我在我的android应用程序中使用SQLite数据库，并为数据库中的每个表设置了帮助程序类。有许多表，这些类只处理每个表的所有crud方法。我按照示例＃6 here
当实例化多个辅助类实例时，我遇到了问题。只有在一次实例化一个帮助程序类时，该数据库才可用。
public class ProjectsHelper {
SQLiteDatabase db;
DataBaseHelper dbHelper;
private String TABLE_NAME = "Projects";
public ProjectsHelper(Context context){
if(dbHelper == null){
dbHelper = new DataBaseHelper(context);
}
}
public void open() throws SQLException {
try {
if(!isOpen()){
db = dbHelper.getWritableDatabase();
}
} catch (Exception e){
e.printStackTrace();
}
}
private boolean isOpen(){
return db != null && db.isOpen();
}
public void close() {
if(isOpen()){
dbHelper.close();
db.close();
dbHelper = null;
db = null;
}
}
public long insert(Project obj){
ContentValues values = new ContentValues();
values.put("ProjName", obj.getProjName().trim());
if(!Utilities.empty(obj.getProjDesc())){
values.put("ProjDesc", obj.getProjDesc().trim());
}
if(!Utilities.empty(obj.getProjNumber())){
values.put("ProjNum", obj.getProjNumber().trim());
}
values.put("ServerID", obj.getServerID());
values.put("GUID", obj.getGUID());
values.put("ModDate", obj.getModDate());
long id = db.insert(TABLE_NAME, null, values);
return id;
}
}
public class DataBaseHelper extends SQLiteOpenHelper {
//The Android's default system path of your application database.
public static String DB_PATH = "/data/data/com.siscoders.arscompnentinspector/databases/";
public static String DB_NAME = "ARSInspection.db";
private SQLiteDatabase myDataBase;
private final Context myContext;
/**
* Constructor
* Takes and keeps a reference of the passed context in order to access to the application assets and resources.
* @param context
*/
public DataBaseHelper(Context context) {
super(context, DB_NAME, null, 1);
this.myContext = context;
try {
createDataBase();
} catch (IOException ioe) {
throw new Error("Unable to create database");
}
try {
//openDataBase();
}catch(SQLException sqle){
throw sqle;
}
}
/**
* Creates a empty database on the system and rewrites it with your own database.
* */
public void createDataBase() throws IOException {
boolean dbExist = checkDataBase();
if(dbExist){
//do nothing - database already exist
}else{
//By calling this method and empty database will be created into the default system path
//of your application so we are gonna be able to overwrite that database with our database.
this.getReadableDatabase();
try {
copyDataBase();
} catch (IOException e) {
throw new Error("Error copying database");
}
}
}
/**
* Check if the database already exist to avoid re-copying the file each time you open the application.
* @return true if it exists, false if it doesn't
*/
private boolean checkDataBase(){
SQLiteDatabase checkDB = null;
try{
String myPath = DB_PATH + DB_NAME;
checkDB = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}catch(SQLiteException e){
//database does't exist yet.
}
if(checkDB != null){
checkDB.close();
}
return checkDB != null ? true : false;
}
/**
* Copies your database from your local assets-folder to the just created empty database in the
* system folder, from where it can be accessed and handled.
* This is done by transfering bytestream.
* */
private void copyDataBase() throws IOException{
//Open your local db as the input stream
InputStream myInput = myContext.getAssets().open(DB_NAME);
// Path to the just created empty db
String outFileName = DB_PATH + DB_NAME;
//Open the empty db as the output stream
OutputStream myOutput = new FileOutputStream(outFileName);
//transfer bytes from the inputfile to the outputfile
byte[] buffer = new byte[1024];
int length;
while ((length = myInput.read(buffer))>0){
myOutput.write(buffer, 0, length);
}
myOutput.flush();
myOutput.close();
myInput.close();
}
@Override
public synchronized void close() {
if(myDataBase != null)
myDataBase.close();
super.close();
}
@Override
public void onCreate(SQLiteDatabase db) {
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
}