8202788: Explicitly reclaim cached thread-local direct buffers at thread exit

Add internal TerminatingThreadLocal and use it to free cached thread-local direct buffers and nio-fs native buffers

Reviewed-by: tonyp, alanb

src/java.base/share/classes/java/lang/Thread.java

@@ -36,6 +36,8 @@ import java.util.HashMap;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 import java.util.concurrent.locks.LockSupport;
+
+import jdk.internal.misc.TerminatingThreadLocal;
 import sun.nio.ch.Interruptible;
 import jdk.internal.reflect.CallerSensitive;
 import jdk.internal.reflect.Reflection;
@@ -838,6 +840,9 @@ class Thread implements Runnable {
      * a chance to clean up before it actually exits.
      */
     private void exit() {
+        if (TerminatingThreadLocal.REGISTRY.isPresent()) {
+            TerminatingThreadLocal.threadTerminated();
+        }
         if (group != null) {
             group.threadTerminated(this);
             group = null;

src/java.base/share/classes/java/lang/ThreadLocal.java

@@ -24,6 +24,8 @@
  */
 
 package java.lang;
+import jdk.internal.misc.TerminatingThreadLocal;
+
 import java.lang.ref.*;
 import java.util.Objects;
 import java.util.concurrent.atomic.AtomicInteger;
@@ -170,6 +172,19 @@ public class ThreadLocal<T> {
         return setInitialValue();
     }
 
+    /**
+     * Returns {@code true} if there is a value in the current thread's copy of
+     * this thread-local variable, even if that values is {@code null}.
+     *
+     * @return {@code true} if current thread has associated value in this
+     *         thread-local variable; {@code false} if not
+     */
+    boolean isPresent() {
+        Thread t = Thread.currentThread();
+        ThreadLocalMap map = getMap(t);
+        return map != null && map.getEntry(this) != null;
+    }
+
     /**
      * Variant of set() to establish initialValue. Used instead
      * of set() in case user has overridden the set() method.
@@ -180,10 +195,14 @@ public class ThreadLocal<T> {
         T value = initialValue();
         Thread t = Thread.currentThread();
         ThreadLocalMap map = getMap(t);
-        if (map != null)
+        if (map != null) {
             map.set(this, value);
-        else
+        } else {
             createMap(t, value);
+        }
+        if (this instanceof TerminatingThreadLocal) {
+            TerminatingThreadLocal.register((TerminatingThreadLocal<?>) this);
+        }
         return value;
     }
 
@@ -199,10 +218,11 @@ public class ThreadLocal<T> {
     public void set(T value) {
         Thread t = Thread.currentThread();
         ThreadLocalMap map = getMap(t);
-        if (map != null)
+        if (map != null) {
             map.set(this, value);
-        else
+        } else {
             createMap(t, value);
+        }
     }
 
     /**
@@ -218,8 +238,9 @@ public class ThreadLocal<T> {
      */
      public void remove() {
          ThreadLocalMap m = getMap(Thread.currentThread());
-         if (m != null)
+         if (m != null) {
              m.remove(this);
+         }
      }
 
     /**