When to use Runtime.maxMemory() and totalMemory()

Question

What is the exact difference between Runtime.maxMemory() and Runtime.totalMemory()? The javadoc is quite vague about this (for me).

What are typical use cases for these two methods, that is, When would it be inappropriate to use the respective other one?

score 7 · Accepted Answer · edited Jun 01 '17 at 17:57

7

The totalMemory() returns how much memory is currently used, while the maxMemory() tells how much the JVM can allocate in total.

Note: that from this follows: totalMemory() <= maxMemory(), and you can also get 'how much memory is left' by maxMemory() - totalMemory()

One use case for this is to diagnose how much memory your program uses, and you'll use totalMemory() for it.
Note: both are referring only to heap memory, and not stack memory.

edited Jun 01 '17 at 17:57

Matthias Braun

32,039
22
142
171

answered Sep 29 '11 at 14:03

amit

175,853
27
231
333

1

"The totalMemory() returns how much memory is currently used" USED?! you mean Allocated? – Yousha Aleayoub Sep 18 '16 at 21:55

score 5 · Answer 2 · answered Sep 29 '11 at 14:06

5

The total memory is the memory that is currently allocated to the JVM. It varies over time. The max memory is the maximum memory that the JVM could ever reach. It's the upper limit of the total memory.

answered Sep 29 '11 at 14:06

JB Nizet

678,734
91
1,224
1,255

score 3 · Answer 3 · answered Oct 01 '11 at 07:40

3

MaxMemory() is the value set by Xmx parameter

answered Oct 01 '11 at 07:40

Nike

312
1
2
9

how to get the Xms parameter after the program started running ? – Dungeon Hunter Jul 03 '12 at 07:50

score 0 · Answer 4 · answered Aug 12 '14 at 19:55

totalMemory() which represent current heap size of JVM which is combination of used memory currently occupied by objects and free memory available for new objects.As per javadoc value returned by totalMemory() may vary over time depending upon environment. JVM totalMemory also equals to initial heap size of JVM

maximum heap space is not going to change over JVM life cycle.Jvm always trying to expand the size of totalMemory() according to no of new object created But not beyond maxMemory() size unles we will get java.lang.OutOfMemoryError.

"may vary over time" more or less says that "equals initial heap size" is unlikely to be reliable in the general case. — Gus, Nov 03 '16 at 02:47

When to use Runtime.maxMemory() and totalMemory()

4 Answers4