Overview of a JAX-RS Application

The following code sample is a very simple example of a root resource class that uses JAX-RS annotations:

package com.dineshonjava.ws.rest;

import javax.ws.rs.GET;
import javax.ws.rs.Produces;
import javax.ws.rs.Path;

// The Java class will be hosted at the URI path "/helloworld"
@Path("/helloworld")
public class HelloWorldResource {
    
    // The Java method will process HTTP GET requests
    @GET
    // The Java method will produce content identified by the MIME Media
    // type "text/plain"
    @Produces("text/plain")
    public String getClichedMessage() {
        // Return some cliched textual content
        return "Hello World!!! Dinesh on Java";
    }
}

The following sections describe the annotations used in this example.

• The @Path annotation’s value is a relative URI path. In the preceding example, the Java class will be hosted at the URI path /helloworld. This is an extremely simple use of the @Path annotation, with a static URI path. Variables can be embedded in the URIs. URI path templates are URIs with variables embedded within the URI syntax.
• The @GET annotation is a request method designator, along with @POST, @PUT, @DELETE, and @HEAD, defined by JAX-RS and corresponding to the similarly named HTTP methods. In the example, the annotated Java method will process HTTP GET requests. The behavior of a resource is determined by the HTTP method to which the resource is responding.
• The @Produces annotation is used to specify the MIME media types a resource can produce and send back to the client. In this example, the Java method will produce representations identified by the MIME media type “text/plain”.
• The @Consumes annotation is used to specify the MIME media types a resource can consume that were sent by the client. The example could be modified to set the message returned by the getClichedMessage method, as shown in this code example:

@POST
@Consumes("text/plain")
public void postClichedMessage(String message) {
    // Store the message
}

The @Path Annotation and URI Path Templates

The @Path annotation identifies the URI path template to which the resource responds and is specified at the class or method level of a resource. The @Path annotation’s value is a partial URI path template relative to the base URI of the server on which the resource is deployed, the context root of the application, and the URL pattern to which the JAX-RS runtime responds.

URI path templates are URIs with variables embedded within the URI syntax. These variables are substituted at runtime in order for a resource to respond to a request based on the substituted URI. Variables are denoted by braces ({ and }). For example, look at the following @Path annotation:

@Path("/users/{username}")

In this kind of example, a user is prompted to type his or her name, and then a JAX-RS web service configured to respond to requests to this URI path template responds. For example, if the user types the user name “Galileo,” the web service responds to the following URL:

http://example.com/users/Galileo

To obtain the value of the user name, the @PathParam annotation may be used on the method parameter of a request method, as shown in the following code example:

@Path("/users/{username}")
public class UserResource {

    @GET
    @Produces("text/xml")
    public String getUser(@PathParam("username") String userName) {
        ...
    }
}

By default, the URI variable must match the regular expression “[^/]+?”. This variable may be customized by specifying a different regular expression after the variable name. For example, if a user name must consist only of lowercase and uppercase alphanumeric characters, override the default regular expression in the variable definition:

@Path("users/{username: [a-zA-Z][a-zA-Z_0-9]*}")

In this example the username variable will match only user names that begin with one uppercase or lowercase letter and zero or more alphanumeric characters and the underscore character. If a user name does not match that template, a 404 (Not Found) response will be sent to the client.

A @Path value isn’t required to have leading or trailing slashes (/). The JAX-RS runtime parses URI path templates the same whether or not they have leading or trailing spaces.

A URI path template has one or more variables, with each variable name surrounded by braces: { to begin the variable name and } to end it. In the preceding example, username is the variable name. At runtime, a resource configured to respond to the preceding URI path template will attempt to process the URI data that corresponds to the location of {username} in the URI as the variable data for username.

For example, if you want to deploy a resource that responds to the URI path template http://example.com/myContextRoot/resources/{name1}/{name2}/, you must deploy the application to a Java EE server that responds to requests to the http://example.com/myContextRoot URI and then decorate your resource with the following @Path annotation:

@Path("/{name1}/{name2}/")
public class SomeResource {
    ...
}

References
1. JAVA REST Web Services
2. Wikipedia for REST Web Service