如果程序是第一次启动，则缓存中没有数据，因此必须从服务器（使用 asyncTask）下载数据并保存到文件中。
public static String filename = "channels.txt";
FileOutputStream fos = null;
ObjectOutputStream out = null;
try {
fos = openFileOutput(filename, Context.MODE_APPEND);
out = new ObjectOutputStream(fos);
out.writeObject(ChannelManager.mInstance);
out.close();
fos.close();
}
主要思想是我序列化对象并将其写入文件以供以后使用。
然后我尝试阅读它：
FileInputStream fis = null;
ObjectInputStream in = null;
try {
fis = openFileInput(filename);
in = new ObjectInputStream(fis);
mChannelManager = (ChannelManager) in.readObject();
Log.e("mChannelManager", mChannelManager.getChannel_list().get(3) + "");
in.close();
fis.close();
}
如果我在写入文件后尝试读取它，它可以正常工作，但是如果我关闭或终止应用程序并尝试从文件中读取，我会得到空指针异常：/
我尝试将文件名保存到共享首选项中并使用它，但仍然出现空指针异常（认为是字符串/对象/内存引用问题）
那么问题在哪里（或可能是）？
错误日志：
10-07 10:54:15.773 10630-10647/? E/CMC->NioClient3﹕ NioClient thread get exception e: I/O exception while read message:
10-07 10:54:30.123 10741-10741/? E/APKInstallReceiver﹕ context = android.app.ReceiverRestrictedContext@41597d68, intent.getAction() = android.intent.action.PACKAGE_REMOVED, intent.getDataString() = package:com.iptviq.mobile.android.skynettv
10-07 10:54:31.393 12624-12624/com.iptviq.mobile.android.skynettv E/IN CACHE﹕ +
10-07 10:54:31.413 12624-12624/com.iptviq.mobile.android.skynettv E/Returning channel_list﹕ null
10-07 10:54:31.423 12624-12624/com.iptviq.mobile.android.skynettv E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.iptviq.mobile.android.skynettv/com.iptviq.mobile.android.engine.SkynetTvMainActivity}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
at android.app.ActivityThread.access$600(ActivityThread.java:141)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:5039)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.iptviq.mobile.android.engine.SkynetTvMainActivity.onCreate(SkynetTvMainActivity.java:202)
at android.app.Activity.performCreate(Activity.java:5104)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
at android.app.ActivityThread.access$600(ActivityThread.java:141)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:5039)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
at dalvik.system.NativeStart.main(Native Method)
编辑2：
@Override
public void onCreate(Bundle bundle) {
super.onCreate(bundle);
setContentView(R.layout.activity_main);
activity = this;
preferences = getSharedPreferences("settings", MODE_PRIVATE);
SERVER_IP = preferences.getString("serverIP", "192.168.37.14");
SERVERPORT = preferences.getString("serverPort", "8221");
areChannelsInCache = preferences.getBoolean("areChannelsInCache", false);
filename = preferences.getString("fileName", filename);
if (!areChannelsInCache) {
Log.e("NOT IN CACHE", "+");
mDownloadChannelsTask = new DownloadChannelsTask();
mDownloadChannelsTask.execute();
} else {
Log.e("IN CACHE", "+");
FileInputStream fis = null;
ObjectInputStream in = null;
try {
fis = openFileInput(filename);
in = new ObjectInputStream(fis);
mChannelManager = (ChannelManager) in.readObject();
Log.e("mChannelManager", mChannelManager.getChannel_list().get(3) + "");
in.close();
fis.close();
} catch (IOException ex) {
Log.e("mChannelManager", "error1 on reading " + ex);
ex.printStackTrace();
} catch (ClassNotFoundException ex) {
Log.e("mChannelManager", "error2 on reading " + ex);
ex.printStackTrace();
}
}
}
AsyncTask 工作正常（下载数据）。完成后，它调用正在保存到文件的处理程序：
FileOutputStream fos = null;
ObjectOutputStream out = null;
try {
fos = openFileOutput(filename, Context.MODE_APPEND);
out = new ObjectOutputStream(fos);
out.writeObject(ChannelManager.mInstance);
out.close();
fos.close();
Log.e("SAVED TO FILE", ":)");
preferences.edit().putBoolean("areChannelsInCache", true).commit();
preferences.edit().putString("fileName", filename).commit();
} catch (IOException ex) {
Log.e("error @ main on saving data to file", ":(( " + ex);
ex.printStackTrace();
}