1 /*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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15 * accompanied this code).
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25
26 package java.lang.management;
27
28 import javax.management.openmbean.CompositeData;
29
30 /**
31 * The management interface for the memory system of
32 * the Java virtual machine.
33 *
34 * <p> A Java virtual machine has a single instance of the implementation
35 * class of this interface. This instance implementing this interface is
36 * an <a href="ManagementFactory.html#MXBean">MXBean</a>
37 * that can be obtained by calling
38 * the {@link ManagementFactory#getMemoryMXBean} 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 * the memory system within an MBeanServer is:
44 * <blockquote>
45 * {@link ManagementFactory#MEMORY_MXBEAN_NAME
46 * <tt>java.lang:type=Memory</tt>}
47 * </blockquote>
48 *
49 * It can be obtained by calling the
50 * {@link PlatformManagedObject#getObjectName} method.
51 *
52 * <h3> Memory </h3>
53 * The memory system of the Java virtual machine manages
54 * the following kinds of memory:
55 *
56 * <h3> 1. Heap </h3>
57 * The Java virtual machine has a <i>heap</i> that is the runtime
58 * data area from which memory for all class instances and arrays
59 * are allocated. It is created at the Java virtual machine start-up.
60 * Heap memory for objects is reclaimed by an automatic memory management
61 * system which is known as a <i>garbage collector</i>.
62 *
63 * <p>The heap may be of a fixed size or may be expanded and shrunk.
64 * The memory for the heap does not need to be contiguous.
65 *
66 * <h3> 2. Non-Heap Memory</h3>
67 * The Java virtual machine manages memory other than the heap
68 * (referred as <i>non-heap memory</i>).
69 *
70 * <p> The Java virtual machine has a <i>method area</i> that is shared
71 * among all threads.
72 * The method area belongs to non-heap memory. It stores per-class structures
73 * such as a runtime constant pool, field and method data, and the code for
74 * methods and constructors. It is created at the Java virtual machine
75 * start-up.
76 *
77 * <p> The method area is logically part of the heap but a Java virtual
78 * machine implementation may choose not to either garbage collect
79 * or compact it. Similar to the heap, the method area may be of a
80 * fixed size or may be expanded and shrunk. The memory for the
81 * method area does not need to be contiguous.
82 *
83 * <p>In addition to the method area, a Java virtual machine
84 * implementation may require memory for internal processing or
85 * optimization which also belongs to non-heap memory.
86 * For example, the JIT compiler requires memory for storing the native
87 * machine code translated from the Java virtual machine code for
88 * high performance.
89 *
90 * <h3>Memory Pools and Memory Managers</h3>
91 * {@link MemoryPoolMXBean Memory pools} and
92 * {@link MemoryManagerMXBean memory managers} are the abstract entities
93 * that monitor and manage the memory system
94 * of the Java virtual machine.
95 *
96 * <p>A memory pool represents a memory area that the Java virtual machine
97 * manages. The Java virtual machine has at least one memory pool
98 * and it may create or remove memory pools during execution.
99 * A memory pool can belong to either the heap or the non-heap memory.
100 *
101 * <p>A memory manager is responsible for managing one or more memory pools.
102 * The garbage collector is one type of memory manager responsible
103 * for reclaiming memory occupied by unreachable objects. A Java virtual
104 * machine may have one or more memory managers. It may
105 * add or remove memory managers during execution.
106 * A memory pool can be managed by more than one memory manager.
107 *
108 * <h3>Memory Usage Monitoring</h3>
109 *
110 * Memory usage is a very important monitoring attribute for the memory system.
111 * The memory usage, for example, could indicate:
112 * <ul>
113 * <li>the memory usage of an application,</li>
114 * <li>the workload being imposed on the automatic memory management system,</li>
115 * <li>potential memory leakage.</li>
116 * </ul>
117 *
118 * <p>
119 * The memory usage can be monitored in three ways:
120 * <ul>
121 * <li>Polling</li>
122 * <li>Usage Threshold Notification</li>
123 * <li>Collection Usage Threshold Notification</li>
124 * </ul>
125 *
126 * Details are specified in the {@link MemoryPoolMXBean} interface.
127 *
128 * <p>The memory usage monitoring mechanism is intended for load-balancing
129 * or workload distribution use. For example, an application would stop
130 * receiving any new workload when its memory usage exceeds a
131 * certain threshold. It is not intended for an application to detect
132 * and recover from a low memory condition.
133 *
134 * <h3>Notifications</h3>
135 *
136 * <p>This <tt>MemoryMXBean</tt> is a
137 * {@link javax.management.NotificationEmitter NotificationEmitter}
138 * that emits two types of memory {@link javax.management.Notification
139 * notifications} if any one of the memory pools
140 * supports a <a href="MemoryPoolMXBean.html#UsageThreshold">usage threshold</a>
141 * or a <a href="MemoryPoolMXBean.html#CollectionThreshold">collection usage
142 * threshold</a> which can be determined by calling the
143 * {@link MemoryPoolMXBean#isUsageThresholdSupported} and
144 * {@link MemoryPoolMXBean#isCollectionUsageThresholdSupported} methods.
145 * <ul>
146 * <li>{@link MemoryNotificationInfo#MEMORY_THRESHOLD_EXCEEDED
147 * usage threshold exceeded notification} - for notifying that
148 * the memory usage of a memory pool is increased and has reached
149 * or exceeded its
150 * <a href="MemoryPoolMXBean.html#UsageThreshold"> usage threshold</a> value.
151 * </li>
152 * <li>{@link MemoryNotificationInfo#MEMORY_COLLECTION_THRESHOLD_EXCEEDED
153 * collection usage threshold exceeded notification} - for notifying that
154 * the memory usage of a memory pool is greater than or equal to its
155 * <a href="MemoryPoolMXBean.html#CollectionThreshold">
156 * collection usage threshold</a> after the Java virtual machine
157 * has expended effort in recycling unused objects in that
158 * memory pool.</li>
159 * </ul>
160 *
161 * <p>
162 * The notification emitted is a {@link javax.management.Notification}
163 * instance whose {@link javax.management.Notification#setUserData
164 * user data} is set to a {@link CompositeData CompositeData}
165 * that represents a {@link MemoryNotificationInfo} object
166 * containing information about the memory pool when the notification
167 * was constructed. The <tt>CompositeData</tt> contains the attributes
168 * as described in {@link MemoryNotificationInfo#from
169 * MemoryNotificationInfo}.
170 *
171 * <hr>
172 * <h3>NotificationEmitter</h3>
173 * The <tt>MemoryMXBean</tt> object returned by
174 * {@link ManagementFactory#getMemoryMXBean} implements
175 * the {@link javax.management.NotificationEmitter NotificationEmitter}
176 * interface that allows a listener to be registered within the
177 * <tt>MemoryMXBean</tt> as a notification listener.
178 *
179 * Below is an example code that registers a <tt>MyListener</tt> to handle
180 * notification emitted by the <tt>MemoryMXBean</tt>.
181 *
182 * <blockquote><pre>
183 * class MyListener implements javax.management.NotificationListener {
184 * public void handleNotification(Notification notif, Object handback) {
185 * // handle notification
186 * ....
187 * }
188 * }
189 *
190 * MemoryMXBean mbean = ManagementFactory.getMemoryMXBean();
191 * NotificationEmitter emitter = (NotificationEmitter) mbean;
192 * MyListener listener = new MyListener();
193 * emitter.addNotificationListener(listener, null, null);
194 * </pre></blockquote>
195 *
196 * @see ManagementFactory#getPlatformMXBeans(Class)
197 * @see <a href="../../../javax/management/package-summary.html">
198 * JMX Specification.</a>
199 * @see <a href="package-summary.html#examples">
200 * Ways to Access MXBeans</a>
201 *
202 * @author Mandy Chung
203 * @since 1.5
204 */
205 public interface MemoryMXBean extends PlatformManagedObject {
206 /**
207 * Returns the approximate number of objects for which
208 * finalization is pending.
209 *
210 * @return the approximate number objects for which finalization
211 * is pending.
212 */
213 public int getObjectPendingFinalizationCount();
214
215 /**
216 * Returns the current memory usage of the heap that
217 * is used for object allocation. The heap consists
218 * of one or more memory pools. The <tt>used</tt>
219 * and <tt>committed</tt> size of the returned memory
220 * usage is the sum of those values of all heap memory pools
221 * whereas the <tt>init</tt> and <tt>max</tt> size of the
222 * returned memory usage represents the setting of the heap
223 * memory which may not be the sum of those of all heap
224 * memory pools.
225 * <p>
226 * The amount of used memory in the returned memory usage
227 * is the amount of memory occupied by both live objects
228 * and garbage objects that have not been collected, if any.
229 *
230 * <p>
231 * <b>MBeanServer access</b>:<br>
232 * The mapped type of <tt>MemoryUsage</tt> is
233 * <tt>CompositeData</tt> with attributes as specified in
234 * {@link MemoryUsage#from MemoryUsage}.
235 *
236 * @return a {@link MemoryUsage} object representing
237 * the heap memory usage.
238 */
239 public MemoryUsage getHeapMemoryUsage();
240
241 /**
242 * Returns the current memory usage of non-heap memory that
243 * is used by the Java virtual machine.
244 * The non-heap memory consists of one or more memory pools.
245 * The <tt>used</tt> and <tt>committed</tt> size of the
246 * returned memory usage is the sum of those values of
247 * all non-heap memory pools whereas the <tt>init</tt>
248 * and <tt>max</tt> size of the returned memory usage
249 * represents the setting of the non-heap
250 * memory which may not be the sum of those of all non-heap
251 * memory pools.
252 *
253 * <p>
254 * <b>MBeanServer access</b>:<br>
255 * The mapped type of <tt>MemoryUsage</tt> is
256 * <tt>CompositeData</tt> with attributes as specified in
257 * {@link MemoryUsage#from MemoryUsage}.
258 *
259 * @return a {@link MemoryUsage} object representing
260 * the non-heap memory usage.
261 */
262 public MemoryUsage getNonHeapMemoryUsage();
263
264 /**
265 * Tests if verbose output for the memory system is enabled.
266 *
267 * @return <tt>true</tt> if verbose output for the memory
268 * system is enabled; <tt>false</tt> otherwise.
269 */
270 public boolean isVerbose();
271
272 /**
273 * Enables or disables verbose output for the memory
274 * system. The verbose output information and the output stream
275 * to which the verbose information is emitted are implementation
276 * dependent. Typically, a Java virtual machine implementation
277 * prints a message whenever it frees memory at garbage collection.
278 *
279 * <p>
280 * Each invocation of this method enables or disables verbose
281 * output globally.
282 *
283 * @param value <tt>true</tt> to enable verbose output;
284 * <tt>false</tt> to disable.
285 *
286 * @exception java.lang.SecurityException if a security manager
287 * exists and the caller does not have
288 * ManagementPermission("control").
289 */
290 public void setVerbose(boolean value);
291
292 /**
293 * Runs the garbage collector.
294 * The call <code>gc()</code> is effectively equivalent to the
295 * call:
296 * <blockquote><pre>
297 * System.gc()
298 * </pre></blockquote>
299 *
300 * @see java.lang.System#gc()
301 */
302 public void gc();
303
304 }