JSR 308 Explained: Java Type Annotations

JSR 308 Explained: Java Type Annotations

JSR 308, Annotations on Java Types, has been incorporated as part of Java SE 8. This JSR builds upon the existing annotation framework, allowing type annotations to become part of the language. Beginning in Java SE 8, annotations can be applied to types in addition to all of their existing uses within Java declarations. This means annotations can now be applied anywhere a type is specified, including during class instance creation, type casting, the implementation of interfaces, and the specification of throws clauses. This allows developers to apply the benefits of annotations in even more places.

Annotations on types can be useful in many cases, most notably to enforce stronger typing, which can help reduce the number of errors within code. Compiler checkers can be written to verify annotated code, enforcing rules by generating compiler warnings when code does not meet certain requirements. Java SE 8 does not provide a default type-checking framework, but it is possible to write custom annotations and processors for type checking. There are also a number of type-checking frameworks that can be downloaded, which can be used as plug-ins to the Java compiler to check and enforce types that have been annotated. Type-checking frameworks comprise type annotation definitions and one or more pluggable modules that are used with the compiler for annotation processing.

This article begins with a brief overview of annotations, and then you'll learn how to apply annotations to Java types, write type annotations, and use compile-time plug-ins for type checking. After reading this article, you'll be able to use type annotations to enforce stronger typing in your Java source code.

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