OBJ13-J. Ensure that references to mutable objects are not exposed

Do not expose references to mutable objects to client code. Never initialize such a field to a client-provided object reference or return the object reference from an accessor. Exposing a public static final object allows clients to modify the contents of the object (although they will not be able to change the object itself, as it is final).

This rule does not address private mutable objects, see rule OBJ05-J. Do not return references to private mutable class members for more information.

Noncompliant Code Example

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

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

This noncompliant code example also violates OBJ01-J. Limit accessibility of fields.

Compliant Solution

One approach is to have a private array and 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 2

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 a client.

Risk Assessment

Having a public static final array is a potential security risk because the array elements may be modified by a client.

Automated Detection

Related Vulnerabilities

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

References