How can I append a query parameter to an existing URL?

I'd like to append key-value pair as a query parameter to an existing URL. While I could do this by checking for the existence of whether the URL has a query part or a fragment part and doing the append by jumping though a bunch of if-clauses but I was wondering if there was clean way if doing this through the Apache Commons libraries or something equivalent.

http://example.com would be http://example.com?name=John

http://example.com#fragment would be http://example.com?name=John#fragment

http://[email protected] would be http://[email protected]&name=John

http://[email protected]#fragment would be http://[email protected]&name=John#fragment

I've run this scenario many times before and I'd like to do this without breaking the URL in any way.

There are plenty of libraries that can help you with URI building (don't reinvent the wheel). Here are three to get you started:

---

Java EE 7

import javax.ws.rs.core.UriBuilder;
...
return UriBuilder.fromUri(url).queryParam(key, value).build();

---

org.apache.httpcomponents:httpclient:4.5.2

import org.apache.http.client.utils.URIBuilder;
...
return new URIBuilder(url).addParameter(key, value).build();

---

org.springframework:spring-web:4.2.5.RELEASE

import org.springframework.web.util.UriComponentsBuilder;
...
return UriComponentsBuilder.fromUriString(url).queryParam(key, value).build().toUri();

---

See also: GIST > URI Builder Tests

This can be done by using the java.net.URI class to construct a new instance using the parts from an existing one, this should ensure it conforms to URI syntax.

The query part will either be null or an existing string, so you can decide to append another parameter with & or start a new query.

public class StackOverflow26177749 {

	public static URI appendUri(String uri, String appendQuery) throws URISyntaxException {
		URI oldUri = new URI(uri);
		
		String newQuery = oldUri.getQuery();
		if (newQuery == null) {
			newQuery = appendQuery;
		} else {
			newQuery += "&" + appendQuery;	
		}
		
		return new URI(oldUri.getScheme(), oldUri.getAuthority(),
				oldUri.getPath(), newQuery, oldUri.getFragment());
	}

	public static void main(String[] args) throws Exception {
		System.out.println(appendUri("http://example.com", "name=John"));
		System.out.println(appendUri("http://example.com#fragment", "name=John"));
		System.out.println(appendUri("http://[email protected]", "name=John"));
		System.out.println(appendUri("http://[email protected]#fragment", "name=John"));
	}
}

Shorter alternative

public static URI appendUri(String uri, String appendQuery) throws URISyntaxException {
    URI oldUri = new URI(uri);
    return new URI(oldUri.getScheme(), oldUri.getAuthority(), oldUri.getPath(),
            oldUri.getQuery() == null ? appendQuery : oldUri.getQuery() + "&" + appendQuery, oldUri.getFragment());
}

Output

http://example.com?name=John
http://example.com?name=John#fragment
http://example.com?email[email protected]&name=John
http://example.com?email[email protected]&name=John#fragment

Use the URI class.

Create a new URI with your existing String to "break it up" to parts, and instantiate another one to assemble the modified url:

URI u = new URI("http://[email protected]&name=John#fragment");

// Modify the query: append your new parameter
StringBuilder sb = new StringBuilder(u.getQuery() == null ? "" : u.getQuery());
if (sb.length() > 0)
    sb.append('&');
sb.append(URLEncoder.encode("paramName", "UTF-8"));
sb.append('=');
sb.append(URLEncoder.encode("paramValue", "UTF-8"));

// Build the new url with the modified query:
URI u2 = new URI(u.getScheme(), u.getAuthority(), u.getPath(),
    sb.toString(), u.getFragment());

For android, Use: https://developer.android.com/reference/android/net/Uri#buildUpon()

URI oldUri = new URI(uri);
Uri.Builder builder = oldUri.buildUpon();
 builder.appendQueryParameter("newParameter", "dummyvalue");
 Uri newUri =  builder.build();

I suggest an improvement of the Adam's answer accepting HashMap as parameter

/**
 * Append parameters to given url
 * @param url
 * @param parameters
 * @return new String url with given parameters
 * @throws URISyntaxException
 */
public static String appendToUrl(String url, HashMap<String, String> parameters) throws URISyntaxException
{
    URI uri = new URI(url);
    String query = uri.getQuery();

    StringBuilder builder = new StringBuilder();

    if (query != null)
    	builder.append(query);

    for (Map.Entry<String, String> entry: parameters.entrySet())
    {
        String keyValueParam = entry.getKey() + "=" + entry.getValue();
        if (!builder.toString().isEmpty())
            builder.append("&");

        builder.append(keyValueParam);
    }
    
    URI newUri = new URI(uri.getScheme(), uri.getAuthority(), uri.getPath(), builder.toString(), uri.getFragment());
    return newUri.toString();
}

Kotlin & clean, so you don't have to refactor before code review:

private fun addQueryParameters(url: String?): String? {
        val uri = URI(url)

        val queryParams = StringBuilder(uri.query.orEmpty())
        if (queryParams.isNotEmpty())
            queryParams.append('&')

        queryParams.append(URLEncoder.encode("$QUERY_PARAM=$param", Xml.Encoding.UTF_8.name))
        return URI(uri.scheme, uri.authority, uri.path, queryParams.toString(), uri.fragment).toString()
    }

An update to Adam's answer considering tryp's answer too. Don't have to instantiate a String in the loop.

public static URI appendUri(String uri, Map<String, String> parameters) throws URISyntaxException {
    URI oldUri = new URI(uri);
    StringBuilder queries = new StringBuilder();

    for(Map.Entry<String, String> query: parameters.entrySet()) {
		queries.append( "&" + query.getKey()+"="+query.getValue());
	}

    String newQuery = oldUri.getQuery();
    if (newQuery == null) {
        newQuery = queries.substring(1);
    } else {
    	newQuery += queries.toString();
    }

    URI newUri = new URI(oldUri.getScheme(), oldUri.getAuthority(),
            oldUri.getPath(), newQuery, oldUri.getFragment());

    return newUri;
}