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Programmers may incorrectly understandexpect 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 (other than through the use of the unsupported {{sun.misc.Unsafe}} class). 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 \[[JLS 2005|AA. Bibliography#JLS 05]\], [Section 4.12.4|http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.12.4], "{{final}} Variables":

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Similarly, a final method parameter obtains an immutable copy of the object reference; once 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. Nevertheless, 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.

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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, as it can no longer change the values of x and y.

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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.

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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 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, items.

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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.

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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.

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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 Vulnerabilities

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

Related Guidelines

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

Bibliography

\[[Bloch 2008|AA. Bibliography#Bloch 08]\] Item 13: Minimize the accessibility of classes and members
\[[Core Java 2004|AA. Bibliography#Core Java 04]\] Chapter 6
\[[JLS 2005|AA. Bibliography#JLS 05]\] Sections [4.12.4 "final Variables"|http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.12.4] and [6.6 "Access Control"|http://java.sun.com/docs/books/jls/third_edition/html/names.html#6.6]

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