Parsing query strings on Android

Java EE has ServletRequest.getParameterValues().

On non-EE platforms, URL.getQuery() simply returns a string.

What's the normal way to properly parse the query string in a URL when not on Java EE?

It is popular in the answers to try and make your own parser. This is very interesting and exciting micro-coding project, but I cannot say that it is a good idea.

The code snippets below are generally flawed or broken. Breaking them is an interesting exercise for the reader. And to the hackers attacking the websites that use them.

Parsing query strings is a well defined problem but reading the spec and understanding the nuances is non-trivial. It is far better to let some platform library coder do the hard work, and do the fixing, for you!

On Android:

import android.net.Uri;

[...]

Uri uri=Uri.parse(url_string);
uri.getQueryParameter("para1");

Since Android M things have got more complicated. The answer of android.net.URI.getQueryParameter() has a bug which breaks spaces before JellyBean. Apache URLEncodedUtils.parse() worked, but was deprecated in L, and removed in M.

So the best answer now is UrlQuerySanitizer. This has existed since API level 1 and still exists. It also makes you think about the tricky issues like how do you handle special characters, or repeated values.

The simplest code is

UrlQuerySanitizer.ValueSanitizer sanitizer = UrlQuerySanitizer.getAllButNullLegal();
// remember to decide if you want the first or last parameter with the same name
// If you want the first call setPreferFirstRepeatedParameter(true);
sanitizer.parseUrl(url);
String value = sanitizer.getValue("paramName");

If you are happy with the default parsing behavior you can do:

new UrlQuerySanitizer(url).getValue("paramName")

but you should make sure you understand what the default parsing behavor is, as it might not be what you want.

On Android, the Apache libraries provide a Query parser:

http://developer.android.com/reference/org/apache/http/client/utils/URLEncodedUtils.html and http://hc.apache.org/httpcomponents-client-ga/httpclient/apidocs/org/apache/http/client/utils/URLEncodedUtils.html

public static Map<String, List<String>> getUrlParameters(String url)
        throws UnsupportedEncodingException {
    Map<String, List<String>> params = new HashMap<String, List<String>>();
    String[] urlParts = url.split("\\?");
    if (urlParts.length > 1) {
        String query = urlParts[1];
        for (String param : query.split("&")) {
            String pair[] = param.split("=", 2);
            String key = URLDecoder.decode(pair[0], "UTF-8");
            String value = "";
            if (pair.length > 1) {
                value = URLDecoder.decode(pair[1], "UTF-8");
            }
            List<String> values = params.get(key);
            if (values == null) {
                values = new ArrayList<String>();
                params.put(key, values);
            }
            values.add(value);
        }
    }
    return params;
}

If you have jetty (server or client) libs on your classpath you can use the jetty util classes (see javadoc), e.g.:

import org.eclipse.jetty.util.*;
URL url = new URL("www.example.com/index.php?foo=bar&bla=blub");
MultiMap<String> params = new MultiMap<String>();
UrlEncoded.decodeTo(url.getQuery(), params, "UTF-8");

assert params.getString("foo").equals("bar");
assert params.getString("bla").equals("blub");

Parsing query strings on Android

Related

Recent Posts