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
a managed bean (MBean) conforming to the JMX Instrumentation
Specification. The MBean can be obtained by calling
the ManagementFactory.getMemoryMBean()
method.
An application can monitor the instrumentation of the
memory system and manage certain characteristics in
any of the following ways:
MBeanServer
.MBean proxy
and call the methods of the MBean proxy.The ManagementFactory.getPlatformMBeanServer()
method returns
the platform MBeanServer registered with all platform MBeans.
The ObjectName for uniquely identifying the MBean for the memory system within an MBeanServer is:
java.lang:type=Memory
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:
MemoryPoolMBean
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 MemoryMBean will emit two types of
MemoryNotification
if any one of the memory pools
supports a usage threshold
or a collection usage
threshold which can be determined by calling the
MemoryPoolMBean.isUsageThresholdSupported()
and
MemoryPoolMBean.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 MemoryMBean object returned by
the ManagementFactory.getMemoryMBean()
method implements
the NotificationEmitter
interface that allows a listener to be registered within the
MemoryMBean as a notification listener.
Below is an example code that registers a MyListener to handle notification emitted by the MemoryMBean.
class MyListener implements javax.management.NotificationListener { public void handleNotification(Notification notif, Object handback) { // handle notification .... } } MemoryMBean mbean = ManagementFactory.getMemoryMBean(); NotificationEmitter emitter = (NotificationEmitter) mbean; MyListener listener = new MyListener(); emitter.addNotificationListener(listener, null, null);
Method Summary | |
---|---|
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. |
Method Detail |
---|
int getObjectPendingFinalizationCount()
MemoryUsage getHeapMemoryUsage()
The amount of used memory in the returned MemoryUsage is the amount of memory occupied by both live objects and garbage objects that have not been collected, if any.
MemoryUsage getNonHeapMemoryUsage()
boolean isVerbose()
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()