How are Java annotations implemented?

The implementation of Java annotations is achieved through the collaboration of the Java compiler and the Java Virtual Machine (JVM).

When we use annotations in Java source code, the Java compiler will parse the annotations and store their information in the compiled bytecode file. This implies that annotations themselves exist in the Java bytecode in an annotated manner.

During runtime, the JVM is able to retrieve annotation information from the bytecode using reflection, and perform relevant actions based on the annotation information. This allows us to dynamically retrieve and process annotation information during runtime.

The implementation of Java annotations requires following certain standards and interfaces. Typically, we can use Java’s meta-annotations to define custom annotations and use annotation processors to handle annotations. Meta-annotations are special annotations used for defining and processing annotations.

Annotation processor is a class that implements the annotation processing API provided by Java (javax.annotation.processing). It scans annotations in the source code and performs actions according to the annotations’ definitions, such as generating additional code or validating the correctness of annotations.

In conclusion, the implementation of Java annotations involves the collaboration between the compiler and JVM. The compiler is responsible for parsing and saving annotation information, while the JVM accesses and processes annotation information through reflection. Additionally, we can define and process custom annotations using meta-annotations and annotation processors.