73. Never confuse the immutability of a reference with that of the referenced object

Immutability helps to support security reasoning. It is safe to share immutable objects without risk that the recipient can modify them [Mettler 2010].

Programmers often incorrectly assume 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 (by normal Java processing). 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 may be mutable. For example, according to the Java Language Specification, §4.12.4, "final Variables" [JLS 2013],

If a final variable holds a reference to an object, then the state of the object may be changed by operations on the object, but the variable will always refer to the same object.

This applies also to arrays because arrays are objects; if a final variable holds a reference to an array, then the components of the array may still 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 doing so 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 to 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();
  }
}

Compliant Solution (final Fields)

When the values of the x and y instance variables 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 variables 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 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 {
    Point point = new Point(1, 2);    // Is not changed in main()
    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 OBJ04-J. Provide mutable classes with copy functionality to safely allow passing instances to untrusted code.)

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.

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. 

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

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

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

Providing direct access to the array objects themselves is safe because String is immutable.

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();
}

Because a copy of the array is returned, 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 OBJ06-J. Defensively copy mutable inputs and mutable internal components for more information.

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

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 OBJ56-J. Provide sensitive mutable classes with unmodifiable wrappers. This solution can also be used when the array contains mutable objects.

Applicability

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

Bibliography