OBJ03-J. Be aware that a final reference may not always refer to immutable data

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 subsequently changed. However, if the variable is a reference to a mutable object, the object's contained data that appears to be immutable may actually be mutable. Consider for example, a final method parameter that is a reference to an object. The argument to this method will use pass-by-value to copy the reference but the referenced data will remain mutable.

According to the Java Language Specification [[JLS 05]], section 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.

Noncompliant Code Example

In this example, the values of instance fields a and b 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 contents that it refers to can still be.

class FinalClass{
  private int a;
  private int b;

  FinalClass(int a, int b){
    this.a = a;
    this.b = b;
  }
  void set_ab(int a, int b){
    this.a = a;
    this.b = b;
  }
  void print_ab(){
    System.out.println("the value a is: "+ this.a);
    System.out.println("the value b is: "+ this.b);
  }
}

public class FinalCaller {
  public static void main(String[] args) {
    final FinalClass fc = new FinalClass(1,2);
    fc.print_ab();
    //now we change the value of a,b.
    fc.set_ab(5, 6);
    fc.print_ab();
  }
}

Compliant Solution

If a and b have to be kept immutable after their initialization, the simplest approach is to declare them as final.

private final int a;
private final int b;

void set_ab(int a, int b){ //But now the compiler complains about set_ab method
  this.a = a;
  this.b = b;
}

The problem with this method is that setter methods cannot be used to alter a and b.

Compliant Solution

An alternative approach is to provide a clone method in the class. The clone method can be used to get a copy of the original object. This new object can be freely used without affecting the original object.

final public class NewFinal implements Cloneable {
  private int a;
  private int b;

  NewFinal(int a, int b){
    this.a = a;
    this.b = b;
  }
  void print_ab(){
    System.out.println("the value a is: "+this.a);
    System.out.println("the value b is: "+this.b);
  }
  void set_ab(int a, int b){
    this.a = a;
    this.b = b;
  }
  public NewFinal clone() throws CloneNotSupportedException{
    NewFinal cloned = (NewFinal) super.clone();
    return cloned;
  }
}

public class NewFinalCaller {
  public static void main(String[] args) {
    final NewFinal nf = new NewFinal(1,2);
    nf.print_ab();
    //get the copy of original object
    try {
      NewFinal nf2 = nf.clone();
      // change the value of a,b of the copy.
      nf2.set_ab(5, 6);
      // original value will not be changed
      nf.print_ab();
    } catch (CloneNotSupportedException e) { /* Forward to handler */ }
  }
}

The class is made final to prevent subclasses from overriding the clone() method. This enables the class to be accessed and used, while preventing the fields from being modified, and complies with OBJ36-J. Provide mutable classes with a clone method to allow passing instances to untrusted code safely.

Noncompliant Code Example

Another common mistake is to use a public static final array. Clients can trivially modify the contents of the array (although they will not be able to change the array itself, as it is final).

With this declaration, SOMETHINGS[1], etc. can be modified by clients of the code.

public static final SomeType [] SOMETHINGS = { ... };

Compliant Solution

One approach is to make use of the above method: first define a private array and then provide a public method that returns a copy of the array.

private static final SomeType [] SOMETHINGS = { ... };
public static final SomeType [] somethings() {
  return SOMETHINGS.clone();
}

Now the original array values cannot be modified by a client.

Compliant Solution

An alternative approach is to have a private array from which a public immutable list is constructed:

private static final SomeType [] THE_THINGS = { ... };
public static final List<SomeType> SOMETHINGS =
Collections.unmodifiableList(Arrays.asList(THE_THINGS));

Now neither the original array values nor the public list can be modified by any client.

Risk Assessment

Using final to declare the reference to a mutable object is a potential security risk 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]] Sections 4.12.4 "final Variables" and 6.6, "Access Control"
[[Bloch 08]] Item 13: Minimize the accessibility of classes and members
[[Core Java 04]] Chapter 6
[[MITRE 09]] CWE ID 607 "Public Static Final Field References Mutable Object"

OBJ02-J. Avoid using finalizers      07. Object Orientation (OBJ)      OBJ04-J. Encapsulate the absence of an object by using a Null Object