Class loaders
- Java course
- Expert
- Lesson 2/46
- 5 min
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.
Practice 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.)
Starter code
public class Main {
public static void main(String[] args) {
// Print the loader of the String class
}
}Sample solution
public class Main {
public static void main(String[] args) {
ClassLoader loader = String.class.getClassLoader();
System.out.println(loader);
}
}Expected output
null
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the Bootstrap loader call
ClassLoader loader = String.class.____();
System.out.println(loader);ClassLoader loader = String.class.getClassLoader();
System.out.println(loader);Explanation: getClassLoader() returns the loader that loaded the class.
2 What does String.class.getClassLoader() return?
- The Application loader
- The Platform loader
- null (Bootstrap loader) Correct answer
- It throws an error
Explanation: Core classes are loaded by Bootstrap, so it returns null.
3 Match the loader tiers
| Term | Meaning |
|---|---|
Bootstrap |
Core Java classes |
Platform |
Java standard library |
Application |
User classes |
getParent() |
Parent loader |