About the parental delegation mechanism and the process of class loading

We wrote java File to final run , He must go through two stages of compilation and class loading , The process of compiling is to put
.java File compiled into .class file , The stage of class loading is to put .class File loading to JVM In memory , Loading finished
You'll get one Class object , We can use new Keyword to instantiate this object , Here's the picture ：

  The loading process of classes needs to involve
To class loader ,JVM When running, three class loaders will be generated , These three class loaders form a hierarchical relationship , Each loader loads different ranges of jar package , for instance Bootstrap Classloader（ Start class loader ）
It is mainly responsible for loading java The loading of the core class library is lib Under the table of contents rt.jar and resources.jar etc. ,Extension ClassLoader（ Extend the classloader ） Main load lib\ext A... In the catalog jar Bao He class file ,Application ClassLoader（ Apply the class loader ）, Mainly responsible for the current application classpath All of the following jar Bao He class file , Besides the class loader provided by the processing system itself, you can also use ClassLoader Class to implement the custom loader , To meet the needs of some scenes .

  The so-called parental delegation model , According to the hierarchical relationship of class loader , Delegate layer by layer , For example, when we need to load a class When you file , First of all, I will take this class The query and loading of files are delegated to the parent loader to execute , If the parent loader fails to load , Try to load this by yourself class

I think such benefits ：

1. Security ： Because this hierarchical relationship actually represents a kind of priority , That is, all class loading priority should be given to the startup class loader , Then there is no way to destroy some classes in the core class library , For example, write a java.lang.String Finally, it is up to the boot class loader , His writing java.lang.String There is no way to cover the classes in the class library .
2. I think this kind of hierarchical design relationship , It can avoid some problems of program confusion caused by repeated loading , If the parent loader has already been loaded , Then there is no need to load the sub loader .

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The running result shows ：