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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 <tt>MBeanServer</tt>} method.
41   *
42   * <p>The <tt>ObjectName</tt> for uniquely identifying the MXBean for
43   * a memory pool within an <tt>MBeanServer</tt> is:
44   * <blockquote>
45   *    {@link ManagementFactory#MEMORY_POOL_MXBEAN_DOMAIN_TYPE
46   *    <tt>java.lang:type=MemoryPool</tt>}<tt>,name=</tt><i>pool's name</i>
47   * </blockquote>
48   *
49   * It can be obtained by calling the
50   * {@link PlatformManagedObject#getObjectName} method.
51   *
52   * <h3>Memory Type</h3>
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   * <h3>Memory Usage Monitoring</h3>
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 name="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 name="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 name="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 name="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 name="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 <tt>MemoryMXBean</tt> 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 <tt>handleNotification</tt> 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 <tt>MemoryMXBean</tt> 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 name="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 <tt>MemoryType</tt> is <tt>String</tt>
378      * and the value is the name of the <tt>MemoryType</tt>.
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 <tt>null</tt>
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 <tt>MemoryUsage</tt> is
403      * <tt>CompositeData</tt> with attributes as specified in
404      * {@link MemoryUsage#from MemoryUsage}.
405      *
406      * @return a {@link MemoryUsage} object; or <tt>null</tt> 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 <tt>null</tt>
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 <tt>MemoryUsage</tt> is
420      * <tt>CompositeData</tt> with attributes as specified in
421      * {@link MemoryUsage#from MemoryUsage}.
422      *
423      * @return a {@link MemoryUsage} object representing the peak
424      * memory usage; or <tt>null</tt> 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 <tt>true</tt> if the memory pool is valid in the running
445      *              Java virtual machine;
446      *         <tt>false</tt> 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 <tt>String</tt> 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 <tt>threshold</tt>
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 <tt>threshold</tt> 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 <tt>true</tt> if the memory usage of
505      * this memory pool reaches or exceeds the threshold value;
506      * <tt>false</tt> 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 <tt>true</tt> if this memory pool supports usage threshold;
529      * <tt>false</tt> 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 <tt>threshold</tt> 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 <tt>threshold</tt> 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 <tt>true</tt> if the memory usage of this memory pool
589      * reaches or exceeds the collection usage threshold value
590      * in the most recent collection;
591      * <tt>false</tt> 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 <tt>null</tt> 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 <tt>MemoryUsage</tt> is
626      * <tt>CompositeData</tt> 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      * <tt>null</tt> 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 <tt>true</tt> if this memory pool supports the
640      * collection usage threshold; <tt>false</tt> otherwise.
641      */
642     public boolean isCollectionUsageThresholdSupported();
643 }