Java tutorial
/* * Copyright (c) 2003, 2019, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ 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 MBeanServer} method. * * <p>The {@code ObjectName} for uniquely identifying the MXBean for * a memory pool within an {@code MBeanServer} is: * <blockquote> * {@link ManagementFactory#MEMORY_POOL_MXBEAN_DOMAIN_TYPE * java.lang:type=MemoryPool}{@code ,name=}<i>pool's name</i> * </blockquote> * * It can be obtained by calling the * {@link PlatformManagedObject#getObjectName} method. * * <h2>Memory Type</h2> * <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> * * <h2>Memory Usage Monitoring</h2> * * 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 id="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 id="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 id="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 id="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 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. * * <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 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 id="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 {@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. * <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 {@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. * * <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 {@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. * </li> * </ol> * * <h3><a id="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 {@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). * * <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 {@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). * * <p> * <b>MBeanServer access</b>:<br> * 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 <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 {@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. * <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 {@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 <a href="#CollectionThreshold">Collection usage threshold</a> */ 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. * * <p> * <b>MBeanServer access</b>:<br> * 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(); }