NullPointerException in Service's constructor

问题

In my Android project, I have a Service :

public class MyService extends Service{

    //I defined a explicite contructor 
    public MyService(){
       //NullPointerException here, why?
       File dir = getDir("my_dir", Context.MODE_PRIVATE);
    }

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

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
    super.onStartCommand(intent, flags, startId);
        ...
    }

}

I know normally, I shouldn't explicitly define a constructor for Android Service. But in my case, I want to use the constructor to create a directory in my internal storage.

When I try it, I got NullPointerException, which I don't understand why? Could anyone explain to me the reason why I got NullPoineterException when call getDir(...) in my explicite constructor ?

回答1:

getDir() is a method in Context.

The service is not set up to be used as a Context until onCreate().

Basically, the framework sets up services (and activities for that matter) the following way:

Create the object
Set up the Context, with appropriate resources for the current configuration
Call onCreate() on the object

You cannot do anything that requires the object to be a properly set up Context at step 1 i.e. during member variable initialization or constructor.

回答2:

You have to use service lifecycle function onCreate(). onCreate function run only once in a service lifecycle and works as constructor. If you want it to update anywhere then you can put it in onStartCommand(Intent intent, int flags, int startId).

回答3:

Probably Context is still null in your service constructor, and getDir is using Context for its internal workings. You can check it in following sources:

http://androidxref.com/4.4.2_r1/xref/frameworks/base/core/java/android/content/ContextWrapper.java#244

243    @Override
244    public File getDir(String name, int mode) {
245        return mBase.getDir(name, mode);
246    }

so if mBase is null then you will get NPE

mBase will be set in constructor:

56    public ContextWrapper(Context base) {
57        mBase = base;
58    }

Service extends ContextWrapper, but service constructor is calling directly this constructor with null value:

http://androidxref.com/4.4.2_r1/xref/frameworks/base/core/java/android/app/Service.java#294

294    public Service() {
295        super(null);
296    }

so you must wait for Service.onCreate(), or (if you can) use some globally known Context like from you aplication object.

回答4:

As @laalto said The service is not set up to be used as a Context until onCreate()..

So make one global variable of Context.

Context ctx;

Also make Constructor for your MyService class should be.

public MyService(Context mContext){

   this.ctx = mContext;
   File dir = ctx.getDir("my_dir", Context.MODE_PRIVATE);
}

来源：https://stackoverflow.com/questions/21088474/nullpointerexception-in-services-constructor

标签

android

android-service