[java] Set the highest released language level through the registry

This will allow changing the language level combobox through the cloud registry. #IDEA-369013 Fixed

GitOrigin-RevId: 1874b9aa5f617795a239f3941572318fc64f1b25

java/idea-ui/src/com/intellij/openapi/roots/ui/configuration/LanguageLevelCombo.kt

@@ -29,8 +29,8 @@ abstract class LanguageLevelCombo @JvmOverloads constructor(
     val items = mutableListOf<Any>()
     items.add(defaultItem ?: "")
 
-    val highestPreviewLevel = LanguageLevel.HIGHEST.getPreviewLevel()
-    val highestWithPreview = highestPreviewLevel ?: LanguageLevel.HIGHEST
+    val highestLanguageLevel = AcceptedLanguageLevelsSettings.getHighest()
+    val highestWithPreview = highestLanguageLevel.getPreviewLevel() ?: highestLanguageLevel
 
     fun MutableList<Any>.filterAndAdd(levels: Collection<LanguageLevel>) = levels.filter(levelFilter).also { addAll(it) }
 

java/java-impl/resources/META-INF/JavaPlugin.xml

@@ -324,6 +324,9 @@
     <registryKey key="java.test.enable.tree.live.time" defaultValue="true"
                  description="Enable live time for tests" restartRequired="true"/>
 
+    <registryKey key="java.highest.language.level" defaultValue="24"
+                 description="Highest released language level" restartRequired="false"/>
+
     <registryKey key="index.ids.from.java.sources.in.jar" defaultValue="true"
                  description="Index ids from class files instead of source files in JARs" restartRequired="false"/>
 

java/java-impl/src/com/intellij/pom/java/AcceptedLanguageLevelsSettings.java

@@ -1,4 +1,4 @@
-// Copyright 2000-2023 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
+// Copyright 2000-2025 JetBrains s.r.o. and contributors. Use of this source code is governed by the Apache 2.0 license.
 package com.intellij.pom.java;
 
 import com.intellij.ide.IdeBundle;
@@ -21,6 +21,7 @@ import com.intellij.openapi.roots.JavaProjectModelModificationService;
 import com.intellij.openapi.roots.LanguageLevelProjectExtension;
 import com.intellij.openapi.ui.DialogWrapper;
 import com.intellij.openapi.util.Disposer;
+import com.intellij.openapi.util.registry.Registry;
 import com.intellij.ui.components.LegalNoticeDialog;
 import com.intellij.util.containers.MultiMap;
 import com.intellij.util.xmlb.XmlSerializerUtil;
@@ -148,7 +149,7 @@ public final class AcceptedLanguageLevelsSettings implements PersistentStateComp
     //allow custom features to appear in EAP
     if (ApplicationManager.getApplication().isEAP()) return true;
     // language levels up to HIGHEST are officially supported
-    return LanguageLevel.HIGHEST.compareTo(languageLevel) >= 0 || getSettings().acceptedNames.contains(languageLevel.name());
+    return getHighest().compareTo(languageLevel) >= 0 || getSettings().acceptedNames.contains(languageLevel.name());
   }
 
   private static void acceptAndRestore(Project project, Collection<? extends Module> modules, LanguageLevel languageLevel) {
@@ -172,8 +173,22 @@ public final class AcceptedLanguageLevelsSettings implements PersistentStateComp
     }
   }
 
-  public static LanguageLevel getHighestAcceptedLevel() {
-    LanguageLevel highest = LanguageLevel.HIGHEST;
+  /**
+   * @return the highest stable language level for Java.
+   */
+  public static @NotNull LanguageLevel getHighest() {
+    LanguageLevel languageLevel = LanguageLevel.forFeature(Registry.intValue("java.highest.language.level"));
+    if (languageLevel == null) throw new IllegalStateException("Highest language level could not be found");
+    return languageLevel;
+  }
+
+
+  /**
+   * @return the highest stable language level for Java, or a preview language level if the user has accepted the legal notice or is using
+   * an EAP version.
+   */
+  public static @NotNull LanguageLevel getHighestAcceptedLevel() {
+    LanguageLevel highest = getHighest();
     for (LanguageLevel level : LanguageLevel.values()) {
       if (isLanguageLevelAccepted(level)) {
         highest = level;
@@ -246,7 +261,7 @@ public final class AcceptedLanguageLevelsSettings implements PersistentStateComp
   private static @NotNull LanguageLevel adjustLanguageLevel(@NotNull LanguageLevel languageLevel) {
     if (isLanguageLevelAccepted(languageLevel)) return languageLevel;
     LanguageLevel highestAcceptedLevel = getHighestAcceptedLevel();
-    return highestAcceptedLevel.isAtLeast(languageLevel) ? LanguageLevel.HIGHEST : highestAcceptedLevel;
+    return highestAcceptedLevel.isAtLeast(languageLevel) ? getHighest() : highestAcceptedLevel;
   }
 
   @TestOnly