PlatformManagedObject
public interface MemoryMXBean extends 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.
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.
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
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.
The memory usage can be monitored in three ways:
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.
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
.
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);
ManagementFactory.getPlatformMXBeans(Class)
,
JMX Specification.,
Ways to Access MXBeansModifier and Type | Method | Description |
---|---|---|
void |
gc() |
Runs the garbage collector.
|
MemoryUsage |
getHeapMemoryUsage() |
Returns the current memory usage of the heap that
is used for object allocation.
|
MemoryUsage |
getNonHeapMemoryUsage() |
Returns the current memory usage of non-heap memory that
is used by the Java virtual machine.
|
int |
getObjectPendingFinalizationCount() |
Returns the approximate number of objects for which
finalization is pending.
|
boolean |
isVerbose() |
Tests if verbose output for the memory system is enabled.
|
void |
setVerbose(boolean value) |
Enables or disables verbose output for the memory
system.
|
getObjectName
int getObjectPendingFinalizationCount()
MemoryUsage getHeapMemoryUsage()
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
.
MemoryUsage
object representing
the heap memory usage.MemoryUsage getNonHeapMemoryUsage()
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
.
MemoryUsage
object representing
the non-heap memory usage.boolean isVerbose()
true
if verbose output for the memory
system is enabled; false
otherwise.void setVerbose(boolean value)
Each invocation of this method enables or disables verbose output globally.
value
- true
to enable verbose output;
false
to disable.SecurityException
- if a security manager
exists and the caller does not have
ManagementPermission("control").void gc()
gc()
is effectively equivalent to the
call:
System.gc()
System.gc()
Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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DRAFT 9-internal+0-adhoc.mlchung.jdk9-jdeps