Denoted by the "@" symbol, annotations are metadata that can be added to classes, methods, and variables for various purposes such as
documentation, code analysis, and runtime processing.
Annotations have retention policies that determine in which context they are retained and available for use. There are three retention policies for
annotations:
-
RetentionPolicy.SOURCE - Annotations are only available during compilation and code analysis. They are not included in the
compiled class file and are not available at runtime. E.G. @Override, @SuppressWarnings
-
RetentionPolicy.CLASS - Annotations are included in the compiled class file providing information to the compiler, but they are
not retained by the JVM at runtime. This is the default retention policy. E.G. @PreviewFeature
-
RetentionPolicy.RUNTIME - Annotations are included in the compiled class file and available at runtime. They can be accessed and
used by the program through reflection. E.G. @FunctionalInterface, @Deprecated
It is important to understand that only annotations having the RUNTIME retention policy can be accessed at runtime using reflection.
For example, the following if condition is true when the method argument is the java.util.function.Function class:
void execute(Class<?> cls) {
if (cls.isAnnotationPresent(FunctionalInterface.class)) {
// ...
}
}
Therefore, it is an issue to use reflection in combination with annotations with the SOURCE or CLASS retention policy
because they are not present at runtime. For example, in the JVM source code, the hashCode() method of the Integer class has
the @Override annotation. However, the following if condition will always be false even if the method argument is the
Integer#hashCode() method because @Override has the SOURCE retention policy:
void execute(Method method) {
if (method.isAnnotationPresent(Override.class)) { // Noncompliant, if condition will always be false because
// @Override is declared with @Retention(RetentionPolicy.SOURCE)
// ...
}
}
This rule detects improper reflective access on annotations having the SOURCE or CLASS retention policy.
