Our Blazor (client-side) app is made up of many components all existing on the UI at the same time. One of these has to do a number of large data calls to Azure SQL. This component does these calls regardless of whether it has UI focus or not. Each of calls these can take up to 3 seconds to return its result during which it renders the UI unresponsive. How can we keep the UI responsive during these calls without using Blazor server-side. Using Task.Run etc does not help in single threaded architecture. Using loading spinners is also not an option as this still leaves the UI unresponsive and may not be visible to the user. Is there any way to achieve this goal in current Blazor 0.9.0?
Running latest Blazor preview release (0.9.0-preview3-19154-02)
You can use Invoke, I modified counter example page to illustrate it, you will need a kind of singleton DI object to avoid running the process for twice.
Remember Blazor is an experimental project. Also, this answer is also an experimental approach.
#page "/counter"
<h1>Counter</h1>
<p>Current count: #currentCount</p>
<button class="btn btn-primary" onclick="#IncrementCount">Click me</button>
#functions {
int currentCount = 0;
void IncrementCount()
{
currentCount++;
}
protected override void OnInit()
{
Invoke(
//here your task.
async () =>
{
for(var i =0; i< 50; i++)
{
await Task.Delay(1000);
currentCount++;
StateHasChanged();
System.Console.WriteLine("Still running ...");
}
});
}
}
If async calls does not help then you can use browser workers, just need to implement some js interop.
I've had success with Task().Start()
put your work in an async Task like so:
async Task MyWork()
{
//sleep 10000
}
now from wherever you don't want this work to block call:
new Task( () => MyWork()).Start() );
I've only used this with Blazor Server, so haven't tested it with Client side Blazor which I hear can have different results because of its running on a single thread.
the suggested answer did not work, for me, I ended up using :
protected override void OnInitialized()
{
InvokeAsync(async () =>
{
myvar = await YourCodeHere();
StateHasChanged();
});
base.OnInitialized();
}
public async Task<MyVarType> YourCodeHere()
{
...
}
Explanation: I have a class called MyVarType that I am using to display values on the Razor component. I have a myVar variable instantiated with that class, and initially it is empty. The component will render its default values immediately. Upon initializing the variable is then also being asynchronously assigned from the OnInitialized method. The YourCodeHere function could take several seconds. The Razor component rerenders when calling StateHasChanged() so it displays the loaded values.
This method throws
java.lang.IllegalStateException: Cannot forward after response has been committed
and I am unable to spot the problem. Any help?
int noOfRows = Integer.parseInt(request.getParameter("noOfRows"));
String chkboxVal = "";
// String FormatId=null;
Vector vRow = new Vector();
Vector vRow1 = new Vector();
String GroupId = "";
String GroupDesc = "";
for (int i = 0; i < noOfRows; i++) {
if ((request.getParameter("chk_select" + i)) == null) {
chkboxVal = "notticked";
} else {
chkboxVal = request.getParameter("chk_select" + i);
if (chkboxVal.equals("ticked")) {
fwdurl = "true";
Statement st1 = con.createStatement();
GroupId = request.getParameter("GroupId" + i);
GroupDesc = request.getParameter("GroupDesc" + i);
ResultSet rs1 = st1
.executeQuery("select FileId,Description from cs2k_Files "
+ " where FileId like 'M%' and co_code = "
+ ccode);
ResultSetMetaData rsm = rs1.getMetaData();
int cCount = rsm.getColumnCount();
while (rs1.next()) {
Vector vCol1 = new Vector();
for (int j = 1; j <= cCount; j++) {
vCol1.addElement(rs1.getObject(j));
}
vRow.addElement(vCol1);
}
rs1 = st1
.executeQuery("select FileId,NotAllowed from cs2kGroupSub "
+ " where FileId like 'M%' and GroupId = '"
+ GroupId + "'" + " and co_code = " + ccode);
rsm = rs1.getMetaData();
cCount = rsm.getColumnCount();
while (rs1.next()) {
Vector vCol2 = new Vector();
for (int j = 1; j <= cCount; j++) {
vCol2.addElement(rs1.getObject(j));
}
vRow1.addElement(vCol2);
}
// throw new Exception("test");
break;
}
}
}
if (fwdurl.equals("true")) {
// throw new Exception("test");
// response.sendRedirect("cs2k_GroupCopiedUpdt.jsp") ;
request.setAttribute("GroupId", GroupId);
request.setAttribute("GroupDesc", GroupDesc);
request.setAttribute("vRow", vRow);
request.setAttribute("vRow1", vRow1);
getServletConfig().getServletContext().getRequestDispatcher(
"/GroupCopiedUpdt.jsp").forward(request, response);
}
forward/sendRedirect/sendError do NOT exit the method!
A common misunderstanding among starters is that they think that the call of a forward(), sendRedirect(), or sendError() would magically exit and "jump" out of the method block, hereby ignoring the remnant of the code. For example:
protected void doXxx() {
if (someCondition) {
sendRedirect();
}
forward(); // This is STILL invoked when someCondition is true!
}
This is thus actually not true. They do certainly not behave differently than any other Java methods (expect of System#exit() of course). When the someCondition in above example is true and you're thus calling forward() after sendRedirect() or sendError() on the same request/response, then the chance is big that you will get the exception:
java.lang.IllegalStateException: Cannot forward after response has been committed
If the if statement calls a forward() and you're afterwards calling sendRedirect() or sendError(), then below exception will be thrown:
java.lang.IllegalStateException: Cannot call sendRedirect() after the response has been committed
To fix this, you need either to add a return; statement afterwards
protected void doXxx() {
if (someCondition) {
sendRedirect();
return;
}
forward();
}
... or to introduce an else block.
protected void doXxx() {
if (someCondition) {
sendRedirect();
}
else {
forward();
}
}
To naildown the root cause in your code, just search for any line which calls a forward(), sendRedirect() or sendError() without exiting the method block or skipping the remnant of the code. This can be inside the same servlet before the particular code line, but also in any servlet or filter which was been called before the particular servlet.
In case of sendError(), if your sole purpose is to set the response status, use setStatus() instead.
Do not write any string before forward/sendRedirect/sendError
Another probable cause is that the servlet writes to the response while a forward() will be called, or has been called in the very same method.
protected void doXxx() {
out.write("<p>some html</p>");
// ...
forward(); // Fail!
}
The response buffer size defaults in most server to 2KB, so if you write more than 2KB to it, then it will be committed and forward() will fail the same way:
java.lang.IllegalStateException: Cannot forward after response has been committed
Solution is obvious, just don't write to the response in the servlet. That's the responsibility of the JSP. You just set a request attribute like so request.setAttribute("data", "some string") and then print it in JSP like so ${data}. See also our Servlets wiki page to learn how to use Servlets the right way.
Do not write any file before forward/sendRedirect/sendError
Another probable cause is that the servlet writes a file download to the response after which e.g. a forward() is called.
protected void doXxx() {
out.write(bytes);
// ...
forward(); // Fail!
}
This is technically not possible. You need to remove the forward() call. The enduser will stay on the currently opened page. If you actually intend to change the page after a file download, then you need to move the file download logic to page load of the target page. Basically: first create a temporary file on disk using the way mentioned in this answer How to save generated file temporarily in servlet based web application, then send a redirect with the file name/identifier as request param, and in the target page conditionally print based on the presence of that request param a <script>window.location='...';</script> which immediately downloads the temporary file via one of the ways mentioned in this answer Simplest way to serve static data from outside the application server in a Java web application.
Do not call forward/sendRedirect/sendError in JSP
Yet another probable cause is that the forward(), sendRedirect() or sendError() methods are invoked via Java code embedded in a JSP file in form of old fashioned way <% scriptlets %>, a practice which was officially discouraged since 2003. For example:
<!DOCTYPE html>
<html lang="en">
<head>
...
</head>
<body>
...
<% sendRedirect(); %>
...
</body>
</html>
The problem here is that JSP internally immediately writes template text (i.e. HTML code) via out.write("<!DOCTYPE html> ... etc ...") as soon as it's encountered. This is thus essentially the same problem as explained in previous section.
Solution is obvious, just don't write Java code in a JSP file. That's the responsibility of a normal Java class such as a Servlet or a Filter. See also our Servlets wiki page to learn how to use Servlets the right way.
See also:
What exactly does "Response already committed" mean? How to handle exceptions then?
Unrelated to your concrete problem, your JDBC code is leaking resources. Fix that as well. For hints, see also How often should Connection, Statement and ResultSet be closed in JDBC?
even adding a return statement brings up this exception, for which only solution is this code:
if(!response.isCommitted())
// Place another redirection
Typically you see this error after you have already done a redirect and then try to output some more data to the output stream. In the cases where I have seen this in the past, it is often one of the filters that is trying to redirect the page, and then still forwards through to the servlet. I cannot see anything immediately wrong with the servlet, so you might want to try having a look at any filters that you have in place as well.
Edit: Some more help in diagnosing the problem…
The first step to diagnosing this problem is to ascertain exactly where the exception is being thrown. We are assuming that it is being thrown by the line
getServletConfig().getServletContext()
.getRequestDispatcher("/GroupCopiedUpdt.jsp")
.forward(request, response);
But you might find that it is being thrown later in the code, where you are trying to output to the output stream after you have tried to do the forward. If it is coming from the above line, then it means that somewhere before this line you have either:
output data to the output stream, or
done another redirect beforehand.
Good luck!
You should add return statement while you are forwarding or redirecting the flow.
Example:
if forwardind,
request.getRequestDispatcher("/abs.jsp").forward(request, response);
return;
if redirecting,
response.sendRedirect(roundTripURI);
return;
This is because your servlet is trying to access a request object which is no more exist..
A servlet's forward or include statement does not stop execution of method block. It continues to the end of method block or first return statement just like any other java method.
The best way to resolve this problem just set the page (where you suppose to forward the request) dynamically according your logic. That is:
protected void doPost(request , response){
String returnPage="default.jsp";
if(condition1){
returnPage="page1.jsp";
}
if(condition2){
returnPage="page2.jsp";
}
request.getRequestDispatcher(returnPage).forward(request,response); //at last line
}
and do the forward only once at last line...
you can also fix this problem using return statement after each forward() or put each forward() in if...else block
I removed
super.service(req, res);
Then it worked fine for me
Bump...
I just had the same error. I noticed that I was invoking super.doPost(request, response); when overriding the doPost() method as well as explicitly invoking the superclass constructor
public ScheduleServlet() {
super();
// TODO Auto-generated constructor stub
}
As soon as I commented out the super.doPost(request, response); from within doPost() statement it worked perfectly...
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
//super.doPost(request, response);
// More code here...
}
Needless to say, I need to re-read on super() best practices :p
After return forward method you can simply do this:
return null;
It will break the current scope.
If you see this on a Spring based web application, make sure you have your method annotated with #ResponseBody or the controller annotated with #RestController instead of #Controller. It will also throw this exception if a method returns JSON, but has not been configured to have that as the response, Spring will instead look for a jsp page to render and throw this exception.
Trying to figure out why I can login with my rest API just fine on the main thread but not in a worker. All communication channels are operating fine and I am able to load it up no problem. However, when it tries to send some data it just hangs.
[Embed(source="../bin/BGThread.swf", mimeType="application/octet-stream")]
private static var BackgroundWorker_ByteClass:Class;
public static function get BackgroundWorker():ByteArray
{
return new BackgroundWorker_ByteClass();
}
On a test script:
public function Main()
{
fBCore.init("secrets", "my-firebase-id");
trace("Init");
//fBCore.auth.addEventListener(FBAuthEvent.LOGIN_SUCCES, hanldeFBSuccess);
fBCore.auth.addEventListener(AuthEvent.LOGIN_SUCCES, hanldeFBSuccess);
fBCore.auth.addEventListener(IOErrorEvent.IO_ERROR, handleIOError);
fBCore.auth.email_login("admin#admin.admin", "password");
}
private function handleIOError(e:IOErrorEvent):void
{
trace("IO error");
trace(e.text); //Nothing here
}
private function hanldeFBSuccess(e:AuthEvent):void
{
trace("Main login success.");
trace(e.message);//Complete success.
}
When triggered by a class via an internal worker channel passed from Main on init:
Primordial:
private function handleLoginClick(e:MouseEvent):void
{
login_mc.buttonMode = false;
login_mc.play();
login_mc.removeEventListener(MouseEvent.CLICK, handleLoginClick);
log("Logging in as " + email_mc.text_txt.text);
commandChannel.send([BGThreadCommand.LOGIN, email_mc.text_txt.text, password_mc.text_txt.text]);
}
Worker:
...
case BGThreadCommand.LOGIN:
log("Logging in with " + message[1] + "::" + message[2]); //Log goes to a progress channel and comes to the main thread reading the outputs successfully.
fbCore.auth.email_login(message[1], message[2]);
fbCore.auth.addEventListener(AuthEvent.LOGIN_SUCCES, loginSuccess); //Nothing
fbCore.auth.addEventListener(IOErrorEvent.IO_ERROR, handleLoginIOError); //Fires
break;
Auth Rest Class: https://github.com/sfxworks/FirebaseREST/blob/master/src/net/sfxworks/firebaseREST/Auth.as
Is this a worker limitation or a security sandbox issue? I have a deep feeling it is the latter of the two. If that's the case how would I load the worker in a way that also gives it the proper permissions to act?
Completely ignored the giveAppPrivelages property in the createWorker function. Sorry Stackoverflow. Sometimes I make bad questions when I get little (or none in this case) sleep the night before.
I'm trying to make my MVC controller action run asynchronously under .NET 4.0. However, none of my attempts have given me the results I want. I have the following action:
public ActionResult ImportXml()
{
try
{
if (_importRunning)
return Content("Already running");
var obj = new object();
lock (obj)
{
_importRunning = true;
Thread.Sleep(20000);
//_employesImportService.ImportXml();
_importRunning = false;
}
return Content("Done");
}
catch (Exception e)
{
return Content(e.Message);
}
}
When I run two browsers simultaneously that call this action, both seem to wait the 20 seconds I set in the Thread.Sleep(20000). I though that using the lock mechanism would block one request and return the "Already running" content immediately. I'm using .NET 4.0 and I don't have the option of using async await. But is there another way of implementing this so that one of the requests responds quickly?
I have 2 windows in swift, one is like a login dialog which sends an NSURLConnection.sendAsynchronousRequest to a server to get authenticated. Once it gets the response, the window is supposed to close.
When I close the Window (either from the login window class or main window clasS) I get these errors:
This application is modifying the autolayout engine from a background thread, which can lead to engine corruption and weird crashes. This will cause an exception in a future release.
I have tried all manner of background threads etc. But I think the issue is that I am closing the window why the asynch NSURLConnection request is still hanging.
My code to send the async request from the login window:
dispatch_async(dispatch_get_main_queue(), {
let queue:NSOperationQueue = NSOperationQueue()
NSURLConnection.sendAsynchronousRequest(request, queue: NSOperationQueue(), completionHandler:{ (response:NSURLResponse!, data: NSData!, error: NSError!) -> Void in
var error: AutoreleasingUnsafeMutablePointer<NSError?> = nil
let jsonResult: NSDictionary! = NSJSONSerialization.JSONObjectWithData(data, options:NSJSONReadingOptions.MutableContainers, error: error) as? NSDictionary
let result: NSString = NSString(data: data, encoding: NSUTF8StringEncoding)!
let expectedString = "special auth string"!
if(result == expectedString) {
self.callback.loginOK()
} else {
self.output.stringValue = result
}
return
})
})
The callback member of the class is the parent view contoller that spawns it, I then close the login window using loginVC.view.window?.close() from the main application window. That causes the error.
The problem is that NSURLConnection.sendAsynchronousRequest will always run in a secondary thread and thus its callback will be called from that secondary thread despite you calling it explicitly from main thread.
You don't need to wrap NSURLConnection.sendAsynchronousRequest in the main thread, instead wrap your ' self.callback.loginOK()' to run in main thread using the dispatch_async to ensure no UI related operations take place in secondary thread. Something like this-
let queue:NSOperationQueue = NSOperationQueue()
NSURLConnection.sendAsynchronousRequest(request, queue: NSOperationQueue(), completionHandler:{ (response:NSURLResponse!, data: NSData!, error: NSError!) -> Void in
var error: AutoreleasingUnsafeMutablePointer<NSError?> = nil
let jsonResult: NSDictionary! = NSJSONSerialization.JSONObjectWithData(data, options:NSJSONReadingOptions.MutableContainers, error: error) as? NSDictionary
let result: NSString = NSString(data: data, encoding: NSUTF8StringEncoding)!
dispatch_async(dispatch_get_main_queue() {
let expectedString = "special auth string"!
if(result == expectedString) {
self.callback.loginOK()
} else {
self.output.stringValue = result
}
})
return
})