Detailed explanation of findloadedclass

problem

AClassLoader Of parent by BClassLoader,BClassLoader Of parent by CClassLoader, When AClassLoader call loadClass() Load class , And finally by CClassLoader Loaded classes , Who loaded it ？ follow-up AClassLoader When loading the same class again , Whether it can be directly from AClassLoader Of findLoadedClass0() Find the class in and return , Or is it another parental assignment, and finally from CClassLoader Of findLoadedClass0() Find the class in and return ？

SystemDictonary

JVM There's a data structure called SystemDictonary, This structure is mainly used to retrieve the class information we often say , In fact, it is private native final Class<?> findLoadedClass0(String name) The logic of the method

The corresponding structure of these class information is klass, Yes SystemDictonary The understanding of the , You can think of it as a Hashtable

key It's a class loader object + The name of the class ,value It's pointing klass The address of

When any of our class loaders loads classes normally , It's going to come to this SystemDictonary To find the , See if there's one like this klass Can return , If there is, go back to it , Otherwise, it will create a new one and put it into the structure

Initial classloader / Define class loader

In the above class loading problem ,AClassLoader When loading a class, it will delegate to BClassLoader To load the ,BClassLoader When loading a class, it will delegate to CClassLoader To load the , When AClassLoader call loadClass() Load class , And finally by CClassLoader load , Then we call CClassLoader For this class Define class loader ,AClassLoader and BClassLoader For this class Initial classloader

In the process ,AClassLoader、BClassLoader and CClassLoader Will be in SystemDictonary Generate records

Then follow up C The child loader of （AClassLoader and BClassLoader） When loading the same class , You can be in findLoadedClass0() Found this class in , There is no need to entrust upward