SCP02-J. Do not expose private members of the outer class from within a nested class

According to [[JLS 05]]:

A nested class is any class whose declaration occurs within the body of another class or interface.

Nested classes are a broad set of classes that are classified as static member and inner classes. "An inner class is a nested class that is not explicitly or implicitly declared static." [JLS 05 Section 8.1.3, Inner Classes and Enclosing Instances]. An inner class may be local, anonymous or non-static.

The use of nested class is prone to error unless the semantics are well understood. A common notion is that only the outer class can access the contents of the nested inner class(es). Not only does the nested class have access to the private fields of the outer class, the same fields can be accessed by another class in the package depending on whether the nested class is declared public or if it contains public methods/constructors.

Also according to [[JLS 05]]:

Note that a private field of a superclass might be accessible to a subclass (for example, if both classes are members of the same class). Nevertheless, a private field is never inherited by a subclass.

Noncompliant Code Example

The code in this noncompliant example illegally exposes the (x,y) coordinates through the getPoint() method of the inner class. The AnotherClass class can as a result illegally access the coordinates which is clearly not desired.

class Coordinates {
  private int x;
  private int y;

  public class Point {
    public void getPoint() {
      System.out.println("(" + x + "," + y + ")");    
    }
  }
}

class AnotherClass {
  public static void main(String[] args) {
    Coordinates c = new Coordinates();
    Coordinates.Point p = c.new Point();
    p.getPoint();
  }        
}

Compliant Solution

Use the private access specifier for declaring the inner class(es) and all contained methods and constructors. The compiler will refuse to compile AnotherClass because of its attempt to access a private nested class.

class Coordinates {
  private int x;
  private int y;

  private class Point {
    private void getPoint() {
      System.out.println("(" + x + "," + y + ")");    
    }
  }
}

class AnotherClass {
  public static void main(String[] args) {
    Coordinates c = new Coordinates();
    Coordinates.Point p = c.new Point();    // fails to compile
    p.getPoint();
  }        
}

Risk Assessment

The Java Language System weakens the access of private entities in inner classes which may result in a security weakness.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[JLS 05]] Section 8.1.3, Inner Classes and Enclosing Instances and 8.3 "Field Declarations"
[[McGraw 00]]
[[Long 05]] Section 2.3, Inner Classes
[[MITRE 09]] CWE ID 492 "Use of Inner Class Containing Sensitive Data"

SCP01-J. Declare sensitive methods private or final, fields private and final, and classes to be final      04. Scope (SCP)      SCP03-J. Do not reuse names