Android NullPointerException and GetDatabaseLocked

问题:

Im trying to store a particular columns data in a method and call that method in another classes String [] where a spinner will access that array to display the columns rows for the user to select which will then get stored in another database.

Im getting nulpointerexception and when I try and open() the database the database gets locked. Sorry for sounding like a complete amateur, relatively new to android. Thank you in advance for any help.

Here is my code when I call getInstance() and getCPnames() in my main class

    String[] carParks = CarParkDb.getInstance().getCpnames();

Here is my code for the database:

package com.example.parkangel;

import java.util.ArrayList;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

public class CarParkDb {


    public static final String KEY_ID = "_id";
    public static final String KEY_CPNAME = "cpname";
    public static final String KEY_COST = "cost";


    private static final String DATABASE_NAME = "CPDB";
    private static final String DATABASE_TABLE = "CPTable";
    private static final int DATABASE_VERSION = 1;

    private CPDbHelper cpdbHelper;

    private Context ourContext;
    private SQLiteDatabase ourDatabase;
    private static CarParkDb instance;


    private static class CPDbHelper extends SQLiteOpenHelper{

        public CPDbHelper(Context context) {
            super(context, DATABASE_NAME, null, DATABASE_VERSION);
            // TODO Auto-generated constructor stub
        }



@Override
        public void onCreate(SQLiteDatabase db) {
            // TODO Auto-generated method stub
            db.execSQL("CREATE TABLE " + DATABASE_TABLE + " (" +
                    KEY_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " +
                    KEY_CPNAME + " TEXT NOT NULL, " + KEY_COST + " TEXTNOT NULL);");

            db.execSQL("INSERT INTO " + DATABASE_TABLE + " Values('1','LearningResource Center','2');");
            db.execSQL("INSERT INTO " + DATABASE_TABLE + " Values ('2','ParkandRide','1');");
            db.execSQL("INSERT INTO " + DATABASE_TABLE + " Values ('3','deHavilland Campus','2');");
            db.execSQL("INSERT INTO " + DATABASE_TABLE + " Values('4','MultiStorey Building','2');");
            db.execSQL("INSERT INTO " + DATABASE_TABLE + " Values('5','Reception','2');");

        }

        public void onOpen(final SQLiteDatabase db) {
            super.onOpen(db);
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            // TODO Auto-generated method stub

            db.execSQL("DROP TABLE IF EXISTS " + DATABASE_TABLE);
            onCreate(db);
        }
    }

        public CarParkDb (final Context c)
        {
            this.cpdbHelper= new CPDbHelper(c);
            establishDb();
            //ourContext = c;
}
        public void establishDb()
        {
            if (this.ourDatabase == null)
            {
                this.ourDatabase = this.cpdbHelper.getWritableDatabase();
            }

        }

        public CarParkDb() {
            // TODO Auto-generated constructor stub
        }
        public CarParkDb open() throws SQLException
        {
            System.out.println ("running open");
            cpdbHelper = new CPDbHelper(ourContext);
            ourDatabase = cpdbHelper.getWritableDatabase();
            return this;
        }

        public void close()
        {
            ourDatabase.close();
        }

        /*public long createEntry(String cpname, String cost){
            ContentValues cv = new ContentValues();
            cv.put(KEY_CPNAME, cpname);
            cv.put(KEY_COST, cost);
            return ourDatabase.insert(DATABASE_TABLE, null, cv);
        }*/

        public String getData() {
            // TODO Auto-generated method stub
            //open();
            String[] columns = new String[] {KEY_ID, KEY_CPNAME, KEY_COST};
            Cursor  c = ourDatabase.query(DATABASE_TABLE, columns, null,null,null, null, null);
            String result = " ";

            int iRow = c.getColumnIndexOrThrow(KEY_ID);
            int iCpname = c.getColumnIndexOrThrow(KEY_CPNAME);
            int iCost = c.getColumnIndexOrThrow(KEY_COST);

            for(c.moveToFirst(); !c.isAfterLast(); c.moveToNext()){
                result = result + c.getString(iRow) + " " +c.getString(iCpname) + " " + c.getString(iCost) + " " + "\n";
                c.close();
                ourDatabase.close();
            }
            return result;
        }

        public static CarParkDb getInstance()
        {
            synchronized(CarParkDb.class)
        {
            if (instance == null)
            {
                instance = new CarParkDb();
            }   
            return instance;
        }
        }

    public String[] getCpnames()
        {
            //open();
            if (ourDatabase == null) System.out.println ("is null");
            Cursor c = null;

            ArrayList<String> list = new ArrayList<String>();
            ourDatabase = cpdbHelper.getReadableDatabase();
            //SQLiteDatabase db = cpdbHelper.getReadableDatabase();
            String query = "SELECT " + KEY_CPNAME + " FROM " + DATABASE_TABLE;



            {
            c = this.ourDatabase.rawQuery(query, null);
            int iCpname = c.getColumnIndexOrThrow(KEY_CPNAME);
            if (c.moveToFirst())
                {
                    do
                    {
                    list.add(c.getString(iCpname));;
                    }           
                    while (c.moveToNext());
                }
                if (c != null && !c.isClosed())
                {
                c.close();
                ourDatabase.close();
                }
                return list.toArray(new String[]{});
            }
        }

} 

**LogCat**

    03-12 01:32:39.759: E/AndroidRuntime(4176): Caused by:java.lang.NullPointerException
    03-12 01:32:39.759: E/AndroidRuntime(4176):     

    at com.example.parkangel.CarParkDb.getCpnames(CarParkDb.java:191)
    03-12 01:32:39.759: E/AndroidRuntime(4176):     

    at com.example.parkangel.BookTicket.<init>(BookTicket.java:22)
    03-12 01:32:39.759: E/AndroidRuntime(4176):     

    at java.lang.Class.newInstanceImpl(Native Method)
    03-12 01:32:39.759: E/AndroidRuntime(4176):     

    at java.lang.Class.newInstance(Class.java:1208)

回答1:

Try updating your getInstance() code with this:

public static CarParkDb getInstance(Context c) // <-- added context as parameter
{
    synchronized(CarParkDb.class)
    {
        if (instance == null)
        {
            instance = new CarParkDb(c); // <-- used context in constructor
        }   
        return instance;
    }
}

The problem seems to be that you're using a constructor (for CarParkDb) that does nothing. You have another constructor that takes a Context as parameter and initializes some of the objects that you're using later.

回答2:

The database doesn't get locked. The "locked" thinkg is just a NPE in a method called getDatabaseLocked() and it's caused by a null Context passed to SQLiteOpenHelper in constructor that manifests itself with getWritableDatabase() or getReadableDatabase().

To fix the NPE in getDatabaseLocked(), make sure ourContext in open() is not null. As of now, you've never initialized ourContext and it's always null. Uncomment the //ourContext = c; in constructor and move it above the establishDb(), remove the other no-argument constructor and pass in a Context for example as suggested by Merlevede.

The NPE stacktrace in your question is when you call getCpNames() with open() commented out and are trying to call a method on a null ourDatabase object reference. Uncomment the open() there once it's fixed. The stacktrace also says you're trying to call getCpNames() in object initialization phase e.g. when initializing a member variable. That might be too early, for example an activity cannot be used as a Context until onCreate().

Some other things in your code you'd hit next:

• in onCreate(): + KEY_COST + " TEXTNOT NULL) - add space between TEXT and NOT

• in getData(): you're closing the cursor and database in the loop where you're accessing the cursor. Do it after the loop. The code shouldn't compile as you're not returning anything from a non-void function in case moveToFirst() returns false.