Develop Reference

Project reactor - not able to use fallback method

I'm new to reactive programming. I was playing around with fallback methods in case of an error scenario. For this, I referred to this doc https://projectreactor.io/docs/core/release/reference/index.html#_fallback_method and created a sample code.
public static void main(String[] args) {
Flux.just("key1", "key2")
.flatMap(k -> callExternalService(k)
.doOnError(e -> System.out.println("Error scenario"))
.onErrorResume(e -> getFromCache(k)))
.subscribe(value -> System.out.println("value = " + value),
error -> System.out.println("error = " + error));
}
private static Mono<String> callExternalService(String input) {
if(input.equals("key2")) throw new RuntimeException("Mocking the exception");
return Mono.just(input + " - " + LocalDateTime.now());
}
private static Mono<String> getFromCache(String input) {
return Mono.just(input + " ^^ " + LocalDateTime.now());
}
Based on whatever I referred so far, doOnError should print the message in case of the ERROR scenario and onErrorResume should fall back to the other method. But I didn't see the expected outcome->
value = key1 - 2022-05-18T15:58:36.364949
error = java.lang.RuntimeException: Mocking the exception
Please correct me if I'm missing anything.

Understanding Mono.when

I'm new to Spring WebFlux and do not fully understand the Mono.when(). The following code does not work as expected:
List<Mono<Void>> results= ....;
String textVar = "my text";
processors.forEach(p -> {
Mono<Object> restResponseMono = client.getSomething();
results.add(restResponseMono.doOnNext(resp -> {
textVar = textVar + resp.getText();
}).then());
});
Mono.when(results).then(
//here it would expect modification of 'textVar'
Mono.just(textVar);
)
After calling the Mono.when(results).then(...) I would expect all my changes to be applied to the textVar because in the docu it is written:
[...Aggregate given publishers into a new Mono that will befulfilled when all of the given Publishers have completed....]
And the restResponseMono.then() should also wait until everything is completed. So I do not know exactly where is my lack of understanding.

Publishers in the when parameter,When subscribe automatically subscribes.
The way it works is simply this.
Flux<Integer> m1 = Flux.just(1,2).doOnNext(e -> System.out.println("M1 doOnNext: " + e));
Mono<Integer> m2 = Mono.just(12).doOnSuccess(e -> System.out.println("M2 doOnSuccess: " + e));
Mono<String> mono = Mono.when(m1,m2).then(Mono.just("STR")).doOnSuccess(e -> System.out.println("_________when doOnSuccess: " + e));
mono.log().subscribe(System.out::println, Exception::new, () -> System.out.println("Completed2."));

Java error loginserver

right now i am working on a ban system for a private game of me.
Everything works expect invalid username.
I am getting syntax error on token else.
How i can fix this issue?
I tried to fix this issue by adding else at the bottom but it keeps telling me that i have a syntax error what i need to do to fix this issue?
I am using the tool eclipse.
Syntax error on token "else", delete this token
#Override
public void handlePacket(LoginRequestPacket packet, LoginServerHandle handle) {
handle.getLog().info("Login request received: " + packet);
Optional<Character> characterOptional = handle.getCharacterByUsername(packet.getUsername());
Character character = characterOptional.get();
int ban = character.getBan();
String ign = character.getIgn();
try {
if (verifyPassword(packet.getPassword(), character.getPasswordHash()) && ban >0) {
handle.send(LoginResponsePacket.result(LoginResponsePacket.RESULT_ID_NO_LONGER_IN_USE));
handle.getLog().info("Login request " + packet + " response: Banned User.");
}
else if (verifyPassword(packet.getPassword(), character.getPasswordHash()) && ign == null) {
handle.getLog().info("Login request " + packet + " response: NEW USER!");
handle.send(LoginResponsePacket.result(LoginResponsePacket.RESULT_CREATE_CHARACTER));
}
else if (verifyPassword(packet.getPassword(), character.getPasswordHash())) { {
handle.send(LoginResponsePacket.success(character.getId(), handle.createSession(character), "127.0.0.1"));
handle.getLog().info("Successful login: " + packet);
}
}
else {
// Incorrect password
handle.send(LoginResponsePacket.result(LoginResponsePacket.RESULT_INVALID_PW));
handle.getLog().info("Login request " + packet + " response: Invalid PW");
}
}
catch (CannotPerformOperationException | InvalidHashException e) {
handle.getLog().error("Error performing password validation. Response: Wrong Data.", e);
handle.send(LoginResponsePacket.result(LoginResponsePacket.RESULT_WRONG_DATA));
}
else {
// Invalid username
handle.getLog().info("Login request " + packet + " response: Invalid ID");
handle.send(LoginResponsePacket.result(LoginResponsePacket.RESULT_INVALID_ID));
}
}
}

Your top level brackets are formed like this
try -> catch -> else
The else is not preceeded by an if, so the compile fails.

AWS SQS Java. Not all messages are retrieved from the SQS queue

I have been trying several approaches to retrieve all messages from the SQS queue by using AWS SDK for Java to no avail. I have read about the distributed nature of the AWS SQS and that messages are stored on the different servers. But what I do not understand is why this architecture is not hidden from the end user. What tricks do I have to apply in Java code to retrieve all messages and be 100% sure that no one was missed?
I tried this with the "Long Polling":
ReceiveMessageRequest receiveMessageRequest = new ReceiveMessageRequest(myQueueUrl);
List<Message> messages = sqs.receiveMessage(receiveMessageRequest).getMessages();
for (Message message : messages) {
System.out.println(" Message");
System.out.println(" MessageId: " + message.getMessageId());
System.out.println(" ReceiptHandle: " + message.getReceiptHandle());
System.out.println(" MD5OfBody: " + message.getMD5OfBody());
System.out.println(" Body: " + message.getBody());
for (Entry<String, String> entry : message.getAttributes().entrySet()) {
System.out.println(" Attribute");
System.out.println(" Name: " + entry.getKey());
System.out.println(" Value: " + entry.getValue());
}
}
System.out.println();
And this with Request Batching / Client-Side Buffering:
// Create the basic Amazon SQS async client
AmazonSQSAsync sqsAsync = new AmazonSQSAsyncClient();
// Create the buffered client
AmazonSQSAsync bufferedSqs = new AmazonSQSBufferedAsyncClient(sqsAsync);
CreateQueueRequest createRequest = new CreateQueueRequest().withQueueName("MyTestQueue");
CreateQueueResult res = bufferedSqs.createQueue(createRequest);
SendMessageRequest request = new SendMessageRequest();
String body = "test message_" + System.currentTimeMillis();
request.setMessageBody( body );
request.setQueueUrl(res.getQueueUrl());
SendMessageResult sendResult = bufferedSqs.sendMessage(request);
ReceiveMessageRequest receiveRq = new ReceiveMessageRequest()
.withMaxNumberOfMessages(10)
.withQueueUrl(queueUrl);
ReceiveMessageResult rx = bufferedSqs.receiveMessage(receiveRq);
List<Message> messages = rx.getMessages();
for (Message message : messages) {
System.out.println(" Message");
System.out.println(" MessageId: " + message.getMessageId());
System.out.println(" ReceiptHandle: " + message.getReceiptHandle());
System.out.println(" MD5OfBody: " + message.getMD5OfBody());
System.out.println(" Body: " + message.getBody());
for (Entry<String, String> entry : message.getAttributes().entrySet()) {
System.out.println(" Attribute");
System.out.println(" Name: " + entry.getKey());
System.out.println(" Value: " + entry.getValue());
}
}
But I am still unable to retrieve all messages.
Any idea?
AWS Forum keeps silence on my post.

When receiving messages from an SQS queue, you need to repeatedly call sqs:ReceiveMessage.
On each call to sqs:ReceiveMessage, you will get 0 or more messages from the queue which you'll need to iterate through. For each message, you'll also need to call sqs:DeleteMessage to remove the message from the queue when you're done processing each message.
Add a loop around your "Long Polling" sample above to receive all messages.
for (;;) {
ReceiveMessageRequest receiveMessageRequest = new ReceiveMessageRequest(myQueueUrl);
List<Message> messages = sqs.receiveMessage(receiveMessageRequest).getMessages();
for (Message message : messages) {
System.out.println(" Message");
System.out.println(" MessageId: " + message.getMessageId());
System.out.println(" ReceiptHandle: " + message.getReceiptHandle());
System.out.println(" MD5OfBody: " + message.getMD5OfBody());
System.out.println(" Body: " + message.getBody());
for (Entry<String, String> entry : message.getAttributes().entrySet()) {
System.out.println(" Attribute");
System.out.println(" Name: " + entry.getKey());
System.out.println(" Value: " + entry.getValue());
}
}
System.out.println();
}
Also note that you may receive the same message more than once. So allow your work to "reprocess" the same message, or detect a repeated message.

I too was facing same issue - only one message was getting returned , then i tried
receiveMessageRequest.setMaxNumberOfMessages(10) , which would help me in retrieving 10 messages in a loop,
since my queue has >500 records what i did was
List<String> messagelist = new ArrayList<>();
try
{
AmazonSQS sqs = new AmazonSQSClient(credentials);
Region usWest2 = Region.getRegion(Regions.US_WEST_2);
sqs.setRegion(usWest2);
boolean flag = true;
while(flag)
{
ReceiveMessageRequest receiveMessageRequest = new ReceiveMessageRequest(queuename);
receiveMessageRequest.setMaxNumberOfMessages(number_of_message_);
receiveMessageRequest.withMaxNumberOfMessages(number_of_message_).withWaitTimeSeconds(wait_time_second_);
List<Message> messages = sqs.receiveMessage(receiveMessageRequest).getMessages();
for (Message message : messages)
{
// System.out.println(" Body: " + message.getBody());
messagelist.add( message.getBody());
String messageReceiptHandle = message.getReceiptHandle();
sqs.deleteMessage(new DeleteMessageRequest().withQueueUrl(queuename).withReceiptHandle(messageReceiptHandle));
}
if(messages.size()==0)
{
flag = false;
}
}
}
catch (AmazonServiceException ase) {
ase.printStackTrace();
} catch (AmazonClientException ace) {
ace.printStackTrace();
}
finally {
return messagelist ;
}
I am reading records from SQS then saving it into a String list and then deletion the record from queue.
so in the end i will have all the data from the queue in a list

An SQS queue is not a database. You can't read all the messages into a list like you are trying to do. There is no beginning and no end to the queue. You poll the queue and ask for some messages, it returns you some messages if they exist.
If you want a method that can return the entire dataset, then sqs is not the right tool - a traditional database might be better in that case.

Long polling will wait if there is no message in Queue. This means that if you call ReceiveMessage with long polling in loop you are guaranteed that you will get all messages. When there is 0 messages received in response, you've already received all messages.
You mentioned that you used also web console. Web console works in same way as calling API with SDK. This means that when you receive and see messages in console, messages are invisible to other clients until visibility timeout expires. That's probably reason why you don't see messages.
See more information about visibility timeout:
http://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/AboutVT.html

Returning JSON response from Servlet to Javascript/JSP page

I think (actually I KNOW!) I'm doing something wrong here I am trying to populate some values into HashMap and add each hasmap to a list which will be added to a JSON object:
JSONObject json = new JSONObject();
try
{
Map address;
List addresses = new ArrayList();
int count = 15;
for (int i=0 ; i<count ; i++)
{
address = new HashMap();
address.put("CustomerName" , "Decepticons" + i);
address.put("AccountId" , "1999" + i);
address.put("SiteId" , "1888" + i);
address.put("Number" , "7" + i);
address.put("Building" , "StarScream Skyscraper" + i);
address.put("Street" , "Devestator Avenue" + i);
address.put("City" , "Megatron City" + i);
address.put("ZipCode" , "ZZ00 XX1" + i);
address.put("Country" , "CyberTron" + i);
addresses.add(address);
}
json.put("Addresses", addresses);
}
catch (JSONException jse)
{
}
response.setContentType("application/json");
response.getWriter().write(json.toString());
My problem is I know this is returning a string, which I cannot seem to parse (which is the problem). My question is how do I return the actual JSON encoded string (or even should I be doing this?) or what is the best method of attack for this type of problem. The JavaScript I am using for this is below:
function getReadyStateHandler(req)
{
// Return an anonymous function that listens to the
// XMLHttpRequest instance
return function ()
{
// If the request's status is "complete"
if (req.readyState == 4)
{
// Check that a successful server response was received
if (req.status == 200)
{
msgBox("JSON Response recieved...");
populateDatagrid(req.responseText.toJSON());
}
else
{
// An HTTP problem has occurred
alert("HTTP error: " + req.status);
}
}
}
}
Note the JSON Response comes back fine, but its a string. Any advice is greatly appreciated. I am also opening to using googles Gson, but don't have too much knowledge on that.

Got it working! I should have been building a JSONArray of JSONObjects and then add the array to a final "Addresses" JSONObject. Observe the following:
JSONObject json = new JSONObject();
JSONArray addresses = new JSONArray();
JSONObject address;
try
{
int count = 15;
for (int i=0 ; i<count ; i++)
{
address = new JSONObject();
address.put("CustomerName" , "Decepticons" + i);
address.put("AccountId" , "1999" + i);
address.put("SiteId" , "1888" + i);
address.put("Number" , "7" + i);
address.put("Building" , "StarScream Skyscraper" + i);
address.put("Street" , "Devestator Avenue" + i);
address.put("City" , "Megatron City" + i);
address.put("ZipCode" , "ZZ00 XX1" + i);
address.put("Country" , "CyberTron" + i);
addresses.add(address);
}
json.put("Addresses", addresses);
}
catch (JSONException jse)
{
}
response.setContentType("application/json");
response.getWriter().write(json.toString());
This worked and returned valid and parse-able JSON. Hopefully this helps someone else in the future. Thanks for your help Marcel

I used JSONObject as shown below in Servlet.
JSONObject jsonReturn = new JSONObject();
NhAdminTree = AdminTasks.GetNeighborhoodTreeForNhAdministrator( connection, bwcon, userName);
map = new HashMap<String, String>();
map.put("Status", "Success");
map.put("FailureReason", "None");
map.put("DataElements", "2");
jsonReturn = new JSONObject();
jsonReturn.accumulate("Header", map);
List<String> list = new ArrayList<String>();
list.add(NhAdminTree);
list.add(userName);
jsonReturn.accumulate("Elements", list);
The Servlet returns this JSON object as shown below:
response.setContentType("application/json");
response.getWriter().write(jsonReturn.toString());
This Servlet is called from Browser using AngularJs as below
$scope.GetNeighborhoodTreeUsingPost = function(){
alert("Clicked GetNeighborhoodTreeUsingPost : " + $scope.userName );
$http({
method: 'POST',
url : 'http://localhost:8080/EPortal/xlEPortalService',
headers: {
'Content-Type': 'application/json'
},
data : {
'action': 64,
'userName' : $scope.userName
}
}).success(function(data, status, headers, config){
alert("DATA.header.status : " + data.Header.Status);
alert("DATA.header.FailureReason : " + data.Header.FailureReason);
alert("DATA.header.DataElements : " + data.Header.DataElements);
alert("DATA.elements : " + data.Elements);
}).error(function(data, status, headers, config) {
alert(data + " : " + status + " : " + headers + " : " + config);
});
};
This code worked and it is showing correct data in alert dialog box:
Data.header.status : Success
Data.header.FailureReason : None
Data.header.DetailElements : 2
Data.Elements : Coma seperated string values i.e. NhAdminTree, userName

I think that what you want to do is turn the JSON string back into an object when it arrives back in your XMLHttpRequest - correct?
If so, you need to eval the string to turn it into a JavaScript object - note that this can be unsafe as you're trusting that the JSON string isn't malicious and therefore executing it. Preferably you could use jQuery's parseJSON