Fixes #5994 - QueuedThreadPool "free" threads

Introduced to QueuedThreadPool:

* getMaxReservedThreads()
* getAvailableReservedThreads()
* getAvailableThreads()
* getReadyThreads()
* getLeasedThreads()

Also few small code cleanups.

Signed-off-by: Simone Bordet <simone.bordet@gmail.com>

jetty-util/src/main/java/org/eclipse/jetty/util/thread/QueuedThreadPool.java

@@ -20,8 +20,6 @@
 
 import java.io.Closeable;
 import java.io.IOException;
-import java.security.AccessController;
-import java.security.PrivilegedAction;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Set;
@@ -51,7 +49,7 @@
 public class QueuedThreadPool extends ContainerLifeCycle implements ThreadFactory, SizedThreadPool, Dumpable, TryExecutor
 {
     private static final Logger LOG = Log.getLogger(QueuedThreadPool.class);
-    private static Runnable NOOP = () ->
+    private static final Runnable NOOP = () ->
     {
     };
 
@@ -158,6 +156,7 @@ public void setThreadPoolBudget(ThreadPoolBudget budget)
     {
         if (budget != null && budget.getSizedThreadPool() != this)
             throw new IllegalArgumentException();
+        updateBean(_budget, budget);
         _budget = budget;
     }
 
@@ -425,16 +424,34 @@ public int getMinThreads()
      * @return number of reserved threads or or -1 for heuristically determined
      * @see #setReservedThreads
      */
-    @ManagedAttribute("the number of reserved threads in the pool")
+    @ManagedAttribute("number of configured reserved threads")
     public int getReservedThreads()
     {
-        if (isStarted())
+        return _reservedThreads;
+    }
+
+    @ManagedAttribute("maximum number of reserved threads")
+    public int getMaxReservedThreads()
+    {
+        TryExecutor tryExecutor = _tryExecutor;
+        if (tryExecutor instanceof ReservedThreadExecutor)
         {
-            ReservedThreadExecutor reservedThreadExecutor = getBean(ReservedThreadExecutor.class);
-            if (reservedThreadExecutor != null)
-                return reservedThreadExecutor.getCapacity();
+            ReservedThreadExecutor reservedThreadExecutor = (ReservedThreadExecutor)tryExecutor;
+            return reservedThreadExecutor.getCapacity();
         }
-        return _reservedThreads;
+        return 0;
+    }
+
+    @ManagedAttribute("number of available reserved threads")
+    public int getAvailableReservedThreads()
+    {
+        TryExecutor tryExecutor = _tryExecutor;
+        if (tryExecutor instanceof ReservedThreadExecutor)
+        {
+            ReservedThreadExecutor reservedThreadExecutor = (ReservedThreadExecutor)tryExecutor;
+            return reservedThreadExecutor.getAvailable();
+        }
+        return 0;
     }
 
     /**
@@ -604,8 +621,26 @@ public int getIdleThreads()
     @ManagedAttribute("number of busy threads in the pool")
     public int getBusyThreads()
     {
-        int reserved = _tryExecutor instanceof ReservedThreadExecutor ? ((ReservedThreadExecutor)_tryExecutor).getAvailable() : 0;
-        return getThreads() - getIdleThreads() - reserved;
+        return getThreads() - getIdleThreads() - getAvailableReservedThreads();
+    }
+
+    @ManagedAttribute("number of potentially available threads in the pool")
+    public int getAvailableThreads()
+    {
+        return getMaxThreads() - getBusyThreads();
+    }
+
+    @ManagedAttribute("number of currently available threads in the pool")
+    public int getReadyThreads()
+    {
+        return getThreads() - getBusyThreads();
+    }
+
+    @ManagedAttribute("number of threads leased to components")
+    public int getLeasedThreads()
+    {
+        ThreadPoolBudget budget = _budget;
+        return budget == null ? 0 : budget.getLeasedThreads();
     }
 
     /**

jetty-util/src/main/java/org/eclipse/jetty/util/thread/ThreadPoolBudget.java

@@ -24,6 +24,8 @@
 import java.util.concurrent.Executor;
 import java.util.concurrent.atomic.AtomicBoolean;
 
+import org.eclipse.jetty.util.annotation.ManagedAttribute;
+import org.eclipse.jetty.util.annotation.ManagedObject;
 import org.eclipse.jetty.util.log.Log;
 import org.eclipse.jetty.util.log.Logger;
 
@@ -32,9 +34,10 @@
  *
  * @see ThreadPool.SizedThreadPool#getThreadPoolBudget()
  */
+@ManagedObject
 public class ThreadPoolBudget
 {
-    static final Logger LOG = Log.getLogger(ThreadPoolBudget.class);
+    private static final Logger LOG = Log.getLogger(ThreadPoolBudget.class);
 
     public interface Lease extends Closeable
     {
@@ -115,6 +118,14 @@ public ThreadPool.SizedThreadPool getSizedThreadPool()
         return pool;
     }
 
+    @ManagedAttribute("the number of threads leased to components")
+    public int getLeasedThreads()
+    {
+        return leases.stream()
+            .mapToInt(Lease::getThreads)
+            .sum();
+    }
+
     public void reset()
     {
         leases.clear();
@@ -146,9 +157,7 @@ public Lease leaseTo(Object leasee, int threads)
      */
     public boolean check(int maxThreads) throws IllegalStateException
     {
-        int required = leases.stream()
-            .mapToInt(Lease::getThreads)
-            .sum();
+        int required = getLeasedThreads();
         int left = maxThreads - required;
         if (left <= 0)
         {