Nest-Based Access Control, introduced in Java 11 via JEP 181, is a mechanism that enhances how nested classes and interfaces interact within the Java programming language, specifically addressing access control for private members. Below is a concise explanation:
What is Nest-Based Access Control?
In Java, classes and interfaces can be nested within each other (e.g., inner classes). These nested types, along with their enclosing class, form a “nest.” Members of the same nest, called “nestmates,” can access each other’s private fields, methods, and constructors without requiring compiler-generated bridge methods, which were necessary before Java 11.
Important Concepts
- Nest: A top-level class or interface and all its nested types (inner classes/interfaces) form a nest. For example, a class Outer with an inner class Inner constitutes a nest where Outer is the nest host, and Inner is a nest member.
- Nestmates: The classes/interfaces within a nest (e.g., Outer and Inner) are nestmates. They have unrestricted access to each other’s private members.
- Purpose: Eliminates the need for the Java compiler to generate synthetic “bridge methods” (e.g., access$000) to broaden access to private members, simplifying bytecode and improving performance.
How It Works
- Before Java 11: The JVM’s access rules didn’t allow private member access between nestmates, so the compiler inserted package-private bridge methods to enable access. This increased application size and could cause confusion.
- After Java 11: The JVM recognizes nestmates and allows direct private member access within the nest. Two new class file attributes were introduced:
- NestHost: Specifies the nest host of a class (the top-level class).
- NestMembers: Lists the nest members in the nest host’s class file.
- Reflection APIs: Java 11 added methods to the java.lang.Class class to query nest-related information:
- getNestHost(): Returns the nest host of a class.
- isNestmateOf(Class<?> c): Checks if a class is a nestmate of another.
- getNestMembers(): Returns an array of all nest members, including the host.
Sample Implementation
import java.util.Arrays;
public class Outer {
private String name = "I'm Outer!";
class Inner {
public void printName() {
System.out.println(name); // Directly accesses Outer's private field
}
}
public static void main(String[] args) {
Outer outer = new Outer();
Inner inner = outer.new Inner();
inner.printName(); // Outputs: I'm Outer!
// Using reflection APIs
System.out.println(Outer.class.getNestHost().getName()); // com.example.Outer
System.out.println(Arrays.toString(Outer.class.getNestMembers())); // [class Outer, class Outer$Inner]
System.out.println(Outer.class.isNestmateOf(Inner.class)); // true
}
}
/*
I'm Outer!
Outer
[class Outer, class Outer$Inner]
true
*/
Code language: JavaScript (javascript)