启动android应用程序/活动时发生意外错误

我遇到了一个问题，当我尝试启动应用程序时，我遇到了一个意外错误。我试图解决它，但我的尝试没有成功

代码如下：

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

import android.app.Activity;
import android.content.Intent;
import android.content.pm.PackageInfo;
import android.content.pm.PackageManager;
import android.content.pm.PackageManager.NameNotFoundException;
import android.content.pm.Signature;
import android.os.Bundle;
import android.util.Base64;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.animation.AlphaAnimation;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

import com.facebook.LoggingBehavior;
import com.facebook.Session;
import com.facebook.SessionState;
import com.facebook.Settings;


public class LoginActivity extends Activity {

    private AlphaAnimation alphaDown;
    private AlphaAnimation alphaUp;
    private Button fbloginbtn;
    private Session.StatusCallback statusCallback = new SessionStatusCallback();

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.home);

        alphaDown = new AlphaAnimation(1.0f, 0.3f);
        alphaUp = new AlphaAnimation(0.3f, 1.0f);
        alphaDown.setDuration(100);
        alphaUp.setDuration(100);
        alphaDown.setFillAfter(true);
        alphaUp.setFillAfter(true);
        fbloginbtn = (Button)findViewById(R.id.fbloginbtn);

        Settings.addLoggingBehavior(LoggingBehavior.INCLUDE_ACCESS_TOKENS);

        // Add Session Callback Here
        Session.getActiveSession().addCallback(statusCallback);

        Session session = Session.getActiveSession();
        if(session == null) {
            if(savedInstanceState != null) {
                session = Session.restoreSession(this, null, statusCallback, savedInstanceState);
            }
            if(session== null) {
                session = new Session(this);
            }
            Session.setActiveSession(session);
            if (session.getState().equals(SessionState.CREATED_TOKEN_LOADED)) {
                session.openForRead(new Session.OpenRequest(this).setCallback(statusCallback));
            }

            // Get Avtive Session again in case of session is null  
            Session session1 = Session.getActiveSession();
        }
    }


    @Override
    public void onStart() {
        super.onStart();
        Session.getActiveSession().addCallback(statusCallback);
    }

    @Override
    public void onStop() {
        super.onStop();
        Session.getActiveSession().removeCallback(statusCallback);
    }

    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        Session.getActiveSession().onActivityResult(this, requestCode, resultCode, data);
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        Session session = Session.getActiveSession();
        Session.saveSession(session, outState);
    }

    private void updateView() {
        final Session session = Session.getActiveSession();    
        fbloginbtn.setOnClickListener(new OnClickListener() {
          public void onClick(View view) {      
           if(session.isOpened()){
              Toast.makeText(getApplicationContext(), "loginfb...", Toast.LENGTH_LONG).show();   
              fblogin(); 
           }
           else{
              Toast.makeText(getApplicationContext(), "onClickLogin...", Toast.LENGTH_LONG).show();
              onClickLogin();
           }
          }
       });
    }

    public void fblogin() {
        Intent intent = new Intent(LoginActivity.this, MoodActivity.class);
        startActivity(intent);

    }

    private void onClickLogin() {
        Session session = Session.getActiveSession();
        if (!session.isOpened() && !session.isClosed()) {
            session.openForRead(new Session.OpenRequest(this).setCallback(statusCallback));
        } else {
            Session.openActiveSession(this, true, statusCallback);
        }
        Intent intent = new Intent(LoginActivity.this, MoodActivity.class);
        startActivity(intent);
    }


    private class SessionStatusCallback implements Session.StatusCallback {
        @Override
        public void call(Session session, SessionState state, Exception exception) {
            updateView();
        }
    }
}

下面是logcat显示的消息

07-18 15:46:12.406: D/dalvikvm(12236): GC_FOR_ALLOC freed 54K, 6% free 2870K/3032K, paused 4ms, total 4ms
07-18 15:46:12.426: I/dalvikvm-heap(12236): Grow heap (frag case) to 25.335MB for 23520012-byte allocation
07-18 15:46:12.434: D/dalvikvm(12236): GC_FOR_ALLOC freed <1K, 1% free 25839K/26004K, paused 7ms, total 7ms
07-18 15:46:12.570: D/dalvikvm(12236): GC_FOR_ALLOC freed 1K, 1% free 27719K/27880K, paused 2ms, total 3ms
07-18 15:46:12.586: D/dalvikvm(12236): GC_FOR_ALLOC freed 2K, 1% free 29051K/29212K, paused 3ms, total 3ms
07-18 15:46:12.598: D/AndroidRuntime(12236): Shutting down VM
07-18 15:46:12.598: W/dalvikvm(12236): threadid=1: thread exiting with uncaught exception (group=0xa4cf1b20)
07-18 15:46:12.598: E/AndroidRuntime(12236): FATAL EXCEPTION: main
07-18 15:46:12.598: E/AndroidRuntime(12236): Process: com.dooba_demo_version, PID: 12236
07-18 15:46:12.598: E/AndroidRuntime(12236): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.dooba_demo_version/com.dooba_demo_version.LoginActivity}: java.lang.NullPointerException
07-18 15:46:12.598: E/AndroidRuntime(12236):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
07-18 15:46:12.598: E/AndroidRuntime(12236):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
07-18 15:46:12.598: E/AndroidRuntime(12236):    at android.app.ActivityThread.access$800(ActivityThread.java:135)
07-18 15:46:12.598: E/AndroidRuntime(12236):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
07-18 15:46:12.598: E/AndroidRuntime(12236):    at android.os.Handler.dispatchMessage(Handler.java:102)
07-18 15:46:12.598: E/AndroidRuntime(12236):    at android.os.Looper.loop(Looper.java:136)
07-18 15:46:12.598: E/AndroidRuntime(12236):    at android.app.ActivityThread.main(ActivityThread.java:5017)
07-18 15:46:12.598: E/AndroidRuntime(12236):    at java.lang.reflect.Method.invokeNative(Native Method)
07-18 15:46:12.598: E/AndroidRuntime(12236):    at java.lang.reflect.Method.invoke(Method.java:515)
07-18 15:46:12.598: E/AndroidRuntime(12236):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
07-18 15:46:12.598: E/AndroidRuntime(12236):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
07-18 15:46:12.598: E/AndroidRuntime(12236):    at dalvik.system.NativeStart.main(Native Method)
07-18 15:46:12.598: E/AndroidRuntime(12236): Caused by: java.lang.NullPointerException
07-18 15:46:12.598: E/AndroidRuntime(12236):    at com.dooba_demo_version.LoginActivity.onCreate(LoginActivity.java:51)
07-18 15:46:12.598: E/AndroidRuntime(12236):    at android.app.Activity.performCreate(Activity.java:5231)
07-18 15:46:12.598: E/AndroidRuntime(12236):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
07-18 15:46:12.598: E/AndroidRuntime(12236):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
07-18 15:46:12.598: E/AndroidRuntime(12236):    ... 11 more

---

提前感谢

此语句中有一个

空点异常

Session.getActiveSession().addCallback(statusCallback);

要解决此问题，在尝试调用

addCallback

方法之前，需要确保会话不为null，因此需要将

else

添加到

if（session==null）

中，并将最后一条语句移动到else中，如下所示

        // Get Avtive Session again in case of session is null  
        Session session1 = Session.getActiveSession();
    } else {
         Session.getActiveSession().addCallback(statusCallback);
    }

---

嗨，奥马尔，谢谢你的意见，我非常感谢。我不确定我是否很好地应用了这些建议，但我仍然收到这样的错误，并已将处理NullPointerException的更新代码更新到初始帖子中；不在更新的代码中复制它删除此注释后的语句//在此处添加会话回调，然后一切都应该正常。我应用了你的建议，解决了这个错误，并让我的应用程序运行。但是，当我删除这一行时，我注意到-Session.getActiveSession（）.addCallback（statusCallback）；当用户已经使用facebook登录时，它会将他们带到下一个活动页面，这很好，但是如果他们还没有登录（或会话尚未激活），我无法调用facebook登录屏幕让他们登录并激活会话。

Session.getActiveSession().addCallback(statusCallback);

        // Get Avtive Session again in case of session is null  
        Session session1 = Session.getActiveSession();
    } else {
         Session.getActiveSession().addCallback(statusCallback);
    }