Module java.management
Package java.lang.management

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:

    Describes the MemoryUsage object content
    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

    • Constructor Summary

      Constructors 
      Constructor Description
      MemoryUsage​(long init, long used, long committed, long max)
      Constructs a MemoryUsage object.

    • Constructor Detail

      • 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.

    • Method Detail

      • 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:
        The attributes and the types the given CompositeData contains
        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.