alvinalexander.com | career | drupal | java | mac | mysql | perl | scala | uml | unix  

Java example source code file (handles.hpp)

This example Java source code file (handles.hpp) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

assert, def_handle, def_metadata_handle, handle, handlearea, handlemark, klass, klasshandle, nohandlemark, null, oop, resetnohandlemark, stackobj, thread

The handles.hpp Java example source code

/*
 * 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.
 *
 * 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.
 *
 */

#ifndef SHARE_VM_RUNTIME_HANDLES_HPP
#define SHARE_VM_RUNTIME_HANDLES_HPP

#include "oops/klass.hpp"

//------------------------------------------------------------------------------------------------------------------------
// In order to preserve oops during garbage collection, they should be
// allocated and passed around via Handles within the VM. A handle is
// simply an extra indirection allocated in a thread local handle area.
//
// A handle is a ValueObj, so it can be passed around as a value, can
// be used as a parameter w/o using &-passing, and can be returned as a
// return value.
//
// oop parameters and return types should be Handles whenever feasible.
//
// Handles are declared in a straight-forward manner, e.g.
//
//   oop obj = ...;
//   Handle h1(obj);              // allocate new handle
//   Handle h2(thread, obj);      // faster allocation when current thread is known
//   Handle h3;                   // declare handle only, no allocation occurs
//   ...
//   h3 = h1;                     // make h3 refer to same indirection as h1
//   oop obj2 = h2();             // get handle value
//   h1->print();                 // invoking operation on oop
//
// Handles are specialized for different oop types to provide extra type
// information and avoid unnecessary casting. For each oop type xxxOop
// there is a corresponding handle called xxxHandle, e.g.
//
//   oop           Handle
//   Method*       methodHandle
//   instanceOop   instanceHandle

//------------------------------------------------------------------------------------------------------------------------
// Base class for all handles. Provides overloading of frequently
// used operators for ease of use.

class Handle VALUE_OBJ_CLASS_SPEC {
 private:
  oop* _handle;

 protected:
  oop     obj() const                            { return _handle == NULL ? (oop)NULL : *_handle; }
  oop     non_null_obj() const                   { assert(_handle != NULL, "resolving NULL handle"); return *_handle; }

 public:
  // Constructors
  Handle()                                       { _handle = NULL; }
  Handle(oop obj);
  Handle(Thread* thread, oop obj);

  // General access
  oop     operator () () const                   { return obj(); }
  oop     operator -> () const                   { return non_null_obj(); }
  bool    operator == (oop o) const              { return obj() == o; }
  bool    operator == (const Handle& h) const          { return obj() == h.obj(); }

  // Null checks
  bool    is_null() const                        { return _handle == NULL; }
  bool    not_null() const                       { return _handle != NULL; }

  // Debugging
  void    print()                                { obj()->print(); }

  // Direct interface, use very sparingly.
  // Used by JavaCalls to quickly convert handles and to create handles static data structures.
  // Constructor takes a dummy argument to prevent unintentional type conversion in C++.
  Handle(oop *handle, bool dummy)                { _handle = handle; }

  // Raw handle access. Allows easy duplication of Handles. This can be very unsafe
  // since duplicates is only valid as long as original handle is alive.
  oop* raw_value()                               { return _handle; }
  static oop raw_resolve(oop *handle)            { return handle == NULL ? (oop)NULL : *handle; }
};

// Specific Handles for different oop types
#define DEF_HANDLE(type, is_a)                   \
  class type##Handle: public Handle {            \
   protected:                                    \
    type##Oop    obj() const                     { return (type##Oop)Handle::obj(); } \
    type##Oop    non_null_obj() const            { return (type##Oop)Handle::non_null_obj(); } \
                                                 \
   public:                                       \
    /* Constructors */                           \
    type##Handle ()                              : Handle()                 {} \
    type##Handle (type##Oop obj) : Handle((oop)obj) {                         \
      assert(is_null() || ((oop)obj)->is_a(),                                 \
             "illegal type");                                                 \
    }                                                                         \
    type##Handle (Thread* thread, type##Oop obj) : Handle(thread, (oop)obj) { \
      assert(is_null() || ((oop)obj)->is_a(), "illegal type");                \
    }                                                                         \
    \
    /* Operators for ease of use */              \
    type##Oop    operator () () const            { return obj(); } \
    type##Oop    operator -> () const            { return non_null_obj(); } \
  };


DEF_HANDLE(instance         , is_instance         )
DEF_HANDLE(array            , is_array            )
DEF_HANDLE(objArray         , is_objArray         )
DEF_HANDLE(typeArray        , is_typeArray        )

//------------------------------------------------------------------------------------------------------------------------

// Metadata Handles.  Unlike oop Handles these are needed to prevent metadata
// from being reclaimed by RedefineClasses.

// Specific Handles for different oop types
#define DEF_METADATA_HANDLE(name, type)          \
  class name##Handle;                            \
  class name##Handle : public StackObj {         \
    type*     _value;                            \
    Thread*   _thread;                           \
   protected:                                    \
    type*        obj() const                     { return _value; } \
    type*        non_null_obj() const            { assert(_value != NULL, "resolving NULL _value"); return _value; } \
                                                 \
   public:                                       \
    /* Constructors */                           \
    name##Handle () : _value(NULL), _thread(NULL) {}   \
    name##Handle (type* obj);                    \
    name##Handle (Thread* thread, type* obj);    \
                                                 \
    name##Handle (const name##Handle &h);        \
    name##Handle& operator=(const name##Handle &s); \
                                                 \
    /* Destructor */                             \
    ~name##Handle ();                            \
    void remove();                               \
                                                 \
    /* Operators for ease of use */              \
    type*        operator () () const            { return obj(); } \
    type*        operator -> () const            { return non_null_obj(); } \
                                                 \
    bool    operator == (type* o) const          { return obj() == o; } \
    bool    operator == (const name##Handle& h) const  { return obj() == h.obj(); } \
                                                 \
    /* Null checks */                            \
    bool    is_null() const                      { return _value == NULL; } \
    bool    not_null() const                     { return _value != NULL; } \
  };


DEF_METADATA_HANDLE(method, Method)
DEF_METADATA_HANDLE(constantPool, ConstantPool)

// Writing this class explicitly, since DEF_METADATA_HANDLE(klass) doesn't
// provide the necessary Klass* <-> Klass* conversions. This Klass
// could be removed when we don't have the Klass* typedef anymore.
class KlassHandle : public StackObj {
  Klass* _value;
 protected:
   Klass* obj() const          { return _value; }
   Klass* non_null_obj() const { assert(_value != NULL, "resolving NULL _value"); return _value; }

 public:
   KlassHandle()                                 : _value(NULL) {}
   KlassHandle(const Klass* obj)                 : _value(const_cast<Klass *>(obj)) {};
   KlassHandle(Thread* thread, const Klass* obj) : _value(const_cast<Klass *>(obj)) {};

   Klass* operator () () const { return obj(); }
   Klass* operator -> () const { return non_null_obj(); }

   bool operator == (Klass* o) const             { return obj() == o; }
   bool operator == (const KlassHandle& h) const { return obj() == h.obj(); }

    bool is_null() const  { return _value == NULL; }
    bool not_null() const { return _value != NULL; }
};

class instanceKlassHandle : public KlassHandle {
 public:
  /* Constructors */
  instanceKlassHandle () : KlassHandle() {}
  instanceKlassHandle (const Klass* k) : KlassHandle(k) {
    assert(k == NULL || k->oop_is_instance(),
           "illegal type");
  }
  instanceKlassHandle (Thread* thread, const Klass* k) : KlassHandle(thread, k) {
    assert(k == NULL || k->oop_is_instance(),
           "illegal type");
  }
  /* Access to klass part */
  InstanceKlass*       operator () () const { return (InstanceKlass*)obj(); }
  InstanceKlass*       operator -> () const { return (InstanceKlass*)obj(); }
};


//------------------------------------------------------------------------------------------------------------------------
// Thread local handle area
class HandleArea: public Arena {
  friend class HandleMark;
  friend class NoHandleMark;
  friend class ResetNoHandleMark;
#ifdef ASSERT
  int _handle_mark_nesting;
  int _no_handle_mark_nesting;
#endif
  HandleArea* _prev;          // link to outer (older) area
 public:
  // Constructor
  HandleArea(HandleArea* prev) : Arena(Chunk::tiny_size) {
    debug_only(_handle_mark_nesting    = 0);
    debug_only(_no_handle_mark_nesting = 0);
    _prev = prev;
  }

  // Handle allocation
 private:
  oop* real_allocate_handle(oop obj) {
#ifdef ASSERT
    oop* handle = (oop*) (UseMallocOnly ? internal_malloc_4(oopSize) : Amalloc_4(oopSize));
#else
    oop* handle = (oop*) Amalloc_4(oopSize);
#endif
    *handle = obj;
    return handle;
  }
 public:
#ifdef ASSERT
  oop* allocate_handle(oop obj);
#else
  oop* allocate_handle(oop obj) { return real_allocate_handle(obj); }
#endif

  // Garbage collection support
  void oops_do(OopClosure* f);

  // Number of handles in use
  size_t used() const     { return Arena::used() / oopSize; }

  debug_only(bool no_handle_mark_active() { return _no_handle_mark_nesting > 0; })
};


//------------------------------------------------------------------------------------------------------------------------
// Handles are allocated in a (growable) thread local handle area. Deallocation
// is managed using a HandleMark. It should normally not be necessary to use
// HandleMarks manually.
//
// A HandleMark constructor will record the current handle area top, and the
// desctructor will reset the top, destroying all handles allocated in between.
// The following code will therefore NOT work:
//
//   Handle h;
//   {
//     HandleMark hm;
//     h = Handle(obj);
//   }
//   h()->print();       // WRONG, h destroyed by HandleMark destructor.
//
// If h has to be preserved, it can be converted to an oop or a local JNI handle
// across the HandleMark boundary.

// The base class of HandleMark should have been StackObj but we also heap allocate
// a HandleMark when a thread is created. The operator new is for this special case.

class HandleMark {
 private:
  Thread *_thread;              // thread that owns this mark
  HandleArea *_area;            // saved handle area
  Chunk *_chunk;                // saved arena chunk
  char *_hwm, *_max;            // saved arena info
  size_t _size_in_bytes;        // size of handle area
  // Link to previous active HandleMark in thread
  HandleMark* _previous_handle_mark;

  void initialize(Thread* thread);                // common code for constructors
  void set_previous_handle_mark(HandleMark* mark) { _previous_handle_mark = mark; }
  HandleMark* previous_handle_mark() const        { return _previous_handle_mark; }

  size_t size_in_bytes() const { return _size_in_bytes; }
 public:
  HandleMark();                            // see handles_inline.hpp
  HandleMark(Thread* thread)                      { initialize(thread); }
  ~HandleMark();

  // Functions used by HandleMarkCleaner
  // called in the constructor of HandleMarkCleaner
  void push();
  // called in the destructor of HandleMarkCleaner
  void pop_and_restore();
  // overloaded operators
  void* operator new(size_t size) throw();
  void* operator new [](size_t size) throw();
  void operator delete(void* p);
  void operator delete[](void* p);
};

//------------------------------------------------------------------------------------------------------------------------
// A NoHandleMark stack object will verify that no handles are allocated
// in its scope. Enabled in debug mode only.

class NoHandleMark: public StackObj {
 public:
#ifdef ASSERT
  NoHandleMark();
  ~NoHandleMark();
#else
  NoHandleMark()  {}
  ~NoHandleMark() {}
#endif
};


class ResetNoHandleMark: public StackObj {
  int _no_handle_mark_nesting;
 public:
#ifdef ASSERT
  ResetNoHandleMark();
  ~ResetNoHandleMark();
#else
  ResetNoHandleMark()  {}
  ~ResetNoHandleMark() {}
#endif
};

#endif // SHARE_VM_RUNTIME_HANDLES_HPP

Other Java examples (source code examples)

Here is a short list of links related to this Java handles.hpp source code file:

... this post is sponsored by my books ...

#1 New Release!

FP Best Seller

 

new blog posts

 

Copyright 1998-2021 Alvin Alexander, alvinalexander.com
All Rights Reserved.

A percentage of advertising revenue from
pages under the /java/jwarehouse URI on this website is
paid back to open source projects.