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

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

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();

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See also: GIST > URI Builder Tests