I am a beginner in web development (self learner),and I am trying to connect my javascript .js file to java servlet through AJAX where I am stuck. It's not making the AJAX call, or not entering the java code, returning to the call back function. Is my url mapping or path specified correct? Or can you see some other error? Thanks!
JS code:
a = parseInt(document.getElementById("num"+ 0).value);
var xhr = new XMLHttpRequest();
xhr.open("GET", "/add?num1=" + a , true ); // true is for Asynchronous request
alert("here3 a=" + a);
xhr.send();
var ret = eval(xhr.responseText); //just trial
alert("eval" + ret);
xhr.onreadystatechange = () => {
if(xhr.readyState == 4 && xhr.status == 200){
document.getElementbyId('ajaxResponse').innerHTML = xhr.responseText;
var ret = eval(xhr.responseText);
alert("Callback1 = " + ret);
}
else(alert("Callback failed"))
};
Java servlet:
public void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException, ServletException {
System.out.println("Add Servlet called");
int i = Integer.parseInt(req.getParameter("num1"));
// int j = Integer.parseInt(req.getParameter("num2"));
// int k = i+j;
PrintWriter out = res.getWriter();
out.println("Result is i=" + i);
res.setContentType("text/plain");
res.getWriter().write(i);
}
Web.xml
(servlet): callJava
com.AddServlet
(servlet-mapping):callJava
/add
It just goes in else condition of callback function ("Callback failed"). Also, does the location/folder structure of the servlet or js file matters, if mapping is done in .xml file? Thanks!
I can't really help you with the server side part as I'm not familiar with the framework that you are using. But the following are my recommendations for the client side code:
Use let instead of var;
true flag to make request async is not necessary because it is the default and will be soon deprecated.
It is preferable to use the new addEventListener API instead of directly adding the handler to the intended target.
Fully setup your request before calling open and send.
Use === operator instead of ==.
const a = parseInt(document.getElementById("num" + 0).value);
let xhr = new XMLHttpRequest();
xhr.addEventListener('readystatechange', (e) => {
if(e.target.readyState === 4){
document.getElementbyId('ajaxResponse').innerHTML = e.target.responseText;
}
});
xhr.open("GET", "/add?num1=" + a);
xhr.send();
Please let me know if it works. If it does, you may want to check if your server is correctly setting the "found" response code (200).
Ok I figured it out, the url should just be "add?num1=" instead of "/add?num1=". It does map through .xml file! Now I am reaching java file and status is 200 :D
Next I am trying to figure out how to return back value any ajax response value from java servlet to javascript file or html file. Should I do it somehow in xhr.onreadystatechange() function? It's still not satisfying both conditions [if(xhr.readyState == 4 && xhr.status == 200)]. Any suggestions?
Related
We want to send a boolean value from python to javascript so we can use it in our html website.
We tried using sockets but thats too complicated for us. Our next thought was to use an api and we know how to get information from an api using javascript. What we want to do is post a python boolean value to an api, and then get the boolean value from the api using javascript.
But we don't know how to do so.
We are using a raspberry pi for all our code and a hardware-button which returns true in python when pressed.
We are currently testing code we found from https://healeycodes.com/javascript/python/beginners/webdev/2019/04/11/talking-between-languages.html
But this code doesnt work for us.
We are also using pycharm as our workspace, is this a problem?
Our current code in javascript:
const request = new XMLHttpRequest();
request.open("GET", url, true);
request.onreadystatechange = function() {
if (request.readyState === 4 && request.status === 200) {
success(JSON.parse(request.responseText));
}
};
request.send();
setInterval(get("button-status.json", receiveStatus), 3000);
}
function receiveStatus(response) {
if (response.status !== status) { // only do something if status has changed
status = response.status;
console.log('button status is now', status);
}
}
let status;
// checks every 100ms
get()
Our python code we're using for testing:
import random
import json
import time
button_status = False
path = (r"C:\Users\Sam\Desktop\pythonProject\pythonflask\emplates") # replace with your actual path
def save_button_status():
with open(path + "/button-status.json", "w") as f:
json.dump({'status': button_status}, f)
while True :
value = random.randrange(1, 10)
if ( value <= 5) :
button_status = True
save_button_status()
time.sleep(3)
else :
button_status = False
save_button_status()
time.sleep(3)
print(button_status)
Javascript within a webpage cannot directly run a Python script on your computer or read information from a local terminal. What you could do is have your Python program output a small json file to your localhost folder which is overwritten when the button is pressed or released, like this:
import json
button_status = False # assuming it is initially off
path = "path/to/your/localhost/folder" # replace with your actual path
def save_button_status():
with open(path + "/button-status.json", "w") as f:
json.dump({'status': button_status}, f)
# Then call save_button_status() whenever the status changes
Then in your javascript, set an interval to periodically call a function that gets this json file and does something based on the value if it has changed:
function get(url, success) {
//--- Get JSON data at the given URL and call `success` if successful
const request = new XMLHttpRequest();
request.open("GET", url, true);
request.onreadystatechange = function() {
if (request.readyState === 4 && request.status === 200) {
success(JSON.parse(request.responseText));
}
};
request.send();
}
function receiveStatus(response) {
if (response.status !== status) { // only do something if status has changed
status = response.status;
console.log('button status is now', status);
}
}
let status;
let interval = setInterval(() => get("button-status.json", receiveStatus), 100); // checks every 100ms
There may be some lag as your local server updates the file.
You can try to set up a SQL Database. Write a SQL statement in Python to receive your boolean. After that make a PHP script on your Web server to receive the SQL data. Then sent a request to the URL of the PHP script using an XHTTP JavasScript request.
I have a list which is pulled from a Postgres database and I need to be able to reference/manipulate it with JavaScript.
I have updated the code as shown below:
Here is the Servlet's doGet method:
protected void doGet(HttpServletRequest req, HttpServletResponse json)
throws ServletException, IOException {
List<Employee> employees = uds.findAll();
req.setAttribute("employees", employees);
json.setContentType("application/json");
json.getWriter().write(String.valueOf(employees));
}
And here is what I currently have in JavaScript:
var ajax = new XMLHttpRequest();
ajax.open("GET", "http://localhost:8080/project1attempt/servlet", true);
// send request
ajax.send();
// event to get response
ajax.onreadystatechange = function() {
// Case state is 4 e o http.status for 200, your request is OK.
if (ajax.readyState == 4 && ajax.status == 200) {
var data = ajax.responseText;
// the return
console.log(data);
}
}
}
The issue I am having at this point is that the client is not receiving data in JSON format. If I log the data as shown above, the log will produce something along the lines of the following:
[Employee{, employee_id='123456', email='lt#gmail.com', firstName='Juan',
lastName='Terri'}, Employee{, employee_id='2', email='sstark#mail.com',
firstName='Sansa', lastName='Stark'}]
This is the correct data, but not in a useful format.
However, if I try to do console.log(JSON.parse(data)), then I receive Uncaught SyntaxError: Unexpected token E in JSON at position 1.
I believe this is a simple syntax error on my part in the servlet, but am unsure of how to fix it.
You should use request.getAttribute():
<%
List<Employee> theEmployees = request.getAttribute("employees");
%>
But if you want to effectively use it in your javascript , it is recommended to convert it to json .
Try change your servlet response to json and get your data with Ajax
This is a sample to do it!
var ajax = new XMLHttpRequest();
ajax.open("GET", "your_url_here", true);
// send request
ajax.send();
// event to get response
ajax.onreadystatechange = function() {
// Case state is 4 e o http.status for 200, your request is OK.
if (ajax.readyState == 4 && ajax.status == 200) {
var data = ajax.responseText;
// the return
console.log(data);
}
}
For other noobs like me, I've compiled the following complete solution for this issue:
The Servlet should look something like this:
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws
ServletException, IOException {
resp.setContentType("application/json");
List<Employee> employees = uds.findAll();
String json = new ObjectMapper().writeValueAsString(employees);
resp.getWriter().write(json);
uds.findAll() is a method which returns a list of objects. ObjectMapper is a Jackson utility (Gson is another option, I believe). This puts the list into JSON format.
The HTML or JSP should look like this:
var ajax = new XMLHttpRequest();
ajax.open("GET", "http://localhost:8080/project1attempt/servlet", true);
// send request
ajax.send();
// event to get response
ajax.onreadystatechange = function() {
// Case state is 4 e o http.status for 200, your request is OK.
if (ajax.readyState == 4 && ajax.status == 200) {
var data = ajax.responseText;
// the return
console.log(JSON.parse(data));
}
}
This will get the list of objects in a usable format which you can then manipulate with JavaScript to do whatever you'd like. Hopefully this will help someone!
Basically I'm just trying to create a simple HTML page that can send a string of text to the server. The server runs on some port on the localhost and receives that string.
I've found code for a simple server that can handle POST requests:
public static void main(String args[]) throws Exception
{
ServerSocket s = new ServerSocket(8080);
while (true) {
Socket remote = s.accept();
System.out.println("Connected");
BufferedReader in = new BufferedReader(new InputStreamReader(remote.getInputStream()));
PrintWriter out = new PrintWriter(remote.getOutputStream());
String str = ".";
while (!str.equals("")) {
str = in.readLine();
if (str.contains("GET")) {
break;
}
}
System.out.println(str);
out.println("HTTP/1.0 200 OK");
out.println("Content-Type: text/html");
out.println("Access-Control-Allow-Origin: null");
out.println("");
out.flush();
}
}
But I don't know what should I do further. I've learned that I need to use a XMLHttpRequest that can send asynchronous requests:
function sendData(data) {
var XHR = new XMLHttpRequest();
var urlEncodedData = "message";
var urlEncodedDataPairs = [];
var name;
for (name in data) {
urlEncodedDataPairs.push(encodeURIComponent(name) + '=' + encodeURIComponent(data[name]));
}
urlEncodedData = urlEncodedDataPairs.join('&').replace(/%20/g, '+');
XHR.open('POST', 'http://localhost:8080', true);
XHR.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
XHR.send(urlEncodedData);
}
So, I'm starting my server, opening the .html file with JS script, and the script connects to the server. How then can I handle the message that the script sends? How can I decode and print it? And, eventually, do I write the message sender in a right way?
If you're simply trying to hit the endpoint you created for testing & continuing to build, try using Postman. You should be able to write a custom body for your POST request.
I am currently trying to write some Javascript to interact with an API that I deployed on GAE (using Python) using XMXMLHttpRequest(). I've had no issue getting a GET, however the PUT is giving me a lot of trouble.
Interestingly, I have no issue touching the PUT request from a test HTTP site (https://www.hurl.it/), however I receive a status value of 0 every time I try from my own Javascript code. Below are snippets of my GAE and Javascript code.
(NOTE - I must use a "put" for this call as a requirement.)
Any guidance would be appreciated!
GAE (Server):
def put(self):
# Save variables for update
cardkey = self.request.get('key', default_value=None)
ident = self.request.get('ident', default_value=None)
brand = self.request.get('brand', default_value=None)
year = self.request.get('year', default_value=None)
player = self.request.get('player', default_value=None)
# If card key is provided then update card
if cardkey:
# Get card
card_to_update = ndb.Key(db_models.Card, int(cardkey)).get()
if ident:
card_to_update.ident = ident
if brand:
card_to_update.brand = brand
if year:
card_to_update.year = year
if player:
card_to_update.player = player
# Save changes and print update to requester
card_to_update.put()
card_dict_format = card_to_update.to_dict()
self.response.write(json.dumps(card_dict_format))
return
# If card key is not provided send error
else:
self.response.write('key not provided. must provide key for update.')
return
And the Javascript from my webpage:
<script>
window.onload = function()
{
var myRequest = new XMLHttpRequest();
var url = 'http://cs496-assignment3-mastrokn.appspot.com/updatecard';
var param = 'key=5636318331666432';
myRequest.open('put', url);
myRequest.onreadystatechange = function()
{
if ((myRequest.readyState == 4) && (myRequest.status == 200))
{
// var myArr = JSON.parse(myRequst.responseText);
// myFunction(myArr);
document.getElementById("viewCards").innerHTML = myRequest.status;
}
else
{
document.getElementById("viewCards").innerHTML = myRequest.status;
}
}
myRequest.send(param);
}
</script>
First, your onreadystatechange() handler should look like this:
myRequest.onreadystatechange = function()
{
if (myRequest.readyState == 4) //Don't do anything until the readyState==4
{
if(myRequest.status == 200) //Check for status==200
{
document.getElementById("viewCards").innerHTML = myRequest.status;
}
else //All other status codes
{
document.getElementById("viewCards").innerHTML =
'readyState='
+ myRequest.readyState
+ ' status='
+ myRequest.status
+ ' status text='
+ myRequest.statusText;
}
}
}
Then, from the docs:
If you end up with an XMLHttpRequest having status=0 and
statusText=null, it means that the request was not allowed to be
performed. It was UNSENT.
To see what went wrong, check the javascript console in your browser for an error, e.g.:
[Error] XMLHttpRequest cannot load
http://cs496-assignment3-mastrokn.appspot.com/updatecard. Origin
http://localhost:4567 is not allowed by Access-Control-Allow-Origin.
(4.htm, line 0)
When I run the code above and send the XMLHttpRequest to my own local server, the PUT request succeeds with a status code of 200.
Lastly, I have doubts about the server code you posted because I don't know of any framework where you return None from a request handler--rather you return some string or a response object. Yet, using other means to make a PUT request to your url returns a 200 status code. Is that really your server code? What framework are you using?
I cannot figure out how to get the following code working in my little demo ASP.NET application, and am hoping someone here can help.
Here is the javascript:
function checkUserName() {
var request = createRequest();
if (request == null) {
alert("Unable to create request.");
} else {
var theName = document.getElementById("username").value;
var userName = escape(theName);
var url = "Default.aspx/CheckName";
request.onreadystatechange = createStateChangeCallback(request);
request.open("GET", url, true);
request.setRequestHeader("Content-Type", "application/json");
//none of my attempts to set the 'values' parameter work
var values = //JSON.stringify({userName:"userName"}); //"{userName:'temp name'}"; //JSON.stringify({ "userName":userName });
request.send(values);
}
}
Here is the method in my *.aspx.cs class:
[WebMethod]
[ScriptMethod(UseHttpGet=true)]
public static string CheckName(string userName)
{
string s = "userName";
return s + " modified backstage";
}
When this code runs I receive this exception:
---------------------------
Message from webpage
---------------------------
{"Message":"Invalid web service call, missing value for parameter: \u0027userName\u0027.","StackTrace":" at System.Web.Script.Services.WebServiceMethodData.CallMethod(Object target, IDictionary`2 parameters)\r\n at System.Web.Script.Services.WebServiceMethodData.CallMethodFromRawParams(Object target, IDictionary`2 parameters)\r\n at System.Web.Script.Services.RestHandler.InvokeMethod(HttpContext context, WebServiceMethodData methodData, IDictionary`2 rawParams)\r\n at System.Web.Script.Services.RestHandler.ExecuteWebServiceCall(HttpContext context, WebServiceMethodData methodData)","ExceptionType":"System.InvalidOperationException"}
---------------------------
OK
---------------------------
I started searching here, then went on to several threads on SO, trying quite a few combinations of quotation marks and key-value pairs, but nothing I've tried has worked.
When I remove the parameter from the C# method and request.send(), I get a response in my JS callback that I can work with. But as soon as I try to do something with parameters, I get the above exception. I'd like to know how to do this without using jQuery, if possible.
Thanks in advance.
FINAL VERSION
Using Alexei's advice, I ended up with the following, which works. The URL was missing the apostrophes on either end of the parameter value; this was keeping the call from going through.
function checkUserName() {
var request = createRequest();
if (request == null) {
alert("Unable to create request.");
} else {
var theName = document.getElementById("username").value;
var userName = encodeURIComponent(theName);
var url = "Default.aspx/CheckName?name='" + theName + "'";
request.onreadystatechange = createStateChangeCallback(request);
request.open("GET", url, true);
request.setRequestHeader("Content-Type", "application/json");
request.send();
}
}
request.send(values);
This won't work with a "GET". Try
request.open("POST", url, true);
http://www.w3schools.com/ajax/ajax_xmlhttprequest_send.asp
You need to:
decide whether you want GET or POST. For GET request you need all parameters to be in Url (and body to be empty), for POST you can use both. As of current code you are expecting GET, but sending POST.
properly add query parameter - name and encoded value. encodeUriComponent is JavaScript function of choice, see Build URL from Form Fields with Javascript or jquery for details
if using POST you need to properly encode parameters there too as well specify correct "content-type" header.
if sending JSON you need to decode JSON server side.
Alternatively you can use hidden form to perform POST/GET as covered in JavaScript post request like a form submit
Side note: jQuery.ajax does most of that for you and good source to look through if you want to do all yourself.
Like Alan said, use the POST method. Or pass your arguments in your URL before opening it, e.g.
var url = "Default.aspx/CheckName?userName=" + values;
EDIT : no, it's probably a bad idea since you want to send JSON, forget what I said.
If you need to go for POST, then you need to send it like this.
var values = JSON.stringify({"'userName':'"+ userName+ "'"});
And you have to change HttpGet to HttpPost
Given that your server side method asks for GET, you need:
request.open("GET", url + "?username=" + userName, true);
request.send();
The works for me:
function checkUserName() {
var request = new XMLHttpRequest();
if (request == null) {
alert("Unable to create request.");
} else {
var userName = "Shaun Luttin";
var url = '#Url.RouteUrl(new{ action="CheckName", controller="Home"})';
request.onreadystatechange = function() {
if (request.readyState == XMLHttpRequest.DONE ) {
if(request.status == 200){
document.getElementById("myDiv").innerHTML = request.responseText;
}
else if(request.status == 400) {
alert('There was an error 400')
}
else {
alert('something else other than 200 was returned')
}
}
}
request.open("GET", url + "?username=" + userName, true);
request.send();
}
}
With this on the server side:
[HttpGet]
public string CheckName(string userName)
{
return userName + " modified backstage";
}