java.lang.management
interface MemoryMXBean

Most common way to construct:

MemoryMXBean mbean = ManagementFactory.getMemoryMXBean();

Based on 134 examples

public interface MemoryMXBean

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 {@link ManagementFactory#getMemoryMXBean} method or from the {@link ManagementFactory#getPlatformMBeanServer platform MBeanServer} method.

The ObjectName for uniquely identifying the MXBean for the memory system within an MBeanServer is:

{@link ManagementFactory#MEMORY_MXBEAN_NAME java.lang:type=Memory}

Memory

1. Heap

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 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

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

• 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

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 {@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 usage threshold or a collection usage threshold which can be determined by calling the {@link MemoryPoolMXBean#isUsageThresholdSupported} and {@link MemoryPoolMXBean#isCollectionUsageThresholdSupported} methods.

• {@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 usage threshold value.
• {@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 collection usage threshold after the Java virtual machine has expended effort in recycling unused objects in that memory pool.

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 CompositeData contains the attributes as described in {@link MemoryNotificationInfo#from MemoryNotificationInfo}.

NotificationEmitter

 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);
 

gc

public void gc()

Runs the garbage collector. The call gc() is effectively equivalent to the call:

 System.gc()
 

getHeapMemoryUsage

public 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. 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 {@link MemoryUsage#from MemoryUsage}.

Returns:

a {@link MemoryUsage} object representing the heap memory usage.

getNonHeapMemoryUsage

public 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. 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 {@link MemoryUsage#from MemoryUsage}.

Returns:

a {@link MemoryUsage} object representing the non-heap memory usage.

getObjectPendingFinalizationCount

public int getObjectPendingFinalizationCount()

Returns the approximate number of objects for which finalization is pending.

Returns:

the approximate number objects for which finalization is pending.

isVerbose

public 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

public 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.