A Java virtual machine has one or more instances of the * implementation class of this interface. An instance * implementing this interface is * an MXBean * that can be obtained by calling * the {@link ManagementFactory#getMemoryPoolMXBeans} method or * from the {@link ManagementFactory#getPlatformMBeanServer * platform MBeanServer} method. * *
The ObjectName for uniquely identifying the MXBean for * a memory pool within an MBeanServer is: *
* {@link ManagementFactory#MEMORY_POOL_MXBEAN_DOMAIN_TYPE * java.lang:type=MemoryPool},name=pool's name ** * It can be obtained by calling the * {@link PlatformManagedObject#getObjectName} method. * *
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: *
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. * *
* 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. *
* 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 youngest generation (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. * *
* The memory usage of the system can be monitored using * polling or * threshold notification mechanisms. * *
* 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. * *
* // 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) {
* ...
* }
* }
*
*
* * 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. * *
* // 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();
* }
* * 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 MemoryMXBean 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. *
* 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 handleNotification 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. * *
* 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.
* ....
*
*
* * There is no guarantee about when the MemoryMXBean 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. *
* The {@link MemoryPoolMXBean#isCollectionUsageThresholdSupported} method can * be used to determine if this functionality is supported. * *
* 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. * *
* 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. * *
* 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 usage threshold * in a similar fashion. * * @see ManagementFactory#getPlatformMXBeans(Class) * @see * JMX Specification. * @see * Ways to Access MXBeans * * @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. * *
* MBeanServer access:
* The mapped type of MemoryType is String
* and the value is the name of the MemoryType.
*
* @return the type of this memory pool.
*/
public MemoryType getType();
/**
* Returns an estimate of the memory usage of this memory pool.
* This method returns null
* if this memory pool is not valid (i.e. no longer exists).
*
*
* 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. * *
This method is designed for use in monitoring system * memory usage and detecting low memory condition. * *
* MBeanServer access:
* The mapped type of MemoryUsage is
* CompositeData with attributes as specified in
* {@link MemoryUsage#from MemoryUsage}.
*
* @return a {@link MemoryUsage} object; or null 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 null
* if this memory pool is not valid (i.e. no longer exists).
*
*
* MBeanServer access:
* The mapped type of MemoryUsage is
* CompositeData with attributes as specified in
* {@link MemoryUsage#from MemoryUsage}.
*
* @return a {@link MemoryUsage} object representing the peak
* memory usage; or null 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 true if the memory pool is valid in the running
* Java virtual machine;
* false 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 String 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 threshold
* 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 threshold 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 Usage threshold
*/
public void setUsageThreshold(long threshold);
/**
* Tests if the memory usage of this memory pool
* reaches or exceeds its usage threshold value.
*
* @return true if the memory usage of
* this memory pool reaches or exceeds the threshold value;
* false 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 true if this memory pool supports usage threshold;
* false 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 threshold 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.
*
* 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 threshold 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 Collection usage threshold */ public void setCollectionUsageThreshold(long threshold); /** * 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 true if the memory usage of this memory pool * reaches or exceeds the collection usage threshold value * in the most recent collection; * false 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 null if the Java virtual * machine does not support this method. * *
* MBeanServer access:
* The mapped type of MemoryUsage is
* CompositeData 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;
* null if this method is not supported.
*/
public MemoryUsage getCollectionUsage();
/**
* Tests if this memory pool supports a collection usage threshold.
*
* @return true if this memory pool supports the
* collection usage threshold; false otherwise.
*/
public boolean isCollectionUsageThresholdSupported();
}