1 /*
   2  * Copyright (c) 2018, 2019, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
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  24 
  25 #ifndef SHARE_OOPS_WEAKHANDLE_HPP
  26 #define SHARE_OOPS_WEAKHANDLE_HPP
  27 
  28 #include "oops/oop.hpp"
  29 #include "runtime/handles.hpp"
  30 
  31 class outputStream;
  32 class OopStorage;
  33 
  34 // A WeakHandle is a pointer to an oop that is stored in an OopStorage that is
  35 // processed weakly by GC.  The runtime structures that point to the oop must
  36 // either peek or resolve the oop, the latter will keep the oop alive for
  37 // the GC cycle.  The runtime structures that reference the oop must test
  38 // if the value is NULL.  If it is NULL, it has been cleaned out by GC.
  39 // This is the vm version of jweak but has different GC lifetimes and policies,
  40 // depending on the type.
  41 
  42 enum WeakHandleType { vm_class_loader_data, vm_string_table_data };
  43 
  44 template <WeakHandleType T>
  45 class WeakHandle {
  46  public:
  47  private:
  48   oop* _obj;
  49 
  50   WeakHandle(oop* w) : _obj(w) {}
  51   static OopStorage* get_storage();
  52  public:
  53   WeakHandle() : _obj(NULL) {} // needed for init
  54 
  55   static WeakHandle create(Handle obj);
  56   inline oop resolve() const;
  57   inline oop peek() const;
  58   void release() const;
  59   bool is_null() const { return _obj == NULL; }
  60 
  61   void replace(oop with_obj);
  62 
  63   void print() const;
  64   void print_on(outputStream* st) const;
  65 };
  66 
  67 typedef WeakHandle<vm_class_loader_data> ClassLoaderWeakHandle;
  68 
  69 #endif // SHARE_OOPS_WEAKHANDLE_HPP