java - onCreate 中的 SQLite 错误

Question

iem 遵循 youtube 上关于在 android 中使用 SQLite 的教程，代码在启动代码时使应用程序崩溃

主 Activity .java

package com.example.asd;

import android.app.Activity;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;

public class MainActivity extends Activity {

VivzHelper vivzHelper;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    vivzHelper = new VivzHelper(this);
    SQLiteDatabase sqLiteDatabase = vivzHelper.getWritableDatabase();
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}
}

消息.java

package com.example.asd;

import android.content.Context;
import android.widget.Toast;

public class Message {

public static void message(Context context, String message) {
    Toast.makeText(context, message, Toast.LENGTH_LONG).show();
}
}

VivzHelper.java

package com.example.asd;

import android.content.Context;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

public class VivzHelper extends SQLiteOpenHelper {

private static final String DATABASE_NAME = "vivzdatabase";
private static final String TABLE_NAME = "VIVZTABLE";
private static final int DATABASE_VERSION = 1;
private static final String UID = "_id";
private static final String NAME = "Name";
private static final String CREATE_TABLE = "CREATE TABLE " + TABLE_NAME + " (" + UID
        + " INTEGER PRIMARY KEY  AUTOINCREMENT, " + NAME + " VARCHAR(255));";
private static final String DROP_TABLE = "DROP TABLE IF EXISTS" + TABLE_NAME;
private Context context;

public VivzHelper(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
    Message.message(context, "constructor called");
}

@Override
public void onCreate(SQLiteDatabase db) {
    try {
        db.execSQL(CREATE_TABLE);
        Message.message(context, "onCreate called");
    } catch (SQLException e) {
        Message.message(context, "" + e);
    }
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    try {
        Message.message(context, "onUpgrade called");
        db.execSQL(DROP_TABLE);
        onCreate(db);
    } catch (SQLException e) {
        Message.message(context, "" + e);
    }
}
}

应用程序崩溃于

SQLiteDatabase sqLiteDatabase = vivzHelper.getWritableDatabase();

错误日志

12-11 19:49:16.459: E/AndroidRuntime(6043): FATAL EXCEPTION: main
12-11 19:49:16.459: E/AndroidRuntime(6043): Process: com.example.asd, PID: 6043
12-11 19:49:16.459: E/AndroidRuntime(6043): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.asd/com.example.asd.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
12-11 19:49:16.459: E/AndroidRuntime(6043):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2695)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2769)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at android.app.ActivityThread.access$900(ActivityThread.java:177)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1430)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at android.os.Handler.dispatchMessage(Handler.java:102)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at android.os.Looper.loop(Looper.java:135)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at android.app.ActivityThread.main(ActivityThread.java:5910)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at java.lang.reflect.Method.invoke(Native Method)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at java.lang.reflect.Method.invoke(Method.java:372)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1405)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1200)
12-11 19:49:16.459: E/AndroidRuntime(6043): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
12-11 19:49:16.459: E/AndroidRuntime(6043):     at android.widget.Toast.<init>(Toast.java:130)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at android.widget.Toast.makeText(Toast.java:427)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at com.example.asd.Message.message(Message.java:9)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at com.example.asd.VivzHelper.onCreate(VivzHelper.java:29)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:251)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:163)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at com.example.asd.MainActivity.onCreate(MainActivity.java:18)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at android.app.Activity.performCreate(Activity.java:6178)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
12-11 19:49:16.459: E/AndroidRuntime(6043):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2648)
12-11 19:49:16.459: E/AndroidRuntime(6043):     ... 10 more

我不知道代码有什么问题，它看起来和视频中的一模一样？？？

Answer 1

问题是什么？

getWritableDatabase调用您的 onCreate()

The first time this is called, the database will be opened and onCreate(SQLiteDatabase), onUpgrade(SQLiteDatabase, int, int) and/or onOpen(SQLiteDatabase) will be called.

在 onCreate() 中

Message.message(context, "onCreate called");

context，此时为null，因为它在顶部被声明为成员变量as

private Context context;

但它从未被初始化。您将 Context 从 Activity 传递给 db helper 类构造函数。初始化成员变量，context，当您将它传递给您的message() 函数时，会为该变量赋值，从而摆脱NPE.

我们是怎么到这里的？

你的堆栈跟踪显示

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 
'android.content.res.Resources android.content.Context.getResources()' 
on a null object reference

所以您正在尝试使用 Context 对象来“做某事”，在本例中，就是创建并显示 Toast 对象。 因为如果我们在堆栈跟踪中再往上看一点，就会找到引用您的项目的第一行

at com.example.asd.Message.message(Message.java:9)

这是哪一行

Toast.makeText(context, message, Toast.LENGTH_LONG).show();

什么可能导致给定堆栈跟踪出现问题？是的，context 为 null。

这让我们回到

The app crashes on SQLiteDatabase sqLiteDatabase = vivzHelper.getWritableDatabase();

正如您在帖子中所说。它在那里崩溃，不是因为该行中的某些内容明显错误，而是因为该行中的某些内容调用了其他正在寻求帮助的内容。