Question

我看到ContentProvider.query（）返回一个Cursor对象。通常光标是SQLiteDatabase查询的结果。如下面的代码片段：

public Cursor query() {     
        try {
            SQLiteDatabase db = this.getReadableDatabase(); 
            Cursor c = db.query(Cfg.table_name, new String[] {"*"}, null, null, null, null, null);
            return c;
        }
        catch(Exception exp) {
        }

        return null;
    }

在代码中，db未关闭。有什么问题吗？

Answer 1

是的，我相信存在问题。您可以将'db'作为参数传递，然后调用者可以在完成光标时关闭它。

Answer 2

您是否在完成时尝试关闭数据库？如果是这样，试试：

public Cursor query() {
        SQLiteDatabase db = null;   
        try {
            SQLiteDatabase db = this.getReadableDatabase(); 
            Cursor c = db.query(Cfg.table_name, new String[] {"*"}, null, null, null, null, null);
            return c;
        }
        catch(Exception exp) {
        }finally{
           try{if(null != db){ db.close(); db = null;}}catch(Exception e){}
        }

        return null;
    }

每次都应该正确关闭数据库。如果你想知道它为什么没有关闭，我会调查一下Exception所说的内容。

顺便说一句，我会用null替换“new String [] {”*“}”。 http://developer.android.com/reference/android/database/sqlite/SQLiteDatabase.html

更新：好的，我现在明白了这个问题。您可能需要重新调整一下您的课程。

TLDR：理想情况下，对我来说，最终应该是两个班级。一个是帮助您打开数据库。其次是将SQLiteDatabase对象作为属性。您可以在创建此类时立即打开连接并初始化DB对象（或创建用于打开数据库的方法）。所有方法仅将仅使用db对象的SCUD。然后，所有db对象关闭或分解都可以放在方法中或覆盖onDestroy。以下示例，但如果您有空闲时间，这是一个很好的教程http://www.vogella.de/articles/AndroidSQLite/article.html。

第一个是DBOpenHelper。此类的目的是管理表并帮助您打开数据库以进行修改。

package com.mondial.th.rsa.db;

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteDatabase.CursorFactory;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

/**
 * DBOpenHelper helps opening database and managing database's upgrade or creation.
 * @author Poohdish Rattanavijai
 *
 */
public class DBOpenHelper extends SQLiteOpenHelper {
    private static final String TAG = DBOpenHelper.class.getSimpleName();
    private static final int DATABASE_VERSION = 2;
    private static final String DATABASE_NAME = "rsa_db";

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

    public DBOpenHelper(Context context, String name, CursorFactory factory,
            int version) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase database) {
        Log.d(TAG, "DBOpenHelper.onCreate");
        // Create table goes here.
    }

    @Override
    public void onUpgrade(SQLiteDatabase database, int oldVersion, int newVersion) {
        // Drop and recreate table goes here.
    }

}

第二个类将使用前面显示的DBOpenHelper为您处理所有查询。

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.util.Log;

import com.mondial.th.rsa.vo.ProfileVO;

/**
 * Database adapter for managing Profile table
 * @author Poohdish Rattanavijai
 *
 */
public class ProfileDBAdapter {
    private static final String TAG = ProfileDBAdapter.class.getName();
    private Context context;
    private SQLiteDatabase db;
    private DBOpenHelper dbHelper;

    public ProfileDBAdapter(Context context){
        this.context = context;
    }

    /**
     * Open writable database.
     * @return writable SQLDatabase
     * @throws SQLException
     */
    public ProfileDBAdapter open() throws SQLException{
        Log.d(TAG, "Open ProfileDBAdapter");
        dbHelper = new DBOpenHelper(context);

        db = dbHelper.getWritableDatabase();
        return this;
    }

    /**
     * Close database; Exception omitted
     */
    public void close(){
        try {
            dbHelper.close();
        } catch (Exception e) {
            //e.printStackTrace();
        }
    }

    /**
     * Creating new record in profile table with given VO.
     * @param profile VO representing data in the new record.
     * @return the row ID of the newly inserted row, or -1 if an error occurred.
     */
    public long createProfile(ProfileVO profile){
        ContentValues values = createContentValues(profile);
        return db.insert(ProfileVO.TABLE_NAME, null, values);
    }

    /**
     * Updating record in profile table with given VO by using ID column.
     * @param profile VO representing new data to updated.
     * @return the number of rows affected .
     */
    public boolean updateProfile(ProfileVO profile){
        ContentValues values = createContentValues(profile);
        return db.update(ProfileVO.TABLE_NAME, values, ProfileVO.COLUMN_ID + "=" + profile.getId(), null) > 0;
    }

    /**
     * Deleting a row representing given VO off profile table by using ID column.
     * @param profile
     * @return
     */
    public boolean deleteProfile(ProfileVO profile){
        return deleteProfile(profile.getId());
    }

    /**
     * Deleting a row off profile table with given ID column.
     * @param profile
     * @return
     */
    public boolean deleteProfile(long rowId){
        return db.delete(ProfileVO.TABLE_NAME, ProfileVO.COLUMN_ID + "=" + rowId, null) > 0;
    }

    /**
     * open cursor representing every records in profile table.
     * @return Cursor representing every records in profile table.
     */
    public Cursor fetchAllProfiles(){
        return db.query(ProfileVO.TABLE_NAME, null, null, null, null, null, null);
    }

    /**
     * open cursor representing a row in profile table with given ID.
     * @return Cursor representing a row in profile table with given ID.
     */
    public Cursor fetchProfile(long rowId){
        Cursor mCursor = db.query(true, ProfileVO.TABLE_NAME, null, ProfileVO.COLUMN_ID + "=" + rowId
                , null, null, null, null, null);

        if(null != mCursor){
            mCursor.moveToFirst();
        }

        return mCursor;
    }

    /**
     * This class translate given VO into ContentValues for ContentResolver to parse data and talk to the database.
     * @param profile VO
     * @return ContentsValues containing given VO's data except ID.
     */
    private ContentValues createContentValues(ProfileVO profile){
        ContentValues values = new ContentValues();

        values.put(ProfileVO.COLUMN_DOB, profile.getDob());
        values.put(ProfileVO.COLUMN_FIRSTNAME, profile.getFirstname());
        values.put(ProfileVO.COLUMN_LASTNAME, profile.getLastname());

        return values;
    }
}

然后你有一个完美的课程充当DAO。对于上面的例子，用法是。

ProfileDBAdapter dbHelper = new ProfileDBAdapter(context);
dbHelper.open();
cursor = dbHelper.fetchAllProfiles();
if(cursor.getCount() > 0){
 //TODO Data exists, do stuff.
}
try {
    if(null != cursor && !cursor.isClosed()){
       cursor.close();
    }
} catch (Exception e) {
    e.printStackTrace();
}

if(null != dbHelper){
    dbHelper.close();
    dbHelper = null;
}

Android SQLite数据库关闭查询（）

2 个答案: