Delegate parents and other loaders

author : TaoYuanming of the Western Wei Dynasty
Blog : https://blog.springlearn.cn/

TaoYuanming of the Western Wei Dynasty
Don't laugh at the young Jianghu dream , Who doesn't dream of Jianghu

• BootStrap ClassLoader： It's called the boot class loader , yes Java The top-level classloader in the class loading hierarchy , Responsible for loading JDK Core class library in , Such as ：rt.jar、resources.jar、charsets.jar etc. , The following procedure can be used to obtain where the class loader loads the relevant jar or class file ：
• Extension ClassLoader： It's called an extended class loader , Responsible for loading Java The extended class library of , The default load JAVA_HOME/jre/lib/ext/ All of this jar.
• App ClassLoader： It's called a system class loader , Responsible for loading the application classpath All under directory jar and class file .

One 、 Class loader

Class is loaded in JVM External implementation of . For any class , Must be established by the class loader that loads it and the class itself Java Uniqueness in virtual machines .JVM Provide medium class loader .

Two 、 Start class loader （Bootstrap ClassLoader）

Responsible for loading JAVA_HOME\lib In the directory , Or through -Xbootclasspath Parameter specifies... In the path , And be Virtual machine approval （ Identify by file name , Such as rt.jar） Class .

3、 ... and 、 Extend the classloader (Extension ClassLoader)

Responsible for loading JAVA_HOME\lib\ext In the directory , Or through java.ext.dirs The system variable specifies the class in the path library .

Four 、 Application class loader (Application ClassLoader)

Responsible for loading user paths （classpath） Class library . JVM Load the class through the parent delegation model , Of course, we can also inherit java.lang.ClassLoader Implement custom class loader .

5、 ... and 、 Explanation of principle

ClassLoader The parent delegation model is used to search for the properties of classes , Every ClassLoader Instance has a reference to the parent loader （ Not an inherited relationship , It's an inclusive relationship ）, Virtual machine built-in classloader （Bootstrap ClassLoader） There is no parent loader in itself , But it can be used for other purposes ClassLoader Instance's parent loader . When one ClassLoader When an instance needs to load a class , It will try to search for a class in person before , Delegate this task to its parent loader first , This process is checked from top to bottom , First, the top-level class loader Bootstrap ClassLoader Attempt to load , If it's not loaded into , Then transfer the task to Extension ClassLoader Attempt to load , If it doesn't load into , Is passed on to App ClassLoader Loading , If it doesn't load either , Is returned to the originator of the delegate , From it to the specified file system or network, etc URL Load the class in . If none of them are loaded into this class , Throw out ClassNotFoundException abnormal . Otherwise, the found class will be generated into a class definition , And load it into memory , Finally, return the in memory Class Instance object .

6、 ... and 、 Why use a parent delegation model ？

Because this can avoid repeated loading , When the father has loaded the class , There is no need ClassLoader Load again . Considering the safety factor , Let's think about , If you don't use this delegation mode , Then we can use the customized String To dynamically replace java The core api Type defined in , There will be a very big security risk , And the way parents entrust , This can be avoided , because String Already booted class loader at boot time （Bootstrcp ClassLoader） load , So user defined ClassLoader Never load a self written String, Unless you change JDK in ClassLoader Default algorithm for search class .

7、 ... and 、 however JVM When searching for classes , How to judge two class It's the same ？

JVM In judging two class Is it the same , It's not just about judging whether two class names are the same , And to determine whether it is loaded by the same classloader instance . Only when both are satisfied ,JVM I think these two class It's the same . Even two class It's the same class Bytecode , If two different ClassLoader Instance loaded ,JVM I think they are two different class.

 /**
         * rt Catalog :
         *  load rt.jar Class loader for 
         */
        ClassLoader rtClassLoader = StringBuffer.class.getClassLoader();
        System.out.println(rtClassLoader);

        /**
         * lib/ext Expansion pack, 
         * [email protected]
         */
        ClassLoader extClassLoader = EventID.class.getClassLoader();
        System.out.println(extClassLoader);

        /**
         *  Current Application Loader 
         * [email protected]
         */
        ClassLoader classLoader = BaseSyntaxTest.class.getClassLoader();
        System.out.println(classLoader);

        /**
         * [email protected]
         */
        ClassLoader currentClassLoader = Thread.currentThread().getContextClassLoader();
        System.out.println(currentClassLoader);

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