Android SQLiteOpenHelper空指针异常

我正在为我的android应用程序实现一个缓存，并使用SQLite数据库存储服务器响应。下面定义了处理数据库的类。它还有一个SQLiteOpenHelper类，用作数据库帮助器

从调用CacheDB类 public static CacheDB CacheDB=new CacheDB（上下文）

我也在调用这个方法 cacheDB.fetchDatafromDB（“歌曲”）

下面给出了这些类的代码

package com.songs.lookup;

import java.util.ArrayList;
import java.util.List;

import android.annotation.SuppressLint;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteDatabase.CursorFactory;
import android.database.sqlite.SQLiteOpenHelper;
import android.database.sqlite.SQLiteStatement;

public class CacheDB {

    public CacheDB(Context context){
        System.out.println("Before constructing ");
        this.context = context;
        this.dbHelper = new CacheDBHelper(context, DATABASE_NAME, null, DATABASE_VERSION);
        System.out.println("After constructing ");

    }
    private Context context;
    private CacheDBHelper dbHelper;
    private static final String DATABASE_NAME = "";
       private static final int DATABASE_VERSION = 1;
       private static final String song_TABLE_NAME = "songs";
       private static final String tune_TABLE_NAME = "tunes";
       private static final String person_TABLE_NAME = "persons";
       private static final String COLUMN_NAME = "name";

       private static final String song_TABLE_CREATE =
                "CREATE TABLE " + song_TABLE_NAME + " (" +
                COLUMN_NAME + " TEXT);";

       private static final String tune_TABLE_CREATE =
                        "CREATE TABLE " + tune_TABLE_NAME + " (" +
                        COLUMN_NAME + " TEXT);";

       private static final String person_TABLE_CREATE =
                        "CREATE TABLE " + person_TABLE_NAME + " (" +
                        COLUMN_NAME + " TEXT);";
   class CacheDBHelper extends SQLiteOpenHelper{    
   public CacheDBHelper(Context context, String name, CursorFactory factory,
            int version) {
       super(context, DATABASE_NAME, null, DATABASE_VERSION);

//     System.out.println("Before the cachedbhelper");
       System.out.println("After the cachedbhelper");

    }

        @Override
        public void onCreate(SQLiteDatabase db) {
            System.out.println("Here inside the oncreate of cacheDBHelper");
            db.execSQL(song_TABLE_CREATE);
            db.execSQL(tune_TABLE_CREATE);
            db.execSQL(person_TABLE_CREATE);

        }

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

   @SuppressLint("NewApi")
public void performOperation(String Operation, String table, ArrayList<String> array1)
   {
       SQLiteDatabase db = dbHelper.getWritableDatabase();

       String INSERT = "insert into "   
                + table + " (" + COLUMN_NAME + ") values (?)";

       String DELETE = "delete from " + table; 

       String FETCH = "select DISTINCT(" + COLUMN_NAME + "from " + table + ")";

       db.beginTransaction();

       SQLiteStatement dbStmt = db.compileStatement(Operation == "INSERT" ? INSERT : DELETE);

       if(Operation == "INSERT")
       {  
            int aSize = array1.size();


                for (int i = 0; i < aSize; i++) {
                    dbStmt.bindString(1, array1.get(i));
                    dbStmt.executeInsert();

            }
       }

       if(Operation == "DELETE")
       {
           dbStmt.executeUpdateDelete();

       }

       if(Operation == "SELECT")
       {
           fetchDatafromDB(table);
       }

       db.setTransactionSuccessful();
       db.endTransaction();


       try {
            db.close();
           } catch (Exception e) {
            e.printStackTrace();
           }
        }

   public List<String> fetchDatafromDB(String table) {
        CacheDBHelper dbHelper = new CacheDBHelper(context, DATABASE_NAME, null, DATABASE_VERSION);
        SQLiteDatabase db = dbHelper.getWritableDatabase();

        List<String> list = new ArrayList<String>();
        // Select All Query
        String selectQuery = "SELECT  * FROM " + table;

        Cursor cursor = db.rawQuery(selectQuery, null);

        // looping through all rows and adding to list
        if (cursor.moveToFirst()) {
            do {
                cursor.getString(0);
            } while (cursor.moveToNext());
        }
        // return contact list
        return list;
    }
}

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问题出在哪里

我之前也遇到过类似的问题，我的问题是因为我试图在一个之前已经打开过的方法中打开一个数据库。我不知道这是否有意义，但我可以在这里看到，在第一次调用数据库后，您可以在不关闭数据库的情况下调用

dbHelper.getWritableDatabase（）

方法两次。 我认为这可能是问题的原因，因为数据库被前面的方法锁定，因此出现错误

at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224) 

---

不太确定我是否100%正确，但希望这有帮助。干杯

您传递给

SQLiteOpenHelper

构造函数的上下文为空。

CacheDB.java:124？Trace into onCreate（），可能它抛出了一个异常，不使用

=

use

equals（）比较字符串。

方法行124是SQLiteDatabase db=dbHelper.getWritableDatabase（）；看起来我的代码没有通过oncreate（）方法。我能够解决这个问题，我认为这是因为没有创建新实例，并且没有像您提到的那样使用两次。很高兴它有所帮助

at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)