为什么在Java EE Application Server中使用单个JVM实例？

Question

I want to ask that if we run a standalone java application using the java command, for each application a new instance of a JVM is created with a separate heap etc.

1. But why in a Java EE application server is there only one JVM instance for each ear or war file deployed on the application server instance?
2. Why we are using separate class loader for each application to distinguish between application instead separate JVM instance.?
3. Does Single JVM instance is required in App server to manage life cycle of servlet,jsp and ejb etc?

Answer 1

Suppose you have start the application server like oracle or jboss it takes one instance of jvm bacause you have configure the jvm tuning properties into the bin folder *.bat or *.sh file with min and max size of heap and all the jvm tuning properties only on the application server. while you deploy the multiple war file it is deployed on above the application server it takes java properties from the server.

But while you used the standalone application than in this case every application has configure in its own jvm properties.

Answer 2

Concerning the class loader - it isolates (in theory) the server from the applications (EAR/WAR), and each application from each other. For example, suppose the server uses Spring 3.x, and app 1 uses Spring 4.x. The isolation allows the server to only see 3.x while app 1 can only see 4.x

But what happens if app 1 doesn't include the Spring 4.x jars in its EAR/WAR? The parent class loader (the server) attempts to load Spring which will of course be 3.x. Then any dependencies are also loaded from the server context. This might actually appear to functionally work (scary), but more likely you end up with MethodNotFound and ClassNotFound exceptions.