[JENKINS-52282] Add isJavaWebStartSupported to JNLPLauncher (#3766)

* [JENKINS-52282] Add isJavaWebStartSupported to JNLPLauncher

Seems cleaner to use JNLPLauncher than to use Functions.  Narrows the
scope of the method so that callers know it is specific to
JNLPLauncher.

* Include hyperlink to jenkins.io Java Web Start redirect page

core/src/main/java/hudson/slaves/JNLPLauncher.java

@@ -221,4 +221,19 @@ public class JNLPLauncher extends ComputerLauncher {
         }
     }
 
+    /**
+     * Returns true if Java Web Start button should be displayed.
+     * Java Web Start is only supported when the Jenkins server is
+     * running with Java 8.  Earlier Java versions are not supported by Jenkins.
+     * Later Java versions do not support Java Web Start.
+     *
+     * This flag is checked in {@code config.jelly} before displaying the
+     * Java Web Start button.
+     * @return {@code true} if Java Web Start button should be displayed.
+     * @since FIXME
+     */
+    @Restricted(NoExternalUse.class) // Jelly use
+    public boolean isJavaWebStartSupported() {
+        return System.getProperty("java.version", "1.8").startsWith("1.8");
+    }
 }

core/src/main/resources/hudson/slaves/JNLPLauncher/main.jelly

@@ -37,14 +37,27 @@ THE SOFTWARE.
           ${%Connect agent to Jenkins one of these ways:}
         </p>
         <ul>
-          <li>
-            <p>
-              <a href="slave-agent.jnlp" id="jnlp-link">
-                <img src="${imagesURL}/webstart.gif" alt="${%launch agent}" />
-              </a>
-              ${%Launch agent from browser}
-            </p>
-          </li>
+          <j:choose>
+            <j:when test="${it.launcher.javaWebStartSupported}">
+              <li>
+                <p>
+                  <a href="slave-agent.jnlp" id="jnlp-link">
+                    <img src="${imagesURL}/webstart.gif" alt="${%launch agent}" />
+                  </a>
+                  ${%Launch agent from browser}
+                </p>
+              </li>
+            </j:when>
+            <j:otherwise>
+              <li>
+                <p>
+                  <a href="https://jenkins.io/redirect/java11-java-web-start">
+                    ${%Java Web Start is not available for the JVM version running Jenkins}
+                  </a>
+                </p>
+              </li>
+            </j:otherwise>
+          </j:choose>
           <j:choose>
             <j:when test="${it.ACL.hasPermission(app.ANONYMOUS, it.CONNECT)}">
               <li>