JAX-RS (Java API for RESTful Web Services) is a set of Java standards and APIs for building RESTful web services in Java. It is part of the Java EE (Enterprise Edition) platform and is designed to simplify the development of RESTful APIs by providing a standardized way to create and interact with web resources using the HTTP protocol. JAX-RS allows you to build web services that can be consumed by a wide range of clients, including web browsers, mobile applications, and other services.

Key features and concepts of JAX-RS include:

1. Resources: In JAX-RS, resources are Java classes that define the endpoints or URIs for your RESTful web service. These classes are annotated with JAX-RS annotations to specify how they handle HTTP requests and responses.

2. Annotations: JAX-RS provides a set of annotations that you can use to define how resources and methods should behave. Common annotations include @Path, @GET, @POST, @PUT, @DELETE, and more.

3. HTTP Methods: JAX-RS maps HTTP methods (GET, POST, PUT, DELETE, etc.) to Java methods in resource classes. For example, a @GET annotation on a Java method indicates that it should respond to HTTP GET requests.

4. Path Parameters: You can use path parameters in JAX-RS to extract values from the URI of the request. These parameters are often used for dynamic resource identification.

5. Query Parameters: JAX-RS supports query parameters, allowing you to extract values from the query string of a URL.

6. Content Negotiation: JAX-RS provides content negotiation capabilities, allowing clients to request data in different formats (e.g., JSON, XML) using HTTP headers.

7. Exception Handling: JAX-RS provides mechanisms for handling exceptions and returning appropriate HTTP status codes and error responses.

8. Filters and Interceptors: JAX-RS allows you to define filters and interceptors to perform preprocessing and postprocessing of HTTP requests and responses.

9. Client API: JAX-RS includes a client API that simplifies the consumption of RESTful web services from Java clients.

10. Security: JAX-RS can be integrated with Java EE security features to secure your web services using standards like OAuth and JWT (JSON Web Tokens).

11. Asynchronous Processing: JAX-RS supports asynchronous processing, allowing resource methods to be executed asynchronously for improved scalability.

Popular JAX-RS implementations include Jersey, which is the reference implementation, and RESTeasy. These implementations provide runtime environments for deploying and running JAX-RS web services.

Here’s a simple example of a JAX-RS resource class:

In this example, the @Path annotation defines the URI path for the resource (“/hello”), and the @GET annotation indicates that the sayHello method should respond to HTTP GET requests.

JAX-RS is a powerful and widely used technology for building RESTful web services in Java, and it simplifies many aspects of web service development by providing a standardized and annotation-driven approach.