Get link relations from response header

acme4j-client/src/main/java/org/shredzone/acme4j/connector/Connection.java

@@ -91,6 +91,15 @@ public interface Connection extends AutoCloseable {
      */
     URI getLocation() throws AcmeException;
 
+    /**
+     * Gets a link relation from the header.
+     *
+     * @param relation
+     *            Link relation
+     * @return Link, or {@code null} if there was no such link relation
+     */
+    URI getLink(String relation) throws AcmeException;
+
     /**
      * Closes the {@link Connection}, releasing all resources.
      */

acme4j-client/src/main/java/org/shredzone/acme4j/impl/DefaultConnection.java

@@ -26,7 +26,9 @@ import java.security.cert.CertificateException;
 import java.security.cert.CertificateFactory;
 import java.security.cert.X509Certificate;
 import java.util.EnumMap;
+import java.util.List;
 import java.util.Map;
+import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
 import org.jose4j.base64url.Base64Url;
@@ -248,6 +250,25 @@ public class DefaultConnection implements Connection {
         }
     }
 
+    @Override
+    public URI getLink(String relation) throws AcmeException {
+        List<String> links = conn.getHeaderFields().get("Link");
+        if (links != null) {
+            Pattern p = Pattern.compile("<(.*?)>\\s*;\\s*rel=\"?"+ Pattern.quote(relation) + "\"?");
+            for (String link : links) {
+                Matcher m = p.matcher(link);
+                if (m.matches()) {
+                    try {
+                        return new URI(m.group(1));
+                    } catch (URISyntaxException ex) {
+                        throw new AcmeException("Bad Link header: " + link);
+                    }
+                }
+            }
+        }
+        return null;
+    }
+
     @Override
     public void close() {
         conn = null;

acme4j-client/src/test/java/org/shredzone/acme4j/impl/DefaultConnectionTest.java

@@ -27,8 +27,10 @@ import java.net.URI;
 import java.net.URISyntaxException;
 import java.security.KeyPair;
 import java.security.cert.X509Certificate;
+import java.util.Arrays;
 import java.util.HashMap;
 import java.util.HashSet;
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
@@ -144,6 +146,31 @@ public class DefaultConnectionTest {
         verifyNoMoreInteractions(mockUrlConnection);
     }
 
+    /**
+     * Test that Link headers are evaluated.
+     */
+    @Test
+    public void testGetLink() throws Exception {
+        Map<String, List<String>> headers = new HashMap<String, List<String>>();
+        headers.put("Content-Type", Arrays.asList("application/json"));
+        headers.put("Location", Arrays.asList("https://example.com/acme/reg/asdf"));
+        headers.put("Link", Arrays.asList(
+                        "<https://example.com/acme/new-authz>;rel=\"next\"",
+                        "<https://example.com/acme/recover-reg>;rel=recover",
+                        "<https://example.com/acme/terms>; rel=\"terms-of-service\""
+                    ));
+
+        when(mockUrlConnection.getHeaderFields()).thenReturn(headers);
+
+        try (DefaultConnection conn = new DefaultConnection(mockHttpConnection)) {
+            conn.conn = mockUrlConnection;
+            assertThat(conn.getLink("next"), is(new URI("https://example.com/acme/new-authz")));
+            assertThat(conn.getLink("recover"), is(new URI("https://example.com/acme/recover-reg")));
+            assertThat(conn.getLink("terms-of-service"), is(new URI("https://example.com/acme/terms")));
+            assertThat(conn.getLink("secret-stuff"), is(nullValue()));
+        }
+    }
+
     /**
      * Test that no Location header returns {@code null}.
      */