1. REST - Representational State Transfer
1.1. Overview
REST is an architectural style which is based on web-standards and the HTTP protocol. REST was first described by Roy Fielding in 2000.
In a REST based architecture everything is a resource. A resource is accessed via a common interface based on the HTTP standard methods.
In a REST based architecture you typically have a REST server which provides access to the resources and a REST client which accesses and modify the REST resources.
Every resource should support the HTTP common operations. Resources are identified by global IDs (which are typically URIs).
REST allows that resources have different representations, e.g. text, xml, json etc. The rest client can ask for specific representation via the HTTP protocol (content negotiation).
1.2. HTTP methods
The PUT, GET, POST and DELETE methods are typical used in REST based architectures.
The following table gives an explanation of these operations.
-
GET defines a reading access of the resource without side-effects. The resource is never changed via a GET request, e.g. the request has no side effects (idempotent).
-
PUT creates a new resource, must also be idempotent.
-
DELETE removes the resources. The operations are idempotent, they can get repeated without leading to different results.
-
POST updates an existing resource or creates a new resource.
1.3. RESTFul webservices
A RESTFul webservices are based on the HTTP methods and the concept of REST. A RESTFul webservice typically defines the base URI for the services, the supported MIME-types (XML, Text, JSON, user-defined,..) and the set of operations (POST, GET, PUT, DELETE) which are supported.
2. JAX-RS with Jersey
2.1. JAX-RS and Jersey
Java defines REST support via the Java Specification Request 311 (JSR). This specificiation is called JAX-RS (The Java API for RESTful Web Services). JAX-RS uses annotations to define the REST relevance of Java classes.
Jersey is the reference implementation for this specification. Jersey contains basically a REST server and a REST client. The core client can be used provides a library to communicate with the server.
On the server side Jersey uses a servlet which scans predefined classes to identify RESTful resources. Via the web.xml configuration file for your web application, registers this servlet which is provided by the Jersey distribution
The base URL of this servlet is:
http:
This servlet analyzes the incoming HTTP request and selects the correct class and method to respond to this request. This selection is based on annotations in the class and methods.
A REST web application consists therefore out of data classes (resources) and services. These two types are typically maintained in different packages as the Jersey servlet will be instructed via the web.xml to scan certain packages for data classes.
JAX-RS supports the creation of XML and JSON via the Java Architecture for XML Binding (JAXB).
JAXB is described in the .
2.2. JAX-RS annotations
The most important annotations in JAX-RS are listed in the following table.
Table 1. JAX-RS annotations
|
Annotation
|
Description
|
|
@PATH(your_path)
|
Sets the path to base URL + /your_path. The base URL is based on your application name, the servlet and the URL pattern from the web.xml" configuration file.
|
|
@POST
|
Indicates that the following method will answer to a HTTP POST request
|
|
@GET
|
Indicates that the following method will answer to a HTTP GET request
|
|
@PUT
|
Indicates that the following method will answer to a HTTP PUT request
|
|
@DELETE
|
Indicates that the following method will answer to a HTTP DELETE request
|
|
@Produces(MediaType.TEXT_PLAIN [, more-types ])
|
@Produces defines which MIME type is delivered by a method annotated with @GET. In the example text ("text/plain") is produced. Other examples would be "application/xml" or "application/json".
|
|
@Consumes(type [, more-types ])
|
@Consumes defines which MIME type is consumed by this method.
|
|
@PathParam
|
Used to inject values from the URL into a method parameter. This way you inject for example the ID of a resource into the method to get the correct object.
|
The complete path to a resource is basd on the base URL and the @PATh annotation in your class.
http:
5. Create your first RESTful Webservice
5.1. Create project with Jersey libraries
Create a new Dynamic Web Project called de.vogella.jersey.first.
Copy all jars from your Jersey download into the WEB-INF/lib folder.
5.2. Java Class
Create the following class.
package de.vogella.jersey.first; import javax.ws.rs.GET; import javax.ws.rs.Path; import javax.ws.rs.Produces; import javax.ws.rs.core.MediaType; @Path("/hello") public class Hello { @GET @Produces(MediaType.TEXT_PLAIN) public String sayPlainTextHello() { return "Hello Jersey";
} @GET @Produces(MediaType.TEXT_XML) public String sayXMLHello() { return "" + " Hello Jersey" + "";
} @GET @Produces(MediaType.TEXT_HTML) public String sayHtmlHello() { return " " + ""</span> + <span class="hl-string" style="color:blue;">"Hello Jersey"</span> + <span class="hl-string" style="color:blue;">"" + ""
+ "Hello Jersey" + "
" + " ";
}
}
This class register itself as a get resource via the @GET annotation. Via the @Produces annotation it defines that it delivers the text and the HTML MIME types. It also defines via the @Path annotation that its service is available under the hello URL.
The browser will always request the html MIME type. To see the text version you can use tool like curl .
5.3. Define Jersey Servlet dispatcher
You need to register Jersey as the servlet dispatcher for REST requests. Open the file web.xml and modify the file to the following.
"1.0" encoding="UTF-8"?>
"" xmlns="" xmlns:web="/web-app_2_5.xsd" xsi:schemaLocation=" /web-app_2_5.xsd" id="WebApp_ID" version="2.5">
de.vogella.jersey.first
Jersey REST Service
class>com.sun.jersey.spi.container.servlet.ServletContainerclass>
com.sun.jersey.config.property.packages
de.vogella.jersey.first
1
Jersey REST Service
/rest
The parameter "com.sun.jersey.config.property.package" defines in which package jersey will look for the web service classes. This property must point to your resources classes. The URL pattern defines part of the base URL your application will be placed.
5.4. Run your rest service
Run you web application in Eclipse. See for details on how to run dynamic web applications.
Test your REST service under: "". This name is derived from the "display-name" defined in the web.xml file, augmented with the servlet-mapping url-pattern and the "hello" @Path annotation from your class file. You should get the message "Hello Jersey".
The browser requests the HTML representation of your resource. In the next chapter we are going to write a client which will read the XML representation.
5.5. Create a client
Jersey contains a REST client library which can be used for testing or to build a real client in Java. Alternative you could use to create a client.
Create a new Java "de.vogella.jersey.first.client" and add the jersey jars to the project and the project build path. Create the following test class.
package de.vogella.jersey.first.client; import java.net.URI; import javax.ws.rs.core.MediaType; import javax.ws.rs.core.UriBuilder; import com.sun.jersey.api.client.Client; import com.sun.jersey.api.client.ClientResponse; import com.sun.jersey.api.client.WebResource; import com.sun.jersey.api.client.config.ClientConfig; import com.sun.jersey.api.client.config.DefaultClientConfig; public class Test { public static void main(String[] args) {
ClientConfig config = new DefaultClientConfig();
Client client = Client.create(config);
WebResource service = client.resource(getBaseURI()); System.out.println(service.path("rest").path("hello").accept(MediaType.TEXT_PLAIN).get(ClientResponse.class).toString()); System.out.println(service.path("rest").path("hello").accept(MediaType.TEXT_PLAIN).get(String.class)); System.out.println(service.path("rest").path("hello").accept(MediaType.TEXT_XML).get(String.class)); System.out.println(service.path("rest").path("hello").accept(MediaType.TEXT_HTML).get(String.class));
} private static URI getBaseURI() { return UriBuilder.fromUri("").build();
}
}
6. Restful webservices and JAXB
JAX-RS supports the automatic creation of XML and JSON via JAXB. For an introduction into XML please see . For an introduction into JAXB please see . You can continue this tutorial without reading these tutorials but they contain more background information.
6.1. Create project
Create a new Dynamic Web Project called de.vogella.jersey.jaxb and copy all jersey jars into the WEB-INF/lib folder.
Create your domain class.
package de.vogella.jersey.jaxb.model; import javax.xml.bind.annotation.XmlRootElement; @XmlRootElement public class Todo { private String summary; private String description; public String getSummary() { return summary;
} public void setSummary(String summary) { this.summary = summary;
} public String getDescription() { return description;
} public void setDescription(String description) { this.description = description;
}
}
Create the following resource class. This class simply return an instance of the Todo class.
package de.vogella.jersey.jaxb; import javax.ws.rs.GET; import javax.ws.rs.Path; import javax.ws.rs.Produces; import javax.ws.rs.core.MediaType; import de.vogella.jersey.jaxb.model.Todo; @Path("/todo") public class TodoResource { @GET @Produces({ MediaType.APPLICATION_XML, MediaType.APPLICATION_JSON }) public Todo getXML() {
Todo todo = new Todo();
todo.setSummary("This is my first todo");
todo.setDescription("This is my first todo"); return todo;
} @GET @Produces({ MediaType.TEXT_XML }) public Todo getHTML() {
Todo todo = new Todo();
todo.setSummary("This is my first todo");
todo.setDescription("This is my first todo"); return todo;
}
}
Change web.xml to the following.
"1.0" encoding="UTF-8"?>
"" xmlns="" xmlns:web="/web-app_2_5.xsd" xsi:schemaLocation=" /web-app_2_5.xsd" id="WebApp_ID" version="2.5">
de.vogella.jersey.jaxb
Jersey REST Service
class>com.sun.jersey.spi.container.servlet.ServletContainerclass>
com.sun.jersey.config.property.packages
de.vogella.jersey.jaxb
1
Jersey REST Service
/rest
Run you web application in Eclipse and validate that you can access your service. Your application should be available under the following URL.
http:
6.2. Create a client
Create a new Java "de.vogella.jersey.jaxb.client" and add the jersey jars to the project and the project build path. Create the following test class.
package de.vogella.jersey.jaxb.client; import java.net.URI; import javax.ws.rs.core.MediaType; import javax.ws.rs.core.UriBuilder; import com.sun.jersey.api.client.Client; import com.sun.jersey.api.client.WebResource; import com.sun.jersey.api.client.config.ClientConfig; import com.sun.jersey.api.client.config.DefaultClientConfig; public class Test { public static void main(String[] args) {
ClientConfig config = new DefaultClientConfig();
Client client = Client.create(config);
WebResource service = client.resource(getBaseURI()); System.out.println(service.path("rest").path("todo").accept(MediaType.TEXT_XML).get(String.class)); System.out.println(service.path("rest").path("todo").accept(MediaType.APPLICATION_JSON).get(String.class)); System.out.println(service.path("rest").path("todo").accept(MediaType.APPLICATION_XML).get(String.class));
} private static URI getBaseURI() { return UriBuilder.fromUri("").build();
}
}
7. CRUD RESTful webservice
This section creates a CRUD (Create, Read, Update, Delete) restful web service. It will allow to maintain a list of todos in your web application via HTTP calls.
7.1. Project
Create a new dynamic project called de.vogella.jersey.todo and add the jersey libs. Change theweb.xml file to the following.
"1.0" encoding="UTF-8"?>
"" xmlns="" xmlns:web="/web-app_2_5.xsd" xsi:schemaLocation=" /web-app_2_5.xsd" id="WebApp_ID" version="2.5">
de.vogella.jersey.todo
Jersey REST Service
class>com.sun.jersey.spi.container.servlet.ServletContainerclass>
com.sun.jersey.config.property.packages
de.vogella.jersey.todo.resources
1
Jersey REST Service
/rest
Create the following data model and a which serves as the data provider for the model. We use the implementation based on an enumeration. Please see the link for details. The Todo class is annotated with a JAXB annotation. See to learn about JAXB.
package de.vogella.jersey.todo.model; import javax.xml.bind.annotation.XmlRootElement; @XmlRootElement public class Todo { private String id; private String summary; private String description; public Todo(){
} public Todo (String id, String summary){ this.id = id; this.summary = summary;
} public String getId() { return id;
} public void setId(String id) { this.id = id;
} public String getSummary() { return summary;
} public void setSummary(String summary) { this.summary = summary;
} public String getDescription() { return description;
} public void setDescription(String description) { this.description = description;
}
}
package de.vogella.jersey.todo.dao; import java.util.HashMap; import java.util.Map; import de.vogella.jersey.todo.model.Todo; public enum TodoDao {
instance; private Map contentProvider = new HashMap(); private TodoDao() {
Todo todo = new Todo("1", "Learn REST");
todo.setDescription("Read ");
contentProvider.put("1", todo);
todo = new Todo("2", "Do something");
todo.setDescription("Read complete ");
contentProvider.put("2", todo);
} public Map getModel(){ return contentProvider;
}
}
7.2. Create a simple HTML form
The rest service can be used via HTML forms. The following HTML form will allow to post new data to the service. Create the following page called create_todo.html in the WebContent folder.
Form to create a <span class="hl-keyword" style="color:#7F0055;font-weight:bold;">new</span> resource
7.3. Rest Service
Create the following classes which will be used as REST resources.
package de.vogella.jersey.todo.resources; import javax.ws.rs.Consumes; import javax.ws.rs.DELETE; import javax.ws.rs.GET; import javax.ws.rs.PUT; import javax.ws.rs.Produces; import javax.ws.rs.core.Context; import javax.ws.rs.core.MediaType; import javax.ws.rs.core.Request; import javax.ws.rs.core.Response; import javax.ws.rs.core.UriInfo; import javax.xml.bind.JAXBElement; import de.vogella.jersey.todo.dao.TodoDao; import de.vogella.jersey.todo.model.Todo; public class TodoResource { @Context UriInfo uriInfo; @Context Request request;
String id; public TodoResource(UriInfo uriInfo, Request request, String id) { this.uriInfo = uriInfo; this.request = request; this.id = id;
} @GET @Produces({MediaType.APPLICATION_XML, MediaType.APPLICATION_JSON}) public Todo getTodo() {
Todo todo = TodoDao.instance.getModel().get(id); if(todo==null) throw new RuntimeException("Get: Todo with " + id + " not found"); return todo;
} @GET @Produces(MediaType.TEXT_XML) public Todo getTodoHTML() {
Todo todo = TodoDao.instance.getModel().get(id); if(todo==null) throw new RuntimeException("Get: Todo with " + id + " not found"); return todo;
} @PUT @Consumes(MediaType.APPLICATION_XML) public Response putTodo(JAXBElement todo) {
Todo c = todo.getValue(); return putAndGetResponse(c);
} @DELETE public void deleteTodo() {
Todo c = TodoDao.instance.getModel().remove(id); if(c==null) throw new RuntimeException("Delete: Todo with " + id + " not found");
} private Response putAndGetResponse(Todo todo) {
Response res; if(TodoDao.instance.getModel().containsKey(todo.getId())) {
res = Response.noContent().build();
} else {
res = Response.created(uriInfo.getAbsolutePath()).build();
}
TodoDao.instance.getModel().put(todo.getId(), todo); return res;
}
}
package de.vogella.jersey.todo.resources; import java.io.IOException; import java.net.URI; import java.util.ArrayList; import java.util.List; import javax.servlet.http.HttpServletResponse; import javax.ws.rs.Consumes; import javax.ws.rs.FormParam; import javax.ws.rs.GET; import javax.ws.rs.POST; import javax.ws.rs.Path; import javax.ws.rs.PathParam; import javax.ws.rs.Produces; import javax.ws.rs.core.Context; import javax.ws.rs.core.MediaType; import javax.ws.rs.core.Request; import javax.ws.rs.core.Response; import javax.ws.rs.core.UriInfo; import de.vogella.jersey.todo.dao.TodoDao; import de.vogella.jersey.todo.model.Todo; @Path("/todos") public class TodosResource { @Context UriInfo uriInfo; @Context Request request; @GET @Produces(MediaType.TEXT_XML) public List getTodosBrowser() {
List todos = new ArrayList();
todos.addAll(TodoDao.instance.getModel().values()); return todos;
} @GET @Produces({MediaType.APPLICATION_XML, MediaType.APPLICATION_JSON}) public List getTodos() {
List todos = new ArrayList();
todos.addAll(TodoDao.instance.getModel().values()); return todos;
} @GET @Path("count") @Produces(MediaType.TEXT_PLAIN) public String getCount() { int count = TodoDao.instance.getModel().size(); return String.valueOf(count);
} @POST @Produces(MediaType.TEXT_HTML) @Consumes(MediaType.APPLICATION_FORM_URLENCODED) public void newTodo(@FormParam("id") String id, @FormParam("summary") String summary, @FormParam("description") String description, @Context HttpServletResponse servletResponse) throws IOException {
Todo todo = new Todo(id,summary); if (description!=null){
todo.setDescription(description);
}
TodoDao.instance.getModel().put(id, todo);
servletResponse.sendRedirect("../create_todo.html");
} @Path("{todo}") public TodoResource getTodo(@PathParam("todo") String id) { return new TodoResource(uriInfo, request, id);
}
}
This TodosResource uses the @PathParam "annotation to define that the id is inserted as parameter.
7.4. Run
Run you web application in Eclipse and test the availability of your REST service under: "". You should see the XML representation of your Todo items.
To see the count of Todo items use "" to see an exiting todo use "/{id}", e.g. "" to see the todo with ID 1. We currently have only todos with the id's 1 and 2, all other requests will result an HTTP error code.
Please note that with the browser you can only issue HTTP GET requests. The next chapter will use the jersey client libraries to issue get, post and delete.
7.5. Create a client
Create a new Java project called de.vogella.jersey.todo.client. Create a lib folder and place all jersey libs in this folder. Add the jars to the classpath of the project.
Create the following class.
package de.vogella.jersey.todo.client; import java.net.URI; import javax.ws.rs.core.MediaType; import javax.ws.rs.core.UriBuilder; import com.sun.jersey.api.client.Client; import com.sun.jersey.api.client.ClientResponse; import com.sun.jersey.api.client.WebResource; import com.sun.jersey.api.client.config.ClientConfig; import com.sun.jersey.api.client.config.DefaultClientConfig; import com.sun.jersey.api.representation.Form; import de.vogella.jersey.todo.model.Todo; public class Tester { public static void main(String[] args) {
ClientConfig config = new DefaultClientConfig();
Client client = Client.create(config);
WebResource service = client.resource(getBaseURI()); Todo todo = new Todo("3", "Blabla");
ClientResponse response = service.path("rest").path("todos")
.path(todo.getId()).accept(MediaType.APPLICATION_XML)
.put(ClientResponse.class, todo); System.out.println(response.getStatus()); System.out.println(service.path("rest").path("todos")
.accept(MediaType.TEXT_XML).get(String.class)); System.out.println(service.path("rest").path("todos")
.accept(MediaType.APPLICATION_JSON).get(String.class)); System.out.println(service.path("rest").path("todos")
.accept(MediaType.APPLICATION_XML).get(String.class)); System.out.println(service.path("rest").path("todos/1")
.accept(MediaType.APPLICATION_XML).get(String.class)); service.path("rest").path("todos/1").delete(); System.out.println(service.path("rest").path("todos")
.accept(MediaType.APPLICATION_XML).get(String.class)); Form form = new Form();
form.add("id", "4");
form.add("summary", "Demonstration of the client lib for forms");
response = service.path("rest").path("todos")
.type(MediaType.APPLICATION_FORM_URLENCODED)
.post(ClientResponse.class, form);
System.out.println("Form response " + response.getEntity(String.class)); System.out.println(service.path("rest").path("todos")
.accept(MediaType.APPLICATION_XML).get(String.class));
} private static URI getBaseURI() { return UriBuilder.fromUri("").build();
}
}
7.6. Using the rest service via HTML page
The above example contains a form which calls a post method of your rest service.
7.7. Using the rest service via X
Usually every programming language provide somewhere libraries for creating HTTP get, post, put and delete requests. For Java the project .
9. Questions and Discussion
If you find errors in this tutorial, please notify me (see the top of the page). Please note that due to the high volume of feedback I receive, I cannot answer questions to your implementation. Ensure you have read the as I don't respond to questions already answered there.
