访问静态字符串会因“空指针异常”而崩溃

【问题标题】：Accessing a static String crashes with "Null Pointer Exception"访问静态字符串会因“空指针异常”而崩溃
【发布时间】：2015-03-18 22:50:46
【问题描述】：

MyService 类包含静态字符串

package com.suprabhatam.alarm;

import android.app.Service;
import android.content.Intent;
import android.media.MediaPlayer;
import android.os.IBinder;
import android.util.Log;

import java.text.SimpleDateFormat;
import java.util.Calendar;

public class MyService extends Service {
    private static final String TAG = "MyService";
    static public MediaPlayer mp;
    final static public String start_pause_stop_audio = "start_pause_stop_audio";
    final static public String start_audio = "start_audio";
    final static public String pause_audio = "pause_audio";
    final static public String stop_audio = "stop_audio";

    @Override
    public int onStartCommand (Intent intent, int flags, int startId) {
        Calendar cal = Calendar.getInstance();
        cal.getTime();
        SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss");
        Log.d(TAG, sdf.format(cal.getTime()));

        if (mp == null) {
            mp = MediaPlayer.create(this, R.raw.venkateshwara_suprabhatam);
        }

        final String s_s_audio = intent.getStringExtra(MyService.start_pause_stop_audio);
        Log.d(TAG, "onStartCommand: " + s_s_audio);
        if (s_s_audio.equals(start_audio)) {
            if (!mp.isPlaying()) {
                mp.start();
            } else {
                Log.d(TAG, "already playing");
            }
        } else if (s_s_audio.equals(pause_audio)) {
            mp.pause();
        } else if (s_s_audio.equals(stop_audio)) {
            mp.seekTo(0);
            mp.pause();
        } else {
            Log.d(TAG, "UNKNOWN: " + s_s_audio);
        }

        return START_STICKY;
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
    }

    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }
}

使应用程序崩溃，并在 adb 输出中显示以下消息。

ANRAppManager﹕ !!! It is not under singleton mode, U can't use it. !!!

只有当应用程序不在内存中（kill）并且 MyReceiver 尝试访问静态字符串时才会发生崩溃。

看起来静态字符串在创建对象之前无效。在 C++ 中，静态字符串进入数据段并在应用程序启动时有效，java 中的规则是否不同？。

更新 1：添加 logcat 消息

01-20 21:53:32.849    2396-2396/com.suprabhatam.alarm D/jdwp﹕ sendBufferedRequest : len=0x45
01-20 21:53:32.854    2396-2396/com.suprabhatam.alarm W/asset﹕ AssetManager-->addDefaultAssets CIP path not exsit!
01-20 21:53:32.875    2396-2396/com.suprabhatam.alarm D/dalvikvm﹕ open_cached_dex_file : /data/app/com.suprabhatam.alarm-1.apk /data/dalvik-cache/data@app@com.suprabhatam.alarm-1.apk@classes.dex
01-20 21:53:32.897    2396-2396/com.suprabhatam.alarm D/MyService﹕ 21:53:32
01-20 21:53:32.959    2396-2396/com.suprabhatam.alarm D/MediaPlayer﹕ Don't notify duration to com.suprabhatam.alarm!
01-20 21:53:32.983    2396-2396/com.suprabhatam.alarm D/AndroidRuntime﹕ Shutting down VM
01-20 21:53:32.983    2396-2396/com.suprabhatam.alarm W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x41bea9a8)
01-20 21:53:32.984    2396-2396/com.suprabhatam.alarm W/dalvikvm﹕ threadid=1: uncaught exception occurred
01-20 21:53:32.984    2396-2396/com.suprabhatam.alarm W/System.err﹕ java.lang.RuntimeException: Unable to start service com.suprabhatam.alarm.MyService@421b4720 with null: java.lang.NullPointerException
01-20 21:53:32.984    2396-2396/com.suprabhatam.alarm W/System.err﹕ at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2867)
01-20 21:53:32.984    2396-2396/com.suprabhatam.alarm W/System.err﹕ at android.app.ActivityThread.access$1900(ActivityThread.java:165)
01-20 21:53:32.984    2396-2396/com.suprabhatam.alarm W/System.err﹕ at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1470)
01-20 21:53:32.984    2396-2396/com.suprabhatam.alarm W/System.err﹕ at android.os.Handler.dispatchMessage(Handler.java:107)
01-20 21:53:32.984    2396-2396/com.suprabhatam.alarm W/System.err﹕ at android.os.Looper.loop(Looper.java:194)
01-20 21:53:32.984    2396-2396/com.suprabhatam.alarm W/System.err﹕ at android.app.ActivityThread.main(ActivityThread.java:5391)
01-20 21:53:32.985    2396-2396/com.suprabhatam.alarm W/System.err﹕ at java.lang.reflect.Method.invokeNative(Native Method)
01-20 21:53:32.985    2396-2396/com.suprabhatam.alarm W/System.err﹕ at java.lang.reflect.Method.invoke(Method.java:525)
01-20 21:53:32.985    2396-2396/com.suprabhatam.alarm W/System.err﹕ at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
01-20 21:53:32.985    2396-2396/com.suprabhatam.alarm W/System.err﹕ at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
01-20 21:53:32.985    2396-2396/com.suprabhatam.alarm W/System.err﹕ at dalvik.system.NativeStart.main(Native Method)
01-20 21:53:32.985    2396-2396/com.suprabhatam.alarm W/System.err﹕ Caused by: java.lang.NullPointerException
01-20 21:53:32.986    2396-2396/com.suprabhatam.alarm W/System.err﹕ at com.suprabhatam.alarm.MyService.onStartCommand(MyService.java:34)
01-20 21:53:32.986    2396-2396/com.suprabhatam.alarm W/System.err﹕ at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2850)
01-20 21:53:32.986    2396-2396/com.suprabhatam.alarm W/System.err﹕ ... 10 more
01-20 21:53:32.986    2396-2396/com.suprabhatam.alarm W/dalvikvm﹕ threadid=1: calling UncaughtExceptionHandler
01-20 21:53:32.987    2396-2396/com.suprabhatam.alarm E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start service com.suprabhatam.alarm.MyService@421b4720 with null: java.lang.NullPointerException
            at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2867)
            at android.app.ActivityThread.access$1900(ActivityThread.java:165)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1470)
            at android.os.Handler.dispatchMessage(Handler.java:107)
            at android.os.Looper.loop(Looper.java:194)
            at android.app.ActivityThread.main(ActivityThread.java:5391)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.suprabhatam.alarm.MyService.onStartCommand(MyService.java:34)
            at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2850)
            at android.app.ActivityThread.access$1900(ActivityThread.java:165)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1470)
            at android.os.Handler.dispatchMessage(Handler.java:107)
            at android.os.Looper.loop(Looper.java:194)
            at android.app.ActivityThread.main(ActivityThread.java:5391)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
            at dalvik.system.NativeStart.main(Native Method)

UPDATE 2 秒崩溃

01-20 21:56:23.332    2793-2799/com.suprabhatam.alarm D/jdwp﹕ handlePacket : cmd=0x1, cmdSet=0xC7, len=0x13, id=0x400000DF, flags=0x0, dataLen=0x8
01-20 21:56:42.336    2793-2805/com.suprabhatam.alarm D/ANRAppManager﹕ !!! It is not under singleton mode, U can't use it. !!!
01-20 21:57:33.091    2793-2793/com.suprabhatam.alarm I/Process﹕ Sending signal. PID: 2793 SIG: 9
01-20 21:57:38.252    3008-3008/com.suprabhatam.alarm D/MediaPlayer﹕ Don't notify duration to com.suprabhatam.alarm!
01-20 21:57:38.260    3008-3008/com.suprabhatam.alarm D/AndroidRuntime﹕ Shutting down VM
01-20 21:57:38.260    3008-3008/com.suprabhatam.alarm W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x41bea9a8)
01-20 21:57:38.260    3008-3008/com.suprabhatam.alarm W/dalvikvm﹕ threadid=1: uncaught exception occurred
01-20 21:57:38.260    3008-3008/com.suprabhatam.alarm W/System.err﹕ java.lang.RuntimeException: Unable to start service com.suprabhatam.alarm.MyService@421af568 with null: java.lang.NullPointerException
01-20 21:57:38.260    3008-3008/com.suprabhatam.alarm W/System.err﹕ at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2867)
01-20 21:57:38.260    3008-3008/com.suprabhatam.alarm W/System.err﹕ at android.app.ActivityThread.access$1900(ActivityThread.java:165)
01-20 21:57:38.260    3008-3008/com.suprabhatam.alarm W/System.err﹕ at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1470)
01-20 21:57:38.260    3008-3008/com.suprabhatam.alarm W/System.err﹕ at android.os.Handler.dispatchMessage(Handler.java:107)
01-20 21:57:38.260    3008-3008/com.suprabhatam.alarm W/System.err﹕ at android.os.Looper.loop(Looper.java:194)
01-20 21:57:38.261    3008-3008/com.suprabhatam.alarm W/System.err﹕ at android.app.ActivityThread.main(ActivityThread.java:5391)
01-20 21:57:38.261    3008-3008/com.suprabhatam.alarm W/System.err﹕ at java.lang.reflect.Method.invokeNative(Native Method)
01-20 21:57:38.261    3008-3008/com.suprabhatam.alarm W/System.err﹕ at java.lang.reflect.Method.invoke(Method.java:525)
01-20 21:57:38.261    3008-3008/com.suprabhatam.alarm W/System.err﹕ at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
01-20 21:57:38.261    3008-3008/com.suprabhatam.alarm W/System.err﹕ at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
01-20 21:57:38.261    3008-3008/com.suprabhatam.alarm W/System.err﹕ at dalvik.system.NativeStart.main(Native Method)
01-20 21:57:38.261    3008-3008/com.suprabhatam.alarm W/System.err﹕ Caused by: java.lang.NullPointerException
01-20 21:57:38.261    3008-3008/com.suprabhatam.alarm W/System.err﹕ at com.suprabhatam.alarm.MyService.onStartCommand(MyService.java:34)
01-20 21:57:38.261    3008-3008/com.suprabhatam.alarm W/System.err﹕ at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2850)
01-20 21:57:38.261    3008-3008/com.suprabhatam.alarm W/System.err﹕ ... 10 more
01-20 21:57:38.261    3008-3008/com.suprabhatam.alarm W/dalvikvm﹕ threadid=1: calling UncaughtExceptionHandler
01-20 21:57:38.262    3008-3008/com.suprabhatam.alarm E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start service com.suprabhatam.alarm.MyService@421af568 with null: java.lang.NullPointerException
            at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2867)
            at android.app.ActivityThread.access$1900(ActivityThread.java:165)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1470)
            at android.os.Handler.dispatchMessage(Handler.java:107)
            at android.os.Looper.loop(Looper.java:194)
            at android.app.ActivityThread.main(ActivityThread.java:5391)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.suprabhatam.alarm.MyService.onStartCommand(MyService.java:34)
            at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2850)
            at android.app.ActivityThread.access$1900(ActivityThread.java:165)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1470)
            at android.os.Handler.dispatchMessage(Handler.java:107)
            at android.os.Looper.loop(Looper.java:194)
            at android.app.ActivityThread.main(ActivityThread.java:5391)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
            at dalvik.system.NativeStart.main(Native Method)
01-20 21:57:58.221    3008-3020/com.suprabhatam.alarm D/ANRAppManager﹕ !!! It is not under singleton mode, U can't use it. !!!
01-20 22:02:38.277    3008-3008/com.suprabhatam.alarm I/Process﹕ Sending signal. PID: 3008 SIG: 9

已确定更新 3 崩溃位置

崩溃发生在 MyService.java 的第 32 行

最终字符串 s_s_audio = intent.getStringExtra(MyService.start_pause_stop_audio)

01-20 22:22:42.230 6257-6257/com.suprabhatam.alarm E/MyService﹕异常 java.lang.NullPointerException 在 com.suprabhatam.alarm.MyService.onStartCommand(MyService.java:32) 在 android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2850) 在 android.app.ActivityThread.access$1900(ActivityThread.java:165)

问题似乎出在 intent.getStringExtra() 方法上

广播接收器发送以下事件。

Intent myIntent = new Intent(context, MyService.class);
myIntent.putExtra(MyService.start_pause_stop_audio, MyService.start_audio);
context.startService(myIntent);

如何调试这个问题。

【问题讨论】：

  • 嗯，我很确定静态字符串不是一个原因......在主线程上做长的事情是相当问题......但如果没有完整的 logcat 日志就很难说跨度>
  • 在阅读了这个线程stackoverflow.com/questions/12482744/…之后，我认为静态字符串可能是个问题，因为“类加载器”不会加载 MyService 类。
  • 我不认为它与静态字符串有关（它们在类加载时和每个进程的基础上加载）。
  • MyService 中是否有任何静态初始化块？还是复杂的类静态变量？ ...
  • What is a Null Pointer Exception, and how do I fix it?的可能重复

标签： android


【解决方案1】：

在进行任何操作之前，您应该检查在onStartCommand() 中收到的意图参数是否为空。 这是因为 Intent 如果服务被杀死然后被系统重新启动并且您不要求重新发送原始意图，则可以为 null。

请深入查看here，并最终将onStartCommand()的结果值更改为START_REDELIVER_INTENT，

【讨论】：

    【解决方案2】：

    更好的解决方案是从一个公共类中获取您的字符串，例如 YourConstants.java 和该类中的所有成员： 公共静态最终字符串；

    然后，当您需要它们时，无论您的项目在哪里，只需调用 YourConstants.NAME_OFF_YOUR_VARIABLE 就不会出现这个问题了。

    我总是将 Constants 类用于我的公共静态最终变量。

    【讨论】：

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