The Last-Modified header field value in HTTP is often used for validating the cached response contents. The Last-Modified entity-header field indicates the date and time at which the entity present in the response body was last modified. A client can use the Last-Modified header field value in combination with the If-Modified-Since or If-Unmodified-Since request headers to perform conditional requests.

The following example illustrates the use of the Last-Modified HTTP header field in the JAX-RS application. The Last-Modified field contains the date when the resource was last changed. When a client requests the same resource next time, it sends the If-Modified-Since header field, with the value set to the date and time at which the resource was last updated on the server. On the server, you can call javax.ws.rs.core.Request::evaluatePreconditions() to check whether the resource has been modified in between the requests. This method evaluates request preconditions on the basis of the passed-in value. If this method returns null, the resource is out of date and needs to be sent back in the response. Otherwise, this method returns the 304 Not Modified HTTP status code to the client. Here is the code snippet for this example:

//Other imports are removed for brevity 
import javax.ws.rs.core.Request;  
import javax.ws.rs.core.Context; 
import javax.ws.rs.core.Response.ResponseBuilder; 
 
@GET 
@Path("departments/{id}") 
@Produces(MediaType.APPLICATION_JSON) 
public Response 
     findDepartmentWithCacheValidationWithLastModifiedDate( 
        @PathParam("id") Short id, @Context Request request) { 
 
    //Reads the latest Department object from DB 
    Department department = entityManager.find(Department.class,  
        id); 
    //Gets the last modified date 
    Date latModifiedDate = department.getModifiedDate(); 
    //Evaluates request preconditions on the basis  
    //of the passed-in value. 
    //evaluatePreconditions() return null If-Modified-Since 
    //check succeeds. This implies that resource is modified 
    ResponseBuilder builder =  
        request.evaluatePreconditions(latModifiedDate); 
 
    //cached resource did change; send new one 
    if (builder == null) { 
        builder = Response.ok(department); 
        builder.lastModified(latModifiedDate); 
 
    }  
    return builder.build(); 
 
}