gcc/libjava/classpath/java/lang/management/MemoryMXBean.java

/* MemoryMXBean.java - Interface for a memory bean
   Copyright (C) 2006 Free Software Foundation

This file is part of GNU Classpath.

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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);

}