Merge pull request #232

src/main/java/org/kohsuke/github/GitHub.java

@@ -200,6 +200,15 @@ public static GitHub connectAnonymously() throws IOException {
         return new GitHubBuilder().build();
     }
 
+    /**
+     * Connects to GitHub Enterprise anonymously.
+     *
+     * All operations that requires authentication will fail.
+     */
+    public static GitHub connectToEnterpriseAnonymously(String apiUrl) throws IOException {
+        return new GitHubBuilder().withEndpoint(apiUrl).build();
+    }
+
     /**
      * Is this an anonymous connection
      * @return {@code true} if operations that require authentication will fail.
@@ -442,6 +451,29 @@ public boolean isCredentialValid() throws IOException {
         }
     }
 
+    private static class GHApiInfo {
+        private String rate_limit_url;
+
+        void check(String apiUrl) throws IOException {
+            if (rate_limit_url==null)
+                throw new IOException(apiUrl+" doesn't look like GitHub API URL");
+
+            // make sure that the URL is legitimate
+            new URL(rate_limit_url);
+        }
+    }
+
+    /**
+     * Ensures that the API URL is valid.
+     *
+     * <p>
+     * This method returns normally if the endpoint is reachable and verified to be GitHub API URL.
+     * Otherwise this method throws {@link IOException} to indicate the problem.
+     */
+    public void checkApiUrlValidity() throws IOException {
+        retrieve().to("/", GHApiInfo.class).check(apiUrl);
+    }
+
     /**
      * Search issues.
      */

src/test/java/org/kohsuke/github/GitHubTest.java

@@ -12,6 +12,7 @@
 import static org.hamcrest.CoreMatchers.notNullValue;
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
 import static org.mockito.Mockito.spy;
 import static org.mockito.Mockito.when;
 
@@ -119,4 +120,10 @@ public void testGitHubEnterpriseDoesNotHaveRateLimit() throws IOException {
         GHRateLimit rateLimit = github.getRateLimit();
         assertThat(rateLimit.getResetDate(), notNullValue());
     }
+
+    @Test
+    public void testGitHubIsApiUrlValid() throws IOException {
+        GitHub github = GitHub.connectAnonymously();
+        github.checkApiUrlValidity();
+    }
 }