Facebook登录无法在我的Android应用中正常工作

我正在尝试在我的应用程序中实现Facebook登录，但是每当我启动我的应用程序时，它就会崩溃.我正在关注 facebook开发人员网站.这是我的代码.

I am trying to implement facebook login in my app but whenever i launch my app it crashes. I am following facebook developer site. Here is my code.

MainActivity.java

MainActivity.java

package com.algor7.samplelogin;
...
public class MainActivity extends Activity {

private LoginButton loginButton;
private CallbackManager callbackManager;
private AccessTokenTracker accessTokenTracker;
private AccessToken accessToken;
private ProfileTracker profileTracker;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    FacebookSdk.sdkInitialize(this.getApplicationContext());
    callbackManager = CallbackManager.Factory.create();

    accessTokenTracker = new AccessTokenTracker() {
        @Override
        protected void onCurrentAccessTokenChanged(
                AccessToken oldAccessToken,
                AccessToken currentAccessToken) {
            // Set the access token using
            // currentAccessToken when it's loaded or set.
        }
    };
    // If the access token is available already assign it.
    accessToken = AccessToken.getCurrentAccessToken();

    profileTracker = new ProfileTracker() {
        @Override
        protected void onCurrentProfileChanged(
                Profile oldProfile,
                Profile currentProfile) {
            // App code
        }
    };

    loginButton.findViewById(R.id.login_button);
    loginButton.setReadPermissions("email");
    loginButton.registerCallback(callbackManager, new FacebookCallback<LoginResult>() {
        @Override
        public void onSuccess(LoginResult loginResult) {
            // App code
        }

        @Override
        public void onCancel() {
            // App code
        }

        @Override
        public void onError(FacebookException exception) {
            // App code
        }
    });

}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    callbackManager.onActivityResult(requestCode, resultCode, data);
}

@Override
public void onDestroy() {
    super.onDestroy();
    accessTokenTracker.stopTracking();
    profileTracker.stopTracking();
}

}

activity_main.xml

activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center"
tools:context=".MainActivity">

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

</RelativeLayout>

strings.xml

strings.xml

<resources>
<string name="app_name">SampleLogin</string>
<string name="facebook_app_id">4932151xxxxxxxx</string>
<string name="fb_login_protocol_scheme">fb4932151xxxxxxxx</string>

AndroidManifest.xml

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.algor7.samplelogin" >

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

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >

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

    <activity
        android:name=".MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

    <activity
        android:name="com.facebook.CustomTabActivity"
        android:exported="true">
        <intent-filter>
            <action android:name="android.intent.action.VIEW" />
            <category android:name="android.intent.category.DEFAULT" />
            <category android:name="android.intent.category.BROWSABLE" />
            <data android:scheme="@string/fb_login_protocol_scheme" />
        </intent-filter>
    </activity>

    <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" />

</application>

build.gradle(项目:)

build.gradle(Project:)

buildscript {
repositories {
    mavenCentral()
}
 dependencies {
    classpath 'com.android.tools.build:gradle:1.3.0'

    // NOTE: Do not place your application dependencies here; they belong
    // in the individual module build.gradle files
 }
}

allprojects {
  repositories {
    jcenter()
  }
}

build.gradle(Module:app)

build.gradle(Module:app)

apply plugin: 'com.android.application'

android {
compileSdkVersion 23
buildToolsVersion "23.0.3"

defaultConfig {
    applicationId "com.algor7.samplelogin"
    minSdkVersion 17
    targetSdkVersion 23
    versionCode 1
    versionName "1.0"
}
buildTypes {
    release {
        minifyEnabled false
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }
}
}

dependencies {
compile fileTree(dir: 'libs', include: ['*.jar'])
compile 'com.android.support:appcompat-v7:24.0.0-alpha2'
compile 'com.facebook.android:facebook-android-sdk:[4,5)'
}

Logcat:

--------- beginning of crash
05-16 15:28:57.915    2478-2478/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.algor7.samplelogin, PID: 2478
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.algor7.samplelogin/com.algor7.samplelogin.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View com.facebook.login.widget.LoginButton.findViewById(int)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
        at android.app.ActivityThread.-wrap11(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:148)
        at android.app.ActivityThread.main(ActivityThread.java:5417)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View com.facebook.login.widget.LoginButton.findViewById(int)' on a null object reference
        at com.algor7.samplelogin.MainActivity.onCreate(MainActivity.java:54)
        at android.app.Activity.performCreate(Activity.java:6251)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
        at android.app.ActivityThread.-wrap11(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:148)
        at android.app.ActivityThread.main(ActivityThread.java:5417)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

请帮助我解决此问题.预先感谢！

Please help me to solve this issue. Thanks in advance!