About Java class loader types

Summarize Java class loader.

boostrap class loader

• bootstrap class loader loads bootstrap class --bootstrap class is a class that exists in jre / lib / rt.jar orjre / lib /under .jar.

• On Mac /Library/Java/JavaVirtualMachines/jdk1.x.x_xxx.jdk/Contents/Home/jre/lib/rt.jar etc. --A class loader that comes with the JVM and is used to load classes when the JVM starts. --The return value of getClassLoader () for the class loaded by bootstrap class loader will be null

Example.java

public class Example
{
    public static void main(String[] args)
    {
        System.out.println(String.class.getClassLoader());
    }
}

$ javac Example.java && java Example
null

extension class loader

• extension class loader loads .jar under jre / lib / ext --The return value of getClassLoader () for the class loaded by extension class loader will be sun.misc.Launcher $ ExtClassLoader. --The return value of getClassLoader (). getParent () will be null, that is, there is no parent

Example.java

import sun.net.spi.nameservice.dns.DNSNameService;

public class Example
{
    public static void main(String[] args)
    {
        ClassLoader extLoader = DNSNameService.class.getClassLoader();
        System.out.println(extLoader);
        System.out.println(extLoader.getParent());
    }
}

$ javac Example.java && java Example
sun.misc.Launcher$ExtClassLoader@75b84c92
null

user class loader

• user class loader loads classes in the CLASSPATH location --Also called system class loader? --The return value of getClassLoader () for the class loaded by user class loader will be sun.misc.Launcher $ AppClassLoader --getClassLoader (). getParent () reveals that sun.misc.Launcher $ ExtClassLoader is the parent

example/A.java

package example;

public class A
{
}

Example.java

import example.A;

public class Example
{
    public static void main(String[] args)
    {
        ClassLoader appLoader = Example.class.getClassLoader();
        System.out.println(appLoader);
        System.out.println(appLoader.getParent());
        System.out.println(appLoader.getParent().getParent());
        System.out.println();

        appLoader = A.class.getClassLoader();
        System.out.println(appLoader);
        System.out.println(appLoader.getParent());
        System.out.println(appLoader.getParent().getParent());
    }
}

$ javac example/A.java && jar cf example/A.jar example/A.class
$ javac Example.java && java -classpath example/A.jar:./ Example
sun.misc.Launcher$AppClassLoader@2a139a55
sun.misc.Launcher$ExtClassLoader@75b84c92
null

sun.misc.Launcher$AppClassLoader@2a139a55
sun.misc.Launcher$ExtClassLoader@75b84c92
null

Reference URL

• How Classes are Found
• Chapter 5. Loading, Linking, and Initializing
• Java Class Unloading
• Class loader memo 1-wayukawa ’s blog
• Let's remember the terms correctly: System class loader and bootstrap class loader --diary of masugata_k