Issue #1841 reduce ServletHolder contention

jetty-servlet/src/main/java/org/eclipse/jetty/servlet/ServletHolder.java

@@ -400,9 +400,11 @@ public class ServletHolder extends Holder<Servlet> implements UserIdentity.Scope
 
         _config=new Config();
 
+        synchronized (this)
+        {
             if (_class!=null && javax.servlet.SingleThreadModel.class.isAssignableFrom(_class))
                 _servlet = new SingleThreadedWrapper();
-
+        }
     }
 
 
@@ -484,6 +486,12 @@ public class ServletHolder extends Holder<Servlet> implements UserIdentity.Scope
      */
     public synchronized Servlet getServlet()
         throws ServletException
+    {
+        Servlet servlet=_servlet;
+        if (servlet!=null && _unavailable==0)
+            return servlet;
+
+        synchronized(this)
         {
             // Handle previous unavailability
             if (_unavailable!=0)
@@ -494,9 +502,23 @@ public class ServletHolder extends Holder<Servlet> implements UserIdentity.Scope
                 _unavailableEx=null;
             }
 
-        if (_servlet==null)
+            servlet=_servlet;
+            if (servlet!=null)
+                return servlet;
+
+            if (isRunning())
+            {
+                if (_class == null)
+                    throw new UnavailableException("Servlet Not Initialized");
+                if (_unavailable != 0 || !_initOnStartup)
                     initServlet();
-        return _servlet;
+                servlet=_servlet;
+                if (servlet == null)
+                    throw new UnavailableException("Could not instantiate " + _class);
+            }
+
+            return servlet;
+        }
     }
 
     /* ------------------------------------------------------------ */
@@ -504,9 +526,15 @@ public class ServletHolder extends Holder<Servlet> implements UserIdentity.Scope
      * @return The servlet or null
      */
     public Servlet getServletInstance()
+    {
+        Servlet servlet=_servlet;
+        if (servlet!=null)
+            return servlet;
+        synchronized(this)
         {
             return _servlet;
         }
+    }
 
     /* ------------------------------------------------------------ */
     /**
@@ -604,7 +632,7 @@ public class ServletHolder extends Holder<Servlet> implements UserIdentity.Scope
     }
 
     /* ------------------------------------------------------------ */
-    private void initServlet()
+    private synchronized void initServlet()
         throws ServletException
     {
         Object old_run_as = null;
@@ -767,36 +795,17 @@ public class ServletHolder extends Holder<Servlet> implements UserIdentity.Scope
     protected void prepare (Request baseRequest, ServletRequest request, ServletResponse response)
     throws ServletException, UnavailableException
     {
-        ensureInstance();
+        getServlet();
         MultipartConfigElement mpce = ((Registration)getRegistration()).getMultipartConfig();
         if (mpce != null)
             baseRequest.setAttribute(Request.__MULTIPART_CONFIG_ELEMENT, mpce);
     }
 
+    @Deprecated
     public Servlet ensureInstance()
             throws ServletException, UnavailableException
     {
-        if (!isStarted())
+        return getServlet();
-            throw new UnavailableException("Servlet not initialized", -1);
-
-        Servlet servlet=_servlet;
-        if (servlet!=null && _unavailable==0)
-            return servlet;
-
-        synchronized(this)
-        {
-            servlet=_servlet;
-            if (servlet!=null)
-                return servlet;
-            if (_class == null)
-                throw new UnavailableException("Servlet Not Initialized");
-            if (_unavailable != 0 || (!_initOnStartup && servlet == null))
-                servlet = getServlet();
-            if (servlet == null)
-                throw new UnavailableException("Could not instantiate " + _class);
-
-            return servlet;
-        }
     }
 
     /* ------------------------------------------------------------ */
@@ -820,7 +829,7 @@ public class ServletHolder extends Holder<Servlet> implements UserIdentity.Scope
         if (_class==null)
             throw new UnavailableException("Servlet Not Initialized");
 
-        Servlet servlet = ensureInstance();
+        Servlet servlet = getServlet();
 
         // Service the request
         Object old_run_as = null;
@@ -865,26 +874,23 @@ public class ServletHolder extends Holder<Servlet> implements UserIdentity.Scope
 
 
     /* ------------------------------------------------------------ */
-    private boolean isJspServlet ()
+    protected boolean isJspServlet ()
     {
-        if (_servlet == null)
+        Servlet servlet = getServletInstance();
-            return false;
+        Class<?> c = servlet==null?_class:servlet.getClass();
 
-        Class<?> c = _servlet.getClass();
+        while (c != null)
-
-        boolean result = false;
-        while (c != null && !result)
         {
-            result = isJspServlet(c.getName());
+            if (isJspServlet(c.getName()))
+                return true;
             c = c.getSuperclass();
         }
-
+        return false;
-        return result;
     }
 
 
     /* ------------------------------------------------------------ */
-    private boolean isJspServlet (String classname)
+    protected boolean isJspServlet (String classname)
     {
         if (classname == null)
             return false;