OBJ09-J. Do not expose sensitive private members of an outer class from within a nested class

A nested class is any class whose declaration occurs within the body of another class or interface [[JLS 2005]]. The use of a nested class is error-prone unless the semantics are well understood. A common notion is that only the outer class can access the contents of the nested class. 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 within the package depending on whether the nested class is declared public or if it contains public methods or constructors. As a result the nested class must not expose the private members of the outer class to external classes or packages.

Also, according to the Java Language Specification [[JLS 2005]], §8.3 "Field Declarations,"

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

This noncompliant code example exposes the sensitive (x,y) coordinates through the getPoint() method of the inner class. Consequently, the AnotherClass class that belongs to the same package can access the coordinates.

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 hiding the inner class and all contained methods and constructors.

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();
  }
}

Compilation of AnotherClass now results in a compilation error because the class attempts to access a private nested class.

Risk Assessment

The Java language system weakens the accessibility of sensitive, private entities in inner classes, which can result in a security weakness.

Automated Detection

Automated detection of non-private nested classes that define non-private members and constructors is straightforward. However, this rule applies only when those classes could potentially expose sensitive data or operations from the outer class. Detection of sensitive data or operations requires programmer assistance.

Related Guidelines

Bibliography

OBJ08-J. Do not leak references to inner class objects when the outer class object maintains sensitive data      04. Object Orientation (OBJ)      OBJ10-J. Do not use public static non-final variables