What class loading mechanisms does the JVM have?

JVM What are the class loading mechanisms ？

1、 Overall responsible for , When a classloader is responsible for loading a certain Class when , The Class Other things that are relied on and quoted Class This kind of loader will also be responsible for loading , Unless it is shown that another classloader is used to load

2、 Parent delegation , First let the parent loader try to load the class , Try to load the class from your own classpath only if the parent loader cannot load the class

3、 Caching mechanisms , The caching mechanism will ensure that all loaded Class Will be cached , When you need to use a Class when , The classloader first looks for this... From the cache Class, Only the cache does not exist , The system will read the binary data corresponding to this class , And turn it into Class object , Store in cache . That's why it's modified Class after , Must restart JVM, Changes to the program will take effect

4、 Parent delegate mechanism ,  If a class loader receives a class load request , It doesn't try to load the class itself first , Instead, the request is delegated to the parent loader to complete , One by one , therefore , All class loading requests should eventually be passed to the top-level boot class loader , Only if the parent loader does not find the required class in its search scope , That is, the load cannot be completed , The child loader will try to load the class by itself .

Parental delegation mechanism process ？

1、 When AppClassLoader Load one class when , It doesn't try to load the class itself first , Instead, delegate the class load request to the parent loader ExtClassLoader To complete .

2、 When ExtClassLoader Load one class when , It doesn't try to load the class itself first , Instead, delegate the class load request to BootStrapClassLoader To complete .

3、 If BootStrapClassLoader Loading failed ( For example, in $JAVA_HOME/jre/lib We didn't find the class), Will use ExtClassLoader To try to load ;

4、 if ExtClassLoader Also failed to load , Will use AppClassLoader To load the , If AppClassLoader Also failed to load , Then an exception will be reported ClassNotFoundException.

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