Allowing Cross domain POST request with JSON or XML content type in Java

Cross domain POST Request

Cross domain POST requests supports only below content types and other content type are not allowed by default.

• application/x-www-form-urlencoded
• multipart/form-data
• text/plain

So, if we try to make an ajax request to such POST URLs with XML or JSON content type, it will not be able to make request and give CORS error.

Allowing Cross domain POST request with JSON or XML Content-Type

We have our service implemented in Java and we will use Servlet Filter & request wrapper to build our solution.
Please see my below POST to see how to implement CORS using java servlets which works well with GET methods or allowed content types with POST method.
https://www.thetechnojournals.com/2020/03/cors-implementation-using-java-filter.html

In this POST I will explain only additional things required for our problem with POST request, so I request you to go through the above link first.

Request wrapper to change/ override the content-type to JSON or XML

Since, POST request is not allowing XML or JSON content-type and other types will fail when it need to match with our REST service's content type. We need to change the content-type at back-end before request reaching to REST service. Below is the code for Request wrapper class. Here we are doing a trick basis on request parameter "json". If this parameter is having value as "true" then we are changing the content type to "application/json" at backend before reaching to our service implementation which only accepts json content.

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;
import java.util.Arrays;
import java.util.Collections;
import java.util.Enumeration;

public class MyCorsServletRequestWrapper extends HttpServletRequestWrapper{
    private String contentType;
    public MyCorsServletRequestWrapper(HttpServletRequest req){
        super(req);
        setContentType("true".equals(req.getParameter("json"))
            ?"application/json" : req.getContentType());
    }
    @Override
    public String getContentType(){
        return contentType;
    }
    
    @Override
    public String getHeader(String name){
        if("Content-Type".equalsIgnoreCase(name))
            return contentType;
        else
            return super.getHeader(name);
    }

    @Override
    public Enumeration<String> getHeaders(String name){
        if("Content-Type".equalsIgnoreCase(name))
            return Collections.enumeration(Arrays.asList(contentType));
        else
            return super.getHeaders(name);
    }

    public void setContentType(String contentType){
        this.contentType = contentType;
    }
}

Changes required to Servlet Filter

Now we need to make below changes in our existing servlet filter which we have created in another post (link mentioned in above section). These changes are made to support preflight requests which made by the client to confirm with server whether server is accepting this CORS request and process properly.

public class CorsFilter extends Filter{
    ...
    ....
    public void doFilter(ServletRequest req, ServletResponse res,
        FilterChain chain) throws ServletException, IOException{

        MyCorsServletRequestWrapper request = new MyCorsServletRequestWrapper((HttpServletRequest) req);

        HttpServletResponse response = (HttpServletResponse) res;
        ....
        .....
        
        if("OPTIONS".equalsIgnoreCase(request.getMethod())){
            response.setStatus(HttpServletResponse.SC_ACCEPTED);
            return;
        }
        chain.doFilter(request, response);
    }
}