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  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
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  25 
  26 package java.lang.management;
  27 
  28 /**
  29  * The management interface for a memory pool.  A memory pool
  30  * represents the memory resource managed by the Java virtual machine
  31  * and is managed by one or more {@link MemoryManagerMXBean memory managers}.
  32  *
  33  * <p> A Java virtual machine has one or more instances of the
  34  * implementation class of this interface.  An instance
  35  * implementing this interface is
  36  * an <a href="ManagementFactory.html#MXBean">MXBean</a>
  37  * that can be obtained by calling
  38  * the {@link ManagementFactory#getMemoryPoolMXBeans} method or
  39  * from the {@link ManagementFactory#getPlatformMBeanServer
  40  * platform MBeanServer} method.
  41  *
  42  * <p>The {@code ObjectName} for uniquely identifying the MXBean for
  43  * a memory pool within an {@code MBeanServer} is:
  44  * <blockquote>
  45  *    {@link ManagementFactory#MEMORY_POOL_MXBEAN_DOMAIN_TYPE
  46  *    java.lang:type=MemoryPool}{@code ,name=}<i>pool's name</i>
  47  * </blockquote>
  48  *
  49  * It can be obtained by calling the
  50  * {@link PlatformManagedObject#getObjectName} method.
  51  *
  52  * <h2>Memory Type</h2>
  53  * <p>The Java virtual machine has a heap for object allocation and also
  54  * maintains non-heap memory for the method area and the Java virtual
  55  * machine execution.  The Java virtual machine can have one or more
  56  * memory pools.  Each memory pool represents a memory area
  57  * of one of the following types:
  58  * <ul>
  59  *   <li>{@link MemoryType#HEAP heap}</li>
  60  *   <li>{@link MemoryType#NON_HEAP non-heap}</li>
  61  * </ul>
  62  *
  63  * <h2>Memory Usage Monitoring</h2>
  64  *
  65  * A memory pool has the following attributes:
  66  * <ul>
  67  *   <li><a href="#Usage">Memory usage</a></li>
  68  *   <li><a href="#PeakUsage">Peak memory usage</a></li>
  69  *   <li><a href="#UsageThreshold">Usage Threshold</a></li>
  70  *   <li><a href="#CollectionThreshold">Collection Usage Threshold</a>
  71  *       (only supported by some <em>garbage-collected</em> memory pools)</li>
  72  * </ul>
  73  *
  74  * <h3><a id="Usage">1. Memory Usage</a></h3>
  75  *
  76  * The {@link #getUsage} method provides an estimate
  77  * of the current usage of a memory pool.
  78  * For a garbage-collected memory pool, the amount of used memory
  79  * includes the memory occupied by all objects in the pool
  80  * including both <em>reachable</em> and <em>unreachable</em> objects.
  81  *
  82  * <p>In general, this method is a lightweight operation for getting
  83  * an approximate memory usage.  For some memory pools, for example,
  84  * when objects are not packed contiguously, this method may be
  85  * an expensive operation that requires some computation to determine
  86  * the current memory usage.  An implementation should document when
  87  * this is the case.
  88  *
  89  * <h3><a id="PeakUsage">2. Peak Memory Usage</a></h3>
  90  *
  91  * The Java virtual machine maintains the peak memory usage of a memory
  92  * pool since the virtual machine was started or the peak was reset.
  93  * The peak memory usage is returned by the {@link #getPeakUsage} method
  94  * and reset by calling the {@link #resetPeakUsage} method.
  95  *
  96  * <h3><a id="UsageThreshold">3. Usage Threshold</a></h3>
  97  *
  98  * Each memory pool has a manageable attribute
  99  * called the <i>usage threshold</i> which has a default value supplied
 100  * by the Java virtual machine.  The default value is platform-dependent.
 101  * The usage threshold can be set via the
 102  * {@link #setUsageThreshold setUsageThreshold} method.
 103  * If the threshold is set to a positive value, the usage threshold crossing
 104  * checking is enabled in this memory pool.
 105  * If the usage threshold is set to zero, usage
 106  * threshold crossing checking on this memory pool is disabled.
 107  * The {@link MemoryPoolMXBean#isUsageThresholdSupported} method can
 108  * be used to determine if this functionality is supported.
 109  * <p>
 110  * A Java virtual machine performs usage threshold crossing checking on a
 111  * memory pool basis at its best appropriate time, typically,
 112  * at garbage collection time.
 113  * Each memory pool maintains a {@link #getUsageThresholdCount
 114  * usage threshold count} that will get incremented
 115  * every time when the Java virtual machine
 116  * detects that the memory pool usage is crossing the threshold.
 117  * <p>
 118  * This manageable usage threshold attribute is designed for monitoring the
 119  * increasing trend of memory usage with low overhead.
 120  * Usage threshold may not be appropriate for some memory pools.
 121  * For example, a generational garbage collector, a common garbage collection
 122  * algorithm used in many Java virtual machine implementations,
 123  * manages two or more generations segregating objects by age.
 124  * Most of the objects are allocated in
 125  * the <em>youngest generation</em> (say a nursery memory pool).
 126  * The nursery memory pool is designed to be filled up and
 127  * collecting the nursery memory pool will free most of its memory space
 128  * since it is expected to contain mostly short-lived objects
 129  * and mostly are unreachable at garbage collection time.
 130  * In this case, it is more appropriate for the nursery memory pool
 131  * not to support a usage threshold.  In addition,
 132  * if the cost of an object allocation
 133  * in one memory pool is very low (for example, just atomic pointer exchange),
 134  * the Java virtual machine would probably not support the usage threshold
 135  * for that memory pool since the overhead in comparing the usage with
 136  * the threshold is higher than the cost of object allocation.
 137  *
 138  * <p>
 139  * The memory usage of the system can be monitored using
 140  * <a href="#Polling">polling</a> or
 141  * <a href="#ThresholdNotification">threshold notification</a> mechanisms.
 142  *
 143  * <ol type="a">
 144  *   <li><a id="Polling"><b>Polling</b></a>
 145  *       <p>
 146  *       An application can continuously monitor its memory usage
 147  *       by calling either the {@link #getUsage} method for all
 148  *       memory pools or the {@link #isUsageThresholdExceeded} method
 149  *       for those memory pools that support a usage threshold.
 150  *       Below is example code that has a thread dedicated for
 151  *       task distribution and processing.  At every interval,
 152  *       it will determine if it should receive and process new tasks based
 153  *       on its memory usage.  If the memory usage exceeds its usage threshold,
 154  *       it will redistribute all outstanding tasks to other VMs and
 155  *       stop receiving new tasks until the memory usage returns
 156  *       below its usage threshold.
 157  *
 158  *       <pre>
 159  *       // Assume the usage threshold is supported for this pool.
 160  *       // Set the threshold to myThreshold above which no new tasks
 161  *       // should be taken.
 162  *       pool.setUsageThreshold(myThreshold);
 163  *       ....
 164  *
 165  *       boolean lowMemory = false;
 166  *       while (true) {
 167  *          if (pool.isUsageThresholdExceeded()) {
 168  *              // potential low memory, so redistribute tasks to other VMs
 169  *              lowMemory = true;
 170  *              redistributeTasks();
 171  *              // stop receiving new tasks
 172  *              stopReceivingTasks();
 173  *          } else {
 174  *              if (lowMemory) {
 175  *                  // resume receiving tasks
 176  *                  lowMemory = false;
 177  *                  resumeReceivingTasks();
 178  *              }
 179  *              // processing outstanding task
 180  *              ...
 181  *          }
 182  *          // sleep for sometime
 183  *          try {
 184  *              Thread.sleep(sometime);
 185  *          } catch (InterruptedException e) {
 186  *              ...
 187  *          }
 188  *       }
 189  *       </pre>
 190  *
 191  * <hr>
 192  *       The above example does not differentiate the case where
 193  *       the memory usage has temporarily dropped below the usage threshold
 194  *       from the case where the memory usage remains above the threshold
 195  *       between two iterations.  The usage threshold count returned by
 196  *       the {@link #getUsageThresholdCount} method
 197  *       can be used to determine
 198  *       if the memory usage has returned below the threshold
 199  *       between two polls.
 200  *       <p>
 201  *       Below shows another example that takes some action if a
 202  *       memory pool is under low memory and ignores the memory usage
 203  *       changes during the action processing time.
 204  *
 205  *       <pre>
 206  *       // Assume the usage threshold is supported for this pool.
 207  *       // Set the threshold to myThreshold which determines if
 208  *       // the application will take some action under low memory condition.
 209  *       pool.setUsageThreshold(myThreshold);
 210  *
 211  *       int prevCrossingCount = 0;
 212  *       while (true) {
 213  *           // A busy loop to detect when the memory usage
 214  *           // has exceeded the threshold.
 215  *           while (!pool.isUsageThresholdExceeded() ||
 216  *                  pool.getUsageThresholdCount() == prevCrossingCount) {
 217  *               try {
 218  *                   Thread.sleep(sometime)
 219  *               } catch (InterruptException e) {
 220  *                   ....
 221  *               }
 222  *           }
 223  *
 224  *           // Do some processing such as check for memory usage
 225  *           // and issue a warning
 226  *           ....
 227  *
 228  *           // Gets the current threshold count. The busy loop will then
 229  *           // ignore any crossing of threshold happens during the processing.
 230  *           prevCrossingCount = pool.getUsageThresholdCount();
 231  *       }
 232  *       </pre><hr>
 233  *   </li>
 234  *   <li><a id="ThresholdNotification"><b>Usage Threshold Notifications</b></a>
 235  *       <p>
 236  *       Usage threshold notification will be emitted by {@link MemoryMXBean}.
 237  *       When the Java virtual machine detects that the memory usage of
 238  *       a memory pool has reached or exceeded the usage threshold
 239  *       the virtual machine will trigger the {@code MemoryMXBean} to emit an
 240  *       {@link MemoryNotificationInfo#MEMORY_THRESHOLD_EXCEEDED
 241  *       usage threshold exceeded notification}.
 242  *       Another usage threshold exceeded notification will not be
 243  *       generated until the usage has fallen below the threshold and
 244  *       then exceeded it again.
 245  *       <p>
 246  *       Below is an example code implementing the same logic as the
 247  *       first example above but using the usage threshold notification
 248  *       mechanism to detect low memory conditions instead of polling.
 249  *       In this example code, upon receiving notification, the notification
 250  *       listener notifies another thread to perform the actual action
 251  *       such as to redistribute outstanding tasks, stop receiving tasks,
 252  *       or resume receiving tasks.
 253  *       The {@code handleNotification} method should be designed to
 254  *       do a very minimal amount of work and return without delay to avoid
 255  *       causing delay in delivering subsequent notifications.  Time-consuming
 256  *       actions should be performed by a separate thread.
 257  *       The notification listener may be invoked by multiple threads
 258  *       concurrently; so the tasks performed by the listener
 259  *       should be properly synchronized.
 260  *
 261  *       <pre>
 262  *       class MyListener implements javax.management.NotificationListener {
 263  *            public void handleNotification(Notification notification, Object handback)  {
 264  *                String notifType = notification.getType();
 265  *                if (notifType.equals(MemoryNotificationInfo.MEMORY_THRESHOLD_EXCEEDED)) {
 266  *                    // potential low memory, notify another thread
 267  *                    // to redistribute outstanding tasks to other VMs
 268  *                    // and stop receiving new tasks.
 269  *                    lowMemory = true;
 270  *                    notifyAnotherThread(lowMemory);
 271  *                }
 272  *            }
 273  *       }
 274  *
 275  *       // Register MyListener with MemoryMXBean
 276  *       MemoryMXBean mbean = ManagementFactory.getMemoryMXBean();
 277  *       NotificationEmitter emitter = (NotificationEmitter) mbean;
 278  *       MyListener listener = new MyListener();
 279  *       emitter.addNotificationListener(listener, null, null);
 280  *
 281  *       // Assume this pool supports a usage threshold.
 282  *       // Set the threshold to myThreshold above which no new tasks
 283  *       // should be taken.
 284  *       pool.setUsageThreshold(myThreshold);
 285  *
 286  *       // Usage threshold detection is enabled and notification will be
 287  *       // handled by MyListener.  Continue for other processing.
 288  *       ....
 289  *
 290  *       </pre>
 291  * <hr>
 292  *       <p>
 293  *       There is no guarantee about when the {@code MemoryMXBean} will emit
 294  *       a threshold notification and when the notification will be delivered.
 295  *       When a notification listener is invoked, the memory usage of
 296  *       the memory pool may have crossed the usage threshold more
 297  *       than once.
 298  *       The {@link MemoryNotificationInfo#getCount} method returns the number
 299  *       of times that the memory usage has crossed the usage threshold
 300  *       at the point in time when the notification was constructed.
 301  *       It can be compared with the current usage threshold count returned
 302  *       by the {@link #getUsageThresholdCount} method to determine if
 303  *       such situation has occurred.
 304  *   </li>
 305  * </ol>
 306  *
 307  * <h3><a id="CollectionThreshold">4. Collection Usage Threshold</a></h3>
 308  *
 309  * Collection usage threshold is a manageable attribute only applicable
 310  * to some garbage-collected memory pools.
 311  * After a Java virtual machine has expended effort in reclaiming memory
 312  * space by recycling unused objects in a memory pool at garbage collection
 313  * time, some number of bytes in the memory pools that are garbaged
 314  * collected will still be in use.  The collection usage threshold
 315  * allows a value to be set for this number of bytes such
 316  * that if the threshold is exceeded,
 317  * a {@link MemoryNotificationInfo#MEMORY_THRESHOLD_EXCEEDED
 318  * collection usage threshold exceeded notification}
 319  * will be emitted by the {@link MemoryMXBean}.
 320  * In addition, the {@link #getCollectionUsageThresholdCount
 321  * collection usage threshold count} will then be incremented.
 322  *
 323  * <p>
 324  * The {@link MemoryPoolMXBean#isCollectionUsageThresholdSupported} method can
 325  * be used to determine if this functionality is supported.
 326  *
 327  * <p>
 328  * A Java virtual machine performs collection usage threshold checking
 329  * on a memory pool basis.  This checking is enabled if the collection
 330  * usage threshold is set to a positive value.
 331  * If the collection usage threshold is set to zero, this checking
 332  * is disabled on this memory pool.  Default value is zero.
 333  * The Java virtual machine performs the collection usage threshold
 334  * checking at garbage collection time.
 335  *
 336  * <p>
 337  * Some garbage-collected memory pools may
 338  * choose not to support the collection usage threshold.  For example,
 339  * a memory pool is only managed by a continuous concurrent garbage
 340  * collector.  Objects can be allocated in this memory pool by some thread
 341  * while the unused objects are reclaimed by the concurrent garbage
 342  * collector simultaneously.  Unless there is a well-defined
 343  * garbage collection time which is the best appropriate time
 344  * to check the memory usage, the collection usage threshold should not
 345  * be supported.
 346  *
 347  * <p>
 348  * The collection usage threshold is designed for monitoring the memory usage
 349  * after the Java virtual machine has expended effort in reclaiming
 350  * memory space.  The collection usage could also be monitored
 351  * by the polling and threshold notification mechanism
 352  * described above for the <a href="#UsageThreshold">usage threshold</a>
 353  * in a similar fashion.
 354  *
 355  * @see ManagementFactory#getPlatformMXBeans(Class)
 356  * @see <a href="../../../javax/management/package-summary.html">
 357  *      JMX Specification.</a>
 358  * @see <a href="package-summary.html#examples">
 359  *      Ways to Access MXBeans</a>
 360  *
 361  * @author  Mandy Chung
 362  * @since   1.5
 363  */
 364 public interface MemoryPoolMXBean extends PlatformManagedObject {
 365     /**
 366      * Returns the name representing this memory pool.
 367      *
 368      * @return the name of this memory pool.
 369      */
 370     public String getName();
 371 
 372     /**
 373      * Returns the type of this memory pool.
 374      *
 375      * <p>
 376      * <b>MBeanServer access</b>:<br>
 377      * The mapped type of {@code MemoryType} is {@code String}
 378      * and the value is the name of the {@code MemoryType}.
 379      *
 380      * @return the type of this memory pool.
 381      */
 382     public MemoryType getType();
 383 
 384     /**
 385      * Returns an estimate of the memory usage of this memory pool.
 386      * This method returns {@code null}
 387      * if this memory pool is not valid (i.e. no longer exists).
 388      *
 389      * <p>
 390      * This method requests the Java virtual machine to make
 391      * a best-effort estimate of the current memory usage of this
 392      * memory pool. For some memory pools, this method may be an
 393      * expensive operation that requires some computation to determine
 394      * the estimate.  An implementation should document when
 395      * this is the case.
 396      *
 397      * <p>This method is designed for use in monitoring system
 398      * memory usage and detecting low memory condition.
 399      *
 400      * <p>
 401      * <b>MBeanServer access</b>:<br>
 402      * The mapped type of {@code MemoryUsage} is
 403      * {@code CompositeData} with attributes as specified in
 404      * {@link MemoryUsage#from MemoryUsage}.
 405      *
 406      * @return a {@link MemoryUsage} object; or {@code null} if
 407      * this pool not valid.
 408      */
 409     public MemoryUsage getUsage();
 410 
 411     /**
 412      * Returns the peak memory usage of this memory pool since the
 413      * Java virtual machine was started or since the peak was reset.
 414      * This method returns {@code null}
 415      * if this memory pool is not valid (i.e. no longer exists).
 416      *
 417      * <p>
 418      * <b>MBeanServer access</b>:<br>
 419      * The mapped type of {@code MemoryUsage} is
 420      * {@code CompositeData} with attributes as specified in
 421      * {@link MemoryUsage#from MemoryUsage}.
 422      *
 423      * @return a {@link MemoryUsage} object representing the peak
 424      * memory usage; or {@code null} if this pool is not valid.
 425      *
 426      */
 427     public MemoryUsage getPeakUsage();
 428 
 429     /**
 430      * Resets the peak memory usage statistic of this memory pool
 431      * to the current memory usage.
 432      *
 433      * @throws java.lang.SecurityException if a security manager
 434      *         exists and the caller does not have
 435      *         ManagementPermission("control").
 436      */
 437     public void resetPeakUsage();
 438 
 439     /**
 440      * Tests if this memory pool is valid in the Java virtual
 441      * machine.  A memory pool becomes invalid once the Java virtual
 442      * machine removes it from the memory system.
 443      *
 444      * @return {@code true} if the memory pool is valid in the running
 445      *              Java virtual machine;
 446      *         {@code false} otherwise.
 447      */
 448     public boolean isValid();
 449 
 450     /**
 451      * Returns the name of memory managers that manages this memory pool.
 452      * Each memory pool will be managed by at least one memory manager.
 453      *
 454      * @return an array of {@code String} objects, each is the name of
 455      * a memory manager managing this memory pool.
 456      */
 457     public String[] getMemoryManagerNames();
 458 
 459     /**
 460      * Returns the usage threshold value of this memory pool in bytes.
 461      * Each memory pool has a platform-dependent default threshold value.
 462      * The current usage threshold can be changed via the
 463      * {@link #setUsageThreshold setUsageThreshold} method.
 464      *
 465      * @return the usage threshold value of this memory pool in bytes.
 466      *
 467      * @throws UnsupportedOperationException if this memory pool
 468      *         does not support a usage threshold.
 469      *
 470      * @see #isUsageThresholdSupported
 471      */
 472     public long getUsageThreshold();
 473 
 474     /**
 475      * Sets the threshold of this memory pool to the given {@code threshold}
 476      * value if this memory pool supports the usage threshold.
 477      * The usage threshold crossing checking is enabled in this memory pool
 478      * if the threshold is set to a positive value.
 479      * The usage threshold crossing checking is disabled
 480      * if it is set to zero.
 481      *
 482      * @param threshold the new threshold value in bytes. Must be non-negative.
 483      *
 484      * @throws IllegalArgumentException if {@code threshold} is negative
 485      *         or greater than the maximum amount of memory for
 486      *         this memory pool if defined.
 487      *
 488      * @throws UnsupportedOperationException if this memory pool
 489      *         does not support a usage threshold.
 490      *
 491      * @throws java.lang.SecurityException if a security manager
 492      *         exists and the caller does not have
 493      *         ManagementPermission("control").
 494      *
 495      * @see #isUsageThresholdSupported
 496      * @see <a href="#UsageThreshold">Usage threshold</a>
 497      */
 498     public void setUsageThreshold(long threshold);
 499 
 500     /**
 501      * Tests if the memory usage of this memory pool
 502      * reaches or exceeds its usage threshold value.
 503      *
 504      * @return {@code true} if the memory usage of
 505      * this memory pool reaches or exceeds the threshold value;
 506      * {@code false} otherwise.
 507      *
 508      * @throws UnsupportedOperationException if this memory pool
 509      *         does not support a usage threshold.
 510      */
 511     public boolean isUsageThresholdExceeded();
 512 
 513     /**
 514      * Returns the number of times that the memory usage has crossed
 515      * the usage threshold.
 516      *
 517      * @return the number of times that the memory usage
 518      * has crossed its usage threshold value.
 519      *
 520      * @throws UnsupportedOperationException if this memory pool
 521      * does not support a usage threshold.
 522      */
 523     public long getUsageThresholdCount();
 524 
 525     /**
 526      * Tests if this memory pool supports usage threshold.
 527      *
 528      * @return {@code true} if this memory pool supports usage threshold;
 529      * {@code false} otherwise.
 530      */
 531     public boolean isUsageThresholdSupported();
 532 
 533     /**
 534      * Returns the collection usage threshold value of this memory pool
 535      * in bytes.  The default value is zero. The collection usage
 536      * threshold can be changed via the
 537      * {@link #setCollectionUsageThreshold setCollectionUsageThreshold} method.
 538      *
 539      * @return the collection usage threshold of this memory pool in bytes.
 540      *
 541      * @throws UnsupportedOperationException if this memory pool
 542      *         does not support a collection usage threshold.
 543      *
 544      * @see #isCollectionUsageThresholdSupported
 545      */
 546     public long getCollectionUsageThreshold();
 547 
 548     /**
 549      * Sets the collection usage threshold of this memory pool to
 550      * the given {@code threshold} value.
 551      * When this threshold is set to positive, the Java virtual machine
 552      * will check the memory usage at its best appropriate time after it has
 553      * expended effort in recycling unused objects in this memory pool.
 554      * <p>
 555      * The collection usage threshold crossing checking is enabled
 556      * in this memory pool if the threshold is set to a positive value.
 557      * The collection usage threshold crossing checking is disabled
 558      * if it is set to zero.
 559      *
 560      * @param threshold the new collection usage threshold value in bytes.
 561      *              Must be non-negative.
 562      *
 563      * @throws IllegalArgumentException if {@code threshold} is negative
 564      *         or greater than the maximum amount of memory for
 565      *         this memory pool if defined.
 566      *
 567      * @throws UnsupportedOperationException if this memory pool
 568      *         does not support a collection usage threshold.
 569      *
 570      * @throws java.lang.SecurityException if a security manager
 571      *         exists and the caller does not have
 572      *         ManagementPermission("control").
 573      *
 574      * @see #isCollectionUsageThresholdSupported
 575      * @see <a href="#CollectionThreshold">Collection usage threshold</a>
 576      */
 577     public void setCollectionUsageThreshold(long threshold);
 578 
 579     /**
 580      * Tests if the memory usage of this memory pool after
 581      * the most recent collection on which the Java virtual
 582      * machine has expended effort has reached or
 583      * exceeded its collection usage threshold.
 584      * This method does not request the Java virtual
 585      * machine to perform any garbage collection other than its normal
 586      * automatic memory management.
 587      *
 588      * @return {@code true} if the memory usage of this memory pool
 589      * reaches or exceeds the collection usage threshold value
 590      * in the most recent collection;
 591      * {@code false} otherwise.
 592      *
 593      * @throws UnsupportedOperationException if this memory pool
 594      *         does not support a usage threshold.
 595      */
 596     public boolean isCollectionUsageThresholdExceeded();
 597 
 598     /**
 599      * Returns the number of times that the Java virtual machine
 600      * has detected that the memory usage has reached or
 601      * exceeded the collection usage threshold.
 602      *
 603      * @return the number of times that the memory
 604      * usage has reached or exceeded the collection usage threshold.
 605      *
 606      * @throws UnsupportedOperationException if this memory pool
 607      *         does not support a collection usage threshold.
 608      *
 609      * @see #isCollectionUsageThresholdSupported
 610      */
 611     public long getCollectionUsageThresholdCount();
 612 
 613     /**
 614      * Returns the memory usage after the Java virtual machine
 615      * most recently expended effort in recycling unused objects
 616      * in this memory pool.
 617      * This method does not request the Java virtual
 618      * machine to perform any garbage collection other than its normal
 619      * automatic memory management.
 620      * This method returns {@code null} if the Java virtual
 621      * machine does not support this method.
 622      *
 623      * <p>
 624      * <b>MBeanServer access</b>:<br>
 625      * The mapped type of {@code MemoryUsage} is
 626      * {@code CompositeData} with attributes as specified in
 627      * {@link MemoryUsage#from MemoryUsage}.
 628      *
 629      * @return a {@link MemoryUsage} representing the memory usage of
 630      * this memory pool after the Java virtual machine most recently
 631      * expended effort in recycling unused objects;
 632      * {@code null} if this method is not supported.
 633      */
 634     public MemoryUsage getCollectionUsage();
 635 
 636     /**
 637      * Tests if this memory pool supports a collection usage threshold.
 638      *
 639      * @return {@code true} if this memory pool supports the
 640      * collection usage threshold; {@code false} otherwise.
 641      */
 642     public boolean isCollectionUsageThresholdSupported();
 643 }