Cannot connect to local Rest Service with AJAX (Status: 0) - ajax

Question

I currently try to connect to a local Rest service with AJAX. However it seems as if I can't get any connection. The Rest Service runs under http://localhost:8080/LernApp/
For Testing I tried to connect to http://localhost:8080/LernApp/user/ which returns a JSON (GET).
I tested this link inside my Browser (works fine and displays the JSON) and with Postman (also works fine and gets the Answer with a status code of 200 (OK)).
However if I try to connect to it with AJAX, it fails (status code 0).
My first try was this:
var xhttp = typeof XMLHttpRequest != 'undefined' ? new XMLHttpRequest() : new ActiveXObject('Microsoft.XMLHTTP');
xhttp.open('GET','http://localhost:8080/LernApp/user/',true);
xhttp.setRequestHeader("Content-type", "application/json");
xhttp.onreadystatechange = function() {
var data;
if(xhttp.readyState==4) {
if(xhttp.status==200) {
alert("OK");
} else if(xhttp.status==404) {
alert("404");
} else {
alert("ERROR CODE: " + xhttp.status);
}
}
}
xhttp.send();
The next thing I tried was this:
$.ajax({
type: 'GET',
url: 'http://localhost:8080/LernApp/user/',
dataType: 'json',
async: true,
success: function(result) {
alert(result.message);
},
error: function(jqXHR, textStatus, errorThrown) {
alert(jqXHR.status + ' ' + jqXHR.responseText);
}
});
And the last thing I tried was this:
$.ajax({
type: "GET",
url: 'http://localhost:8080/LernApp/user/',
headers: {
Accept : "application/json"
}
})
.done(function (data, textStatus, jqXHR) {
alert(data);
})
.fail(function(jqXHR, textStatus, errorThrown) {
alert("STATUS: " + jqXHR.status);
});
None of these seem to work. Every function returns the status code 0.
Does anyone have any idea what is happening here? Like I've said: Browser and Postman can connect to the url without and problems and retrieve the correct data.
Thanks for your help.

Answer 1

I finally found the solution. I should have mentioned more about my server (but since the server did well in the Browser and in Postman, I assumed that the problem would be on the client side).
Well... it was of course a server problem. The server is running on Apache Tomcat 9 with an embedded H2 database and JAX-RS (for REST Service).
If anyone else faces my problem: the solution is called CORS. If you are having the same problem, first google CORS, so that you know what it is and then you can make all of the needed settings inside the web.xml and/or context.xml of your Tomcat Project.