Java 将SQLite数据库转换为csv

我一直在尝试将SQLite数据库列值导出到log.csv文件。到目前为止，我已经到了一个地步，文件导出，但没有任何类型的数据。执行该方法后，我在执行rawQuery的行上收到一个java.lang.NullPointerException。有人能在这件事上帮助我吗

感谢您提前抽出时间、发表看法并回答此问题

数据库代码：

public class happinessDb  extends SQLiteOpenHelper {

public static final String KEY_ROWID = "_id";
public static final String KEY_DATE = "date";
public static final String KEY_TIME = "time";
public static final String KEY_HAPPY = "happy";
public static final String KEY_NORMAL = "normal";
public static final String KEY_SAD = "sad";

public static final String DATABASE_NAME = "happinesMeter";
public static final String DATABASE_TABLE = "practiceReport";
public static final int DATABASE_VERSION = 2;

private DbHelper ourHelper;
private final Context ourContext;
private SQLiteDatabase ourDatabase;

   private static class DbHelper extends SQLiteOpenHelper{

    public DbHelper (Context context){
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE " + DATABASE_TABLE + " (" +
                        KEY_ROWID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
                        KEY_DATE + " TEXT NOT NULL, " +
                        KEY_TIME + " TEXT NOT NULL, " +
                        KEY_HAPPY + " TEXT NOT NULL, " +
                        KEY_NORMAL + " TEXT NOT NULL, " +
                        KEY_SAD + " TEXT NOT NULL);"
        );

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS " + DATABASE_TABLE);
        onCreate(db);
    }
}

    public happinessDb(Context c){ //Error
        ourContext = c;
}
public happinessDb open() throws SQLException{
    ourHelper = new DbHelper(ourContext);
    ourDatabase = ourHelper.getWritableDatabase();
    return this;
}
public void close(){
    ourHelper.close();
}

 public boolean convertHappiness() {
      String FILENAME = "log.csv";
      File directoryDownload = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS);
      File logDir = new File(directoryDownload, FILENAME);
      try {
          logDir.createNewFile();
          CSVWriter csvWriter = new CSVWriter(new FileWriter(logDir));
          Cursor curCSV = ourDatabase.rawQuery("SELECT * FROM practiceReport", null);
          csvWriter.writeNext(curCSV.getColumnNames());
          while (curCSV.moveToNext()) {
              String arrStr[] = { curCSV.getString(1)+ ",", curCSV.getString(2)+ ",",
                      curCSV.getString(3)+ ",", curCSV.getString(4)+ ",", curCSV.getString(5)+ ","};
              csvWriter.writeNext(arrStr);
          }
          csvWriter.close();
          curCSV.close();
          return true;
      } catch (Exception e) {
          Log.e("MainActivity", e.getMessage(), e);
          return false;
      }

转换数据库的方法：

public class happinessDb  extends SQLiteOpenHelper {

public static final String KEY_ROWID = "_id";
public static final String KEY_DATE = "date";
public static final String KEY_TIME = "time";
public static final String KEY_HAPPY = "happy";
public static final String KEY_NORMAL = "normal";
public static final String KEY_SAD = "sad";

public static final String DATABASE_NAME = "happinesMeter";
public static final String DATABASE_TABLE = "practiceReport";
public static final int DATABASE_VERSION = 2;

private DbHelper ourHelper;
private final Context ourContext;
private SQLiteDatabase ourDatabase;

   private static class DbHelper extends SQLiteOpenHelper{

    public DbHelper (Context context){
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE " + DATABASE_TABLE + " (" +
                        KEY_ROWID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
                        KEY_DATE + " TEXT NOT NULL, " +
                        KEY_TIME + " TEXT NOT NULL, " +
                        KEY_HAPPY + " TEXT NOT NULL, " +
                        KEY_NORMAL + " TEXT NOT NULL, " +
                        KEY_SAD + " TEXT NOT NULL);"
        );

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS " + DATABASE_TABLE);
        onCreate(db);
    }
}

    public happinessDb(Context c){ //Error
        ourContext = c;
}
public happinessDb open() throws SQLException{
    ourHelper = new DbHelper(ourContext);
    ourDatabase = ourHelper.getWritableDatabase();
    return this;
}
public void close(){
    ourHelper.close();
}

 public boolean convertHappiness() {
      String FILENAME = "log.csv";
      File directoryDownload = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS);
      File logDir = new File(directoryDownload, FILENAME);
      try {
          logDir.createNewFile();
          CSVWriter csvWriter = new CSVWriter(new FileWriter(logDir));
          Cursor curCSV = ourDatabase.rawQuery("SELECT * FROM practiceReport", null);
          csvWriter.writeNext(curCSV.getColumnNames());
          while (curCSV.moveToNext()) {
              String arrStr[] = { curCSV.getString(1)+ ",", curCSV.getString(2)+ ",",
                      curCSV.getString(3)+ ",", curCSV.getString(4)+ ",", curCSV.getString(5)+ ","};
              csvWriter.writeNext(arrStr);
          }
          csvWriter.close();
          curCSV.close();
          return true;
      } catch (Exception e) {
          Log.e("MainActivity", e.getMessage(), e);
          return false;
      }

---

}

在使用该命令之前，必须初始化数据库变量：

ourDatabase = this.getWritableDatabase();//you missed this line..
Cursor curCSV = ourDatabase.rawQuery("SELECT * FROM practiceReport", null);

---

谢谢你关于漏线的提示，但是仍然没有数据。GetWritableDatabase（）；用红色文本标记，在android studio中为我提供选项：创建getter、创建属性或创建方法。这是指向getWritableDatabase（）的错误；10-27 15:42:29.470 22382-22382/com.sportelitemanagement.animus.happyapp E/main活动﹕ null java.lang.NullPointerException位于com.sportelitemanagement.animus.happyapp.happinessDb.convertHappiness（happinessDb.java:143），如果android studio为您提供创建getter或setter的选项。。。这意味着它无法识别此方法。您可以使用

this.getWriteDatabase（）

在类内使用

SQLiteOpenHelper

扩展，例如

public类DBHelper扩展SQLiteOpenHelper{

由于

此

指示

SQLiteOpenHelper

其他情况，您必须从预定义的外部传递方法中的db变量….

public boolean convertHappines（SQLiteOpenHelper db）{

我不知道您的

convertHappines（）

函数的确切位置…我的convertHappines（）位于不同的活动中。在该不同的活动中按下onClick按钮后，SQLite应转换为csv并自动附加到电子邮件。这就是我需要转换它的原因。public void exportCsv（视图）{//TODO-创建一个方法，该方法将读取数据库happinessDb convertHappinessdb=new happinessDb（this）；convertHappiness.convertHappiness（）；readFromFile（）；attachreportandsendmail（）；}