OBJ02-J. Never conflate immutability of a reference with that of the referenced object

Immutability helps to supporting security reasoning. It is safe to share immutable objects, without risk that the recipient can modify something that we are relying upon [[Mettler 2010B]].

Programmers could incorrectly expect that declaring a field or variable final makes the referenced object immutable. Declaring variables that have a primitive type to be final does prevent changes to their values after initialization (unless the unsupported sun.misc.Unsafe class is used). However, when the variable has a reference type, the presence of a final clause in the declaration only makes the reference itself immutable. The final clause has no effect on the referenced object. Consequently, the fields of the referenced object can be mutable. For example, according to the Java Language Specification [[JLS 2005]], §4.12.4, "final Variables,"

... if a final variable holds a reference to an array, then the components of the array may be changed by operations on the array, but the variable will always refer to the same array.

Similarly, a final method parameter obtains an immutable copy of the object reference. Again, this has no effect on the mutability of the referenced data.

Noncompliant Code Example (Mutable Class, final Reference)

In this noncompliant code example, the programmer has declared the reference to the point instance to be final, under the incorrect assumption that this prevents modification of the values of the instance fields x and y. The values of the instance fields can be changed after their initialization because the final clause applies only to the reference to the point instance and not the referenced object.

class Point {
  private int x;
  private int y;

  Point(int x, int y) {
    this.x = x;
    this.y = y;
  }
  void set_xy(int x, int y) {
    this.x = x;
    this.y = y;
  }
  void print_xy() {
    System.out.println("the value x is: " + this.x);
    System.out.println("the value y is: " + this.y);
  }
}

public class PointCaller {
  public static void main(String[] args) {
    final Point point = new Point(1, 2);
    point.print_xy();

    // change the value of x, y
    point.set_xy(5, 6);
    point.print_xy();
  }
}

When an object reference is declared final, it signifies only that the reference cannot be changed; the mutability of the referenced object is unaffected.

Compliant Solution (final Fields)

When the values of the x and y members must remain immutable after their initialization, they should be declared final. However, this invalidates a set_xy() method because it can no longer change the values of x and y.

class Point {
  private final int x;
  private final int y;

  Point(int x, int y) {
    this.x = x;
    this.y = y;
  }
  void print_xy() {
    System.out.println("the value x is: " + this.x);
    System.out.println("the value y is: " + this.y);
  }

  // set_xy(int x, int y) no longer possible
}

With this modification, the values of the instance fields become immutable and, consequently, match the programmer's intended usage model.

Compliant Solution (Provide Copy Functionality)

If the class must remain mutable, another compliant solution is to provide copy functionality. This compliant solution provides a clone() method in the final class Point, avoiding the elimination of the setter method.

final public class Point implements Cloneable {
  private int x;
  private int y;

  Point(int x, int y) {
    this.x = x;
    this.y = y;
  }
  void set_xy(int x, int y) {
    this.x = x;
    this.y = y;
  }
  void print_xy() {
    System.out.println("the value x is: "+ this.x);
    System.out.println("the value y is: "+ this.y);
  }
  public Point clone() throws CloneNotSupportedException{
    Point cloned = (Point) super.clone();
    // No need to clone x and y as they are primitives
    return cloned;
  }
}

public class PointCaller {
  public static void main(String[] args) throws CloneNotSupportedException {
    final Point point = new Point(1, 2);
    point.print_xy();
    
    // Get the copy of original object
    Point pointCopy = point.clone();
    // pointCopy now holds a unique reference to the newly cloned Point instance

    // Change the value of x,y of the copy.
    pointCopy.set_xy(5, 6);

    // Original value remains unchanged
    point.print_xy();
  }
}

The clone() method returns a copy of the original object that reflects the state of the original object at the moment of cloning. This new object can be used without exposing the original object. Because the caller holds the only reference to the newly cloned instance, the instance fields cannot be changed without the caller's cooperation. This use of the clone() method allows the class to remain securely mutable. (See guideline "OBJ08-J. Provide mutable classes with copy functionality to allow passing instances to untrusted code safely.")

The Point class is declared final to prevent subclasses from overriding the clone() method. This enables the class to be suitably used without any inadvertent modifications of the original object. This solution also complies with guideline "OBJ08-J. Provide mutable classes with copy functionality to allow passing instances to untrusted code safely."

Noncompliant Code Example (Arrays)

This noncompliant code example uses a public static final array, items.

public static final String[] items = {/* ... */};

Clients can trivially modify the contents of the array, even though declaring the array reference to be final prevents modification of the reference itself.

Compliant Solution (Index Getter)

This compliant solution makes the array private and provides public methods to get individual items and array size. Providing direct access to the array objects themselves is safe because String is immutable.

private static final String[] items = {/* ... */};

public static final String getItem(int index) {
  return items[index];
}

public static final int getItemCount() {
  return items.length;
}

Compliant Solution (Clone the Array)

This compliant solution defines a private array and a public method that returns a copy of the array.

private static final String[] items = {/* ... */};

public static final String[] getItems() {
  return items.clone();
}

As a result, the original array values cannot be modified by a client. Note that a manual deep copy could be required when dealing with arrays of objects. This generally happens when the objects do not export a clone() method. Refer to guideline "FIO00-J. Defensively copy mutable inputs and mutable internal components" for more information.

As before, this method provides direct access to the array objects themselves, which is safe because String is immutable. If the array contained mutable objects, the getItems() method could return a cloned array of cloned objects.

Compliant Solution (Unmodifiable Wrappers)

This compliant solution declares a private array from which a public immutable list is constructed.

private static final String[] items = { ... };

public static final List<String> itemsList =
Collections.unmodifiableList(Arrays.asList(items));

Neither the original array values nor the public list can be modified by a client. For more details about unmodifiable wrappers, refer to guideline "SEC14-J. Provide sensitive mutable classes with unmodifiable wrappers." This solution still applies if the array contains mutable items instead of String.

Risk Assessment

Incorrectly assuming that final references cause the contents of the referenced object to remain mutable can result in an attacker modifying an object thought by the programmer to be immutable.

Related Guidelines

MITRE CWE: CWE-607 "Public Static Final Field References Mutable Object"

Bibliography

[[Bloch 2008]] Item 13: Minimize the accessibility of classes and members
[[Core Java 2004]] Chapter 6
[[JLS 2005]] §4.12.4 "final Variables" and §6.6 "Access Control"
[[Mettler 2010B]]

OBJ01-J. Declare data members as private and provide accessible wrapper methods      04. Object Orientation (OBJ)      OBJ03-J. Sensitive classes must not let themselves be copied