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 {@code ObjectName} for uniquely identifying the MXBean for + * a memory pool within an {@code MBeanServer} is: + *
+ * {@link ManagementFactory#MEMORY_POOL_MXBEAN_DOMAIN_TYPE + * java.lang:type=MemoryPool}{@code ,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 dedicated 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 {@code 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 {@code 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 {@code 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 {@code MemoryType} is {@code String}
+ * and the value is the name of the {@code 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 {@code 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 {@code MemoryUsage} is
+ * {@code CompositeData} with attributes as specified in
+ * {@link MemoryUsage#from MemoryUsage}.
+ *
+ * @return a {@link MemoryUsage} object; or {@code 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 {@code null}
+ * if this memory pool is not valid (i.e. no longer exists).
+ *
+ *
+ * MBeanServer access:
+ * The mapped type of {@code MemoryUsage} is
+ * {@code CompositeData} with attributes as specified in
+ * {@link MemoryUsage#from MemoryUsage}.
+ *
+ * @return a {@link MemoryUsage} object representing the peak
+ * memory usage; or {@code 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 {@code true} if the memory pool is valid in the running
+ * Java virtual machine;
+ * {@code 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 {@code 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 {@code 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 {@code 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 {@code true} if the memory usage of
+ * this memory pool reaches or exceeds the threshold value;
+ * {@code 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 {@code true} if this memory pool supports usage threshold;
+ * {@code 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 {@code 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 {@code 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 {@code true} if the memory usage of this memory pool + * reaches or exceeds the collection usage threshold value + * in the most recent collection; + * {@code 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 {@code null} if the Java virtual + * machine does not support this method. + * + *
+ * MBeanServer access:
+ * The mapped type of {@code MemoryUsage} is
+ * {@code 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;
+ * {@code null} if this method is not supported.
+ */
+ public MemoryUsage getCollectionUsage();
+
+ /**
+ * Tests if this memory pool supports a collection usage threshold.
+ *
+ * @return {@code true} if this memory pool supports the
+ * collection usage threshold; {@code false} otherwise.
+ */
+ public boolean isCollectionUsageThresholdSupported();
+}