How to obtain the query string in a GET with Java HttpServer/HttpExchange?

The following: httpExchange.getRequestURI().getQuery()

will return string in format similar to this: "field1=value1&field2=value2&field3=value3..."

so you could simply parse string yourself, this is how function for parsing could look like:

public Map<String, String> queryToMap(String query) {
    if(query == null) {
        return null;
    }
    Map<String, String> result = new HashMap<>();
    for (String param : query.split("&")) {
        String[] entry = param.split("=");
        if (entry.length > 1) {
            result.put(entry[0], entry[1]);
        }else{
            result.put(entry[0], "");
        }
    }
    return result;
}

And this is how you could use it:

Map<String, String> params = queryToMap(httpExchange.getRequestURI().getQuery()); 
System.out.println("param A=" + params.get("A"));

Building on the answer by @anon01, this is how to do it in Groovy:

Map<String,String> getQueryParameters( HttpExchange httpExchange )
{
    def query = httpExchange.getRequestURI().getQuery()
    return query.split( '&' )
            .collectEntries {
        String[] pair = it.split( '=' )
        if (pair.length > 1)
        {
            return [(pair[0]): pair[1]]
        }
        else
        {
            return [(pair[0]): ""]
        }
    }
}

And this is how to use it:

def queryParameters = getQueryParameters( httpExchange )
def parameterA = queryParameters['A']

This answer, contrary to annon01's, properly decodes the keys and values. It does not use String.split, but scans the string using indexOf, which is faster.

public static Map<String, String> parseQueryString(String qs) {
    Map<String, String> result = new HashMap<>();
    if (qs == null)
        return result;

    int last = 0, next, l = qs.length();
    while (last < l) {
        next = qs.indexOf('&', last);
        if (next == -1)
            next = l;

        if (next > last) {
            int eqPos = qs.indexOf('=', last);
            try {
                if (eqPos < 0 || eqPos > next)
                    result.put(URLDecoder.decode(qs.substring(last, next), "utf-8"), "");
                else
                    result.put(URLDecoder.decode(qs.substring(last, eqPos), "utf-8"), URLDecoder.decode(qs.substring(eqPos + 1, next), "utf-8"));
            } catch (UnsupportedEncodingException e) {
                throw new RuntimeException(e); // will never happen, utf-8 support is mandatory for java
            }
        }
        last = next + 1;
    }
    return result;
}