Question

我想知道是否有人可以提供帮助。每次我运行我的Android应用程序项目时都会崩溃。我把它缩小到一个单一的功能，但我不知道它为什么会崩溃！

ERROR：

05-07 17:38:22.059 3952-3986/com.tfs.adihub W/System: ClassLoader referenced         unknown path: /data/data/com.tfs.adihub/lib
05-07 17:38:22.122 3952-3952/com.tfs.adihub D/AndroidRuntime: Shutting down     VM
05-07 17:38:22.123 3952-3952/com.tfs.adihub E/AndroidRuntime: FATAL EXCEPTION: main
                                                          Process: com.tfs.adihub, PID: 3952
                                                          java.lang.RuntimeException: Unable to start activity ComponentInfo{com.tfs.adihub/com.tfs.adihub.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' 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.handleRelaunchActivity(ActivityThread.java:4077)
                                                              at android.app.ActivityThread.-wrap15(ActivityThread.java)
                                                              at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1350)
                                                              at android.os.Handler.dispatchMessage(Handler.java:102)
                                                              at android.os.Looper.loop(Looper.java:148)
                                                              at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                              at java.lang.reflect.Method.invoke(Native Method)
                                                              at com.android.internal.os.ZygoteInit$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 'java.lang.String android.content.Context.getPackageName()' on a null object reference
                                                              at android.content.ContextWrapper.getPackageName(ContextWrapper.java:133)
                                                              at android.app.Activity.getLocalClassName(Activity.java:5205)
                                                              at android.app.Activity.getPreferences(Activity.java:5239)
                                                              at com.tfs.adihub.helpers.isLoggedIn(helpers.java:99)
                                                              at com.tfs.adihub.MainActivity$override.onCreate(MainActivity.java:35)
                                                              at com.tfs.adihub.MainActivity$override.access$dispatch(MainActivity.java)
                                                              at com.tfs.adihub.MainActivity.onCreate(MainActivity.java:0)
                                                              at android.app.Activity.performCreate(Activity.java:6237)
                                                              at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                                                              at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                                                              at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                                                              at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:4077) 
                                                              at android.app.ActivityThread.-wrap15(ActivityThread.java) 
                                                              at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1350) 
                                                              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) 
05-07 17:38:30.716 3952-3952/com.tfs.adihub I/Process: Sending signal. PID: 3952 SIG: 9

MainActivity.java：

package com.tfs.adihub;

import android.content.Intent;
import android.net.Uri;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

import com.google.android.gms.appindexing.Action;
import com.google.android.gms.appindexing.AppIndex;
import com.google.android.gms.common.api.GoogleApiClient;

public class MainActivity extends AppCompatActivity {

/**
 * ATTENTION: This was auto-generated to implement the App Indexing API.
 * See https://g.co/AppIndexing/AndroidStudio for more information.
 */
private GoogleApiClient client;

Button LoginButton, RegisterButton;

@Override

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    final helpers helpers = new helpers();
    client = new GoogleApiClient.Builder(this).addApi(AppIndex.API).build();

    Button LoginButton = (Button) findViewById(R.id.button_login);
    Button RegisterButton = (Button) findViewById(R.id.button_register);

    if (helpers.isLoggedIn()) {
        startActivity(new Intent(MainActivity.this, Dashboard.class));
    }

    RegisterButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            startActivity(new Intent(MainActivity.this, Register.class));
        }
    });

    LoginButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            startActivity(new Intent(MainActivity.this, Login.class));
        }
    });
}

@Override
public void onStart() {
    super.onStart();

    // ATTENTION: This was auto-generated to implement the App Indexing API.
    // See https://g.co/AppIndexing/AndroidStudio for more information.
    client.connect();
    Action viewAction = Action.newAction(
            Action.TYPE_VIEW, // TODO: choose an action type.
            "Main Page", // TODO: Define a title for the content shown.
            // TODO: If you have web page content that matches this app activity's content,
            // make sure this auto-generated web page URL is correct.
            // Otherwise, set the URL to null.
            Uri.parse("http://host/path"),
            // TODO: Make sure this auto-generated app URL is correct.
            Uri.parse("android-app://com.tfs.adihub/http/host/path")
    );
    AppIndex.AppIndexApi.start(client, viewAction);
}

@Override
public void onStop() {
    super.onStop();

    // ATTENTION: This was auto-generated to implement the App Indexing API.
    // See https://g.co/AppIndexing/AndroidStudio for more information.
    Action viewAction = Action.newAction(
            Action.TYPE_VIEW, // TODO: choose an action type.
            "Main Page", // TODO: Define a title for the content shown.
            // TODO: If you have web page content that matches this app activity's content,
            // make sure this auto-generated web page URL is correct.
            // Otherwise, set the URL to null.
            Uri.parse("http://host/path"),
            // TODO: Make sure this auto-generated app URL is correct.
            Uri.parse("android-app://com.tfs.adihub/http/host/path")
    );
    AppIndex.AppIndexApi.end(client, viewAction);
    client.disconnect();
}
}

Login.Java：

package com.tfs.adihub;

import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.net.Uri;
import android.os.Bundle;
import android.preference.PreferenceManager;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.google.android.gms.appindexing.Action;
import com.google.android.gms.appindexing.AppIndex;
import com.google.android.gms.common.api.GoogleApiClient;
import com.kosalgeek.asynctask.AsyncResponse;
import com.kosalgeek.asynctask.PostResponseAsyncTask;

import org.json.JSONException;
import org.json.JSONObject;

import java.util.HashMap;

import static android.app.PendingIntent.getActivity;

public class Login extends AppCompatActivity implements AsyncResponse {

    Button LoginSubmitButton, LoginRegisterButton;
    EditText LoginEmail, LoginPassword;
    /**
     * ATTENTION: This was auto-generated to implement the App Indexing API.
     * See https://g.co/AppIndexing/AndroidStudio for more information.
     */
    private GoogleApiClient client;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);

        final helpers helpers = new helpers();

        final EditText LoginEmail = (EditText) findViewById(R.id.login_email);
        final EditText LoginPassword = (EditText) findViewById(R.id.login_password);
        Button LoginSubmitButton = (Button) findViewById(R.id.login_submit);
        Button LoginRegisterButton = (Button) findViewById(R.id.login_register_button);

        LoginRegisterButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                startActivity(new Intent(Login.this, Register.class));
            }
        });

        assert LoginSubmitButton != null;
        LoginSubmitButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                final Context context = getApplicationContext();
                int duration = Toast.LENGTH_SHORT;

                if (helpers.isEmpty(LoginEmail)) {
                    CharSequence text = "Please enter your email address.";

                    Toast toast = Toast.makeText(context, text, duration);
                    toast.show();
                } else {
                    if (helpers.isEmpty(LoginPassword)) {
                        CharSequence text = "Please enter your password.";

                        Toast toast = Toast.makeText(context, text, duration);
                        toast.show();
                    } else {
                        if (helpers.isValidEmail(LoginEmail))
                            if (helpers.isValidPassword(LoginPassword)) {
                                HashMap<String, String> postData = new HashMap<String, String>();
                                postData.put("emailaddress", LoginEmail.getText().toString());
                                postData.put("password", LoginPassword.getText().toString());
                                final PostResponseAsyncTask loginTask = new PostResponseAsyncTask(Login.this, postData, new AsyncResponse() {
                                    @Override
                                    public void processFinish(String s) {
                                        JSONObject results = null;
                                        String result_status = null;
                                        String MID = null;
                                        String MT = null;

                                        try {
                                            results = new JSONObject(s);
                                            result_status = results.getString("STATUS");
                                            MID = results.getString("MID");
                                            MT = results.getString("MT");
                                        } catch (JSONException e) {
                                            e.printStackTrace();
                                        }

                                        if (result_status.equalsIgnoreCase("success")) {
                                            SharedPreferences sharedPref = Login.this.getPreferences(Context.MODE_PRIVATE);
                                            SharedPreferences.Editor editor = sharedPref.edit();
                                            editor.putString("MID", MID);
                                            editor.putString("MTOK", MT);
                                            editor.commit();
                                            startActivity(new Intent(Login.this, Dashboard.class));
                                        } else {
                                            Context context = getApplicationContext();
                                            int duration = Toast.LENGTH_SHORT;
                                            CharSequence text = result_status;
                                            Toast toast = Toast.makeText(context, text, duration);
                                            toast.show();
                                        }
                                    }
                                });
                                loginTask.execute("http://192.168.1.64/app/test.php");
                            } else {
                                CharSequence text = "Invalid password.";

                                Toast toast = Toast.makeText(context, text, duration);
                                toast.show();
                            }
                        else {
                            CharSequence text = "Invalid email address.";

                            Toast toast = Toast.makeText(context, text, duration);
                            toast.show();
                        }
                    }
                }
            }
        });
        // ATTENTION: This was auto-generated to implement the App Indexing API.
        // See https://g.co/AppIndexing/AndroidStudio for more information.
        client = new GoogleApiClient.Builder(this).addApi(AppIndex.API).build();
    }

    @Override
    public void processFinish(String s) {

    }

    @Override
    public void onStart() {
        super.onStart();

        // ATTENTION: This was auto-generated to implement the App Indexing API.
        // See https://g.co/AppIndexing/AndroidStudio for more information.
        client.connect();
        Action viewAction = Action.newAction(
                Action.TYPE_VIEW, // TODO: choose an action type.
                "Login Page", // TODO: Define a title for the content shown.
                // TODO: If you have web page content that matches this app activity's content,
                // make sure this auto-generated web page URL is correct.
                // Otherwise, set the URL to null.
                Uri.parse("http://host/path"),
                // TODO: Make sure this auto-generated app URL is correct.
                Uri.parse("android-app://com.tfs.adihub/http/host/path")
        );
        AppIndex.AppIndexApi.start(client, viewAction);
    }

    @Override
    public void onStop() {
        super.onStop();

        // ATTENTION: This was auto-generated to implement the App Indexing API.
        // See https://g.co/AppIndexing/AndroidStudio for more information.
        Action viewAction = Action.newAction(
                Action.TYPE_VIEW, // TODO: choose an action type.
                "Login Page", // TODO: Define a title for the content shown.
                // TODO: If you have web page content that matches this app activity's content,
                // make sure this auto-generated web page URL is correct.
                // Otherwise, set the URL to null.
                Uri.parse("http://host/path"),
                // TODO: Make sure this auto-generated app URL is correct.
                Uri.parse("android-app://com.tfs.adihub/http/host/path")
        );
        AppIndex.AppIndexApi.end(client, viewAction);
        client.disconnect();
    }
}

Helpers.Java：

package com.tfs.adihub;

import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.net.Uri;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.text.TextUtils;
import android.util.Patterns;
import android.widget.EditText;
import android.widget.Toast;

import com.google.android.gms.appindexing.Action;
import com.google.android.gms.appindexing.AppIndex;
import com.google.android.gms.common.api.GoogleApiClient;
import com.kosalgeek.asynctask.AsyncResponse;
import com.kosalgeek.asynctask.PostResponseAsyncTask;

import org.json.JSONException;
import org.json.JSONObject;

import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.net.URLConnection;
import java.util.HashMap;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * Created by Offic on 03/05/2016.
 */
public class helpers extends AppCompatActivity implements AsyncResponse {

    /**
     * ATTENTION: This was auto-generated to implement the App Indexing API.
     * See https://g.co/AppIndexing/AndroidStudio for more information.
     */
    private GoogleApiClient client;

    boolean isEmpty(EditText etText) {
        return etText.getText().toString().trim().length() <= 0;
    }

    public boolean isValidEmail(EditText email) {
        if (TextUtils.isEmpty(email.getText())) {
            return false;
        } else {
            return Patterns.EMAIL_ADDRESS.matcher(email.getText()).matches();
        }
    }

    public boolean isValidPassword(EditText password) {
        String regExpn = "^[a-z0-9_]{6,24}$";
        CharSequence inputStr = password.getText();
        Pattern pattern = Pattern.compile(regExpn, Pattern.CASE_INSENSITIVE);
        Matcher matcher = pattern.matcher(inputStr);

        if (matcher.matches()) {
            return true;
        } else {
            return false;
        }
    }

    private boolean verifySession(String MID, String MTOK) {
        HashMap<String, String> postData = new HashMap<String, String>();
        postData.put("MID", MID);
        postData.put("MTOK", MTOK);
        final Boolean[] Ret = {false};
        final PostResponseAsyncTask loginTask = new PostResponseAsyncTask(this, postData, new AsyncResponse() {
            @Override
            public void processFinish(String s) {
                JSONObject results = null;
                String result_status = null;

                try {
                    results = new JSONObject(s);
                    result_status = results.getString("STATUS");
                } catch (JSONException e) {
                    e.printStackTrace();
                }

                if (result_status == "success") {
                    Ret[0] = true;
                } else {
                    Ret[0] = false;
                }
            }
        });
        loginTask.execute("http://192.168.1.64/app/test.php");

        return Ret[0];
    }

    public boolean isLoggedIn() {
        SharedPreferences sharedPref = this.getPreferences(Context.MODE_PRIVATE);
        String MID = sharedPref.getString("MID", null);
        String MT = sharedPref.getString("MTOK", null);

        if (MID == null || MT == null) {
            return false;
        } else {
            return verifySession(MID, MT);
        }
    }

    @Override
    public void processFinish(String s) {

    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        client = new GoogleApiClient.Builder(this).addApi(AppIndex.API).build();
    }

    @Override
    public void onStart() {
        super.onStart();
        client.connect();
        Action viewAction = Action.newAction(
                Action.TYPE_VIEW, // TODO: choose an action type.
                "helpers Page", // TODO: Define a title for the content shown.
                Uri.parse("http://host/path"),
                Uri.parse("android-app://com.tfs.adihub/http/host/path")
        );
        AppIndex.AppIndexApi.start(client, viewAction);
    }

    @Override
    public void onStop() {
        super.onStop();
        Action viewAction = Action.newAction(
                Action.TYPE_VIEW, // TODO: choose an action type.
                "helpers Page", // TODO: Define a title for the content shown.
                // TODO: If you have web page content that matches this app activity's content,
                // make sure this auto-generated web page URL is correct.
                // Otherwise, set the URL to null.
                Uri.parse("http://host/path"),
                // TODO: Make sure this auto-generated app URL is correct.
                Uri.parse("android-app://com.tfs.adihub/http/host/path")
        );
        AppIndex.AppIndexApi.end(client, viewAction);
        client.disconnect();
    }
}

任何帮助将不胜感激！我对这种语言非常非常新，所以我仍然要掌握这一切是如何运作的。

非常感谢

Answer 1

您的设计存在重大缺陷。错误很明显。在这一行

final helpers helpers = new helpers();

您正在尝试创建活动的对象，这是一种非常糟糕的做法。你不能简单地创建这样的活动对象。 Android中的所有活动都必须通过活动生命周期，以便它们附加有效的上下文。在这种情况下，有效的上下文不会附加到帮助程序实例。因此，行

SharedPreferences sharedPref = this.getPreferences(Context.MODE_PRIVATE);

导致空指针，因为'this'为null。（为什么？因为这是指由于未使用startActivity（intent）启动活动而尚未创建的上下文）

另一件事虽然这不是错误，但您必须使用以大写字母开头的活动名称。遵循惯例，将使您的代码更具可读性。由于大写和小写名称的奇怪使用，您的代码非常混乱。您使用小写字母声明您的类，而在某些地方，变量是大写的。这会造成混乱。规则是类，接口等应该以大写字母和变量作为小写字母开始。

所以在这种情况下你做什么？

您永远不会开始您的帮助程序活动，我认为您不需要向用户显示它。这就是它需要在没有用户交互的情况下完成一些工作。你应该把它变成一种服务。然后使用startService(intent).启动它或者您也可以将其创建为Java类，具体取决于您的用例。

编辑：顺便说一下，我回答了another类似问题，但问题相同，但今天晚些时候涉及服务。

Android应用程序在运行时崩溃：'java.lang.String android.content.Context.getPackageName（）'

1 个答案: