We are writing an interface servlet which has data passed to it, parses the data and creates the JSON, then sends the JSON to Salesforce. I'm having an issue authenticating. My authorization is as follows:

public String getAuth() throws HttpException, IOException, JSONException {
    	//post.addParameter("code",code);
    	HttpClient httpClient = new HttpClient();
    	ServletConfig context = getServletConfig();
    	post = new PostMethod(context.getInitParameter("environment"));
    	
    	System.out.println("environment: " + context.getInitParameter("environment"));
    	System.out.println("clientId: " + context.getInitParameter("clientId"));
    	System.out.println("clientSecret: " + context.getInitParameter("clientSecret"));
    	System.out.println("redirectUri: " + context.getInitParameter("redirectUri"));

    	
        post.addParameter("grant_type","authorization_code");
        post.addParameter("client_id",context.getInitParameter("clientId"));    	
    	post.addParameter("client_secret",context.getInitParameter("clientSecret"));
    	post.addParameter("redirect_uri",context.getInitParameter("redirectUri"));
    	   
    	httpClient = new HttpClient();
    	httpClient.executeMethod(post);
   
    	String responseBody = post.getResponseBodyAsString();
    	
    	System.out.println("response body: " + responseBody);
    	   
    	JSONObject json = null;
    	json = new JSONObject(responseBody);
    	String accessToken = json.getString("access_token");
        //String issuedAt = json.getString("issued_at");
              
       /*HttpServletResponse httpResponse = (HttpServletResponse)response;
       Cookie session = new Cookie("ACCESS_TOKEN", accessToken);
       session.setMaxAge(-1); //cookie not persistent, destroyed on browser exit
       httpResponse.addCookie(session);*/
        
       return accessToken;
    }

I recieve this error back:

{"error":"invalid_grant","error_description":"invalid authorization code"}