Class loaders

Learn Bootstrap, Platform and Application class loaders.

Class loaders

The class loader finds a class's bytecode from a file and loads it into memory. The JVM uses a three-tier loading model: Bootstrap, Platform and Application loaders.

The Bootstrap loader loads core Java classes (for example java.lang.String) and is written in C++; the getClassLoader() call returns null. The Platform loader loads Java standard libraries; the Application loader loads user classes.

This layered design provides security and modularity; each loader handles its own responsibility.

The getParent chain

Every class loader has a parent loader. The getParent() call returns the parent loader. Application's parent is Platform, Platform's parent is Bootstrap.

ClassLoader app = Main.class.getClassLoader();
ClassLoader plat = app.getParent();
System.out.println(plat);

This chain ensures that a class is first looked up in the parent loader (parent-delegation model). Core classes are thus always loaded from a trusted source.

Exercise Print the String class loader

Define a Main class. In main, assign the result of String.class.getClassLoader() to a ClassLoader variable. Print it with System.out.println. (Bootstrap classes return null.)

Show example solution

public class Main {
    public static void main(String[] args) {
        ClassLoader loader = String.class.getClassLoader();
        System.out.println(loader);
    }
}

Test yourself

Complete the Bootstrap loader call

ClassLoader loader = String.class.____();
System.out.println(loader);

Show answer

ClassLoader loader = String.class.getClassLoader();
System.out.println(loader);

What does String.class.getClassLoader() return?
- The Application loader
- The Platform loader
- null (Bootstrap loader)
- It throws an error
Show answer

null (Bootstrap loader) — Core classes are loaded by Bootstrap, so it returns null.
Match the loader tiers

Bootstrap

Core Java classes
Platform

Java standard library
Application

User classes
getParent()

Parent loader

Next lesson: Bytecode

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