The MemoryMXBean.java Java example source code
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package java.lang.management;
import javax.management.openmbean.CompositeData;
/**
* The management interface for the memory system of
* the Java virtual machine.
*
* <p> A Java virtual machine has a single instance of the implementation
* class of this interface. This instance implementing this interface is
* an <a href="ManagementFactory.html#MXBean">MXBean
* that can be obtained by calling
* the {@link ManagementFactory#getMemoryMXBean} method or
* from the {@link ManagementFactory#getPlatformMBeanServer
* platform <tt>MBeanServer} method.
*
* <p>The ObjectName for uniquely identifying the MXBean for
* the memory system within an MBeanServer is:
* <blockquote>
* {@link ManagementFactory#MEMORY_MXBEAN_NAME
* <tt>java.lang:type=Memory}
* </blockquote>
*
* It can be obtained by calling the
* {@link PlatformManagedObject#getObjectName} method.
*
* <h3> Memory
* The memory system of the Java virtual machine manages
* the following kinds of memory:
*
* <h3> 1. Heap
* The Java virtual machine has a <i>heap that is the runtime
* data area from which memory for all class instances and arrays
* are allocated. It is created at the Java virtual machine start-up.
* Heap memory for objects is reclaimed by an automatic memory management
* system which is known as a <i>garbage collector.
*
* <p>The heap may be of a fixed size or may be expanded and shrunk.
* The memory for the heap does not need to be contiguous.
*
* <h3> 2. Non-Heap Memory
* The Java virtual machine manages memory other than the heap
* (referred as <i>non-heap memory).
*
* <p> The Java virtual machine has a method area that is shared
* among all threads.
* The method area belongs to non-heap memory. It stores per-class structures
* such as a runtime constant pool, field and method data, and the code for
* methods and constructors. It is created at the Java virtual machine
* start-up.
*
* <p> The method area is logically part of the heap but a Java virtual
* machine implementation may choose not to either garbage collect
* or compact it. Similar to the heap, the method area may be of a
* fixed size or may be expanded and shrunk. The memory for the
* method area does not need to be contiguous.
*
* <p>In addition to the method area, a Java virtual machine
* implementation may require memory for internal processing or
* optimization which also belongs to non-heap memory.
* For example, the JIT compiler requires memory for storing the native
* machine code translated from the Java virtual machine code for
* high performance.
*
* <h3>Memory Pools and Memory Managers
* {@link MemoryPoolMXBean Memory pools} and
* {@link MemoryManagerMXBean memory managers} are the abstract entities
* that monitor and manage the memory system
* of the Java virtual machine.
*
* <p>A memory pool represents a memory area that the Java virtual machine
* manages. The Java virtual machine has at least one memory pool
* and it may create or remove memory pools during execution.
* A memory pool can belong to either the heap or the non-heap memory.
*
* <p>A memory manager is responsible for managing one or more memory pools.
* The garbage collector is one type of memory manager responsible
* for reclaiming memory occupied by unreachable objects. A Java virtual
* machine may have one or more memory managers. It may
* add or remove memory managers during execution.
* A memory pool can be managed by more than one memory manager.
*
* <h3>Memory Usage Monitoring
*
* Memory usage is a very important monitoring attribute for the memory system.
* The memory usage, for example, could indicate:
* <ul>
* <li>the memory usage of an application,
* <li>the workload being imposed on the automatic memory management system,
* <li>potential memory leakage.
* </ul>
*
* <p>
* The memory usage can be monitored in three ways:
* <ul>
* <li>Polling
* <li>Usage Threshold Notification
* <li>Collection Usage Threshold Notification
* </ul>
*
* Details are specified in the {@link MemoryPoolMXBean} interface.
*
* <p>The memory usage monitoring mechanism is intended for load-balancing
* or workload distribution use. For example, an application would stop
* receiving any new workload when its memory usage exceeds a
* certain threshold. It is not intended for an application to detect
* and recover from a low memory condition.
*
* <h3>Notifications
*
* <p>This MemoryMXBean is a
* {@link javax.management.NotificationEmitter NotificationEmitter}
* that emits two types of memory {@link javax.management.Notification
* notifications} if any one of the memory pools
* supports a <a href="MemoryPoolMXBean.html#UsageThreshold">usage threshold
* or a <a href="MemoryPoolMXBean.html#CollectionThreshold">collection usage
* threshold</a> which can be determined by calling the
* {@link MemoryPoolMXBean#isUsageThresholdSupported} and
* {@link MemoryPoolMXBean#isCollectionUsageThresholdSupported} methods.
* <ul>
* <li>{@link MemoryNotificationInfo#MEMORY_THRESHOLD_EXCEEDED
* usage threshold exceeded notification} - for notifying that
* the memory usage of a memory pool is increased and has reached
* or exceeded its
* <a href="MemoryPoolMXBean.html#UsageThreshold"> usage threshold value.
* </li>
* <li>{@link MemoryNotificationInfo#MEMORY_COLLECTION_THRESHOLD_EXCEEDED
* collection usage threshold exceeded notification} - for notifying that
* the memory usage of a memory pool is greater than or equal to its
* <a href="MemoryPoolMXBean.html#CollectionThreshold">
* collection usage threshold</a> after the Java virtual machine
* has expended effort in recycling unused objects in that
* memory pool.</li>
* </ul>
*
* <p>
* The notification emitted is a {@link javax.management.Notification}
* instance whose {@link javax.management.Notification#setUserData
* user data} is set to a {@link CompositeData CompositeData}
* that represents a {@link MemoryNotificationInfo} object
* containing information about the memory pool when the notification
* was constructed. The <tt>CompositeData contains the attributes
* as described in {@link MemoryNotificationInfo#from
* MemoryNotificationInfo}.
*
* <hr>
* <h3>NotificationEmitter
* The <tt>MemoryMXBean object returned by
* {@link ManagementFactory#getMemoryMXBean} implements
* the {@link javax.management.NotificationEmitter NotificationEmitter}
* interface that allows a listener to be registered within the
* <tt>MemoryMXBean as a notification listener.
*
* Below is an example code that registers a <tt>MyListener to handle
* notification emitted by the <tt>MemoryMXBean.
*
* <blockquote> * class MyListener implements javax.management.NotificationListener {
* public void handleNotification(Notification notif, Object handback) {
* // handle notification
* ....
* }
* }
*
* MemoryMXBean mbean = ManagementFactory.getMemoryMXBean();
* NotificationEmitter emitter = (NotificationEmitter) mbean;
* MyListener listener = new MyListener();
* emitter.addNotificationListener(listener, null, null);
* </pre>
*
* @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 MemoryMXBean extends PlatformManagedObject {
/**
* Returns the approximate number of objects for which
* finalization is pending.
*
* @return the approximate number objects for which finalization
* is pending.
*/
public int getObjectPendingFinalizationCount();
/**
* Returns the current memory usage of the heap that
* is used for object allocation. The heap consists
* of one or more memory pools. The <tt>used
* and <tt>committed size of the returned memory
* usage is the sum of those values of all heap memory pools
* whereas the <tt>init and max size of the
* returned memory usage represents the setting of the heap
* memory which may not be the sum of those of all heap
* memory pools.
* <p>
* The amount of used memory in the returned memory usage
* is the amount of memory occupied by both live objects
* and garbage objects that have not been collected, if any.
*
* <p>
* <b>MBeanServer access:
* The mapped type of <tt>MemoryUsage is
* <tt>CompositeData with attributes as specified in
* {@link MemoryUsage#from MemoryUsage}.
*
* @return a {@link MemoryUsage} object representing
* the heap memory usage.
*/
public MemoryUsage getHeapMemoryUsage();
/**
* Returns the current memory usage of non-heap memory that
* is used by the Java virtual machine.
* The non-heap memory consists of one or more memory pools.
* The <tt>used and committed size of the
* returned memory usage is the sum of those values of
* all non-heap memory pools whereas the <tt>init
* and <tt>max size of the returned memory usage
* represents the setting of the non-heap
* memory which may not be the sum of those of all non-heap
* memory pools.
*
* <p>
* <b>MBeanServer access:
* The mapped type of <tt>MemoryUsage is
* <tt>CompositeData with attributes as specified in
* {@link MemoryUsage#from MemoryUsage}.
*
* @return a {@link MemoryUsage} object representing
* the non-heap memory usage.
*/
public MemoryUsage getNonHeapMemoryUsage();
/**
* Tests if verbose output for the memory system is enabled.
*
* @return <tt>true if verbose output for the memory
* system is enabled; <tt>false otherwise.
*/
public boolean isVerbose();
/**
* Enables or disables verbose output for the memory
* system. The verbose output information and the output stream
* to which the verbose information is emitted are implementation
* dependent. Typically, a Java virtual machine implementation
* prints a message whenever it frees memory at garbage collection.
*
* <p>
* Each invocation of this method enables or disables verbose
* output globally.
*
* @param value <tt>true to enable verbose output;
* <tt>false to disable.
*
* @exception java.lang.SecurityException if a security manager
* exists and the caller does not have
* ManagementPermission("control").
*/
public void setVerbose(boolean value);
/**
* Runs the garbage collector.
* The call <code>gc() is effectively equivalent to the
* call:
* <blockquote> * System.gc()
* </pre>
*
* @see java.lang.System#gc()
*/
public void gc();
}
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