我试图在android中为SqliteHelper写下正确的单例访问权限。从这里到那里得到了点点滴滴，形成最终版本，但不确定我是否在这里遗漏了一些东西。
public class DatabaseHelper extends SQLiteOpenHelper {
private static final String DATABASE_NAME = "EffiaSoft";
private static final int DATABASE_VERSION = 1;
private static SQLiteOpenHelper sInstance;
private Semaphore semaphore = new Semaphore(1);
private Context mCxt;
/**
* Constructor should be private to prevent direct instantiation.
* make call to static method "getInstance()" instead.
*/
private DatabaseHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
this.mCxt = context;
}
public static synchronized SQLiteOpenHelper getInstance(Context context) {
if (sInstance == null) {
sInstance = new DatabaseHelper(context.getApplicationContext());
}
return sInstance;
}
private static <T> void setFields(Cursor cursor, T returnObject, Field[] declaredFields, String[] cursorColumns) throws IllegalAccessException {
for (Field declaredField : declaredFields) {
String fieldName = declaredField.getName();
boolean hasColumn = false;
for (String columnName : cursorColumns) {
if (columnName.equals(fieldName))
hasColumn = true;
else
continue;
break;
}
if (hasColumn) {
declaredField.setAccessible(true);
int columnIndex = cursor.getColumnIndex(fieldName);
if (columnIndex >= 0) {
if (declaredField.getType() == Character.TYPE) {
String value = cursor.getString(columnIndex);
if (!ValidationHelper.isNullOrEmpty(value) && value.length() == 1) {
char c = value.charAt(0);
declaredField.set(returnObject, c);
}
} else if (declaredField.getType() == Short.TYPE) {
declaredField.set(returnObject, cursor.getShort(columnIndex));
} else if (declaredField.getType() == Integer.TYPE) {
declaredField.set(returnObject, cursor.getInt(columnIndex));
} else if (declaredField.getType() == Long.TYPE) {
declaredField.set(returnObject, cursor.getLong(columnIndex));
} else if (declaredField.getType() == Float.TYPE) {
declaredField.set(returnObject, cursor.getFloat(columnIndex));
} else if (declaredField.getType() == Double.TYPE) {
declaredField.set(returnObject, cursor.getDouble(columnIndex));
} else if (declaredField.getType() == Boolean.TYPE) {
String temp = cursor.getString(columnIndex);
declaredField.setBoolean(returnObject, temp.equalsIgnoreCase("Y"));
} else if (Date.class.equals(declaredField.getType())) {
if (!ValidationHelper.isNullOrEmpty(cursor.getString(columnIndex))) {
Date date = ValueFormatter.formatDateTime(cursor.getString(columnIndex));
declaredField.set(returnObject, date);
}
} else if (BigDecimal.class.equals(declaredField.getType())) {
declaredField.set(returnObject, BigDecimal.valueOf(cursor.getDouble(columnIndex)));
} else {
declaredField.set(returnObject, cursor.getString(columnIndex));
}
}
}
}
}
private void Disconnect() {
semaphore.release();
}
private void Connect() {
try {
semaphore.acquire();
} catch (InterruptedException e) {
LogHelper.writeExceptionLog(e);
}
}
@Override
public void onCreate(SQLiteDatabase db) {
}
public <T> ArrayList<T> getData(Context context, String tableName, String columnName, String filterCondition, String orderBy,
String groupBy, Class<T> type, SQLiteDatabase database) {
Connect();
ArrayList<T> arrayList = null;
try {
if (context != null && !ValidationHelper.isNullOrEmpty(tableName)) {
SQLiteDatabase db;
if (database == null) {
db = getWritableDatabase();
} else
db = database;
arrayList = new ArrayList<>();
if (db != null) {
StringBuilder selectQuery = new StringBuilder();
selectQuery.append("SELECT ");
if (columnName != null && columnName.length() > 0)
selectQuery.append(columnName);
else
selectQuery.append("*");
selectQuery.append(" FROM ");
selectQuery.append(tableName);
if (!ValidationHelper.isNullOrEmpty(filterCondition)) {
selectQuery.append(" WHERE ");
selectQuery.append(filterCondition);
}
if (!ValidationHelper.isNullOrEmpty(orderBy)) {
selectQuery.append(" ORDER BY ");
selectQuery.append(orderBy);
}
if (!ValidationHelper.isNullOrEmpty(groupBy)) {
selectQuery.append(" GROUP BY ");
selectQuery.append(groupBy);
}
Cursor cursor;
cursor = db.rawQuery(selectQuery.toString(), null);
if (cursor != null) {
String[] cursorColumn = cursor.getColumnNames();
if (cursor.moveToFirst()) {
do {
T returnObject = type.newInstance();
Class<?> returnClass = returnObject.getClass();
if (!returnClass.getSuperclass().equals(Object.class)) {
setFields(cursor, returnObject, returnClass.getSuperclass().getDeclaredFields(), cursorColumn);
}
setFields(cursor, returnObject, returnClass.getDeclaredFields(), cursorColumn);
arrayList.add(returnObject);
} while (cursor.moveToNext());
}
}
}
}
} catch (Exception e) {
LogHelper.writeExceptionLog(e);
} finally {
Disconnect();
}
return arrayList;
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
@Override
protected void finalize() throws Throwable {
this.close();
super.finalize();
Disconnect();
}
}
答案 0 :(得分：0)
你只需要这样定义：
private static DatabaseHelper mInstance = null;
public static DatabaseHelper getInstance(Context context) {
if (mInstance == null) {
mInstance = new DatabaseHelper(context);
}
return mInstance;
}
在您的活动中：
DatabaseHelper db = DatabaseHelper.getInstance(context);