How many JVM instances will create when we issue java command in different command prompts?

Question

Can anyone please tell me how many jvm instances will created when we issue the following commands in different commands.

- command 1: c:\java -Xms32m -Xmx512m -Xss254k AccountSalary
- command 2: d:\java -Xms32m -Xmx512m -Xss254k AccountSalary
- command 3: c:\java -Xms32m -Xmx512m -Xss254k AccountSalary

Whether they will use save jvm heap size or for each java process different heap size?

Answer 1

Each time you run the java command, you create a new JVM instances. (And each JVM instance will be a distinct process.)

So, in your example, the number of JVM instances will be 3.

JVM instances do not share heaps. Each one will have its own heap. There is no saving of heap space by creating multiple heaps. (In fact you probably use more heap space by creating multiple JVMs ... compared with running 3 instance of your application in the same JVM using multi-threading.)

The heap sizes are independent. They are determined by the individual JVMs' command line options ...

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With most JVMs, the only memory that is shared between JVM instances is the read-only segment containing the JVM code, and (possibly) shared native libraries.

Historically, there have been JVMs where compiled Java code can be shared between JVMs. However, it is complicated, and (AFAIK) current generation JVMs don't support this.