Runtime类中的内存方法是什么意思？

Question

Runtime.getRuntime().totalMemory()

I imagine this is the amount of memory I am currently using - the amount of allocated memory. More objects, more memory, yes?

Runtime.getRuntime().freeMemory()

And this would be, huh, the amount of not-allocated memory I can use?

Therefore, the TOTAL amount of memory available to my program should be totalMemory() + freeMemory() ? The naming convention doesn't help me much.

But then, what is this

Runtime.getRuntime().maxMemory()

exactly?

Answer 1

Javadoc indeed give a very clear definition. However it seems to me that OP is a bit confused on how JVM works.

First, definition from Javadoc:

freeMemory() : Returns the amount of free memory in the Java Virtual Machine.
maxMemory() : Returns the maximum amount of memory that the Java virtual machine will attempt to use.
totalMemory() : Returns the total amount of memory in the Java virtual machine.

Explanation on how JVM allocate memory (this is of course a very brief explanation but it should give you the picture):

In most, if not all, JVM implementation, there is usually a upper limit of memory that a JVM can request from OS. (eg in Sun/Oracle JVM, it is affected by -Xmx ). If you set the limit to 2GB, then maxMemory() is going to return you 2GB.

However, a JVM start, it is allowed to allocate less memory than its allowed upper limit. For example, even you have set the upper limit to 2GB, when the JVM start, it can request for only 500MB from OS. When there is more and more object created, and when it need more memory, it can then request more memory from the OS, until it hit the upper limit.

In the above case, when JVM has allocated only 500MB from OS, totalMemory() is going to return 500MB.

If within that 500MB, 400MB is already used (for objects created etc), then freeMemory() will return you 100MB.

Answer 2

I believe this is the Maximum menory allocated to your Java programm, if you start your program from Console, you can specify the max memory ( java -Xms & -Xmx ). Example would be java -jar -Xms1024MB -Xmx1024MB myjar.jar .

How it helps :D

Answer 3

Total memory is the current amount of memory managed by the virtual machine. This does not reflect how much is actually used, but how much is managed. Free memory is the amount of total memory that is currently free. The amount used would roughly be total - free. Max memory is the max that the virtual machine would ever attempt to grow itself to (ie Xmx).