8320687: sun.jvmstat.monitor.MonitoredHost.getMonitoredHost() throws unexpected exceptions when invoked concurrently

Reviewed-by: alanb, kevinw

src/jdk.internal.jvmstat/share/classes/sun/jvmstat/monitor/MonitoredHost.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2004, 2021, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2004, 2023, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -50,7 +50,7 @@ import sun.jvmstat.monitor.event.HostListener;
  * @see HostListener
  */
 public abstract class MonitoredHost {
-    private static Map<HostIdentifier, MonitoredHost> monitoredHosts =
+    private static final Map<HostIdentifier, MonitoredHost> monitoredHosts =
                 new HashMap<HostIdentifier, MonitoredHost>();
 
     /*
@@ -133,13 +133,6 @@ public abstract class MonitoredHost {
         return getMonitoredHost(hostId);
     }
 
-
-    /*
-     * Load the MonitoredHostServices
-     */
-    private static ServiceLoader<MonitoredHostService> monitoredHostServiceLoader =
-        ServiceLoader.load(MonitoredHostService.class, MonitoredHostService.class.getClassLoader());
-
     /**
      * Factory method to construct a MonitoredHost instance to manage the
      * connection to the host indicated by {@code hostId}.
@@ -167,9 +160,12 @@ public abstract class MonitoredHost {
 
         hostId = resolveHostId(hostId);
 
-        for (MonitoredHostService mhs : monitoredHostServiceLoader) {
+        ServiceLoader<MonitoredHostService> services = ServiceLoader.load(
+                MonitoredHostService.class, MonitoredHostService.class.getClassLoader());
+        for (MonitoredHostService mhs : services) {
             if (mhs.getScheme().equals(hostId.getScheme())) {
                 mh = mhs.getMonitoredHost(hostId);
+                break;
             }
         }
 

test/jdk/sun/jvmstat/monitor/MonitoredVm/ConcurrentGetMonitoredHost.java

@@ -0,0 +1,90 @@
+/*
+ * Copyright (c) 2023, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+
+import sun.jvmstat.monitor.MonitoredHost;
+import sun.jvmstat.monitor.VmIdentifier;
+
+/*
+ * @test
+ * @bug 8320687
+ * @summary verify that sun.jvmstat.monitor.MonitoredHost.getMonitoredHost() doesn't
+ *          unexpectedly throw an exception when invoked by concurrent threads
+ *
+ * @run main/othervm ConcurrentGetMonitoredHost
+ */
+public class ConcurrentGetMonitoredHost {
+
+    /*
+     * Launches multiple concurrent threads and invokes MonitoredHost.getMonitoredHost()
+     * in each of these threads and expects the call to return successfully without any
+     * exceptions.
+     */
+    public static void main(final String[] args) throws Exception {
+        final String pidStr = "12345";
+        final VmIdentifier vmid = new VmIdentifier(pidStr);
+        final int numTasks = 100;
+        final List<Task> tasks = new ArrayList<>();
+        for (int i = 0; i < numTasks; i++) {
+            tasks.add(new Task(vmid));
+        }
+        System.out.println("Submitting " + numTasks + " concurrent tasks to" +
+                " get MonitoredHost for " + vmid);
+        try (ExecutorService executor = Executors.newCachedThreadPool()) {
+            // wait for all tasks to complete
+            final List<Future<MonitoredHost>> results = executor.invokeAll(tasks);
+            // verify each one successfully completed and each of
+            // the returned MonitoredHost is not null
+            for (final Future<MonitoredHost> result : results) {
+                final MonitoredHost mh = result.get();
+                if (mh == null) {
+                    throw new AssertionError("MonitoredHost.getMonitoredHost() returned" +
+                            " null for vmid " + vmid);
+                }
+            }
+        }
+        System.out.println("All " + numTasks + " completed successfully");
+    }
+
+    // a task which just calls MonitoredHost.getMonitoredHost(VmIdentifier) and
+    // returns the resultant MonitoredHost
+    private static final class Task implements Callable<MonitoredHost> {
+        private final VmIdentifier vmid;
+
+        private Task(final VmIdentifier vmid) {
+            this.vmid = Objects.requireNonNull(vmid);
+        }
+
+        @Override
+        public MonitoredHost call() throws Exception {
+            return MonitoredHost.getMonitoredHost(this.vmid);
+        }
+    }
+}