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package java.lang.management;

import javax.management.openmbean.CompositeData;

/**

 * The management interface for the memory system of

 * the Java virtual machine.

 *

 * <p> A Java virtual machine has a single instance of the implementation

 * class of this interface.  This instance implementing this interface is

 * an <a href="ManagementFactory.html#MXBean">MXBean</a>

 * that can be obtained by calling

 * the {@link ManagementFactory#getMemoryMXBean} method or

 * from the {@link ManagementFactory#getPlatformMBeanServer

 *

 * the memory system within an MBeanServer is:

 * <blockquote>

 *    {@link ManagementFactory#MEMORY_MXBEAN_NAME

 * </blockquote>

 *

 * It can be obtained by calling the

 * {@link PlatformManagedObject#getObjectName} method.

 *

 * <h3> Memory </h3>

 * The memory system of the Java virtual machine manages

 * the following kinds of memory:

 *

 * <h3> 1. Heap </h3>

 * The Java virtual machine has a <i>heap</i> that is the runtime

 * data area from which memory for all class instances and arrays

 * are allocated.  It is created at the Java virtual machine start-up.

 * Heap memory for objects is reclaimed by an automatic memory management

 * system which is known as a <i>garbage collector</i>.

 *

 * <p>The heap may be of a fixed size or may be expanded and shrunk.

 * The memory for the heap does not need to be contiguous.

 *

 * <h3> 2. Non-Heap Memory</h3>

 * The Java virtual machine manages memory other than the heap

 * (referred as <i>non-heap memory</i>).

 *

 * <p> The Java virtual machine has a <i>method area</i> that is shared

 * among all threads.

 * The method area belongs to non-heap memory.  It stores per-class structures

 * such as a runtime constant pool, field and method data, and the code for

 * methods and constructors.  It is created at the Java virtual machine

 * start-up.

 *

 * <p> The method area is logically part of the heap but a Java virtual

 * machine implementation may choose not to either garbage collect

 * or compact it.  Similar to the heap, the method area may be of a

 * fixed size or may be expanded and shrunk.  The memory for the

 * method area does not need to be contiguous.

 *

 * <p>In addition to the method area, a Java virtual machine

 * implementation may require memory for internal processing or

 * optimization which also belongs to non-heap memory.

 * For example, the JIT compiler requires memory for storing the native

 * machine code translated from the Java virtual machine code for

 * high performance.

 *

 * <h3>Memory Pools and Memory Managers</h3>

 * {@link MemoryPoolMXBean Memory pools} and

 * {@link MemoryManagerMXBean memory managers} are the abstract entities

 * that monitor and manage the memory system

 * of the Java virtual machine.

 *

 * <p>A memory pool represents a memory area that the Java virtual machine

 * manages.  The Java virtual machine has at least one memory pool

 * and it may create or remove memory pools during execution.

 * A memory pool can belong to either the heap or the non-heap memory.

 *

 * <p>A memory manager is responsible for managing one or more memory pools.

 * The garbage collector is one type of memory manager responsible

 * for reclaiming memory occupied by unreachable objects.  A Java virtual

 * machine may have one or more memory managers.   It may

 * add or remove memory managers during execution.

 * A memory pool can be managed by more than one memory manager.

 *

 * <h3>Memory Usage Monitoring</h3>

 *

 * Memory usage is a very important monitoring attribute for the memory system.

 * The memory usage, for example, could indicate:

 * <ul>

 *   <li>the memory usage of an application,</li>

 *   <li>the workload being imposed on the automatic memory management system,</li>

 *   <li>potential memory leakage.</li>

 * </ul>

 *

 * <p>

 * The memory usage can be monitored in three ways:

 * <ul>

 *   <li>Polling</li>

 *   <li>Usage Threshold Notification</li>

 *   <li>Collection Usage Threshold Notification</li>

 * </ul>

 *

 * Details are specified in the {@link MemoryPoolMXBean} interface.

 *

 * <p>The memory usage monitoring mechanism is intended for load-balancing

 * or workload distribution use.  For example, an application would stop

 * receiving any new workload when its memory usage exceeds a

 * certain threshold. It is not intended for an application to detect

 * and recover from a low memory condition.

 *

 * <h3>Notifications</h3>

 *

 * {@link javax.management.NotificationEmitter NotificationEmitter}

 * that emits two types of memory {@link javax.management.Notification

 * notifications} if any one of the memory pools

 * supports a <a href="MemoryPoolMXBean.html#UsageThreshold">usage threshold</a>

 * or a <a href="MemoryPoolMXBean.html#CollectionThreshold">collection usage

 * threshold</a> which can be determined by calling the

 * {@link MemoryPoolMXBean#isUsageThresholdSupported} and

 * {@link MemoryPoolMXBean#isCollectionUsageThresholdSupported} methods.

 * <ul>

 *   <li>{@link MemoryNotificationInfo#MEMORY_THRESHOLD_EXCEEDED

 *       usage threshold exceeded notification} - for notifying that

 *       the memory usage of a memory pool is increased and has reached

 *       or exceeded its

 *       <a href="MemoryPoolMXBean.html#UsageThreshold"> usage threshold</a> value.

 *       </li>

 *   <li>{@link MemoryNotificationInfo#MEMORY_COLLECTION_THRESHOLD_EXCEEDED

 *       collection usage threshold exceeded notification} - for notifying that

 *       the memory usage of a memory pool is greater than or equal to its

 *       <a href="MemoryPoolMXBean.html#CollectionThreshold">

 *       collection usage threshold</a> after the Java virtual machine

 *       has expended effort in recycling unused objects in that

 *       memory pool.</li>

 * </ul>

 *

 * <p>

 * The notification emitted is a {@link javax.management.Notification}

 * instance whose {@link javax.management.Notification#setUserData

 * user data} is set to a {@link CompositeData CompositeData}

 * that represents a {@link MemoryNotificationInfo} object

 * containing information about the memory pool when the notification

 * as described in {@link MemoryNotificationInfo#from

 * MemoryNotificationInfo}.

 *

 * <hr>

 * <h3>NotificationEmitter</h3>

 * {@link ManagementFactory#getMemoryMXBean} implements

 * the {@link javax.management.NotificationEmitter NotificationEmitter}

 * interface that allows a listener to be registered within the

 *

 *

 * <blockquote><pre>

 * class MyListener implements javax.management.NotificationListener {

 *     public void handleNotification(Notification notif, Object handback) {

 *         // handle notification

 *         ....

 *     }

 * }

 *

 * MemoryMXBean mbean = ManagementFactory.getMemoryMXBean();

 * NotificationEmitter emitter = (NotificationEmitter) mbean;

 * MyListener listener = new MyListener();

 * emitter.addNotificationListener(listener, null, null);

 * </pre></blockquote>

 *

 * @see ManagementFactory#getPlatformMXBeans(Class)

 * @see <a href="../../../javax/management/package-summary.html">

 *      JMX Specification.</a>

 * @see <a href="package-summary.html#examples">

 *      Ways to Access MXBeans</a>

 *

 * @author  Mandy Chung

 * @since   1.5

 */

public interface MemoryMXBean extends PlatformManagedObject {

    /**

     * Returns the approximate number of objects for which

     * finalization is pending.

     *

     * @return the approximate number objects for which finalization

     * is pending.

     */

    public int getObjectPendingFinalizationCount();

    /**

     * Returns the current memory usage of the heap that

     * is used for object allocation.  The heap consists

     * usage is the sum of those values of all heap memory pools

     * returned memory usage represents the setting of the heap

     * memory which may not be the sum of those of all heap

     * memory pools.

     * <p>

     * The amount of used memory in the returned memory usage

     * is the amount of memory occupied by both live objects

     * and garbage objects that have not been collected, if any.

     *

     * <p>

     * <b>MBeanServer access</b>:<br>

     * {@link MemoryUsage#from MemoryUsage}.

     *

     * @return a {@link MemoryUsage} object representing

     * the heap memory usage.

     */

    public MemoryUsage getHeapMemoryUsage();

    /**

     * Returns the current memory usage of non-heap memory that

     * is used by the Java virtual machine.

     * The non-heap memory consists of one or more memory pools.

     * returned memory usage is the sum of those values of

     * represents the setting of the non-heap

     * memory which may not be the sum of those of all non-heap

     * memory pools.

     *

     * <p>

     * <b>MBeanServer access</b>:<br>

     * {@link MemoryUsage#from MemoryUsage}.

     *

     * @return a {@link MemoryUsage} object representing

     * the non-heap memory usage.

     */

    public MemoryUsage getNonHeapMemoryUsage();

    /**

     * Tests if verbose output for the memory system is enabled.

     *

     */

    public boolean isVerbose();

    /**

     * Enables or disables verbose output for the memory

     * system.  The verbose output information and the output stream

     * to which the verbose information is emitted are implementation

     * dependent.  Typically, a Java virtual machine implementation

     * prints a message whenever it frees memory at garbage collection.

     *

     * <p>

     * Each invocation of this method enables or disables verbose

     * output globally.

     *

     *

     * @exception  java.lang.SecurityException if a security manager

     *             exists and the caller does not have

     *             ManagementPermission("control").

     */

    public void setVerbose(boolean value);

    /**

     * Runs the garbage collector.

     * The call <code>gc()</code> is effectively equivalent to the

     * call:

     * <blockquote><pre>

     * System.gc()

     * </pre></blockquote>

     *

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

     */

    public void gc();

}