I read that each application runs in its own JVM. Why is it so ? Why don't they make one JVM run 2 or more apps ?
I read a SO post, but could not get the answers there. Is there one JVM per Java application?
I am talking about applications launched via a public static void main(String[]) method ...)
Yes,you can install more than one jvm in your PC, because OS loads an instance of jvm (not whole jvm) in RAM. We can call different jvm like JDK 1.4 or JDK 1.6 by setting its path. Multiple JRE (Java Runtime Enviroment) is very possible. Multiple number of JVMs can run on a single machine.
There's one JVM per Java application. There shouldn't be any connection between them unless you establish one, e.g. with networking. If you're working inside of an IDE, the code you write generally runs in a separate JVM. The IDE will typically connect the separate JVM for debugging.
Each Java application uses an independent JVM. Each JVM is a separate process, and that means there is no sharing of stacks, heaps, etcetera.
Supports running applications (objects with main methods) and ScalaTest tests in multiple JVMs at the same time. Useful for integration testing where multiple systems communicate with each other.
(I assume you are talking about applications launched via a public static void main(String[])
method ...)
In theory you can run multiple applications in a JVM. In practice, they can interfere with each other in various ways. For example:
System.in/out/err
, one default encoding, one default locale, one set of system properties, and so on. If one application changes these, it affects all applications.System.exit()
will effectively kill all applications.In short, there are lots of problems. People have tried hard to make this work, but they have never really succeeded. One example is the Echidna library, though that project has been quiet for ~10 years. JNode is another example, though they (actually we) "cheated" by hacking core Java classes (like java.lang.System
) so that each application got what appeared to be independent versions of System.in/out/err
, the System properties and so on1.
1 - This ("proclets") was supposed to be an interim hack, pending a proper solution using true "isolates". But isolates support stalled, primarily because the JNode architecture used a single address space with no obvious way to separate "system" and "user" stuff. So while we could create APIs that matched the isolate APIs, key isolate functionality (like cleanly killing an isolate) was virtually impossible to implement. Or at least, that was/is my view.
For isolating execution contexts.
If one of the processes hangs, or fails, or it's security is compromised, the others don't get affected.
I think having separate runtimes also helps GC, because it has less references to handle than if it was altogether.
Besides, why would you run them all in one JVM?
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