Instances of JVM

If you started Sun's java.exe from their JDK/JRE version 1.6 from the same source path twice, you would get two separate and distinct JVM instances. There would be no sharing between them unless you configured it via your applications. If you wanted two different JVM's running you would have to start a java.exe of one type (say 1.5) from one location and a java.exe (version 1.6) from another.

JVM is Java Virtual Machine, a memory space where classes (code) are loaded and objects (data) are shared. JVM is equivalent to an Operating System process.

When you type java... in you command line you are executing an independent process that loads Java classes in memory, the base classes from Java and yours (from the .class files or .jar you indicate).

Another java... command will load a different process with its own memory and will load classes by itself.

Instance word confusion: when you say 'two instances of the same JVM'. It's usual to say instance of a JVM to a separate process, it is, to a loaded independent JVM. If you are saying: two process are running JVM 1.5, OK, it's the same JVM in the sense it's the same version but they are different processes, different 'instances', independent in all sense.

Webapp confusion: A webapp (by example) is simply a bunch of classes and objects instantiated, attending some URL in a web server. You can start Tomcat with 10 different apps -it is, 10 different bunches of classes and objects each of them attending different request, but in fact they share the same memory space (OS process). A webapp cannot touch other webapp's objects because nobody gives it a reference to the other objects (and classes are in some way hidden but that's another story called: class-loading).

What is the difference in your question? I would say: two different JVM instances. :)

Each run of the the java command does invoke a new JVM instance. The running java application could run new Java Threads (like a Tomcat does with web applications).

Two separate JVMs. You can run lots of stuff inside the same JVM (say 10 webapps served up by the same Tomcat instance), but there is only 1 java command to start tomcat.