Interface MemoryMXBean
- All Superinterfaces:
PlatformManagedObject
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
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
TheMemoryMXBean
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);
- Since:
- 1.5
- See Also:
-
Method Summary
Modifier and TypeMethodDescriptionvoid
gc()
Runs the garbage collector.Returns the current memory usage of the heap that is used for object allocation.Returns the current memory usage of non-heap memory that is used by the Java virtual machine.int
Deprecated.Finalization has been deprecated for removal.boolean
Tests if verbose output for the memory system is enabled.void
setVerbose
(boolean value) Enables or disables verbose output for the memory system.Methods declared in interface java.lang.management.PlatformManagedObject
getObjectName
-
Method Details
-
getObjectPendingFinalizationCount
Deprecated.Finalization has been deprecated for removal. SeeObject.finalize()
for details.Returns the approximate number of objects for which finalization is pending.- Returns:
- the approximate number objects for which finalization is pending. If this MemoryMXBean contains information about a JVM in which finalization has been disabled or removed, this method always returns zero.
-
getHeapMemoryUsage
MemoryUsage getHeapMemoryUsage()Returns the current memory usage of the heap that is used for object allocation. The heap consists of one or more memory pools. Theused
andcommitted
size of the returned memory usage is the sum of those values of all heap memory pools whereas theinit
andmax
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 ofMemoryUsage
isCompositeData
with attributes as specified inMemoryUsage
.- Returns:
- a
MemoryUsage
object representing the heap memory usage.
-
getNonHeapMemoryUsage
MemoryUsage getNonHeapMemoryUsage()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. Theused
andcommitted
size of the returned memory usage is the sum of those values of all non-heap memory pools whereas theinit
andmax
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 ofMemoryUsage
isCompositeData
with attributes as specified inMemoryUsage
.- Returns:
- a
MemoryUsage
object representing the non-heap memory usage.
-
isVerbose
boolean isVerbose()Tests if verbose output for the memory system is enabled.- Returns:
true
if verbose output for the memory system is enabled;false
otherwise.
-
setVerbose
void setVerbose(boolean value) 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.
- Parameters:
value
-true
to enable verbose output;false
to disable.- Throws:
SecurityException
- if a security manager exists and the caller does not have ManagementPermission("control").
-
gc
void gc()Runs the garbage collector. The callgc()
is effectively equivalent to the call:System.gc()
- See Also:
-