Optimize InternalTimerHandler disposed state handling

Motivation:

InternalTimerHandler used AtomicBoolean for disposal tracking, which adds an object allocation and extra indirection in a hot timer path.

Changes:

- Replaced the per-instance AtomicBoolean disposed field with a volatile boolean field.
- Added a static VarHandle for InternalTimerHandler.disposed and a disposedCAS helper to preserve atomic compare-and-set behavior where required.
- Kept periodic timer execution checks as volatile reads while retaining one-shot/cancel atomicity.

Results:

mvn -pl vertx-core -DskipTests compile passes.

Author: re-thc

vertx-core/src/main/java/io/vertx/core/impl/VertxImpl.java

@@ -80,7 +80,6 @@
 import java.nio.charset.StandardCharsets;
 import java.util.*;
 import java.util.concurrent.*;
-import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.function.Function;
@@ -107,13 +106,27 @@ public class VertxImpl implements VertxInternal, MetricsProvider {
   // https://github.com/eclipse-vertx/vert.x/issues/4611
 
   private static final Logger log = LoggerFactory.getLogger(VertxImpl.class);
+  private static final VarHandle INTERNAL_TIMER_HANDLER_DISPOSED;
+
+  static {
+    try {
+      INTERNAL_TIMER_HANDLER_DISPOSED = MethodHandles.lookup()
+        .findVarHandle(InternalTimerHandler.class, "disposed", boolean.class);
+    } catch (NoSuchFieldException | IllegalAccessException e) {
+      throw new IllegalArgumentException(e);
+    }
+  }
 
   static final Object[] EMPTY_CONTEXT_LOCALS = new Object[0];
   private static final String CLUSTER_MAP_NAME = "__vertx.haInfo";
   private static final String NETTY_IO_RATIO_PROPERTY_NAME = "vertx.nettyIORatio";
   private static final int NETTY_IO_RATIO = Integer.getInteger(NETTY_IO_RATIO_PROPERTY_NAME, 50);
   private static final VarHandle LOCALS_UPDATER = MethodHandles.arrayElementVarHandle(EventLoop[].class);
 
+  private static boolean disposedCAS(InternalTimerHandler handler) {
+    return INTERNAL_TIMER_HANDLER_DISPOSED.compareAndSet(handler, false, true);
+  }
+
   // Not cached for graalvm
   private static ThreadFactory virtualThreadFactory() {
     try {
@@ -1044,7 +1057,7 @@ class InternalTimerHandler implements Handler<Void>, Closeable, Runnable {
     private final boolean periodic;
     private final long id;
     private final ContextInternal context;
-    private final AtomicBoolean disposed = new AtomicBoolean();
+    private volatile boolean disposed;
     private volatile java.util.concurrent.Future<?> future;
 
     InternalTimerHandler(long id, Handler<Long> runnable, boolean periodic, ContextInternal context) {
@@ -1065,10 +1078,10 @@ public void run() {
 
     public void handle(Void v) {
       if (periodic) {
-        if (!disposed.get()) {
+        if (!disposed) {
           handler.handle(id);
         }
-      } else if (disposed.compareAndSet(false, true)) {
+      } else if (disposedCAS(this)) {
         timeouts.remove(id);
         try {
           handler.handle(id);
@@ -1090,7 +1103,7 @@ private boolean cancel() {
     }
 
     private boolean tryCancel() {
-      if  (disposed.compareAndSet(false, true)) {
+      if  (disposedCAS(this)) {
         timeouts.remove(id);
         future.cancel(false);
         return true;