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package java.lang.management;

/**

 * The management interface for a memory pool.  A memory pool

 * represents the memory resource managed by the Java virtual machine

 * and is managed by one or more {@link MemoryManagerMXBean memory managers}.

 *

 * <p> A Java virtual machine has one or more instances of the

 * implementation class of this interface.  An instance

 * implementing this interface is

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

 * that can be obtained by calling

 * the {@link ManagementFactory#getMemoryPoolMXBeans} method or

 * from the {@link ManagementFactory#getPlatformMBeanServer

 * platform <tt>MBeanServer</tt>} method.

 *

 * <p>The <tt>ObjectName</tt> for uniquely identifying the MXBean for

 * a memory pool within an <tt>MBeanServer</tt> is:

 * <blockquote>

 *    {@link ManagementFactory#MEMORY_POOL_MXBEAN_DOMAIN_TYPE

 *    <tt>java.lang:type=MemoryPool</tt>}<tt>,name=</tt><i>pool's name</i>

 * </blockquote>

 *

 * It can be obtained by calling the

 * {@link PlatformManagedObject#getObjectName} method.

 *

 * <h3>Memory Type</h3>

 * <p>The Java virtual machine has a heap for object allocation and also

 * maintains non-heap memory for the method area and the Java virtual

 * machine execution.  The Java virtual machine can have one or more

 * memory pools.  Each memory pool represents a memory area

 * of one of the following types:

 * <ul>

 *   <li>{@link MemoryType#HEAP heap}</li>

 *   <li>{@link MemoryType#NON_HEAP non-heap}</li>

 * </ul>

 *

 * <h3>Memory Usage Monitoring</h3>

 *

 * A memory pool has the following attributes:

 * <ul>

 *   <li><a href="#Usage">Memory usage</a></li>

 *   <li><a href="#PeakUsage">Peak memory usage</a></li>

 *   <li><a href="#UsageThreshold">Usage Threshold</a></li>

 *   <li><a href="#CollectionThreshold">Collection Usage Threshold</a>

 *       (only supported by some <em>garbage-collected</em> memory pools)</li>

 * </ul>

 *

 * <h3><a name="Usage">1. Memory Usage</a></h3>

 *

 * The {@link #getUsage} method provides an estimate

 * of the current usage of a memory pool.

 * For a garbage-collected memory pool, the amount of used memory

 * includes the memory occupied by all objects in the pool

 * including both <em>reachable</em> and <em>unreachable</em> objects.

 *

 * <p>In general, this method is a lightweight operation for getting

 * an approximate memory usage.  For some memory pools, for example,

 * when objects are not packed contiguously, this method may be

 * an expensive operation that requires some computation to determine

 * the current memory usage.  An implementation should document when

 * this is the case.

 *

 * <h3><a name="PeakUsage">2. Peak Memory Usage</a></h3>

 *

 * The Java virtual machine maintains the peak memory usage of a memory

 * pool since the virtual machine was started or the peak was reset.

 * The peak memory usage is returned by the {@link #getPeakUsage} method

 * and reset by calling the {@link #resetPeakUsage} method.

 *

 * <h3><a name="UsageThreshold">3. Usage Threshold</a></h3>

 *

 * Each memory pool has a manageable attribute

 * called the <i>usage threshold</i> which has a default value supplied

 * by the Java virtual machine.  The default value is platform-dependent.

 * The usage threshold can be set via the

 * {@link #setUsageThreshold setUsageThreshold} method.

 * If the threshold is set to a positive value, the usage threshold crossing

 * checking is enabled in this memory pool.

 * If the usage threshold is set to zero, usage

 * threshold crossing checking on this memory pool is disabled.

 * The {@link MemoryPoolMXBean#isUsageThresholdSupported} method can

 * be used to determine if this functionality is supported.

 * <p>

 * A Java virtual machine performs usage threshold crossing checking on a

 * memory pool basis at its best appropriate time, typically,

 * at garbage collection time.

 * Each memory pool maintains a {@link #getUsageThresholdCount

 * usage threshold count} that will get incremented

 * every time when the Java virtual machine

 * detects that the memory pool usage is crossing the threshold.

 * <p>

 * This manageable usage threshold attribute is designed for monitoring the

 * increasing trend of memory usage with low overhead.

 * Usage threshold may not be appropriate for some memory pools.

 * For example, a generational garbage collector, a common garbage collection

 * algorithm used in many Java virtual machine implementations,

 * manages two or more generations segregating objects by age.

 * Most of the objects are allocated in

 * the <em>youngest generation</em> (say a nursery memory pool).

 * The nursery memory pool is designed to be filled up and

 * collecting the nursery memory pool will free most of its memory space

 * since it is expected to contain mostly short-lived objects

 * and mostly are unreachable at garbage collection time.

 * In this case, it is more appropriate for the nursery memory pool

 * not to support a usage threshold.  In addition,

 * if the cost of an object allocation

 * in one memory pool is very low (for example, just atomic pointer exchange),

 * the Java virtual machine would probably not support the usage threshold

 * for that memory pool since the overhead in comparing the usage with

 * the threshold is higher than the cost of object allocation.

 *

 * <p>

 * The memory usage of the system can be monitored using

 * <a href="#Polling">polling</a> or

 * <a href="#ThresholdNotification">threshold notification</a> mechanisms.

 *

 * <ol type="a">

 *   <li><a name="Polling"><b>Polling</b></a>

 *       <p>

 *       An application can continuously monitor its memory usage

 *       by calling either the {@link #getUsage} method for all

 *       memory pools or the {@link #isUsageThresholdExceeded} method

 *       for those memory pools that support a usage threshold.

 *       task distribution and processing.  At every interval,

 *       it will determine if it should receive and process new tasks based

 *       on its memory usage.  If the memory usage exceeds its usage threshold,

 *       it will redistribute all outstanding tasks to other VMs and

 *       stop receiving new tasks until the memory usage returns

 *       below its usage threshold.

 *

 *       <pre>

 *       // Assume the usage threshold is supported for this pool.

 *       // Set the threshold to myThreshold above which no new tasks

 *       // should be taken.

 *       pool.setUsageThreshold(myThreshold);

 *       ....

 *

 *       boolean lowMemory = false;

 *       while (true) {

 *          if (pool.isUsageThresholdExceeded()) {

 *              // potential low memory, so redistribute tasks to other VMs

 *              lowMemory = true;

 *              redistributeTasks();

 *              // stop receiving new tasks

 *              stopReceivingTasks();

 *          } else {

 *              if (lowMemory) {

 *                  // resume receiving tasks

 *                  lowMemory = false;

 *                  resumeReceivingTasks();

 *              }

 *              // processing outstanding task

 *              ...

 *          }

 *          // sleep for sometime

 *          try {

 *              Thread.sleep(sometime);

 *          } catch (InterruptedException e) {

 *              ...

 *          }

 *       }

 *       </pre>

 *

 * <hr>

 *       The above example does not differentiate the case where

 *       the memory usage has temporarily dropped below the usage threshold

 *       between two iterations.  The usage threshold count returned by

 *       the {@link #getUsageThresholdCount} method

 *       can be used to determine

 *       if the memory usage has returned below the threshold

 *       between two polls.

 *       <p>

 *       Below shows another example that takes some action if a

 *       memory pool is under low memory and ignores the memory usage

 *       changes during the action processing time.

 *

 *       <pre>

 *       // Assume the usage threshold is supported for this pool.

 *       // Set the threshold to myThreshold which determines if

 *       // the application will take some action under low memory condition.

 *       pool.setUsageThreshold(myThreshold);

 *

 *       int prevCrossingCount = 0;

 *       while (true) {

 *           // A busy loop to detect when the memory usage

 *           // has exceeded the threshold.

 *           while (!pool.isUsageThresholdExceeded() ||

 *                  pool.getUsageThresholdCount() == prevCrossingCount) {

 *               try {

 *                   Thread.sleep(sometime)

 *               } catch (InterruptException e) {

 *                   ....

 *               }

 *           }

 *

 *           // Do some processing such as check for memory usage

 *           // and issue a warning

 *           ....

 *

 *           // Gets the current threshold count. The busy loop will then

 *           // ignore any crossing of threshold happens during the processing.

 *           prevCrossingCount = pool.getUsageThresholdCount();

 *       }

 *       </pre><hr>

 *   </li>

 *   <li><a name="ThresholdNotification"><b>Usage Threshold Notifications</b></a>

 *       <p>

 *       Usage threshold notification will be emitted by {@link MemoryMXBean}.

 *       When the Java virtual machine detects that the memory usage of

 *       a memory pool has reached or exceeded the usage threshold

 *       the virtual machine will trigger the <tt>MemoryMXBean</tt> to emit an

 *       {@link MemoryNotificationInfo#MEMORY_THRESHOLD_EXCEEDED

 *       usage threshold exceeded notification}.

 *       Another usage threshold exceeded notification will not be

 *       generated until the usage has fallen below the threshold and

 *       then exceeded it again.

 *       <p>

 *       Below is an example code implementing the same logic as the

 *       first example above but using the usage threshold notification

 *       mechanism to detect low memory conditions instead of polling.

 *       In this example code, upon receiving notification, the notification

 *       listener notifies another thread to perform the actual action

 *       such as to redistribute outstanding tasks, stop receiving tasks,

 *       or resume receiving tasks.

 *       The <tt>handleNotification</tt> method should be designed to

 *       do a very minimal amount of work and return without delay to avoid

 *       causing delay in delivering subsequent notifications.  Time-consuming

 *       actions should be performed by a separate thread.

 *       The notification listener may be invoked by multiple threads

 *       concurrently; so the tasks performed by the listener

 *       should be properly synchronized.

 *

 *       <pre>

 *       class MyListener implements javax.management.NotificationListener {

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

 *                String notifType = notification.getType();

 *                if (notifType.equals(MemoryNotificationInfo.MEMORY_THRESHOLD_EXCEEDED)) {

 *                    // potential low memory, notify another thread

 *                    // to redistribute outstanding tasks to other VMs

 *                    // and stop receiving new tasks.

 *                    lowMemory = true;

 *                    notifyAnotherThread(lowMemory);

 *                }

 *            }

 *       }

 *

 *       // Register MyListener with MemoryMXBean

 *       MemoryMXBean mbean = ManagementFactory.getMemoryMXBean();

 *       NotificationEmitter emitter = (NotificationEmitter) mbean;

 *       MyListener listener = new MyListener();

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

 *

 *       // Assume this pool supports a usage threshold.

 *       // Set the threshold to myThreshold above which no new tasks

 *       // should be taken.

 *       pool.setUsageThreshold(myThreshold);

 *

 *       // Usage threshold detection is enabled and notification will be

 *       // handled by MyListener.  Continue for other processing.

 *       ....

 *

 *       </pre>

 * <hr>

 *       <p>

 *       There is no guarantee about when the <tt>MemoryMXBean</tt> will emit

 *       a threshold notification and when the notification will be delivered.

 *       When a notification listener is invoked, the memory usage of

 *       the memory pool may have crossed the usage threshold more

 *       than once.

 *       The {@link MemoryNotificationInfo#getCount} method returns the number

 *       of times that the memory usage has crossed the usage threshold

 *       at the point in time when the notification was constructed.

 *       It can be compared with the current usage threshold count returned

 *       by the {@link #getUsageThresholdCount} method to determine if

 *       such situation has occurred.

 *   </li>

 * </ol>

 *

 * <h3><a name="CollectionThreshold">4. Collection Usage Threshold</a></h3>

 *

 * Collection usage threshold is a manageable attribute only applicable

 * to some garbage-collected memory pools.

 * After a Java virtual machine has expended effort in reclaiming memory

 * space by recycling unused objects in a memory pool at garbage collection

 * time, some number of bytes in the memory pools that are garbaged

 * collected will still be in use.  The collection usage threshold

 * allows a value to be set for this number of bytes such

 * that if the threshold is exceeded,

 * a {@link MemoryNotificationInfo#MEMORY_THRESHOLD_EXCEEDED

 * collection usage threshold exceeded notification}

 * will be emitted by the {@link MemoryMXBean}.

 * In addition, the {@link #getCollectionUsageThresholdCount

 * collection usage threshold count} will then be incremented.

 *

 * <p>

 * The {@link MemoryPoolMXBean#isCollectionUsageThresholdSupported} method can

 * be used to determine if this functionality is supported.

 *

 * <p>

 * A Java virtual machine performs collection usage threshold checking

 * on a memory pool basis.  This checking is enabled if the collection

 * usage threshold is set to a positive value.

 * If the collection usage threshold is set to zero, this checking

 * is disabled on this memory pool.  Default value is zero.

 * The Java virtual machine performs the collection usage threshold

 * checking at garbage collection time.

 *

 * <p>

 * Some garbage-collected memory pools may

 * choose not to support the collection usage threshold.  For example,

 * a memory pool is only managed by a continuous concurrent garbage

 * collector.  Objects can be allocated in this memory pool by some thread

 * while the unused objects are reclaimed by the concurrent garbage

 * collector simultaneously.  Unless there is a well-defined

 * garbage collection time which is the best appropriate time

 * to check the memory usage, the collection usage threshold should not

 * be supported.

 *

 * <p>

 * The collection usage threshold is designed for monitoring the memory usage

 * after the Java virtual machine has expended effort in reclaiming

 * memory space.  The collection usage could also be monitored

 * by the polling and threshold notification mechanism

 * described above for the <a href="#UsageThreshold">usage threshold</a>

 * in a similar fashion.

 *

 * @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 MemoryPoolMXBean extends PlatformManagedObject {

    /**

     * Returns the name representing this memory pool.

     *

     * @return the name of this memory pool.

     */

    public String getName();

    /**

     * Returns the type of this memory pool.

     *

     * <p>

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

     * The mapped type of <tt>MemoryType</tt> is <tt>String</tt>

     * and the value is the name of the <tt>MemoryType</tt>.

     *

     * @return the type of this memory pool.

     */

    public MemoryType getType();

    /**

     * Returns an estimate of the memory usage of this memory pool.

     * This method returns <tt>null</tt>

     * if this memory pool is not valid (i.e. no longer exists).

     *

     * <p>

     * This method requests the Java virtual machine to make

     * a best-effort estimate of the current memory usage of this

     * memory pool. For some memory pools, this method may be an

     * expensive operation that requires some computation to determine

     * the estimate.  An implementation should document when

     * this is the case.

     *

     * <p>This method is designed for use in monitoring system

     * memory usage and detecting low memory condition.

     *

     * <p>

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

     * The mapped type of <tt>MemoryUsage</tt> is

     * <tt>CompositeData</tt> with attributes as specified in

     * {@link MemoryUsage#from MemoryUsage}.

     *

     * @return a {@link MemoryUsage} object; or <tt>null</tt> if

     * this pool not valid.

     */

    public MemoryUsage getUsage();

    /**

     * Returns the peak memory usage of this memory pool since the

     * Java virtual machine was started or since the peak was reset.

     * This method returns <tt>null</tt>

     * if this memory pool is not valid (i.e. no longer exists).

     *

     * <p>

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

     * The mapped type of <tt>MemoryUsage</tt> is

     * <tt>CompositeData</tt> with attributes as specified in

     * {@link MemoryUsage#from MemoryUsage}.

     *

     * @return a {@link MemoryUsage} object representing the peak

     * memory usage; or <tt>null</tt> if this pool is not valid.

     *

     */

    public MemoryUsage getPeakUsage();

    /**

     * Resets the peak memory usage statistic of this memory pool

     * to the current memory usage.

     *

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

     *         exists and the caller does not have

     *         ManagementPermission("control").

     */

    public void resetPeakUsage();

    /**

     * Tests if this memory pool is valid in the Java virtual

     * machine.  A memory pool becomes invalid once the Java virtual

     * machine removes it from the memory system.

     *

     * @return <tt>true</tt> if the memory pool is valid in the running

     *              Java virtual machine;

     *         <tt>false</tt> otherwise.

     */

    public boolean isValid();

    /**

     * Returns the name of memory managers that manages this memory pool.

     * Each memory pool will be managed by at least one memory manager.

     *

     * @return an array of <tt>String</tt> objects, each is the name of

     * a memory manager managing this memory pool.

     */

    public String[] getMemoryManagerNames();

    /**

     * Returns the usage threshold value of this memory pool in bytes.

     * Each memory pool has a platform-dependent default threshold value.

     * The current usage threshold can be changed via the

     * {@link #setUsageThreshold setUsageThreshold} method.

     *

     * @return the usage threshold value of this memory pool in bytes.

     *

     * @throws UnsupportedOperationException if this memory pool

     *         does not support a usage threshold.

     *

     * @see #isUsageThresholdSupported

     */

    public long getUsageThreshold();

    /**

     * Sets the threshold of this memory pool to the given <tt>threshold</tt>

     * value if this memory pool supports the usage threshold.

     * The usage threshold crossing checking is enabled in this memory pool

     * if the threshold is set to a positive value.

     * The usage threshold crossing checking is disabled

     * if it is set to zero.

     *

     * @param threshold the new threshold value in bytes. Must be non-negative.

     *

     * @throws IllegalArgumentException if <tt>threshold</tt> is negative

     *         or greater than the maximum amount of memory for

     *         this memory pool if defined.

     *

     * @throws UnsupportedOperationException if this memory pool

     *         does not support a usage threshold.

     *

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

     *         exists and the caller does not have

     *         ManagementPermission("control").

     *

     * @see #isUsageThresholdSupported

     * @see <a href="#UsageThreshold">Usage threshold</a>

     */

    public void setUsageThreshold(long threshold);

    /**

     * Tests if the memory usage of this memory pool

     * reaches or exceeds its usage threshold value.

     *

     * @return <tt>true</tt> if the memory usage of

     * this memory pool reaches or exceeds the threshold value;

     * <tt>false</tt> otherwise.

     *

     * @throws UnsupportedOperationException if this memory pool

     *         does not support a usage threshold.

     */

    public boolean isUsageThresholdExceeded();

    /**

     * Returns the number of times that the memory usage has crossed

     * the usage threshold.

     *

     * @return the number of times that the memory usage

     * has crossed its usage threshold value.

     *

     * @throws UnsupportedOperationException if this memory pool

     * does not support a usage threshold.

     */

    public long getUsageThresholdCount();

    /**

     * Tests if this memory pool supports usage threshold.