Android 尝试从资产访问复制数据库中的DatabaseHelper到data\data时出现空指针异常\
我有一个数据库助手类(代码如下)。 此帮助程序的类任务是将数据库从应用程序附带的“我的资产”文件夹复制到data\data。。。我的应用程序,以便我可以使用它 一旦我将数据库放入data\data(我能够)。。我想添加它并执行CRUD操作,该数据库将保留在应用程序中,直到用户删除该应用程序 但是,复制完成后,我将数据库设置为data\data。。我尝试在不同的活动(我要从中添加数据的活动)中创建DatabaseHelper的实例。。 当我这样做的时候,,,当代码到达时,我会得到一个空指针异常错误 DatabaseHelper dbh=新的DatabaseHelper(此); 我知道造成这种情况的可能原因可能是错误的上下文或空数据库。。。但就我而言,一切似乎都很好 下面是代码Android 尝试从资产访问复制数据库中的DatabaseHelper到data\data时出现空指针异常\,android,mysql,Android,Mysql,我有一个数据库助手类(代码如下)。 此帮助程序的类任务是将数据库从应用程序附带的“我的资产”文件夹复制到data\data。。。我的应用程序,以便我可以使用它 一旦我将数据库放入data\data(我能够)。。我想添加它并执行CRUD操作,该数据库将保留在应用程序中,直到用户删除该应用程序 但是,复制完成后,我将数据库设置为data\data。。我尝试在不同的活动(我要从中添加数据的活动)中创建DatabaseHelper的实例。。 当我这样做的时候,,,当代码到达时,我会得到一个空指针异常错误
DatabaseHelper.java
public class DatabaseHelper extends SQLiteOpenHelper
{
String DB_PATH = null;
// assign the database givens, such as name and context.
private static String DB_NAME = "offline";
private SQLiteDatabase myDataBase;
private final Context myContext;
static int count = 0;
/**
* 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;
DB_PATH = "/data/data/" + context.getPackageName() + "/" + "databases/";
}
// ///////////////////////////////////////////////////////////////////////////////////////
public void createDataBase() throws IOException
{
boolean dbExist = checkDataBase();
if (dbExist)
{
// do nothing - database already exist
} else
{
this.getWritableDatabase();
try
{
copyDataBase();
} catch (IOException e)
{
throw new Error("Error copying database");
}
}
}
// ///////////////////////////////////////////////////////////////////////////////////////
private boolean checkDataBase()
{
SQLiteDatabase checkDB = null;
try
{
String myPath = DB_PATH + DB_NAME;
checkDB = SQLiteDatabase.openDatabase(myPath, null,SQLiteDatabase.OPEN_READWRITE);
} catch (SQLiteException e)
{
// database does't exist yet.
}
if (checkDB != null)
{
checkDB.close();
}
return checkDB != null ? true : false;
}
// ///////////////////////////////////////////////////////////////////////////////////////
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);
}
// Close the streams
myOutput.flush();
myOutput.close();
myInput.close();
}
// ///////////////////////////////////////////////////////////////////////////////////////
public void openDataBase() throws SQLException
{
// Open the database
String myPath = DB_PATH + DB_NAME;
myDataBase = SQLiteDatabase.openDatabase(myPath, null,SQLiteDatabase.OPEN_READWRITE);
}
// ///////////////////////////////////////////////////////////////////////////////////////
@Override
public synchronized void close()
{
if (myDataBase != null)
myDataBase.close();
super.close();
}
// return cursor
public Cursor query(String table, String[] columns, String selection,
String[] selectionArgs, String groupBy, String having,
String orderBy)
{
return myDataBase.query("myTable", null, null, null, null, null, null);
}
// //////////////////////////////////////////////////////////////////////////////////
@Override
public void onCreate(SQLiteDatabase db)
{
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
{
}
// //////////////////////////////////////////////////////////////////////////////////
public long insertPuzzle(String q,String a,String c1,String c2,String c3,String c4)
{
ContentValues initialValues = new ContentValues();
initialValues.put("q", q);
initialValues.put("a",a);
initialValues.put("c1",c1);
initialValues.put("c2",c2);
initialValues.put("c3",c3);
initialValues.put("c4",c4);
return myDataBase.insert("myTable", null, initialValues);
}
每当我在活动中
DatabaseHelper db1 = new DatabaseHelper(this);
db1.openDatabase();
db1.insertPuzzle("q","a","1","2","3","4");
我得到了空指针错误
希望你能帮忙,
关于,将此文件放入您的软件包中
public class DBAdapter extends SQLiteOpenHelper {
private static String DB_PATH = "";
private static final String DB_NAME = "temp.sqlite";//your database name with extention any
private SQLiteDatabase myDataBase;
private final Context myContext;
private static DBAdapter mDBConnection;
/**
* Constructor
* Takes and keeps a reference of the passed context in order to access to the application assets and resources.
* @param context
*/
private DBAdapter(Context context) {
super(context, DB_NAME, null, 1);
this.myContext = context;
DB_PATH = "/data/data/"
+ context.getApplicationContext().getPackageName()
+ "/databases/";
// The Android's default system path of your application database is
// "/data/data/mypackagename/databases/"
}
/**
* getting Instance
* @param context
* @return DBAdapter
*/
public static synchronized DBAdapter getDBAdapterInstance(Context context) {
if (mDBConnection == null) {
mDBConnection = new DBAdapter(context);
}
return mDBConnection;
}
/**
* Creates an 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 following method
// 1) an empty database will be created into the default system path of your application
// 2) than we 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);
}
// Close the streams
myOutput.flush();
myOutput.close();
myInput.close();
}
/**
* Open the database
* @throws SQLException
*/
public SQLiteDatabase openDataBase() throws SQLException {
String myPath = DB_PATH + DB_NAME;
myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READWRITE);
return myDataBase;
}
/**
* Close the database if exist
*/
@Override
public synchronized void close() {
if (myDataBase != null)
myDataBase.close();
super.close();
}
/**
* Call on creating data base for example for creating tables at run time
*/
@Override
public void onCreate(SQLiteDatabase db) {
}
/**
* can used for drop tables then call onCreate(db) function to create tables again - upgrade
*/
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
}
活动onCreate方法
private DBAdapter dba;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.profile);
dba = DBAdapter.getDBAdapterInstance(this);
try {
dba.createDataBase();
} catch (IOException e) {
Log.e("log_tag", e.getLocalizedMessage());
}
}
和用于插入代码
SQLiteDatabase sqldb = dba.openDataBase();
ContentValues cv = new ContentValues();
cv.put("field", fieldvalue);
sqldb.insert(table, nullColumnHack, cv);
sqldb.close();
dba.close();
cv = null;
把这个文件放在你的包里
public class DBAdapter extends SQLiteOpenHelper {
private static String DB_PATH = "";
private static final String DB_NAME = "temp.sqlite";//your database name with extention any
private SQLiteDatabase myDataBase;
private final Context myContext;
private static DBAdapter mDBConnection;
/**
* Constructor
* Takes and keeps a reference of the passed context in order to access to the application assets and resources.
* @param context
*/
private DBAdapter(Context context) {
super(context, DB_NAME, null, 1);
this.myContext = context;
DB_PATH = "/data/data/"
+ context.getApplicationContext().getPackageName()
+ "/databases/";
// The Android's default system path of your application database is
// "/data/data/mypackagename/databases/"
}
/**
* getting Instance
* @param context
* @return DBAdapter
*/
public static synchronized DBAdapter getDBAdapterInstance(Context context) {
if (mDBConnection == null) {
mDBConnection = new DBAdapter(context);
}
return mDBConnection;
}
/**
* Creates an 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 following method
// 1) an empty database will be created into the default system path of your application
// 2) than we 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);
}
// Close the streams
myOutput.flush();
myOutput.close();
myInput.close();
}
/**
* Open the database
* @throws SQLException
*/
public SQLiteDatabase openDataBase() throws SQLException {
String myPath = DB_PATH + DB_NAME;
myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READWRITE);
return myDataBase;
}
/**
* Close the database if exist
*/
@Override
public synchronized void close() {
if (myDataBase != null)
myDataBase.close();
super.close();
}
/**
* Call on creating data base for example for creating tables at run time
*/
@Override
public void onCreate(SQLiteDatabase db) {
}
/**
* can used for drop tables then call onCreate(db) function to create tables again - upgrade
*/
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
}
活动onCreate方法
private DBAdapter dba;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.profile);
dba = DBAdapter.getDBAdapterInstance(this);
try {
dba.createDataBase();
} catch (IOException e) {
Log.e("log_tag", e.getLocalizedMessage());
}
}
和用于插入代码
SQLiteDatabase sqldb = dba.openDataBase();
ContentValues cv = new ContentValues();
cv.put("field", fieldvalue);
sqldb.insert(table, nullColumnHack, cv);
sqldb.close();
dba.close();
cv = null;
工作并完成it所需的工作。但是,如果数据库在onCreate()中打开,在onDestroy()中关闭,为什么会抛出空指针呢?它可以工作,并完成需要它完成的任务。但是,如果数据库在onCreate()中打开,在onDestroy()中关闭,为什么会抛出空指针?