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<title>javax.management.monitor package</title>
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Copyright 1999-2006 Sun Microsystems, Inc. All Rights Reserved.
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FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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version 2 for more details (a copy is included in the LICENSE file that
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accompanied this code).
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You should have received a copy of the GNU General Public License version
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</head>
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<body bgcolor="white">
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<p>Provides the definition of the monitor classes. A Monitor is
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an MBean that periodically observes the value of an attribute in
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one or more other MBeans. If the attribute meets a certain
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condition, the Monitor emits a {@link
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javax.management.monitor.MonitorNotification
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MonitorNotification}. When the monitor MBean periodically calls
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{@link javax.management.MBeanServer#getAttribute getAttribute}
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to retrieve the value of the attribute being monitored it does
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so within the access control context of the
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{@link javax.management.monitor.Monitor#start} caller.</p>
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<p>The value being monitored can be a simple value contained within a
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complex type. For example, the {@link java.lang.management.MemoryMXBean
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MemoryMXBean} defined in <tt>java.lang.management</tt> has an attribute
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<tt>HeapMemoryUsage</tt> of type {@link java.lang.management.MemoryUsage
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MemoryUsage}. To monitor the amount of <i>used</i> memory, described by
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the <tt>used</tt> property of <tt>MemoryUsage</tt>, you could monitor
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"<tt>HeapMemoryUsage.used</tt>". That string would be the argument to
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{@link javax.management.monitor.MonitorMBean#setObservedAttribute(String)
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setObservedAttribute}.</p>
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<p>The rules used to interpret an <tt>ObservedAttribute</tt> like
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<tt>"HeapMemoryUsage.used"</tt> are as follows. Suppose the string is
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<i>A.e</i> (so <i>A</i> would be <tt>"HeapMemoryUsage"</tt> and <i>e</i>
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would be <tt>"used"</tt> in the example).</p>
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<p>First the value of the attribute <i>A</i> is obtained. Call it
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<i>v</i>. A value <i>x</i> is extracted from <i>v</i> as follows:</p>
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<ul>
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<li>If <i>v</i> is a {@link javax.management.openmbean.CompositeData
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CompositeData} and if <i>v</i>.{@link
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javax.management.openmbean.CompositeData#get(String) get}(<i>e</i>)
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returns a value then <i>x</i> is that value.</li>
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<li>If <i>v</i> is an array and <i>e</i> is the string <tt>"length"</tt>
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then <i>x</i> is the length of the array.</li>
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<li>If the above rules do not produce a value, and if {@link
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java.beans.Introspector#getBeanInfo(Class) Introspector.getBeanInfo}
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for the class of <i>v</i> (<i>v</i>.<tt>getClass()</tt>) contains a
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{@link java.beans.PropertyDescriptor PropertyDescriptor} with the name
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<i>e</i>, then <i>x</i> is the result of calling the property's {@link
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java.beans.PropertyDescriptor#getReadMethod() read method} on
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<i>v</i>.</li>
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</ul>
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<p>The third rule means for example that if the attribute
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<tt>HeapMemoryUsage</tt> is a <tt>MemoryUsage</tt>, monitoring
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<tt>"HeapMemoryUsage.used"</tt> will obtain the observed value by
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calling <tt>MemoryUsage.getUsed()</tt>.</p>
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<p>If the <tt>ObservedAttribute</tt> contains more than one period,
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for example <tt>"ConnectionPool.connectionStats.length"</tt>, then the
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above rules are applied iteratively. Here, <i>v</i> would initially be
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the value of the attribute <tt>ConnectionPool</tt>, and <i>x</i> would
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be derived by applying the above rules with <i>e</i> equal to
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<tt>"connectionStats"</tt>. Then <i>v</i> would be set to this <i>x</i>
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and a new <i>x</i> derived by applying the rules again with <i>e</i>
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equal to <tt>"length"</tt>.</p>
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<p>Although it is recommended that attribute names be valid Java
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identifiers, it is possible for an attribute to be called
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<tt>HeapMemoryUsage.used</tt>. This means that an
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<tt>ObservedAttribute</tt> that is <tt>HeapMemoryUsage.used</tt>
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could mean that the value to observe is either an attribute of that
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name, or the property <tt>used</tt> within an attribute called
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<tt>HeapMemoryUsage</tt>. So for compatibility reasons, when the
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<tt>ObservedAttribute</tt> contains a period (<tt>.</tt>), the monitor
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will check whether an attribute exists whose name is the full
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<tt>ObservedAttribute</tt> string (<tt>HeapMemoryUsage.used</tt> in the
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example). It does this by calling {@link
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javax.management.MBeanServer#getMBeanInfo(javax.management.ObjectName)
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getMBeanInfo} for the observed MBean and looking for a contained {@link
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javax.management.MBeanAttributeInfo MBeanAttributeInfo} with the given
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name. If one is found, then that is what is monitored. If more than one
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MBean is being observed, the behavior is unspecified if some of them have
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a <tt>HeapMemoryUsage.used</tt> attribute and others do not. An
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implementation may therefore call <tt>getMBeanInfo</tt> on just one of
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the MBeans in this case. The behavior is also unspecified if the result
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of the check changes while the monitor is active.</p>
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<p>The exact behavior of monitors is detailed in the
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<a href="#spec">JMX Specification</a>. What follows is a
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summary.</p>
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<p>There are three kinds of Monitors:</p>
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<ul>
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<li>
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<p>A {@link javax.management.monitor.CounterMonitor
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CounterMonitor} observes attributes of integer
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type. The attributes are assumed to be non-negative, and
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monotonically increasing except for a possible
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<em>roll-over</em> at a specified <em>modulus</em>. Each
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observed attribute has an associated <em>threshold</em>
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value. A notification is sent when the attribute exceeds
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its threshold.</p>
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<p>An <em>offset</em> value can be specified. When an
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observed value exceeds its threshold, the threshold is
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incremented by the offset, or by a multiple of the offset
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sufficient to make the threshold greater than the new
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observed value.</p>
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<p>A <code>CounterMonitor</code> can operate in
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<em>difference mode</em>. In this mode, the value
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compared against the threshold is the difference between
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two successive observations of an attribute.</p>
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</li>
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<li>
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<p>A {@link javax.management.monitor.GaugeMonitor
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GaugeMonitor} observes attributes of numerical type. Each
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observed attribute has an associated <em>high
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threshold</em> and <em>low threshold</em>.</p>
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<p>When an observed attribute crosses the high threshold, if
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the <em>notify high</em> flag is true, then a notification
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is sent. Subsequent crossings of the high threshold value
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will not trigger further notifications until the gauge value
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becomes less than or equal to the low threshold.</p>
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<p>When an observed attribute crosses the low threshold, if
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the <em>notify low</em> flag is true, then a notification
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is sent. Subsequent crossings of the low threshold value
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will not trigger further notifications until the gauge
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value becomes greater than or equal to the high
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threshold.</p>
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<p>Typically, only one of the notify high and notify low
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flags is set. The other threshold is used to provide a
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<em>hysteresis</em> mechanism to avoid the repeated
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triggering of notifications when an attribute makes small
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oscillations around the threshold value.</p>
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<p>A <code>GaugeMonitor</code> can operate in <em>difference
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mode</em>. In this mode, the value compared against the
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high and low thresholds is the difference between two
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successive observations of an attribute.</p>
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</li>
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<li>
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<p>A {@link javax.management.monitor.StringMonitor
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StringMonitor} observes attributes of type
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<code>String</code>. A notification is sent when an
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observed attribute becomes equal and/or not equal to a
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given string.</p>
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</li>
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</ul>
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<p id="spec">
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@see <a href="{@docRoot}/../technotes/guides/jmx/">
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Java SE 6 Platform documentation on JMX technology</a>,
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in particular the
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<a href="{@docRoot}/../technotes/guides/jmx/JMX_1_4_specification.pdf">
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JMX Specification, version 1.4(pdf).</a>
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@since 1.5
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</BODY>
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