Add support for custom prefix

src/main/java/org/jvnet/hudson/test/RealJenkinsRule.java

@@ -243,6 +243,9 @@ public final class RealJenkinsRule implements TestRule {
     private SSLSocketFactory sslSocketFactory;
     private X509Certificate rootCA;
 
+    @NonNull
+    private String prefix = "/jenkins";
+
     public RealJenkinsRule() {
         home = new AtomicReference<>();
     }
@@ -390,6 +393,26 @@ public RealJenkinsRule withHost(String host) {
         return this;
     }
 
+    /**
+     * Sets a custom prefix for the Jenkins root URL.
+     * <p>
+     * By default, the prefix defaults to {@code /jenkins}.
+     * <p>
+     * If not empty, must start with '/' and not end with '/'.
+     */
+    public RealJenkinsRule withPrefix(@NonNull String prefix) {
+        if (!prefix.isEmpty()) {
+            if (!prefix.startsWith("/")) {
+                throw new IllegalArgumentException("Prefix must start with a leading slash.");
+            }
+            if (prefix.endsWith("/")) {
+                throw new IllegalArgumentException("Prefix must not end with a trailing slash.");
+            }
+        }
+        this.prefix = prefix;
+        return this;
+    }
+
     /**
      * Sets a custom WAR file to be used by the rule instead of the one in the path or {@code war/target/jenkins.war} in case of core.
      */
@@ -820,12 +843,14 @@ public <T extends Serializable> T then(Step2<T> s) throws Throwable {
 
     /**
      * Similar to {@link JenkinsRule#getURL}. Requires Jenkins to be started before using {@link #startJenkins()}.
+     * <p>
+     * Always ends with a '/'.
      */
     public URL getUrl() throws MalformedURLException {
         if (port == 0) {
             throw new IllegalStateException("This method must be called after calling #startJenkins.");
         }
-        return new URL(https ? "https" : "http", host, port, "/jenkins/");
+        return new URL(https ? "https" : "http", host, port, prefix + "/");
     }
 
     /**
@@ -1008,8 +1033,10 @@ public void startJenkins() throws Throwable {
         argv.addAll(List.of(
                 "-jar", war.getAbsolutePath(),
                 "--enable-future-java",
-                "--httpListenAddress=" + httpListenAddress,
-                "--prefix=/jenkins"));
+                "--httpListenAddress=" + httpListenAddress));
+        if (!prefix.isEmpty()) {
+            argv.add("--prefix=" + prefix);
+        }
         argv.addAll(getPortOptions());
         if (https) {
             argv.add("--httpsKeyStore=" + keyStoreManager.getPath().toAbsolutePath());

src/test/java/org/jvnet/hudson/test/RealJenkinsRuleTest.java

@@ -27,6 +27,8 @@
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.arrayContainingInAnyOrder;
 import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.emptyString;
+import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.hasSize;
 import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.is;
@@ -36,6 +38,7 @@
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
+import static org.junit.Assume.assumeThat;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
@@ -75,7 +78,6 @@
 import javax.servlet.ServletResponse;
 import jenkins.model.Jenkins;
 import jenkins.model.JenkinsLocationConfiguration;
-import static org.junit.Assume.assumeThat;
 import org.junit.AssumptionViolatedException;
 import org.junit.Ignore;
 import org.junit.Rule;
@@ -100,15 +102,41 @@ private static void _smokes(JenkinsRule r) throws Throwable {
 
     @Test public void testReturnObject() throws Throwable {
         rr.startJenkins();
-        assertEquals(rr.getUrl().toExternalForm(), rr.runRemotely(RealJenkinsRuleTest::_getJenkinsUrlFromRemote));
+        assertThatLocalAndRemoteUrlEquals();
+    }
+
+    @Test public void customPrefix() throws Throwable {
+        rr.withPrefix("/foo").startJenkins();
+        assertThat(rr.getUrl().getPath(), equalTo("/foo/"));
+        assertThatLocalAndRemoteUrlEquals();
+    }
+
+    @Test public void complexPrefix() throws Throwable {
+        rr.withPrefix("/foo/bar").startJenkins();
+        assertThat(rr.getUrl().getPath(), equalTo("/foo/bar/"));
+        assertThatLocalAndRemoteUrlEquals();
+    }
+
+    @Test public void noPrefix() throws Throwable {
+        rr.withPrefix("").startJenkins();
+        assertThat(rr.getUrl().getPath(), equalTo("/"));
+        assertThatLocalAndRemoteUrlEquals();
+    }
+
+    @Test public void invalidPrefixes() {
+        assertThrows(IllegalArgumentException.class, () -> rr.withPrefix("foo"));
+        assertThrows(IllegalArgumentException.class, () -> rr.withPrefix("/foo/"));
     }
 
     @Test public void ipv6() throws Throwable {
         // Use -Djava.net.preferIPv6Addresses=true if dualstack
         assumeThat(InetAddress.getLoopbackAddress(), instanceOf(Inet6Address.class));
         rr.withHost("::1").startJenkins();
-        var externalForm = rr.getUrl().toExternalForm();
-        assertEquals(externalForm, rr.runRemotely(RealJenkinsRuleTest::_getJenkinsUrlFromRemote));
+        assertThatLocalAndRemoteUrlEquals();
+    }
+
+    private void assertThatLocalAndRemoteUrlEquals() throws Throwable {
+        assertEquals(rr.getUrl().toExternalForm(), rr.runRemotely(RealJenkinsRuleTest::_getJenkinsUrlFromRemote));
     }
 
     @Test public void testThrowsException() {