/*
* Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package java.lang.management;
import javax.management.openmbean.CompositeData;
The management interface for the memory system of
the Java virtual machine.
A Java virtual machine has a single instance of the implementation
class of this interface. This instance implementing this interface is
an MXBean that can be obtained by calling the ManagementFactory.getMemoryMXBean method or from the
platform MBeanServer method.
The ObjectName for uniquely identifying the MXBean for the memory system within an MBeanServer is:
java.lang:type=Memory
It can be obtained by calling the PlatformManagedObject.getObjectName method. Memory
The memory system of the Java virtual machine manages
the following kinds of memory:
1. Heap
The Java virtual machine has a 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 garbage collector.
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.
2. Non-Heap Memory
The Java virtual machine manages memory other than the heap
(referred as non-heap memory).
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.
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.
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.
Memory Pools and Memory Managers
Memory pools and memory managers are the abstract entities that monitor and manage the memory system of the Java virtual machine. 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.
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.
Memory Usage Monitoring
Memory usage is a very important monitoring attribute for the memory system.
The memory usage, for example, could indicate:
- the memory usage of an application,
- the workload being imposed on the automatic memory management system,
- potential memory leakage.
The memory usage can be monitored in three ways:
- Polling
- Usage Threshold Notification
- Collection Usage Threshold Notification
Details are specified in the MemoryPoolMXBean interface. 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.
Notifications
This MemoryMXBean is a NotificationEmitter that emits two types of memory
notifications if any one of the memory pools supports a usage threshold
or a collection usage
threshold which can be determined by calling the MemoryPoolMXBean.isUsageThresholdSupported and MemoryPoolMXBean.isCollectionUsageThresholdSupported methods.
usage threshold exceeded notification - for notifying that the memory usage of a memory pool is increased and has reached or exceeded its usage threshold value.
collection usage threshold exceeded notification - for notifying that the memory usage of a memory pool is greater than or equal to its
collection usage threshold after the Java virtual machine
has expended effort in recycling unused objects in that
memory pool.
The notification emitted is a Notification instance whose
user data is set to a CompositeData that represents a MemoryNotificationInfo object containing information about the memory pool when the notification was constructed. The CompositeData contains the attributes as described in
MemoryNotificationInfo.
NotificationEmitter
The MemoryMXBean object returned by ManagementFactory.getMemoryMXBean implements the NotificationEmitter interface that allows a listener to be registered within the MemoryMXBean as a notification listener. Below is an example code that registers a MyListener to handle notification emitted by the MemoryMXBean. 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);
Author: Mandy Chung See Also: Since: 1.5
/**
* 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</a>
* that can be obtained by calling
* the {@link ManagementFactory#getMemoryMXBean} method or
* from the {@link ManagementFactory#getPlatformMBeanServer
* platform MBeanServer} method.
*
* <p>The {@code ObjectName} for uniquely identifying the MXBean for
* the memory system within an MBeanServer is:
* <blockquote>
* {@link ManagementFactory#MEMORY_MXBEAN_NAME
* java.lang:type=Memory}
* </blockquote>
*
* It can be obtained by calling the
* {@link PlatformManagedObject#getObjectName} method.
*
* <h3> Memory </h3>
* The memory system of the Java virtual machine manages
* the following kinds of memory:
*
* <h3> 1. Heap </h3>
* The Java virtual machine has a <i>heap</i> 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</i>.
*
* <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</h3>
* The Java virtual machine manages memory other than the heap
* (referred as <i>non-heap memory</i>).
*
* <p> The Java virtual machine has a <i>method area</i> 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</h3>
* {@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</h3>
*
* 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>
* <li>the workload being imposed on the automatic memory management system,</li>
* <li>potential memory leakage.</li>
* </ul>
*
* <p>
* The memory usage can be monitored in three ways:
* <ul>
* <li>Polling</li>
* <li>Usage Threshold Notification</li>
* <li>Collection Usage Threshold Notification</li>
* </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</h3>
*
* <p>This {@code 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</a>
* 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</a> 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 {@code CompositeData} contains the attributes
* as described in {@link MemoryNotificationInfo#from
* MemoryNotificationInfo}.
*
* <hr>
* <h3>NotificationEmitter</h3>
* The {@code MemoryMXBean} object returned by
* {@link ManagementFactory#getMemoryMXBean} implements
* the {@link javax.management.NotificationEmitter NotificationEmitter}
* interface that allows a listener to be registered within the
* {@code MemoryMXBean} as a notification listener.
*
* Below is an example code that registers a {@code MyListener} to handle
* notification emitted by the {@code MemoryMXBean}.
*
* <blockquote><pre>
* 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></blockquote>
*
* @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.
Returns: the approximate number objects for which finalization
is pending.
/**
* 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 used and committed size of the returned memory usage is the sum of those values of all heap memory pools whereas the 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.
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.
MBeanServer access:
The mapped type of MemoryUsage is CompositeData with attributes as specified in MemoryUsage.
Returns: a MemoryUsage object representing the heap memory usage.
/**
* Returns the current memory usage of the heap that
* is used for object allocation. The heap consists
* of one or more memory pools. The {@code used}
* and {@code committed} size of the returned memory
* usage is the sum of those values of all heap memory pools
* whereas the {@code init} and {@code 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</b>:<br>
* The mapped type of {@code MemoryUsage} is
* {@code 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 used and committed size of the returned memory usage is the sum of those values of all non-heap memory pools whereas the init and 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.
MBeanServer access:
The mapped type of MemoryUsage is CompositeData with attributes as specified in MemoryUsage.
Returns: a MemoryUsage object representing the non-heap memory usage.
/**
* 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 {@code used} and {@code committed} size of the
* returned memory usage is the sum of those values of
* all non-heap memory pools whereas the {@code init}
* and {@code 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</b>:<br>
* The mapped type of {@code MemoryUsage} is
* {@code 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.
Returns: true if verbose output for the memory system is enabled; false otherwise.
/**
* Tests if verbose output for the memory system is enabled.
*
* @return {@code true} if verbose output for the memory
* system is enabled; {@code 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.
Each invocation of this method enables or disables verbose
output globally.
Params: - value –
true to enable verbose output; false to disable.
Throws: - SecurityException – if a security manager
exists and the caller does not have
ManagementPermission("control").
/**
* 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 {@code true} to enable verbose output;
* {@code 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 gc() is effectively equivalent to the
call:
System.gc()
See Also: - gc.gc()
/**
* Runs the garbage collector.
* The call <code>gc()</code> is effectively equivalent to the
* call:
* <blockquote><pre>
* System.gc()
* </pre></blockquote>
*
* @see java.lang.System#gc()
*/
public void gc();
}