src/java.base/share/classes/java/lang/ref/Finalizer.java

*** 1,7 ****
  /*
!  * Copyright (c) 1997, 2013, 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.  Oracle designates this
--- 1,7 ----
  /*
!  * Copyright (c) 1997, 2015, 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.  Oracle designates this
*** 30,39 ****
--- 30,44 ----
  import sun.misc.JavaLangAccess;
  import sun.misc.ManagedLocalsThread;
  import sun.misc.SharedSecrets;
  import sun.misc.VM;
  
+ import java.util.Map;
+ import java.util.HashMap;
+ import java.util.Map.Entry;
+ import java.util.Arrays;
+ 
  final class Finalizer extends FinalReference<Object> { /* Package-private; must be in
                                                            same package as the Reference
                                                            class */
  
      private static ReferenceQueue<Object> queue = new ReferenceQueue<>();
*** 217,226 ****
--- 222,275 ----
                  }
              }
          }
      }
  
+     // Support for GC.finalizer_info DCMD
+ 
+     // Fileds of the class below is accessed directly by VM
+     // using hardcoded field offsets, so don't change
+     // class layout.
+     static class FinalizerHistogramEntry {
+         private final int instanceCount;
+         private final String className;
+ 
+         int getInstanceCount() { return instanceCount; }
+ 
+         FinalizerHistogramEntry(Map.Entry<String, int[]> e) {
+             className = e.getKey();
+             instanceCount =  e.getValue()[0];
+         }
+     };
+ 
+     // This method called by VM from FinalizerInfoDCmd and
+     @SuppressWarnings({"unchecked","rawtypes"})
+     static Object[] getFinalizerHistogram() {
+         Map<String, int[]> countMap = new HashMap<>();
+         queue.forEach(r -> {
+             Object referent = r.get();
+             if (referent != null) {
+                 countMap.computeIfAbsent(
+                     referent.getClass().getName(), k -> new int[1])[0]++;
+                 /* Clear stack slot containing this variable, to decrease
+                    the chances of false retention with a conservative GC */
+                 referent = null;
+               }
+         });
+ 
+         // Copy HashMap entry set to array of objects accessible from VM
+         FinalizerHistogramEntry fhe[] = new FinalizerHistogramEntry[countMap.size()];
+         int i = 0;
+         for (Map.Entry<String, int[]> e : countMap.entrySet()) {
+              fhe[i++] = new  FinalizerHistogramEntry(e);
+         }
+ 
+         Arrays.sort(fhe, (e1, e2) -> Integer.compare(e2.getInstanceCount(), e1.getInstanceCount()));
+         return fhe;
+     }
+ 
+ 
      static {
          ThreadGroup tg = Thread.currentThread().getThreadGroup();
          for (ThreadGroup tgn = tg;
               tgn != null;
               tg = tgn, tgn = tg.getParent());