Webapp classpaths and classes being loaded into heap

If I have two webapps, both of which have the same third party library jars in their web-inf/lib directories, say log4j.....when the first webapp is loaded and a log4j class is created, this class is loaded into the heap. When the second webapp is loaded and tries to load a log4j class, will it find the class in the heap and use that one? Or will it load its own copy of the c开发者_StackOverflow社区lass into the heap?

Hmm I think it is mostly a ClassLoader problem here, it still depend of the application server you are using, but I guess most of them use a single JVM and reserve a ClassLoader per running webapp so that you can have different webapp with different version of the same jar/clases running alltogether.

In tomcat for instance, if you need to have some shared libraries, you can use the /tomcat/shared/lib folder where you put all the jars that will be accessible by all your webapps.

Otherwise yes, different webapp won't share the same heap, it would mean webapp could access object created by other webapp running in the same application server

No, it shouldn't.

The fact classes are loaded on the heap mean nothing here. as each class loader maintains its own list of classes it loaded.

However, Classloaders are also organised into a tree and they are supposed to ask their parent classloaders to attempt to load the class first, as described in the javadoc of the ClassLoader class.

The ClassLoader class uses a delegation model to search for classes and resources. Each instance of ClassLoader has an associated parent class loader. When requested to find a class or resource, a ClassLoader instance will delegate the search for the class or resource to its parent class loader before attempting to find the class or resource itself.

However, webservers typically don't follow this delegation model, to avoid libraries used by the webserver itself being picked up by webapps. (This behaviour is sometimes configurable, but it depends on the webserver you're using.)

So in practice every webapp should have its own separate class space, independent of all the other webapps, therefore they can even use two different versions of the same library without any problems.

The other important lesson is that the same class file loaded by two different class loader will actually be two separate classes in the heap and objects from one class will not be compatible with the other class.