }
    return cause;
}

默认cause等于this, 当创建的Throwable带有cause的话, 则返回的便是新的Throwable对象.

2.3 打印Caused by

[-> Throwable.java]

private void printStackTrace(Appendable err, String indent, StackTraceElement[] parentStack)
        throws IOException {
    err.append(toString());
    err.append("\n");

    StackTraceElement[] stack = getInternalStackTrace();
    if (stack != null) {
        //获取栈帧的重复个数 [见小节2.3.1]
        int duplicates = parentStack != null ? countDuplicates(stack, parentStack) : 0;
        // 从栈顶往下依次输出 调用栈 直到跟parentStack重复的栈为止.
        for (int i = 0; i < stack.length - duplicates; i++) {
            err.append(indent);
            err.append("\tat ");
            err.append(stack[i].toString());
            err.append("\n");
        }

        if (duplicates > 0) {
            //最终以省略号结尾, 并带上栈帧重复的个数
            err.append(indent);
            err.append("\t... ");
            err.append(Integer.toString(duplicates));
            err.append(" more\n");
        }
    }

    if (suppressedExceptions != null) {
        ...
    }

    Throwable cause = getCause();
    if (cause != null) {
        ... //第二次cause为空,一般为空. 也存在级联情况
    }
}

2.3.1 countDuplicates

private static int countDuplicates(StackTraceElement[] currentStack,
        StackTraceElement[] parentStack) {
    int duplicates = 0;
    int parentIndex = parentStack.length;
    for (int i = currentStack.length; --i >= 0 && --parentIndex >= 0;) {
        StackTraceElement parentFrame = parentStack[parentIndex];
        //比如栈帧相同的个数
        if (parentFrame.equals(currentStack[i])) {
            duplicates++;
        } else {
            break;
        }
    }
    return duplicates;
}

比较parentStack和当前currentStack, 分别从栈底开始比较, 最终得到栈帧重复个数duplicates

三. 实例

举例说明:

java.lang.RuntimeException: Error receiving broadcast Intent{}
in com.android.server.notification.NotificationManagerService$3@ef5346c
at android.app.LoadedApk$ReceiverDispatcher$Args.run(LoadedApk.java:1134)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at com.android.server.SystemServer.run(SystemServer.java:352)
at com.android.server.SystemServer.main(SystemServer.java:220)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:874)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:764)

Caused by: java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.ArrayList.get(ArrayList.java:411)
at com.android.server.notification.NotificationManagerService$NotificationRankers.onUserSwitched(NotificationManagerService.java:3912)
at com.android.server.notification.NotificationManagerService$3.onReceive(NotificationManagerService.java:804)
at android.app.LoadedApk$ReceiverDispatcher$Args.run(LoadedApk.java:1124)
... 8 more

可以看出, Args.run()的1134行抛出异常RuntimeException; 造成这个异常的原因是该方法的1124行代码出现IndexOutOfBoundsException(数组越界). 这两个stack栈底重复的栈帧个数为8个. 既然是重复了8个, 那么展开就等价于:

java.lang.RuntimeException: Error receiving broadcast Intent{}
in com.android.server.notification.NotificationManagerService$3@ef5346c
at android.app.LoadedApk$ReceiverDispatcher$Args.run(LoadedApk.java:1134)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at com.android.server.SystemServer.run(SystemServer.java:352)
at com.android.server.SystemServer.main(SystemServer.java:220)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:874)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:764)

Caused by: java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.ArrayList.get(ArrayList.java:411)
at com.android.server.notification.NotificationManagerService$NotificationRankers.onUserSwitched(NotificationManagerService.java:3912)
at com.android.server.notification.NotificationManagerService$3.onReceive(NotificationManagerService.java:804)
at android.app.LoadedApk$ReceiverDispatcher$Args.run(LoadedApk.java:1124)
//以下部分是duplicates部分
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at com.android.server.SystemServer.run(SystemServer.java:352)
at com.android.server.SystemServer.main(SystemServer.java:220)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:874)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:764)
微信公众号 Gityuan | 微博 weibo.com/gityuan | 博客 留言区交流
CATALOG
    标签
    android 组件系列 else debug 权限 web tool java performance app algorithm 进程系列 虚拟机 memory jvm linux binder ipc handler process power 系统启动 AMS PMS 自学编程 stability 组件 art graphic NativeDebug 实战案例 flutter