java - 委托(delegate)类中 getApplicationContext() 的 NullPointerException

Question

我有第二个类，我从中调用一些方法来处理应用程序更新。其中有一个进度对话框，因此我必须将当前应用程序上下文传递给其构造函数。当直接在 MainActivity onCreate() 中调用时，代码工作正常，但当我将代码委托(delegate)给外部类时，代码会崩溃。出了什么问题？

OnCreate() 中的方法调用:

private AppUpdateHelper appUpdateHelper = new AppUpdateHelper(getApplicationContext());

@Override
    protected void onCreate(Bundle savedInstanceState) {
    appUpdateHelper.handleAppUpdate();
    }

委托(delegate)类:

public class AppUpdateHelper {

    private Context mContext;

    public AppUpdateHelper(Context mContext) {
        this.mContext = mContext;
    }

    public void handleAppUpdate() {
        String versionCode = getVersionCode(); // Get app's current version code

        // Is app update to date?
        if (isAppCurrent(versionCode)) {
            promptAppUpdate();
        }
    }


    private String getVersionCode() {
        String versionCode = null;
        try {
            PackageInfo pInfo = mContext.getPackageManager().getPackageInfo(mContext.getPackageName(), 0);
            versionCode = pInfo.versionName;
            // Log.w(mContext.getClass().getSimpleName(), "Current Version: " + versionCode);
        } catch (PackageManager.NameNotFoundException e) {
            e.printStackTrace();
        }
        return versionCode;
    }


    private boolean isAppCurrent(String versionCode) {
        ParseInstallation installation = ParseInstallation.getCurrentInstallation();
        String userAppVersion = installation.getString("appVersion");
        return !userAppVersion.equals(versionCode);
    }


    private void promptAppUpdate() {
        SweetAlertDialog pDialog = new SweetAlertDialog(mContext, SweetAlertDialog.WARNING_TYPE);
        pDialog.setTitleText("Update Available!");
        pDialog.setContentText("You must update to continue using Yeet Club!");
        pDialog.setConfirmClickListener(sDialog -> {
            final String appPackageName = mContext.getPackageName(); // getPackageName() from Context or Activity object
            try {
                mContext.startActivity(new Intent(Intent.ACTION_VIEW, Uri.parse("market://details?id=" + appPackageName)));
            } catch (ActivityNotFoundException anfe) {
                mContext.startActivity(new Intent(Intent.ACTION_VIEW, Uri.parse("https://play.google.com/store/apps/details?id=" + appPackageName)));
            }

            sDialog.dismissWithAnimation();
        });
        pDialog.setCancelable(false);
        pDialog.showCancelButton(false);
        pDialog.show();
    }

}

异常(exception):

08 - 20 15: 43: 43.874 21456 - 21456 / com.app.android E / AndroidRuntime: FATAL EXCEPTION: main
Process: com.app.android, PID: 21456
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo {
 com.app.android / com.app.android.activity.MainActivity
}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()'
on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java: 2458)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java: 2613)
at android.app.ActivityThread.access$900(ActivityThread.java: 180)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java: 1473)
at android.os.Handler.dispatchMessage(Handler.java: 111)
at android.os.Looper.loop(Looper.java: 207)
at android.app.ActivityThread.main(ActivityThread.java: 5710)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java: 900)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java: 761)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()'
on a null object reference
at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java: 117)
at com.app.android.activity.MainActivity. < init > (MainActivity.java: 77)
at java.lang.Class.newInstance(Native Method)
at android.app.Instrumentation.newActivity(Instrumentation.java: 1072)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java: 2448)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java: 2613) 
at android.app.ActivityThread.access$900(ActivityThread.java: 180) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java: 1473) 
at android.os.Handler.dispatchMessage(Handler.java: 111) 
at android.os.Looper.loop(Looper.java: 207) 
at android.app.ActivityThread.main(ActivityThread.java: 5710) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java: 900) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java: 761) 

Answer 1

这很简单。

调用OnCreate后可以获得上下文实例

private AppUpdateHelper appUpdateHelper;

@Override
     protected void onCreate(Bundle savedInstanceState) {
     appUpdateHelper = new AppUpdateHelper(getApplicationContext());
     appUpdateHelper.handleAppUpdate();
    }