001/* MemoryMXBean.java - Interface for a memory bean
002   Copyright (C) 2006 Free Software Foundation
003
004This file is part of GNU Classpath.
005
006GNU Classpath is free software; you can redistribute it and/or modify
007it under the terms of the GNU General Public License as published by
008the Free Software Foundation; either version 2, or (at your option)
009any later version.
010
011GNU Classpath is distributed in the hope that it will be useful, but
012WITHOUT ANY WARRANTY; without even the implied warranty of
013MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014General Public License for more details.
015
016You should have received a copy of the GNU General Public License
017along with GNU Classpath; see the file COPYING.  If not, write to the
018Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
01902110-1301 USA.
020
021Linking this library statically or dynamically with other modules is
022making a combined work based on this library.  Thus, the terms and
023conditions of the GNU General Public License cover the whole
024combination.
025
026As a special exception, the copyright holders of this library give you
027permission to link this library with independent modules to produce an
028executable, regardless of the license terms of these independent
029modules, and to copy and distribute the resulting executable under
030terms of your choice, provided that you also meet, for each linked
031independent module, the terms and conditions of the license of that
032module.  An independent module is a module which is not derived from
033or based on this library.  If you modify this library, you may extend
034this exception to your version of the library, but you are not
035obligated to do so.  If you do not wish to do so, delete this
036exception statement from your version. */
037
038package java.lang.management;
039
040/**
041 * <p>
042 * Provides access to information about the memory used 
043 * by the virtual machine.  An instance of this bean is
044 * obtained by calling
045 * {@link ManagementFactory#getMemoryMXBean()}.
046 * </p>
047 * <p>
048 * The Java virtual machine uses two types of memory:
049 * heap memory and non-heap memory.  The heap is the
050 * storage location for class and array instances, and is
051 * thus the main source of memory associated with running
052 * Java programs.  The heap is created when the virtual
053 * machine is started, and is periodically scanned by the
054 * garbage collector(s), in order to reclaim memory which
055 * is no longer used (e.g. because an object reference has
056 * gone out of scope).
057 * </p>
058 * <p>
059 * Non-heap memory is used by the virtual machine in order to
060 * perform its duties.  Thus, it mainly acts as the storage
061 * location for structures created as a result of parsing Java
062 * bytecode, such as the constant pool and constructor/method
063 * declarations.  When a Just-In-Time (JIT) compiler is in
064 * operation, this will use non-heap memory to store compiled
065 * bytecode.
066 * </p>
067 * <p>
068 * Both types of memory may be non-contiguous.  During the
069 * lifetime of the virtual machine, the size of both may
070 * either change (either expanding or contracting) or stay
071 * the same.
072 * </p>
073 * <h2>Notifications</h2>
074 * <p>
075 * Implementations of this interface also conform to the
076 * {@link javax.management.NotificationEmitter} interface,
077 * and supply two notifications reflecting memory usage.
078 * These notifications occur when a usage threshold is
079 * exceeded; for more details of these, see the documentation
080 * of {@link MemoryPoolMXBean}.  If threshold monitoring
081 * is supported, then a notification will be emitted each time
082 * the threshold is crossed.  Another notification will not
083 * be emitted unless the usage level has dropped below the
084 * threshold again in the meantime.
085 * </p>
086 * <p>
087 * The emitted notifications are instances of
088 * {@link javax.management.Notification}, with a type of
089 * either
090 * {@link java.lang.management.MemoryNotificationInfo#MEMORY_THRESHOLD_EXCEEDED}
091 * or
092 * {@link java.lang.management.MemoryNotificationInfo#MEMORY_COLLECTION_THRESHOLD_EXCEEDED}
093 * (depending on whether the notification refers to the general
094 * usage threshold or the garbage collection threshold) and an instance
095 * of {@link java.lang.management.MemoryNotificationInfo} contained
096 * in the user data section.  This is wrapped inside an instance
097 * of {@link javax.management.openmbean.CompositeData}, as explained
098 * in the documentation for
099 * {@link java.lang.management.MemoryNotificationInfo}.
100 * </p>
101 *
102 * @author Andrew John Hughes (gnu_andrew@member.fsf.org)
103 * @since 1.5
104 */
105public interface MemoryMXBean
106{
107  
108  /**
109   * Instigates a garbage collection cycle.  The virtual
110   * machine's garbage collector should make the best
111   * attempt it can at reclaiming unused memory.  This
112   * is equivalent to invoking {@link java.lang.System.gc()}.
113   *
114   * @see java.lang.System#gc()
115   */
116  void gc();
117
118  /**
119   * Returns a {@link MemoryUsage} object representing the
120   * current state of the heap.  This incorporates various
121   * statistics on both the initial and current memory
122   * allocations used by the heap.
123   *
124   * @return a {@link MemoryUsage} object for the heap.
125   */
126  MemoryUsage getHeapMemoryUsage();
127
128  /**
129   * Returns a {@link MemoryUsage} object representing the
130   * current state of non-heap memory.  This incorporates
131   * various statistics on both the initial and current
132   * memory allocations used by non-heap memory..
133   *
134   * @return a {@link MemoryUsage} object for non-heap
135   *         memory.
136   */
137  MemoryUsage getNonHeapMemoryUsage();
138  
139  /**
140   * Returns the number of objects which are waiting to
141   * be garbage collected (finalized).  An object is
142   * finalized when the garbage collector determines that
143   * there are no more references to that object are in
144   * use.
145   *
146   * @return the number of objects awaiting finalization.
147   */
148  int getObjectPendingFinalizationCount();
149
150  /**
151   * Returns true if the virtual machine will emit additional
152   * information when memory is allocated and deallocated.  The
153   * format of the output is left up to the virtual machine.
154   *
155   * @return true if verbose memory output is on.
156   */
157  boolean isVerbose();
158
159  /**
160   * Turns on or off the emission of additional information
161   * when memory is allocated and deallocated.  The format of the
162   * output is left up to the virtual machine.  This method
163   * may be called by multiple threads concurrently, but there
164   * is only one global setting of verbosity that is affected.
165   *
166   * @param verbose the new setting for verbose memory output.
167   * @throws SecurityException if a security manager exists and
168   *                           denies ManagementPermission("control").
169   */
170  void setVerbose(boolean verbose);
171
172}