facebook登录空指针异常android [英] facebook login null pointer exception android

问题描述

我通过 stackoverflow 搜索了很多，但找不到解决我问题的答案.因此我发布了这个.

I searched a lot through stackoverflow but I couldn't find an answer that resolves my issue. Hence I am posting this.

我有一个示例 Android 应用程序，我在其中尝试使用 facebook 登录.以下是我遵循的步骤:

I have a sample Android App in which I'm trying to use facebook Login. Here are the steps that I followed:

环境:Android工作室Android SDK 版本:22Facebook SDK:4.0

Environment: Android studio Android SDK version : 22 Facebook SDK : 4.0

按照步骤在模拟器上安装FB APK，生成Developer hash key并在app->设置中更新它.我的 android manifest 有这些代码

Followed the steps to install FB APK on the emulator, generated the Developer hash key and updated it in app-> settings. My android manifest has these code

<uses-permission android:name="android.permission.INTERNET"/>

<activity android:name="com.facebook.FacebookActivity"
        android:configChanges="keyboard|keyboardHidden|screenLayout|screenSize|orientation"
        android:theme="@android:style/Theme.Translucent.NoTitleBar" android:label="@string/app_name" />

<meta-data android:name="com.facebook.sdk.ApplicationId" android:value="@string/facebook_app_id"/>

在我的activity_main.xml布局文件中，我有这段代码

In my activity_main.xml layout file, I have this piece of code

<com.facebook.login.widget.LoginButton
    android:id="@+id/fblogin_button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center_horizontal"
    android:layout_marginTop="30dp"
    android:layout_marginBottom="30dp" />

最后在我的主要活动中我有这个

And finally in my main Activity I have this

public class MainActivity extends Activity {
LoginButton loginButton;
CallbackManager callbackManager;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    FacebookSdk.sdkInitialize(getApplicationContext());
    callbackManager = CallbackManager.Factory.create();
    loginButton = (LoginButton) findViewById(R.id.fblogin_button);
    loginButton.registerCallback(callbackManager, new FacebookCallback<LoginResult>() {
        @Override
        public void onSuccess(LoginResult loginResult) {
            Toast.makeText(getApplicationContext(),"Fb Login Success",Toast.LENGTH_LONG);
        }

        @Override
        public void onCancel() {
            Toast.makeText(getApplicationContext(),"Fb on cancel",Toast.LENGTH_LONG);
        }

        @Override
        public void onError(FacebookException e) {
            Toast.makeText(getApplicationContext(),"Fb Login Error",Toast.LENGTH_LONG);
        }
    });
    setContentView(R.layout.activity_main);

}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data){
    super.onActivityResult(requestCode, resultCode, data);
    Log.d("Inside Fb login","on Activiry result");
    callbackManager.onActivityResult(requestCode, resultCode, data);
}}

我收到一个空指针异常，如下所示

I am getting a null pointer exception as shown below

 java.lang.RuntimeException: Unable to start activity componentInfo{PROJECT.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.facebook.login.widget.LoginButton.registerCallback(com.facebook.CallbackManager, com.facebook.FacebookCallback)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
        at android.app.ActivityThread.access$800(ActivityThread.java:144)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5221)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

我在这里错过了什么?

推荐答案

看起来你得到了一个 NullPointerException 因为你在调用之前调用了 findViewById()setContentView()，所以当你调用loginButton.registerCallback()时loginButton为空.

Looks like you're getting a NullPointerException because you're calling findViewById() before you call setContentView(), so loginButton is null when you call loginButton.registerCallback().

只需将调用 setContentView() 移动到顶部:

Just move the call to setContentView() to the top:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    FacebookSdk.sdkInitialize(getApplicationContext());

    setContentView(R.layout.activity_main); //Moved up here to resolve NPE

    callbackManager = CallbackManager.Factory.create();
    loginButton = (LoginButton) findViewById(R.id.fblogin_button);
    loginButton.registerCallback(callbackManager, new FacebookCallback<LoginResult>() {
        @Override
        public void onSuccess(LoginResult loginResult) {
            Toast.makeText(getApplicationContext(),"Fb Login Success",Toast.LENGTH_LONG);
        }

        @Override
        public void onCancel() {
            Toast.makeText(getApplicationContext(),"Fb on cancel",Toast.LENGTH_LONG);
        }

        @Override
        public void onError(FacebookException e) {
            Toast.makeText(getApplicationContext(),"Fb Login Error",Toast.LENGTH_LONG);
        }
    });

}

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