Parental delegation mechanism

Every time ⼀ Both classes have ⼀ A corresponding ClassLoader Class loader , The classloader is in ⼯ It will default to ⽤ Parent delegate mechanism ;
ClassLoader The specific role of will be class The bytecode file is loaded into jvm Go to the virtual machine , The program will run correctly ;
however ,jvm Starting does not load all at once class file , It's loading dynamically as needed ;

The so-called parental delegation mechanism , It's called ClassLoader Of loadClass( ) The rules of ：

The specific process ：

1. The first step is the application class loader , call findLoadedClass( ) Check whether this class loader has loaded this class ,
2. If not , Find out if there is parent The superior , If there is a superior, the superior's loadclass( ) Continue to find , It's like recursion
3. until parent The superior is null（ Start class loader C++）, Delegate the start class loader to call findBootstrapClassOrNull( ) stay jre/lib In the catalog jar Start looking for classes in the package
4. If the boot class loader is not found , Will report a mistake ClassNotFoundException, Expand the class loader catch Exceptions are caught , Just before the extension class loader calls findClass()（ Rewritten per loader ） Look up the directory jre/lib/ext;
   If not , Will report a mistake ClassNotFoundException, Application class loader's catch Exceptions are caught , Finally, call the application class loader's own findClass( ) To classpath Find the class under the directory

Why check from bottom to top ？

To avoid repeated loading ;

In some cases, the parental delegation mechanism will be broken ：

JDBC Of DriverManager By default, it is loaded by the boot class loader , But there is no... In the boot class loader Mysql Drive class , So the call to be displayed Class Of forName Method uses an application class loader to load Mysql Driver class