Q:

HttpURLConnection Invalid HTTP method: PATCH

When I try to use a non-standard HTTP Method like PATCH with URLConnection:
    HttpURLConnection conn = (HttpURLConnection) new URL("http://example.com").openConnection();
    conn.setRequestMethod("PATCH");

I get an exception:
java.net.ProtocolException: Invalid HTTP method: PATCH
at java.net.HttpURLConnection.setRequestMethod(HttpURLConnection.java:440)

Using a higher level API like Jersey generates the same error. Is there a workaround to issue a PATCH HTTP request?

A:

Yes there is workaround for this. Use 

X-HTTP-Method-Override

. This header can be used in a POST request to “fake” other HTTP methods. Simply set the value of the X-HTTP-Method-Override header to the HTTP method you would like to actually perform.
So use following code.
conn.setRequestProperty("X-HTTP-Method-Override", "PATCH");
conn.setRequestMethod("POST");

A:

There are a lot of good answers, so here is mine (not work in jdk12):
import java.io.IOException;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.Arrays;
import java.util.LinkedHashSet;
import java.util.Set;

public class SupportPatch {
    public static void main(String... args) throws IOException {
        allowMethods("PATCH");

        HttpURLConnection conn = (HttpURLConnection) new URL("http://example.com").openConnection();
        conn.setRequestMethod("PATCH");
    }

    private static void allowMethods(String... methods) {
        try {
            Field methodsField = HttpURLConnection.class.getDeclaredField("methods");

            Field modifiersField = Field.class.getDeclaredField("modifiers");
            modifiersField.setAccessible(true);
            modifiersField.setInt(methodsField, methodsField.getModifiers() & ~Modifier.FINAL);

            methodsField.setAccessible(true);

            String[] oldMethods = (String[]) methodsField.get(null);
            Set<String> methodsSet = new LinkedHashSet<>(Arrays.asList(oldMethods));
            methodsSet.addAll(Arrays.asList(methods));
            String[] newMethods = methodsSet.toArray(new String[0]);

            methodsField.set(null/*static field*/, newMethods);
        } catch (NoSuchFieldException | IllegalAccessException e) {
            throw new IllegalStateException(e);
        }
    }
}

It also uses reflection, but instead of hacking into every connection object we're hacking HttpURLConnection#methods static field which is used in the checks internally.

A:

There is a Won't Fix bug in OpenJDK for this: https://bugs.openjdk.java.net/browse/JDK-7016595
However, with Apache Http-Components Client 4.2+ this is possible. It has a custom networking implementation, thus using non-standard HTTP methods like PATCH is possible. It even has a HttpPatch class supporting the patch method.
CloseableHttpClient httpClient = HttpClients.createDefault();
HttpPatch httpPatch = new HttpPatch(new URI("http://example.com"));
CloseableHttpResponse response = httpClient.execute(httpPatch);

Maven Coordinates:
<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpclient</artifactId>
    <version>4.2+</version>
</dependency>

