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/* MemoryMXBean.java - Interface for a memory bean

   Copyright (C) 2006 Free Software Foundation

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

the Free Software Foundation; either version 2, or (at your option)

any later version.

GNU Classpath is distributed in the hope that it will be useful, but

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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

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02110-1301 USA.

Linking this library statically or dynamically with other modules is

making a combined work based on this library.  Thus, the terms and

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As a special exception, the copyright holders of this library give you

permission to link this library with independent modules to produce an

executable, regardless of the license terms of these independent

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terms of your choice, provided that you also meet, for each linked

independent module, the terms and conditions of the license of that

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or based on this library.  If you modify this library, you may extend

this exception to your version of the library, but you are not

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exception statement from your version. */

package java.lang.management;

/**

 * <p>

 * Provides access to information about the memory used

 * by the virtual machine.  An instance of this bean is

 * obtained by calling

 * {@link ManagementFactory#getMemoryMXBean()}.

 * </p>

 * <p>

 * The Java virtual machine uses two types of memory:

 * heap memory and non-heap memory.  The heap is the

 * storage location for class and array instances, and is

 * thus the main source of memory associated with running

 * Java programs.  The heap is created when the virtual

 * machine is started, and is periodically scanned by the

 * garbage collector(s), in order to reclaim memory which

 * is no longer used (e.g. because an object reference has

 * gone out of scope).

 * </p>

 * <p>

 * Non-heap memory is used by the virtual machine in order to

 * perform its duties.  Thus, it mainly acts as the storage

 * location for structures created as a result of parsing Java

 * bytecode, such as the constant pool and constructor/method

 * declarations.  When a Just-In-Time (JIT) compiler is in

 * operation, this will use non-heap memory to store compiled

 * bytecode.

 * </p>

 * <p>

 * Both types of memory may be non-contiguous.  During the

 * lifetime of the virtual machine, the size of both may

 * either change (either expanding or contracting) or stay

 * the same.

 * </p>

 * <h2>Notifications</h2>

 * <p>

 * Implementations of this interface also conform to the

 * {@link javax.management.NotificationEmitter} interface,

 * and supply two notifications reflecting memory usage.

 * These notifications occur when a usage threshold is

 * exceeded; for more details of these, see the documentation

 * of {@link MemoryPoolMXBean}.  If threshold monitoring

 * is supported, then a notification will be emitted each time

 * the threshold is crossed.  Another notification will not

 * be emitted unless the usage level has dropped below the

 * threshold again in the meantime.

 * </p>

 * <p>

 * The emitted notifications are instances of

 * {@link javax.management.Notification}, with a type of

 * either

 * {@link java.lang.management.MemoryNotificationInfo#MEMORY_THRESHOLD_EXCEEDED}

 * or

 * {@link java.lang.management.MemoryNotificationInfo#MEMORY_COLLECTION_THRESHOLD_EXCEEDED}

 * (depending on whether the notification refers to the general

 * usage threshold or the garbage collection threshold) and an instance

 * of {@link java.lang.management.MemoryNotificationInfo} contained

 * in the user data section.  This is wrapped inside an instance

 * of {@link javax.management.openmbean.CompositeData}, as explained

 * in the documentation for

 * {@link java.lang.management.MemoryNotificationInfo}.

 * </p>

 *

 * @author Andrew John Hughes (gnu_andrew@member.fsf.org)

 * @since 1.5

 */

public interface MemoryMXBean

{

  /**

   * Instigates a garbage collection cycle.  The virtual

   * machine's garbage collector should make the best

   * attempt it can at reclaiming unused memory.  This

   * is equivalent to invoking {@link java.lang.System.gc()}.

   *

   * @see java.lang.System#gc()

   */

  void gc();

  /**

   * Returns a {@link MemoryUsage} object representing the

   * current state of the heap.  This incorporates various

   * statistics on both the initial and current memory

   * allocations used by the heap.

   *

   * @return a {@link MemoryUsage} object for the heap.

   */

  MemoryUsage getHeapMemoryUsage();

  /**

   * Returns a {@link MemoryUsage} object representing the

   * current state of non-heap memory.  This incorporates

   * various statistics on both the initial and current

   * memory allocations used by non-heap memory..

   *

   * @return a {@link MemoryUsage} object for non-heap

   *         memory.

   */

  MemoryUsage getNonHeapMemoryUsage();

  /**

   * Returns the number of objects which are waiting to

   * be garbage collected (finalized).  An object is

   * finalized when the garbage collector determines that

   * there are no more references to that object are in

   * use.

   *

   * @return the number of objects awaiting finalization.

   */

  int getObjectPendingFinalizationCount();

  /**

   * Returns true if the virtual machine will emit additional

   * information when memory is allocated and deallocated.  The

   * format of the output is left up to the virtual machine.

   *

   * @return true if verbose memory output is on.

   */

  boolean isVerbose();

  /**

   * Turns on or off the emission of additional information

   * when memory is allocated and deallocated.  The format of the

   * output is left up to the virtual machine.  This method

   * may be called by multiple threads concurrently, but there

   * is only one global setting of verbosity that is affected.

   *

   * @param verbose the new setting for verbose memory output.

   * @throws SecurityException if a security manager exists and

   *                           denies ManagementPermission("control").

   */

  void setVerbose(boolean verbose);

}