[FIXED] I keep getting this error, XML Parsing error: syntax error but still the website runs fine

Issue

I am new to developing websites. I know that I have to map servlets in the web.xml file. The web.xml file is this

    <?xml version="1.0" encoding="UTF-8"?>
    <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" version="3.0">
    <display-name>TestApp</display-name>
    <welcome-file-list>
     <welcome-file>index.html</welcome-file>
     <welcome-file>index.htm</welcome-file>
     <welcome-file>index.jsp</welcome-file>
     <welcome-file>default.html</welcome-file>
     <welcome-file>default.htm</welcome-file>
     <welcome-file>default.jsp</welcome-file>
    </welcome-file-list>
    <servlet>
     <servlet-name>Serve</servlet-name>
     <servlet-class>Serve</servlet-class>
    </servlet>
    <servlet-mapping>
     <servlet-name>Serve</servlet-name>
     <url-pattern>/TestApp</url-pattern>
    </servlet-mapping>
   </web-app>

But when I call the jquery $.ajax() function, I get this error.

XML Parsing Error: syntax error Location: http://localhost:8080/TestApp/Serve Line Number 1, Column 1:

The AJAX Call is

    $.ajax({
    url: "Serve",
    type: "POST",
    success: function(out){
        alert(out);
    },
    error: function(){
        alert("No");
    }
});

The problem is the servlet still runs fine. The alert(out) works as expected. Please explain why the browser is showing the error and please tell me a solution.

If this is a duplicate question please give a link to the original question.

Solution

I recently encountered the same issue. jQuery appeared to be handling the data and the dataType correctly, but instead it was Firefox returning the syntax error, which explains why your code was executing as intended but still printing an error to the console.

If you look in the developer console, you can see that Firefox is interpreting the plain text data as another format (likely XML). Firefox tires to parse the data as XML, but can't because it's not valid XML which results in "Syntax error" being printed to the console.

Fixing this problem for me involved editing the server so it returned the following header:

Content-Type: "text/plain"

This only appeared to be an issue with Firefox, Chrome did not encounter this issue. There is a Firefox bug here which seems to touch on the issue.

source

Answered By - Cholowao