DCL04-J. Do not apply final to public constants whose value might change in the future

The final keyword is used to identify constant values. That is, final indicates fields that may not be changed during an invocation of a program.

The value of public final fields is permitted to be inserted inline into any compilation unit that reads the value. This means that if the field's value ever changes, then a compilation unit that depends on the value may still have the old value until re-compiled.

Another pitfall arises when static-final is used inappropriately to declare mutable data. See guideline OBJ01-J. Do not assume that declaring a reference to be final causes the referenced object to be immutable.

Noncompliant Code Example

In this noncompliant code example, class Foo stores a number representing the version of the software being used. It is subsequently accessed by class Bar, which lives in a separate compilation unit.

Foo.java:

class Foo {
  static public final int VERSION = 1;
  // ...
}

Bar.java:

class Bar {
  public static void main(String[] args) {
    printf("You are using version " + Foo.VERSION);
  }
}

When compiled and run, the software correctly prints:

    You are using version 1

However, a subtle flaw is possible in the future. When the version number must be upgraded, suppose a developer modifies Foo.java, and changes VERSION to have the value 2. The developer then recompiles Foo.java without recompiling Bar.java. Now the software incorrectly prints:

    You are using version 2

because Bar.java still thinks that Foo.VERSION is 1.

While recompiling Bar.java will solve this problem, a better solution is available.

Compliant Solution

According to the Java Language Specification [[JLS 2005]], Section 13.4.9, "final Fields and Constants"

Other than for true mathematical constants, we recommend that source code make very sparing use of class variables that are declared static and final. If the read-only nature of final is required, a better choice is to declare a private static variable and a suitable accessor method to get its value.

Thus a compliant solution would be:

Foo.java:

class Foo {
  static private final int version = 1;
  static public String getVersion() {
    return version;
  }

  // ...
}

Bar.java:

class Bar {
  public static void main(String[] args) {
    printf("You are using version " + Foo.getVersion());
  }
}

The private version value can therefore not be copied into the Bar class when it is compiled, thus preventing the bug.

Exceptions

DCL04-EX1: According to the Java Language Specification [[JLS 2005]], Section 9.3 "Field (Constant) Declarations," "Every field declaration in the body of an interface is implicitly public, static, and final. It is permitted to redundantly specify any or all of these modifiers for such fields."

DCL04-EX2: Constants declared using the enum type may violate this guideline.

DCL04-EX3: Constants that never change their values throughout the lifetime of the software may indeed be declared final. For instance, the JLS recommends that mathematical constants be declared final.

Risk Assessment

Failing to declare mathematical constants static and final can lead to thread safety issues as well as to inconsistent behavior.

Automated Detection

Static checking of this guideline is not feasible in the general case.

Related Vulnerabilities

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

Related Guidelines

C Secure Coding Standard: DCL00-C. Const-qualify immutable objects

Bibliography

[[JLS 2005]] "13.4.9 final Fields and Constants", "9.3 Field (Constant) Declarations", "4.12.4 final Variables", "8.3.1.1 static Fields"

DCL03-J. Properly encode relationships in constant definitions      03. Declarations and Initialization (DCL)      DCL05-J. Do not attempt to assign to the loop variable in an enhanced for loop