Android attachBaseContext方法中获取ge

首先我们来看下Application的继承关系

package android.app;
public class Application extends ContextWrapper implements ComponentCallbacks2 {
}

从这里可以看出Application继承自ContextWrapper，所以当我们在调用Application的getApplicationContext时，实际上调用的是ContextWapper的getApplicationContext

package android.content;
public class ContextWrapper extends Context {
    Context mBase;
    protected void attachBaseContext(Context base) {
        if (mBase != null) {
            throw new IllegalStateException("Base context already set");
        }
        mBase = base;
    }
    
    @Override
    public Context getApplicationContext() {
        return mBase.getApplicationContext();
    }
}

那这个mBase.getApplicationContext()获取的是啥？因为mBase是个接口，它的实现类是ContextImpl，所以我们来具体看看mBase.getApplicationContext()的实现

package android.app;
class ContextImpl extends Context {
    final @NonNull LoadedApk mPackageInfo;
    @Override
    public Context getApplicationContext() {
        return (mPackageInfo != null) ? mPackageInfo.getApplication() : mMainThread.getApplication();
    }
}

看到这里疑问又来了，这完全是俄罗斯套娃啊！mPackageInfo.getApplication()获取的又是啥？

package android.app;
public final class LoadedApk {
    private Application mApplication;
    Application getApplication() {
        return mApplication;
    }
}

搜嘎，原来getApplicationContext()获取到的就是Application啊！

休息会儿让我喝口水歇歇......

上面讲到getApplicationContext()获取到的就是Application的，那为啥在attachBaseContext方法中获取getApplicationContext()为空呢？嘿嘿，我也想问......

接下来我们继续来分析看看attachBaseContext是在什么时候调用的

package android.app;
public class Application extends ContextWrapper implements ComponentCallbacks2 {
    public LoadedApk mLoadedApk;
    final void attach(Context context) {
        attachBaseContext(context);
        mLoadedApk = ContextImpl.getImpl(context).mPackageInfo;
    }
}

最后根据上面一步步的分析，getApplicationContext()获取到的就是Application对象，而Application对象是通过mPackageInfo.getApplication()获取的，而mPackageInfo(mLoadedApk)是在attachBaseContext之后才被赋值的，所以你在attachBaseContext方法中获取getApplicationContext()当然是null啦！！！