Classloader and parental delegation mechanism

One 、 Class loader model

There is a class Car, Created 3 An example ：

Print 3 individual car class hashcode, The address is different （356573597,1735600054,
21685669）; But through this, this 3 An example , Get the original class , The address is the same （1956725890）, Explain this 3 All classes come from the same template .

Two 、 Parent delegate mechanism

Let's talk about the class loader type first , Altogether 4 Kind of ：
1. Virtual machine loader
2. Start class （ root ） loader
3. Extend the classloader
4. Applications （ System class ） loader

To build a java.lang Under the same name String Method , Print test , Tips main No way to find

This is java A mechanism to ensure security ,java Runtime , Find the class loader in the following order ：
1. Class loader received class load request
2. Delegate this class loading request upward to the parent class loader to complete , All the way up , Until you start the class loader
3. Start the class loader to check whether the current class can be loaded , If you can load it, it's over , Use current loader ; otherwise , Throw an exception , Notify the subclass loader to load
4. Repeat step 3
5. All loaders failed to load , Throw an exception ：Class Not Found

What we found this time is jre\lib\rt.jar Under the bag java.lang.String.toString() Method execution , This method is not main Methodical , So wrong reporting , In this way, the programmer can avoid misoperation and modification java Standard package for , Ensure the security of the system .