java - How Tomcat Classloader separates different Webapps object scope in same JVM?

Question

Since Tomcat can load more than one webapp at once, and those webapps can work separately, and do not disturb each other, and they work in same JVM. So I am very confused about how tomcat handle The Object scope in the same JVM.

For example I have a singleton object in both of the two different Webapps, and tomcat will generate two different singleton Object for each. I always thought that the singleton object have only one object in the same JVM, but in tomcat JVM there maybe has two or more.

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I have read some info about the ClassLoader, Tomcat has its own WebAppClassLoader to load webapps. So does it mean the Object Scope here is the ClassLoader or am I wrong. Does anyone know about this or can give me some info about tomcat work memory layout?

Answer 1

All the secrets is behind those ClassLoader instances.

The state of the class (like all static variables, byte code and so on) is scoped by the class loader which loads that class (a class is identified in JVM by its fully qualiflied name and the class loader loading the class. This is not exactly a scope, but thinking as scope usually helps understanding this better).

So if a class is loaded by two different class loaders, this class exists twice within the VM, it has two sets of static fields, can have different byte code (like different method implementations) and all such. Note that these two objects cannot be cast to each other even their names are identical. "Normal" Java applications have all classes loaded by a class loader hierarchy and every class is only loaded once.

For more complex scenarios, you will need different behaviours. Sometimes you want to isolate a library from messing with your code (like plugins in eclipse or web applications in an application server).

The basic idea to isolate your program from other classes is to load those with an extra class loader and use a lot of reflection. If you want to read up on this have a look at Oracle's documentation on ClassLoaders or OSGI.

Tomcat (and a lot of other web containers / application servers) load the application with separate ClassLoader hierarchies. This isolates all classes against other (web) applications and thus also makes sure, that singletons, different class versions and all this stuff does not collide.