Revert "Require Java 21 or newer (jenkinsci#26018)"

Changed in the jenkins.io documentation with pull request:

* jenkins-infra/jenkins.io#8680

Removes the backport of pull request:

* jenkins-infra/jenkins.io#8675

Issue report:

* jenkinsci#26039

This reverts commit 5afa4b4.

Author: MarkEWaite

core/src/main/java/jenkins/monitor/JavaVersionRecommendationAdminMonitor.java

@@ -78,6 +78,7 @@ public class JavaVersionRecommendationAdminMonitor extends AdministrativeMonitor
 
     static {
         NavigableMap<Integer, LocalDate> supportedVersions = new TreeMap<>();
+        supportedVersions.put(17, LocalDate.of(2026, 3, 31)); // Temurin: 2027-10-31
         supportedVersions.put(21, LocalDate.of(2027, 9, 30)); // Temurin: 2029-09-30
         SUPPORTED_JAVA_VERSIONS = Collections.unmodifiableNavigableMap(supportedVersions);
     }

core/src/main/java/jenkins/security/stapler/StaplerDispatchValidator.java

@@ -311,6 +311,7 @@ private void loadWhitelist(@NonNull List<String> whitelistLines) {
             }
         }
 
+        @SuppressFBWarnings(value = "SIC_INNER_SHOULD_BE_STATIC", justification = "TODO needs triage")
         private class Validator {
             // lazy load parents to avoid trying to load potentially unavailable classes
             private final Supplier<Collection<Validator>> parentsSupplier;

pom.xml

@@ -28,7 +28,7 @@ THE SOFTWARE.
   <parent>
     <groupId>org.jenkins-ci</groupId>
     <artifactId>jenkins</artifactId>
-    <version>2.1326.v00b_e26755312</version>
+    <version>1.142</version>
     <relativePath />
   </parent>
 

war/src/main/java/executable/Main.java

@@ -76,7 +76,7 @@ public class Main {
      * This list must remain synchronized with the one in {@code
      * JavaVersionRecommendationAdminMonitor}.
      */
-    private static final NavigableSet<Integer> SUPPORTED_JAVA_VERSIONS = new TreeSet<>(List.of(21, 25));
+    private static final NavigableSet<Integer> SUPPORTED_JAVA_VERSIONS = new TreeSet<>(List.of(17, 21, 25));
 
     /**
      * Sets custom session cookie name.

war/src/test/java/executable/MainTest.java

@@ -13,26 +13,26 @@ void unsupported() {
         assertJavaCheckFails(8, true);
         assertJavaCheckFails(11, false);
         assertJavaCheckFails(11, true);
-        assertJavaCheckFails(17, false);
-        assertJavaCheckFails(17, true);
     }
 
     @Test
     void supported() {
+        assertJavaCheckPasses(17, false);
+        assertJavaCheckPasses(17, true);
         assertJavaCheckPasses(21, false);
         assertJavaCheckPasses(21, true);
-        assertJavaCheckPasses(25, false);
-        assertJavaCheckPasses(25, true);
     }
 
     @Test
     void future() {
+        assertJavaCheckFails(18, false);
+        assertJavaCheckFails(19, false);
+        assertJavaCheckFails(20, false);
         assertJavaCheckFails(22, false);
-        assertJavaCheckFails(23, false);
-        assertJavaCheckFails(24, false);
+        assertJavaCheckPasses(18, true);
+        assertJavaCheckPasses(19, true);
+        assertJavaCheckPasses(20, true);
         assertJavaCheckPasses(22, true);
-        assertJavaCheckPasses(23, true);
-        assertJavaCheckPasses(24, true);
     }
 
     private static void assertJavaCheckFails(int releaseVersion, boolean enableFutureJava) {