The {{final}} keyword identifies constant values. That is, {{final}} indicates fields whose value cannot change during an invocation of a program.
The JLS allows implementations to insert the value of public final fields inline intoin any compilation unit that reads the field. Consequently, if the declaring class is edited such that the new version gives a different value for the field, compilation units that read the public final field may still see the old value until they are themselves re-compiled.
A related error can arise when a programmer declares a {{static final}} reference to a mutable object; see guideline [OBJ01-J. Do not assume that declaring a reference to be final causes the referenced object to be immutable] for additional information.
h2. Noncompliant Code Example
In this noncompliant code example, class {{Foo}} declares a field whose value represents the version of the software being. The field is subsequently accessed by class {{Bar}}, which lives in a separate compilation unit.
Foo.java:
{code:bgColor=#ffcccc}
class Foo {
static public final int VERSION = 1;
// ...
}
{code}
Bar.java:
{code:bgColor=#ffcccc}
class Bar {
public static void main(String[] args) {
printf("You are using version " + Foo.VERSION);
}
}
{code}
When compiled and run, the software correctly prints:
{code}
You are using version 1
{code}
However, a subtle flaw is possible in the future. Suppose a developer updates the version number by modifying Foo.java, changing the value of {{VERSION}} to be 2. The developer then recompiles Foo.java, but fails to recompile Bar.java. Now the software incorrectly prints:
{code}
You are using version 1
{code}
because Bar.java still thinks that {{Foo.VERSION}} is 1.
Although recompiling Bar.java will solve this problem, a better solution is available.
h2. Compliant Solution
According to the Java Language Specification \[[JLS 2005|AA. Bibliography#JLS 05]\], Section 13.4.9, "{{final}} Fields and Constants"
{quote}
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.
{quote}
Thus a compliant solution would be:
Foo.java:
{code:bgColor=#ccccff}
class Foo {
static private final int version = 1;
static public String getVersion() {
return version;
}
// ...
}
{code}
Bar.java:
{code:bgColor=#ccccff}
class Bar {
public static void main(String[] args) {
printf("You are using version " + Foo.getVersion());
}
}
{code}
The private version value can therefore not be copied into the Bar class when it is compiled, thus preventing the bug. Note that most JITs are capable of inlining the {getVersion()} method at runtime; consequently there is little or no performance penalty incurred.
h2. Exceptions
*DCL04-EX1*: According to the Java Language Specification \[[JLS 2005|AA. Bibliography#JLS 05]\], 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.
h2. Risk Assessment
Failing to declare mathematical constants {{static}} and {{final}} can lead to thread safety issues as well as to inconsistent behavior.
|| Guideline || Severity || Likelihood || Remediation Cost || Priority || Level ||
| DCL04-J | low | probable | medium | {color:green}{*}P2{*}{color} | {color:green}{*}L3{*}{color} |
h3. Automated Detection
Static checking of this guideline is not feasible in the general case.
h3. Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this guideline on the [CERT website|https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+DCL31-J].
h2. Related Guidelines
C Secure Coding Standard: [seccode:DCL00-C. Const-qualify immutable objects]
h2. Bibliography
\[[JLS 2005|AA. Bibliography#JLS 05]\] "13.4.9 final Fields and Constants", "9.3 Field (Constant) Declarations", "4.12.4 final Variables", "8.3.1.1 static Fields"
----
[!The CERT Oracle Secure Coding Standard for Java^button_arrow_left.png!|DCL03-J. Properly encode relationships in constant definitions] [!The CERT Oracle Secure Coding Standard for Java^button_arrow_up.png!|03. Declarations and Initialization (DCL)] [!The CERT Oracle Secure Coding Standard for Java^button_arrow_right.png!|DCL05-J. Do not attempt to assign to the loop variable in an enhanced for loop]
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