Class MemoryUsage

public class MemoryUsage
extends Object

A MemoryUsage object represents a snapshot of memory usage. Instances of the MemoryUsage class are usually constructed by methods that are used to obtain memory usage information about individual memory pool of the Java virtual machine or the heap or non-heap memory of the Java virtual machine as a whole.

A MemoryUsage object contains four values:

Value Description
init represents the initial amount of memory (in bytes) that the Java virtual machine requests from the operating system for memory management during startup. The Java virtual machine may request additional memory from the operating system and may also release memory to the system over time. The value of init may be undefined.
used represents the amount of memory currently used (in bytes).
committed represents the amount of memory (in bytes) that is guaranteed to be available for use by the Java virtual machine. The amount of committed memory may change over time (increase or decrease). The Java virtual machine may release memory to the system and committed could be less than init. committed will always be greater than or equal to used.
max represents the maximum amount of memory (in bytes) that can be used for memory management. Its value may be undefined. The maximum amount of memory may change over time if defined. The amount of used and committed memory will always be less than or equal to max if max is defined. A memory allocation may fail if it attempts to increase the used memory such that used > committed even if used <= max would still be true (for example, when the system is low on virtual memory).
Below is a picture showing an example of a memory pool: 
+----------------------------------------------+
        +////////////////           |                  +
        +////////////////           |                  +
        +----------------------------------------------+

        |--------|
           init
        |---------------|
               used
        |---------------------------|
                  committed
        |----------------------------------------------|
                            max

MXBean Mapping

MemoryUsage is mapped to a CompositeData with attributes as specified in the from method.
Since:
1.5

Constructors

Constructor Description
MemoryUsage​(long init, long used, long committed, long max)

Constructs a MemoryUsage object.

Methods

Modifier and Type Method Description
static MemoryUsage from​(CompositeData cd)

Returns a MemoryUsage object represented by the given CompositeData.
long getCommitted()

Returns the amount of memory in bytes that is committed for the Java virtual machine to use.
long getInit()

Returns the amount of memory in bytes that the Java virtual machine initially requests from the operating system for memory management.
long getMax()

Returns the maximum amount of memory in bytes that can be used for memory management.
long getUsed()

Returns the amount of used memory in bytes.
String toString()

Returns a descriptive representation of this memory usage.

Methods declared in class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructors

MemoryUsage

public MemoryUsage(long init,
                   long used,
                   long committed,
                   long max)

Constructs a MemoryUsage object.

Parameters:
init - the initial amount of memory in bytes that the Java virtual machine allocates; or -1 if undefined.
used - the amount of used memory in bytes.
committed - the amount of committed memory in bytes.
max - the maximum amount of memory in bytes that can be used; or -1 if undefined.
Throws:
IllegalArgumentException - if
  • the value of init or max is negative but not -1; or
  • the value of used or committed is negative; or
  • used is greater than the value of committed; or
  • committed is greater than the value of max max if defined.

Methods

getInit

public long getInit()

Returns the amount of memory in bytes that the Java virtual machine initially requests from the operating system for memory management. This method returns -1 if the initial memory size is undefined.

Returns:
the initial size of memory in bytes; -1 if undefined.

getUsed

public long getUsed()

Returns the amount of used memory in bytes.

Returns:
the amount of used memory in bytes.

getCommitted

public long getCommitted()

Returns the amount of memory in bytes that is committed for the Java virtual machine to use. This amount of memory is guaranteed for the Java virtual machine to use.

Returns:
the amount of committed memory in bytes.

getMax

public long getMax()

Returns the maximum amount of memory in bytes that can be used for memory management. This method returns -1 if the maximum memory size is undefined.

This amount of memory is not guaranteed to be available for memory management if it is greater than the amount of committed memory. The Java virtual machine may fail to allocate memory even if the amount of used memory does not exceed this maximum size.

Returns:
the maximum amount of memory in bytes; -1 if undefined.

toString

public String toString()

Returns a descriptive representation of this memory usage.

Overrides:
toString in class Object
Returns:
a string representation of the object.

from

public static MemoryUsage from(CompositeData cd)

Returns a MemoryUsage object represented by the given CompositeData. The given CompositeData must contain the following attributes:

Attribute Name Type
init java.lang.Long
used java.lang.Long
committed java.lang.Long
max java.lang.Long

Parameters:
cd - CompositeData representing a MemoryUsage
Returns:
a MemoryUsage object represented by cd if cd is not null; null otherwise.
Throws:
IllegalArgumentException - if cd does not represent a MemoryUsage with the attributes described above.

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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
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https://docs.oracle.com/en/java/javase/11/docs/api/java.management/java/lang/management/MemoryUsage.html