Add warning for string deduplication with genertional zgc.

src/main/java/org/github/joa/JvmOptions.java

@@ -4665,9 +4665,12 @@ public void doAnalysis() {
                 addAnalysis(Analysis.INFO_JFR_FLIGHT_RECORDER_DISABLED);
             }
             // String deduplication
-            if (jvmContext.getGarbageCollectors().contains(GarbageCollector.G1) || useG1Gc != null) {
+            if (jvmContext.getGarbageCollectors().contains(GarbageCollector.G1) || useG1Gc != null
+                    || jvmContext.getVersionMajor() > 18) {
                 if (JdkUtil.isOptionDisabled(useStringDeduplication)) {
                     addAnalysis(Analysis.INFO_USE_STRING_DEDUPLICATION_REDUNDANT);
+                } else if (jvmContext.getGarbageCollectors().contains(GarbageCollector.ZGC_GENERATIONAL)) {
+                    addAnalysis(Analysis.WARN_USE_STRING_DEDUPLICATION_Z_GENERATIONAL);
                 }
             } else if (JdkUtil.isOptionEnabled(useStringDeduplication)) {
                 addAnalysis(Analysis.INFO_USE_STRING_DEDUPLICATION_UNSUPPORTED);

src/main/java/org/github/joa/domain/JvmContext.java

@@ -61,8 +61,8 @@ public class JvmContext {
     private boolean container = false;
 
     /**
-     * Identified garbage collectors (not just expected). It should agree with the JVM options unless options are
-     * overriden or ignored for some reason (e.g. nonsense values).
+     * Identified garbage collectors (not just expected). It should agree with the expected garbage collector(s)
+     * determined from JVM options unless options are overriden or ignored for some reason (e.g. nonsense values).
      */
     List<GarbageCollector> garbageCollectors = new ArrayList<>();
 

src/main/java/org/github/joa/util/Analysis.java

@@ -1094,6 +1094,11 @@ public enum Analysis {
      */
     WARN_USE_MEMBAR("warn.use.membar"),
 
+    /**
+     * Property key for -XX:+ZGenerational -XX:+UseStringDeduplication.
+     */
+    WARN_USE_STRING_DEDUPLICATION_Z_GENERATIONAL("warn.use.string.deduplication.z.generational"),
+
     /**
      * Property key for disabling the Java Thread API with -XX:-UseThreadPriorities.
      */

src/main/resources/org/github/joa/analysis.properties

@@ -204,6 +204,7 @@ warn.thread.stack.size.small=Thread stack size is less than 128K.
 warn.thread.stack.size.tiny=Thread stack size is less than 1K. Were units (e.g. 'K') left off by mistake? 
 warn.use.cond.card.mark=Conditional dirty card marking enabled with -XX:+UseCondCardMark. This is not a standard recommended option and can negatively impact performance (e.g. on single cpu/cores). Unless testing has shown this improves performance, consider removing this option.
 warn.use.membar=The -XX:+UseMembar option is used. This was a workaround for a bug in old versions of JDK 1.6 (see https://bugs.openjdk.org/browse/JDK-6822370) and is not necessary with modern JVMs. If there is not a good use case for using this option, remove it, as it causes the JVM to use a memory barrier that slows down the JVM.
+warn.use.string.deduplication.z.generational=Generational ZGC is affected by the following bug when string deduplication is enabled: https://bugs.openjdk.org/browse/JDK-8347337. Otherwise short-lived strings are kept artificially alive and promoted to the old generation, filling up the old generation and causing long old generation collections.
 warn.use.thread.priorities.disabled=The Java Thread API is disabled with -XX:+UseThreadPriorities.
 warn.verify.none=Class verification during loading is disabled with -Xverify:none. This is unsupported and can result in corrupt or invalid classes being loaded, causing undetermined JVM behavior and crashes.
 warn.zgc.heap.32g.lt=ZGC and heap < 32G. ZGC is generally for larger heaps (e.g. it does not support compressed object references). 

src/test/java/org/github/joa/util/TestAnalysis.java

@@ -2921,7 +2921,20 @@ void testUseStringCache() {
     }
 
     @Test
-    void testUseStringDeduplicationDisabledCMS() {
+    void testUseStringDeduplicationDisabledG1() {
+        String opts = "-Xss128k -XX:-UseStringDeduplication -Xmx2048M";
+        JvmContext context = new JvmContext(opts);
+        JvmOptions jvmOptions = new JvmOptions(context);
+        context.getGarbageCollectors().add(GarbageCollector.G1);
+        jvmOptions.doAnalysis();
+        assertTrue(jvmOptions.hasAnalysis(Analysis.INFO_USE_STRING_DEDUPLICATION_REDUNDANT.getKey()),
+                Analysis.INFO_USE_STRING_DEDUPLICATION_REDUNDANT + " analysis not identified.");
+        assertNull(jvmOptions.getUnaccountedDisabledOptions(),
+                "-XX:-UseStringDeduplication incorrectly identified as an unaccounted disabled option.");
+    }
+
+    @Test
+    void testUseStringDeduplicationEnabledCMS() {
         String opts = "-Xss128k -XX:+UseStringDeduplication -Xmx2048M";
         JvmContext context = new JvmContext(opts);
         JvmOptions jvmOptions = new JvmOptions(context);
@@ -2932,7 +2945,7 @@ void testUseStringDeduplicationDisabledCMS() {
     }
 
     @Test
-    void testUseStringDeduplicationDisabledDefaultCollectorJdk11() {
+    void testUseStringDeduplicationEnabledDefaultCollectorJdk11() {
         String opts = "-Xss128k -XX:+UseStringDeduplication -Xmx2048M";
         JvmContext context = new JvmContext(opts);
         JvmOptions jvmOptions = new JvmOptions(context);
@@ -2943,7 +2956,7 @@ void testUseStringDeduplicationDisabledDefaultCollectorJdk11() {
     }
 
     @Test
-    void testUseStringDeduplicationDisabledDefaultCollectorJdk8() {
+    void testUseStringDeduplicationEnabledDefaultCollectorJdk8() {
         String opts = "-Xss128k -XX:+UseStringDeduplication -Xmx2048M";
         JvmContext context = new JvmContext(opts);
         JvmOptions jvmOptions = new JvmOptions(context);
@@ -2954,16 +2967,15 @@ void testUseStringDeduplicationDisabledDefaultCollectorJdk8() {
     }
 
     @Test
-    void testUseStringDeduplicationDisabledG1() {
-        String opts = "-Xss128k -XX:-UseStringDeduplication -Xmx2048M";
+    void testUseStringDeduplicationEnabledZgcGenerational() {
+        String opts = "-XX:+UseStringDeduplication -Xmx32g";
         JvmContext context = new JvmContext(opts);
         JvmOptions jvmOptions = new JvmOptions(context);
-        context.getGarbageCollectors().add(GarbageCollector.G1);
+        context.setVersionMajor(21);
+        context.getGarbageCollectors().add(GarbageCollector.ZGC_GENERATIONAL);
         jvmOptions.doAnalysis();
-        assertTrue(jvmOptions.hasAnalysis(Analysis.INFO_USE_STRING_DEDUPLICATION_REDUNDANT.getKey()),
-                Analysis.INFO_USE_STRING_DEDUPLICATION_REDUNDANT + " analysis not identified.");
-        assertNull(jvmOptions.getUnaccountedDisabledOptions(),
-                "-XX:-UseStringDeduplication incorrectly identified as an unaccounted disabled option.");
+        assertTrue(jvmOptions.hasAnalysis(Analysis.WARN_USE_STRING_DEDUPLICATION_Z_GENERATIONAL.getKey()),
+                Analysis.WARN_USE_STRING_DEDUPLICATION_Z_GENERATIONAL + " analysis not identified.");
     }
 
     @Test