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Sometimes, when a variable is declared final, it is believed to be immutable. If the variable is a primitive type, declaring it final means that its value cannot be changed after initialization (other than through the use of the unsupported sun.misc.Unsafe class).

However, if the variable is a reference to a mutable object, the object's members that appear to be immutable, may in fact be mutable. For example, a final field that stores a reference to an object does not imply immutability of the object itself. Similarly, a final method parameter obtains a an immutable copy of the object reference through pass-by-value, but the referenced data remains mutable.

According to the Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\], section 4.12.4 "{{final}} Variables":

...

. 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 a and b x and y. The values of the instance fields can be changed even after their initialization . When an object reference is declared final, it only signifies that the reference cannot be changed, whereas the referenced contents can still be alteredbecause the final clause applies only to the reference to the point instance and not to the referenced object.

class FinalClassPoint {
  private int ax;
  private int by;

  FinalClassPoint(int ax, int by) {
    this.ax = ax;
    this.by = by;
  }
  void set_abxy(int ax, int by) {
    this.ax = ax;
    this.by = by;
  }
  void print_abxy() {
    System.out.println("the value ax is: " + this.ax);
    System.out.println("the value by is: " + this.by);
  }
}

public class FinalCallerPointCaller {
  public static void main(String[] args) {
    final FinalClassPoint fcpoint = new FinalClassPoint(1, 2);
    fcpoint.print_abxy();

    // changeChange the value of ax,b. y
    fcpoint.set_abxy(5, 6);
    fcpoint.print_abxy();
  }
}

Compliant Solution (final

...

Fields)

If a and b When the values of the x and y instance variables must remain immutable after their initialization, they should be declared as final. However, this requires the elimination of the setter method invalidates a set_abxy(). method because it can no longer change the values of x and y:

class FinalClassPoint {
  private final int ax;
  private final int y;

  Point(int b 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 final class and does not require the the class Point, avoiding the elimination of the setter method. :

final public class FinalClassPoint implements Cloneable {
  private int ax;
  private int b;

  FinalClass(int a int y;

  Point(int x, int y) {
    this.x = x;
    this.y = y;
  }
  void set_xy(int x, int by) {
    this.ax = ax;
    this.by = by;
  }
  void print_abxy() {
    System.out.println("the value ax is: "+ this.ax);
    System.out.println("the value by is: "+ this.by);
  }
  void set_ab(int a, int b)public Point clone() throws CloneNotSupportedException{
    this.aPoint cloned = a;
    this.b = b(Point) super.clone();
  }
  public// FinalClassNo clone()need throwsto CloneNotSupportedException{
clone x and y FinalClassas clonedthey = (FinalClass) super.clone();are primitives
    return cloned;
  }
}

public class FinalCallerPointCaller {
  public static void main(String[] args) 
      throws CloneNotSupportedException {
    finalPoint FinalClasspoint nf = new FinalClassPoint(1, 2);    // Is not changed in main()
    nfpoint.print_abxy();
    
    // Get the copy of original object
  
  Point  FinalClasspointCopy nf2 = nfpoint.clone();
    // pointCopy now holds a unique reference to the 
    // newly cloned Point instance

    // Change the value of ax,by of the copy.
    nf2pointCopy.set_abxy(5, 6);

    // Original value will not be changedremains unchanged
    nfpoint.print_abxy();
  }
}

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 freely used without exposing the original object. Using . 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. (OBJ10See OBJ04-J. Provide mutable classes with copy functionality to safely allow passing instances to untrusted code safely.)

The FinalClass 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 compliant solution complies with OBJ10-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. Clients can trivially modify the contents of the array (although they are unable to change the array reference, as it is {{final}}). In this noncompliant code example, the elements of the {{items\[\]}} array, are modifiable.

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

Compliant Solution

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[] somethings() {
  return items.clone();
}

As a result, the original array values cannot be modified by a client.

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 SEC14-J. Provide sensitive mutable classes with unmodifiable wrappers.

Risk Assessment

Using final to declare the reference to a mutable object is potentially misleading because the contents of the object can still be changed.

Related Vulnerabilities

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

Related Vulnerabilities

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

References

\[[JLS 05|AA. Java References#JLS 05]\] Sections 4.12.4 "final Variables" and 6.6, "Access Control"
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 13: Minimize the accessibility of classes and members
\[[Core Java 04|AA. Java References#Core Java 04]\] Chapter 6
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 607|http://cwe.mitre.org/data/definitions/607.html] "Public Static Final Field References Mutable Object"

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

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Image Added Image Added Image AddedOBJ00-J. Declare data members as private and provide accessible wrapper methods      08. Object Orientation (OBJ)      OBJ02-J. Do not ignore return values of methods that operate on immutable objects