Why do you need to specify the Java heap size?

Question

I have always wondered, why does Java require you set the heap size manually? I was under the impression that programs written in other languages would just allocate as much memory as needed as the program run until the OS could allocate no more.

In the Java world we need to set the heap, stack, and permgen size. While this is simple enough forgetting to increase these to "large enough" numbers is the #1 reason I have seen servers go down.

Why is it not possible to tell Java to grow the heap/stack/permgen as much as it needs over time?

Answer 1

My interpretation is that Sun is a company focused on selling Big Boxes and they like that the sysadmins can do things on these boxes. Running a system to fill all the memory is a good way to run it into the ground and be unable to efficiently take action to restore operation because you cannot allocate memory to create a login shell for example.

Answer 2

Three reasons:

1. Because Java was intended to be a language for writing web apps. It is generally not thought to be a good idea to let a web app take over all of the machine resources.
2. Because Java is garbage collected. As well as specifying an upper limit of memory, the heap size triggers garbage collection too. Essentially you are saying "you can use this much, but when you hit that limit you've got to tidy up".
3. There are many classes of applications where you do want to limit the amount of memory used by the Java process, e.g. where it is more important that other processes continue when there is insufficient memory for both.
4. Because being able to limit heap size is a feature. If you like the approach where the process can have unlimited memory, (as on the other languages mentioned) then you can always set the heap limit to be more than it could possibly acquire. If you want to limit heap size, you can do that. Other languages have only one of those possible behaviours, making them less flexible.

Answer 3

I think because actual memory of computer is finite. So, in a sense, JVM allows you to detect memleaks before all memory is wasted.
Plus, what to do if you run several JVMs on the same machine -- in this case how do you allow each of these JVM to grow "as much as it needs"?

Answer 4

Because growing the heap, stack, and permgen size dynamically just would make the server go down anyway. In such a world, the server would eventually leak away all of its resources.

Also, developing automatic memory management wasn't as mature as it is today. Having virtual limits makes things easier if you pick the wrong defaults, but it can make the coding a bit less efficient on the back end.

Yes, these are guesses, but reasonable ones; especially when you consider Java's / Oak's origins of embedded set top box programming. It isn't like an embedded system is going to have swap or disk backed virtual memory, so why make the JVM act as if it were available?