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/**
* 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.
*
* <h4>Memory Type</h4>
* <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>
*
* <h4>Memory Usage Monitoring</h4>
*
* 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>
*
* <h4><a name="Usage">1. Memory Usage</a></h4>
*
* 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.
*
* <h4><a name="PeakUsage">2. Peak Memory Usage</a></h4>
*
* 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.
*
* <h4><a name="UsageThreshold">3. Usage Threshold</a></h4>
*
* 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.
* Below is example code that has a thread delicated for
* 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
* from the case where the memory usage remains above the threshould
* 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>
*
* <h4><a name="CollectionThreshold">4. Collection Usage Threshold</a></h4>
*
* 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
*/
/**
* Returns the name representing this memory pool.
*
* @return the name of this memory pool.
*/
/**
* 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.
*/
/**
* 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.
*/
/**
* 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.
*
*/
/**
* 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.
*/
/**
* 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>
*/
/**
* 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.
*
* @return <tt>true</tt> if this memory pool supports usage threshold;
* <tt>false</tt> otherwise.
*/
public boolean isUsageThresholdSupported();
/**
* Returns the collection usage threshold value of this memory pool
* in bytes. The default value is zero. The collection usage
* threshold can be changed via the
* {@link #setCollectionUsageThreshold setCollectionUsageThreshold} method.
*
* @return the collection usage threshold of this memory pool in bytes.
*
* @throws UnsupportedOperationException if this memory pool
* does not support a collection usage threshold.
*
* @see #isCollectionUsageThresholdSupported
*/
public long getCollectionUsageThreshold();
/**
* Sets the collection usage threshold of this memory pool to
* the given <tt>threshold</tt> value.
* When this threshold is set to positive, the Java virtual machine
* will check the memory usage at its best appropriate time after it has
* expended effort in recycling unused objects in this memory pool.
* <p>
* The collection usage threshold crossing checking is enabled
* in this memory pool if the threshold is set to a positive value.
* The collection usage threshold crossing checking is disabled
* if it is set to zero.
*
* @param threshold the new collection usage 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 collection usage threshold.
*
* @throws java.lang.SecurityException if a security manager
* exists and the caller does not have
* ManagementPermission("control").
*
* @see #isCollectionUsageThresholdSupported
* @see <a href="#CollectionThreshold">Collection usage threshold</a>
*/
/**
* Tests if the memory usage of this memory pool after
* the most recent collection on which the Java virtual
* machine has expended effort has reached or
* exceeded its collection usage threshold.
* This method does not request the Java virtual
* machine to perform any garbage collection other than its normal
* automatic memory management.
*
* @return <tt>true</tt> if the memory usage of this memory pool
* reaches or exceeds the collection usage threshold value
* in the most recent collection;
* <tt>false</tt> otherwise.
*
* @throws UnsupportedOperationException if this memory pool
* does not support a usage threshold.
*/
public boolean isCollectionUsageThresholdExceeded();
/**
* Returns the number of times that the Java virtual machine
* has detected that the memory usage has reached or
* exceeded the collection usage threshold.
*
* @return the number of times that the memory
* usage has reached or exceeded the collection usage threshold.
*
* @throws UnsupportedOperationException if this memory pool
* does not support a collection usage threshold.
*
* @see #isCollectionUsageThresholdSupported
*/
public long getCollectionUsageThresholdCount();
/**
* Returns the memory usage after the Java virtual machine
* most recently expended effort in recycling unused objects
* in this memory pool.
* This method does not request the Java virtual
* machine to perform any garbage collection other than its normal
* automatic memory management.
* This method returns <tt>null</tt> if the Java virtual
* machine does not support this method.
*
* <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} representing the memory usage of
* this memory pool after the Java virtual machine most recently
* expended effort in recycling unused objects;
* <tt>null</tt> if this method is not supported.
*/
/**
* Tests if this memory pool supports a collection usage threshold.
*
* @return <tt>true</tt> if this memory pool supports the
* collection usage threshold; <tt>false</tt> otherwise.
*/
public boolean isCollectionUsageThresholdSupported();
}