Issue #5859 Make safe creation of threads a utility class.

Signed-off-by: Jan Bartel <janb@webtide.com>

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

@@ -687,19 +687,15 @@ private boolean addCounts(int deltaThreads, int deltaIdle)
     @Override
     public Thread newThread(Runnable runnable)
     {
-        return (AccessController.doPrivileged(new PrivilegedAction<Thread>()
+        return ThreadCreator.create(() ->
         {
-            @Override
-            public Thread run()
-            {
-                Thread thread = new Thread(_threadGroup, runnable);
-                thread.setDaemon(isDaemon());
-                thread.setPriority(getThreadsPriority());
-                thread.setName(_name + "-" + thread.getId());
-                thread.setContextClassLoader(this.getClass().getClassLoader());
-                return thread;
-            }
-        }));
+            Thread thread = new Thread(_threadGroup, runnable);
+            thread.setDaemon(isDaemon());
+            thread.setPriority(getThreadsPriority());
+            thread.setName(_name + "-" + thread.getId());
+            thread.setContextClassLoader(this.getClass().getClassLoader());
+            return thread;
+        });
     }
 
     protected void removeThread(Thread thread)

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

@@ -38,15 +38,10 @@
 public class ShutdownThread extends Thread
 {
     private static final Logger LOG = Log.getLogger(ShutdownThread.class);
-    private static final ShutdownThread _thread = AccessController.doPrivileged(new PrivilegedAction<ShutdownThread>()
-        {
-            @Override
-            public ShutdownThread run()
-            {
-                return new ShutdownThread();
-            }
-
-        });
+    private static final ShutdownThread _thread = ThreadCreator.create(() ->
+    {
+        return new ShutdownThread();
+    });
 
     private boolean _hooked;
     private final List<LifeCycle> _lifeCycles = new CopyOnWriteArrayList<LifeCycle>();

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

@@ -0,0 +1,53 @@
+//
+//  ========================================================================
+//  Copyright (c) 1995-2020 Mort Bay Consulting Pty Ltd and others.
+//  ------------------------------------------------------------------------
+//  All rights reserved. This program and the accompanying materials
+//  are made available under the terms of the Eclipse Public License v1.0
+//  and Apache License v2.0 which accompanies this distribution.
+//
+//      The Eclipse Public License is available at
+//      http://www.eclipse.org/legal/epl-v10.html
+//
+//      The Apache License v2.0 is available at
+//      http://www.opensource.org/licenses/apache2.0.php
+//
+//  You may elect to redistribute this code under either of these licenses.
+//  ========================================================================
+//
+
+package org.eclipse.jetty.util.thread;
+
+import java.security.AccessController;
+import java.security.PrivilegedAction;
+
+/**
+ * ThreadCreator
+ *
+ * Convenience class to ensure that a new Thread is created
+ * inside a privileged block. This prevents the Thread constructor
+ * from pinning the caller's context classloader. This happens
+ * when the Thread constructor takes a snapshot of the current
+ * calling context - which contains ProtectionDomains that may
+ * reference the context classloader - and remembers it for the
+ * lifetime of the Thread.
+ */
+class ThreadCreator
+{
+    interface Factory<T extends Thread>
+    {
+        T newThread();
+    }
+
+    static <T extends Thread> T create(Factory<T> maker)
+    {
+        return AccessController.doPrivileged(new PrivilegedAction<T>()
+        {
+            @Override
+            public T run()
+            {
+                return maker.newThread();
+            }
+        });
+    }
+}

jetty-util/src/test/java/org/eclipse/jetty/util/thread/QueuedThreadPoolTest.java

@@ -840,10 +840,6 @@ public void testDump() throws Exception
     public void testContextClassLoader() throws Exception
     {
         QueuedThreadPool tp = new QueuedThreadPool();
-        tp.setMinThreads(1);
-        tp.setMaxThreads(3);
-        tp.setIdleTimeout(1000);
-        tp.setThreadsPriority(Thread.NORM_PRIORITY - 1);
         try (StacklessLogging stackless = new StacklessLogging(QueuedThreadPool.class))
         {
             //change the current thread's classloader to something else
@@ -859,8 +855,6 @@ public void testContextClassLoader() throws Exception
 
             //new thread should be set to the classloader of the QueuedThreadPool
             assertThat(t.getContextClassLoader(), Matchers.equalTo(QueuedThreadPool.class.getClassLoader()));
-
-            t.start();
         }
     }