When you declare a variable final, you do not want anyone to change it. If the type of variable is primitive types, you can undoubtedly make it. Unfortunately, if the variable is a reference to an object, the "final" stuff you think may be not final!
Non-compliant Code Example
In this code, the value of a and b has been changed, which means that when you declare a reference final, it only means that the reference can not be changed but the contents it refer to can still be changed!
| Code Block | ||
|---|---|---|
| ||
class Test{
 
 Test(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);
 }
 private int a;
 private int b;
}
public class TestFinal1 {
 
 public static void main(String[] args) {
        final Test mytest = new Test(1,2);
        mytest.print_ab();
        //now we change the value of a,b.
        mytest.set_ab(5, 6);
        mytest.print_ab();
       
    }
}
|
Non-Compliant Code Example
If you do not want to change a and b after they are initialized, the simplest approach is to declare a and b final:_
| Code Block | ||
|---|---|---|
| ||
 void set_ab(int a, int b){ //But now compiler complains about set_ab method!
  this.a = a;
  this.b = b;
 }
 private final int a;
 private final int b;
|
But now you can not have setter methods of a and b.
Compliant Solution
An alternative approach is to provide the clone method in the class. When you want do something about the object, you can use clone method to get a copy of original object. Now, you can do everything to this new object, and the original one will be never changed.
| Code Block | ||
|---|---|---|
| ||
class NewFinal implements Cloneable
{
 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;
 }
 private int a;
 private int b;
}
public class Test2 {
 
 public static void main(String[] args) {
        final NewFinal mytest = new NewFinal(1,2);
        mytest.print_ab();
        //get the copy of original object
   try {
  NewFinal mytest2 = mytest.clone();
        //now we change the value of a,b of the copy.
   mytest2.set_ab(5, 6);
   //but the original value will not be changed
        mytest.print_ab();
  } catch (CloneNotSupportedException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
 }
}
|
One common mistake about this is to use public static final array, clients will be able to modify the contents of the array (although they will not be able to change the array itself, as it is final).
Non-compliant Code Example
| Wiki Markup |
|---|
With this declaration, {{SOMETHINGS\[1\]}}, etc. can be modified by clients of the code. |
| Code Block | ||
|---|---|---|
| ||
 public static final SomeType [] SOMETHINGS = { ... };
|
Compliant Solution
One approach is to make use of above method: first define a private array and then provide a public method that returns a copy of the array:
| Code Block | ||
|---|---|---|
| ||
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 contructed:
| Code Block | ||
|---|---|---|
| ||
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 a client.
Risk Assessment
Using final to declare the reference to an object is a potential security risk, because the contents of the object can still be changed.
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
OBJ03-J | medium | likely | medium | P9 | L2 |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
Chapter 6, Core Java⢠2 Volume I - Fundamentals, Seventh Edition by Cay S. Horstmann, Gary Cornell
Publisher:Prentice Hall PTR;Pub Date:August 17, 2004
OBJ02-J. Avoid using finalizers 06. Objects Orientation (OBJ) OBJ31-J. Misusing public static variables