Droidparts library has helper-method for execute serial statements in one transaction:
executeStatements(SQLiteDatabase db, ArrayList<String> statements)
But in real life some intermediate inspection can be occur such as:
if(some_check = some_result) { do statement_1 }
else { do statement_2 }
How one can do in these cases?
Well, I got it. One should do it in this manner:
Create Callable task
Put it in PersistUtils.executeInTransaction(SQLiteDatabase db, Callable task)
Pseudocode:
/* Create Callable task */
Callable<Boolean> task = new Callable<Boolean>() {
#Override
public Boolean call() throws Exception {
... Some operations ...
return Boolean.TRUE (or FALSE);
}
};
/* Check result */
Boolean result = PersistUtils.executeInTransaction(getDB(), task);
Related
Here I want to call n threads and execute my function padrDao.saveGuidanceDetails(sgd) which is a DAO method performing insert operation and return a long value as shown in below code.
Im using Callable, but it asks me to return some value but I'm not familiar with threads to use Runnable for the same job. Can someone pls validate if code is right or any modifications to be done? I feel code is wrong since there is a return statement inside callable and that will take me outside the main method for the first task itself.
int totalThreadsNeeded=listForguidanceItems.size();
ExecutorService executor = Executors.newFixedThreadPool(totalThreadsNeeded);
List<Callable<Void>> runnableTasks = new ArrayList<>();
final PriceLineItemsResultExt response1=response;
for(final ProductLineItemResultExt item: listForguidanceItems)
{
int counter=0;
final SavedGuidanceDetailsDto sgd=list.get(counter);
Callable<Void> task1 = new Callable() {
public Void call() {
if (sgd.hasGuidance())
{
if (response1.isSaveGuidance()) {
long guidanceDetailsId = padrDao.saveGuidanceDetails(sgd);
item.setGuidanceDetailsId(String.valueOf(guidanceDetailsId));
}
}
return null;
}};
counter++;
runnableTasks.add(task1);
}
try {
executor.invokeAll(runnableTasks);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
logger.info("Thread fail exception " + e);
}
executor.shutdown();
Pls suggest me modifications with the right code? Thanks in advance
To use Runnable you can simply replace these :
Callable<Void> task1 = new Callable() {
public Void call() {
...
With
Runnable task1 = new Runnable {
public void run() {
...
And with runnable you wouldn't have to return anything.
Of course you'd also need to modify you runnableTasks to be a List<Runnable> if tou still want to store these in a Collection (possibly not), and also change the way you submit them in the ExecutorService as :
executor.submit(your_Runnable_object)
I have a scenario where two threads invoke a method and this method generated a sequence using postgres nextval(test_sequence).
test_sequence is initailly assigned to 1.
public String createNotification() {
logger.info("createNotification ENTRY");
Future<String> futRes = this.threadPool.submit(new Callable<String>() {
#Override
public String call() {
String notificationID = getNotificationId();//DB CALL TO GENERATE THE NEXT SEQUENCE.
boolean isInsertSuccess = notificationDaoService.insertNotificationIntoDB(notificationID);
if (isInsertSuccess == true) {
return notificationID;
} else {
return null;
}
}
});
try {
return futRes.get(5, TimeUnit.SECONDS);
} catch (Exception e) {
logger.error("Issue while getting value from future with exception :", e);
return null;
}
}
So in the above snippet, getNotificationId() will generate the sequence and insertNotificationIntoDB() wil insert the generated notification id to the table.
I some times observing the primary key voilation exception when multiple threads try to invoke createNotification().
So i am thinking to synchronise the db calls as mentioned below,
synchronised(object)
{
String notificationID = getNotificationId();
boolean isInsertSuccess = notificationDaoService.insertNotificationIntoDB(notificationID);
}
is this solution ok?
and also i want to ask if i can generalise that if multiple threads are accessing a function and if that function has DB calls that does basic CRUD, then all the DB calls needs to be synchronised. Is this right inference?
Is it possible to abort a Task in JavaFX? My Task could run into situations where I want to cancel the rest of the operations within it.
I would need to return a value, somehow, so I can handle the cause of the abort in the JFX Application Thread.
Most of the related answers I've seen refer to handling an already-canceled Task, but now how to manually cancel it from within the Task itself.
The cancel() method seems to have no effect as both messages below are displayed:
public class LoadingTask<Void> extends Task {
#Override
protected Object call() throws Exception {
Connection connection;
// ** Connect to server ** //
updateMessage("Contacting server ...");
try {
connection = DataFiles.getConnection();
} catch (SQLException ex) {
updateMessage("ERROR: " + ex.getMessage());
ex.printStackTrace();
cancel();
return null;
}
// ** Check user access ** //
updateMessage("Verifying user access ...");
try {
String username = System.getProperty("user.name");
ResultSet resultSet = connection.createStatement().executeQuery(
SqlQueries.SELECT_USER.replace("%USERNAME%", username));
// If user doesn't exist, block access
if (!resultSet.next()) {
}
} catch (SQLException ex) {
}
return null;
}
}
And example would be greatly appreciated.
Why not just let the task go into a FAILED state if it fails? All you need (I also corrected the errors with the type of the task and return type of the call method) is
public class LoadingTask extends Task<Void> {
#Override
protected Void call() throws Exception {
Connection connection;
// ** Connect to server ** //
updateMessage("Contacting server ...");
connection = DataFiles.getConnection();
// ** Check user access ** //
updateMessage("Verifying user access ...");
String username = System.getProperty("user.name");
ResultSet resultSet = connection.createStatement().executeQuery(
SqlQueries.SELECT_USER.replace("%USERNAME%", username));
// I am not at all sure what this is supposed to do....
// If user doesn't exist, block access
if (!resultSet.next()) {
}
return null;
}
}
Now if an exception is thrown by DataFiles.getConnection(), the call method terminates immediately with an exception (the remained is not executed) and the task enters a FAILED state. If you need access to the exception in the case that something goes wrong, you can do:
LoadingTask loadingTask = new LoadingTask();
loadingTask.setOnFailed(e -> {
Throwable exc = loadingTask.getException();
// do whatever you need with exc, e.g. log it, inform user, etc
});
loadingTask.setOnSucceeded(e -> {
// whatever you need to do when the user logs in...
});
myExecutor.execute(loadingTask);
I'm pretty new with concurency and I'v hit the wall several times already.
Code pretty much describes everything, but just to clarify: user press the button, application send query to db and in the meantime statusLabel is set to:
Veryfing.
200ms
Veryfing..
200ms
Veryfing...
200ms
Result of query
I'v managed to achieve that, but now, I need to use result of query in another class (if it succeed, another window is opened), but It never does. I came to conclusion that it just checks the result before Task is finished so result is always false, I have no idea how to work around this, so another class checks condition once Task is done.
First, my Authorization class
public class Authorization {
private static String query = "";
private static boolean isValid;
private static Task<Void> task;
public static void verifyLogin(String username, String password) throws SQLException{
Status.get().unbind();
isValid = false;
task = new Task<Void>() {
#Override
protected Void call() throws SQLException {
while(!isCancelled()) {
try {
updateMessage("Weryfikacja.");
Thread.sleep(200);
updateMessage("Weryfikacja..");
Thread.sleep(200);
updateMessage("Weryfikacja...");
Thread.sleep(200);
if(username.equals("") || password.equals("")) {
task.cancel();
updateMessage("Pola nie mogą być puste");
} else {
query = "SELECT login FROM users WHERE login = ?";
Query.execute(query, username);
if(!Query.resultSet.next()) {
task.cancel();
updateMessage("Nie ma takiego użytkownika");
} else {
query = "SELECT password FROM users WHERE login = ?";
Query.execute(query, username);
if(Query.resultSet.next()) {
String passwordValue = Query.resultSet.getString(1);
if(!password.equals(passwordValue)) {
task.cancel();
updateMessage("Podane hasło jest błędne");
} else {
task.cancel();
updateMessage("");
isValid = true;
}
}
}
}
} catch(InterruptedException e) {
if(isCancelled()) {
break;
}
}
}
return null;
}
};
Status.get().bind(task.messageProperty());
new Thread(task).start();
}
public static boolean isValid() {
return isValid;
}
}
called from another class
private void login() {
if( SqlConnection.isConnected()) {
try{
Authorization.verifyLogin(String.valueOf(loginInput.getText()), String.valueOf(passwordInput.getText()));
if(Authorization.isValid()) {
//should go to next menu
//but never does
}
} catch (SQLException e) {
e.printStackTrace();
Debug.log(e.toString());
}
}
}
edit#
Sorry for polish in updateMessage().
Your verifyLogin() method simply starts the verification process in another thread, and then exits immediately. The isValid flag will not be changed until that thread completes, which happens quite a lot later. If you want to do the verification process and then do something else, it doesn't really make sense to manage the threads in verifyLogin().
I don't really understand a lot of what your code is supposed to be doing; you have a while(...) loop, which as far as I can tell can only be executed once (so is redundant). You also seem to execute two SQL queries which are essentially the same. (The first checks to see if there is a row with a certain condition, then if there is, the second retrieves that row. Why not just retrieve the row and check if it is there?)
I would refactor this so that the validateLogin() method doesn't handle the threading at all, and just returns the result of the validation (e.g. a status string, but maybe something else would be appropriate).
/**
* #return An empty string if the login is valid, or an error message otherwise
*/
public static String verifyLogin(String username, String password) throws SQLException{
isValid = false ;
if(username.equals("") || password.equals("")) {
return "Pola nie mogą być puste";
}
query = "SELECT login, password FROM users WHERE login = ?";
Query.execute(query, username);
if(!Query.resultSet.next()) {
return "Nie ma takiego użytkownika";
}
String passwordValue = Query.resultSet.getString(2);
if(!password.equals(passwordValue)) {
return "Podane hasło jest błędne" ;
}
isValid = true;
return "" ;
}
Now I would manage the threads from the login() method. That way you can use the task's onSucceeded handler to execute code when the task completes:
private void login() {
if( SqlConnection.isConnected()) {
Task<String> verifyTask = new Task<String>() {
#Override
protected String call() throws SQLException {
return Authorization.verifyLogin(loginInput.getText(), passwordInput.getText());
}
};
// probably better to use a progress indicator or similar here, but:
Animation animation = new Timeline(
new KeyFrame(Duration.ZERO, e -> Status.get().set("Weryfikacja.")),
new KeyFrame(Duration.millis(200), e -> Status.get().set("Weryfikacja..")),
new KeyFrame(Duration.millis(400), e -> Status.get().set("Weryfikacja...")),
new KeyFrame(Duration.millis(600)));
animation.setCycleCount(Animation.INDEFINITE);
verifyTask.setOnSucceeded(event -> {
animation.stop();
Status.get().set(verifyTask.getValue());
if(Authorization.isValid()) { // or if (verifyTask.getValue().isEmpty())
// go to next menu
}
});
verifyTask.setOnFailed(event -> {
animation.stop();
verifyTask.getException().printStackTrace();
Debug.log(verifyTask.getException().toString());
}
animation.play();
new Thread(verifyTask()).start();
}
}
Say I have a bunch of transformations on an Observable:
operation()
.flatMap(toSomething())
.map(toSomethingElse())
.flatMap(toYetSomethingElse())
.subscribeOn(Schedulers.newThread())
.observeOn(AdroidSchedulers.mainThread())
.subscribe(observer);
Are all of these operations synchronous except for the last call to flatMap()? Or are all of the operations run on the thread that I told it to subscribe on?
I figured this out, with a test. The following test passes (which means the emissions on the Observable are all on the same background thread):
volatile long observableThreadId;
#Test
public void transformedObservables_shouldRunInSameThread() {
Observable.from(new String[]{"a", "b", "c"}) //
.flatMap(new Func1<String, Observable<Object>>() {
#Override public Observable<Object> call(String s) {
observableThreadId = Thread.currentThread().getId();
return Observable.from((Object) s);
}
}) //
.map(new Func1<Object, String>() {
#Override public String call(Object o) {
long id = Thread.currentThread().getId();
if (id != observableThreadId) {
throw new RuntimeException("Thread ID mismatch");
}
return (String) o;
}
}) //
.flatMap(new Func1<String, Observable<String>>() {
#Override public Observable<String> call(String s) {
long id = Thread.currentThread().getId();
if (id != observableThreadId) {
throw new RuntimeException("Thread ID mismatch");
}
return Observable.from(s);
}
}) //
.subscribeOn(Schedulers.newThread()) //
.observeOn(Schedulers.currentThread()) //
.subscribe(new Observer<String>() {
#Override public void onCompleted() {
assertThat(Thread.currentThread().getId()).isNotEqualTo(observableThreadId);
}
#Override public void onError(Throwable throwable) {
}
#Override public void onNext(String s) {
}
});
System.out.println("blah");
}
===============================
UPDATE:
A better answer can actually be found in the ReactiveX documentation on Scheduler:
By default, an Observable and the chain of operators that you apply to
it will do its work, and will notify its observers, on the same thread
on which its Subscribe method is called. The SubscribeOn operator
changes this behavior by specifying a different Scheduler on which the
Observable should operate. The ObserveOn operator specifies a
different Scheduler that the Observable will use to send notifications
to its observers.
... the SubscribeOn operator designates which thread the Observable will
begin operating on, no matter at what point in the chain of operators
that operator is called. ObserveOn, on the other hand, affects the
thread that the Observable will use below where that operator appears.
For this reason, you may call ObserveOn multiple times at various
points during the chain of Observable operators in order to change on
which threads certain of those operators operate.