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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." \[[JLS 05 Section 8.1.3, Inner Classes and Enclosing Instances|http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.1.3]\]. An inner class may be local, anonymous or non-static.

Nested class usage is prone to error A nested class is any class whose declaration occurs within the body of another class or interface [JLS 2015]. The use of a nested class is error prone unless the semantics are well understood. A common notion is that only the outer nested class can may access the contents of the nested inner outer class(es). Not only does the nested class have access to the private fields of the outer class, but the same fields can be accessed by another any other class in within the package depending on whether when the nested class is declared public or if it contains public methods /constructorsor constructors. As a result, the nested class must not expose the private members of the outer class to external classes or packages.

According to The Java Language Specification (JLS), §8.3, "Field Declarations" [JLS 2015]:

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 This noncompliant code in this noncompliant example illegally exposes the private (x,y) coordinates through the getPoint() method of the inner class. The Consequently, the AnotherClass class can as a result illegally that belongs to the same package can also 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 to hide 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();    // failsFails 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 language system weakens the access accessibility of private entities in inner classes which may result in a security weaknessmembers of an outer class when a nested inner class is present, which can result in an information leak.

Automated Detection

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TODO

Related Vulnerabilities

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

References

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Automated detection of nonprivate inner classes that define nonprivate members and constructors that leak private data from the outer class is straightforward.

Tool	Version	Checker	Description
CodeSonar	Include Page CodeSonar_V CodeSonar_V	JAVA.CLASS.ICSBS	Inner Class Should be Static (Java)
Parasoft Jtest	Include Page Parasoft_V Parasoft_V	CERT.OBJ08.INNER	Make all member classes "private"

Related Guidelines

Bibliography

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