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RESTful Web Services---a very good artical

WebSOAPRESTXMLSQL Server 
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In software engineering, the term software architectural style generally refers to "a set of design rules that identify the kinds of components and connectors that may be used to compose a system or subsystem."* Some common examples of architectural styles include the Pipe and Filter, Layered, Push Based, and so on. In the web services world, REpresentational State Transfer (REST) is a key design idiom that embraces a stateless client-server architecture in which the web services are viewed as resources and can be identified by their URLs. Web service clients that want to use these resources access a particular representation by transferring application content using a small globally defined set of remote methods that describe the action to be performed on the resource. REST is an analytical description of the existing web architecture, and thus the interplay between the style and the underlying HTTP protocol appears seamless.

The HTTP methods such as GET and POST are the verbs that the developer can use to describe the necessary create, read, update, and delete (CRUD) actions to be performed. Some may see an analogy to operations in SQL, which also relies on a few common verbs, as shown in Table 1. However, the REST style and HTTP protocol are mutually exclusive, and REST does not require HTTP.

Table 1: Relationships Between SQL and HTTP Verbs
 
 
Action
SQL
HTTP
<!---->
Create
Insert
PUT
Read
Select
GET
Update
Update
POST
Delete
Delete
DELETE
 
When to Use REST

Architects and developers need to decide when this particular style is an appropriate choice for their applications. A RESTFul design may be appropriate when

A SOAP-based design may be appropriate when

  • A formal contract must be established to describe the interface that the web service offers. The Web Services Description Language (WSDL) describes the details such as messages, operations, bindings, and location of the web service.

  • The architecture must address complex nonfunctional requirements. Many web services specifications address such requirements and establish a common vocabulary for them. Examples include Transactions, Security, Addressing, Trust, Coordination, and so on. Most real-world applications go beyond simple CRUD operations and require contextual information and conversational state to be maintained. With the RESTful approach, developers must build this plumbing into the application layer themselves.

  • The architecture needs to handle asynchronous processing and invocation. In such cases, the infrastructure provided by standards such as WSRM and APIs such as JAX-WS with their client-side asynchronous invocation support can be leveraged out of the box.

RESTful Support in JAX-WS

The Java API for XML Web Services (JAX-WS) provides full support for building and deploying RESTful web services. The API was developed through the Java Community Process program as JSR 224. It is tightly integrated with the Java Architecture for XML Binding (JAXB) for binding XML to Java technology data and is included in both the Java Platform, Standard Edition (Java SE) 6 and the Java Platform, Enterprise Edition (Java EE) 5.

Building RESTful Services

The example used in this section builds on the purchase order service described in the Java EE BluePrints catalog as the "Patterns and Design" white papers (Parts 1, 2, and 3). The original endpoint contained only one method -- acceptPO -- that accepted a PurchaseOrder and returned a PurchaseOrderStatus, both of which were defined by individual schemas.

Table 2 shows some more CRUD operations to add to this service.

Table 2: Additional CRUD Operations
 
 
Description
Java Technology Method Signature
<!---->
Create a new order
public Purchase OrderStatus acceptPO(PurchaseOrder order)
Retrieve an existing order
public PurchaseOrder retrievePO (String orderID)
Modify an existing order
public PurchaseOrder updatePO(PurchaseOrder order)
Cancel an order already submitted
public void cancelPO(String orderID)
 

JAX-WS enables building RESTful endpoints through a javax.xml.ws.Provider interface in the API. Provider is a generic interface that can be implemented by a class as a dynamic alternative to a service endpoint interface (SEI), and a service implementing this interface can be deployed in a Java EE container or published in a stand-alone mode through the JAX-WS Endpoint API. The Provider interface contains a single method with the following signature:

T invoke(T request)
 

Provider is a low-level generic API, but using it requires the endpoint to have an intimate knowledge of the desired message or payload structure being passed to the service. Depending on how the provider is implemented, the supported types for T and their uses are the following:

  • javax.xml.transform.Source. Allows the provider to generate and consume XML directly
  • javax.activation.DataSource. Works with MIME-typed messages
  • javax.xml.soap.SOAPMessage. Conveniently works with and manipulates the entire SOAP message

The ServiceMode annotation is used to configure the messaging mode of a Provider instance. With @ServiceMode(value=MESSAGE), the provider will receive and return entire protocol messages -- for example, a SOAP message when using the SOAP binding. With @ServiceMode(value=PAYLOAD), the runtime will pass only the message payload on to the provider -- for example, the contents of a SOAP Body element when using the SOAP binding. This is useful when you want to build a WSDL-based web service but want to directly access the XML in the payload and return XML directly as well. For details, refer to the "Switching off Data Binding" strategy in this previous article.

Code Sample 1 shows a simple stand-alone provider that, when accessed by browsing to http://127.0.0.1:8084/hello/world, returns the XML message <p>hello world</p>. Note: The URLs provided in this article are for demonstration purposes only and are not live links.

Code Sample 1

@WebServiceProvider
@ServiceMode(value=Service.Mode.PAYLOAD)
public class MyProvider implements Provider<Source> {
    public Source invoke(Source source) {
         String replyElement = new String("<p>hello world</p>");
         StreamSource reply = new StreamSource(
                                  new StringReader(replyElement));
         return reply;
      }

public static void main(String args[]) {
       Endpoint e = Endpoint.create( HTTPBinding.HTTP_BINDING,
                                     new MyProvider());
      e.publish("http://127.0.0.1:8084/hello/world");
       // Run forever  e.stop();
 }
}
 

Code Sample 2 shows an extract for the PurchaseOrderService class that implements a Provider interface. The service processes the four main HTTP methods and invokes business operations in their context. Table 3 lists the operation invoked, the sample HTTP request, the HTTP response, and the Java technology method. It is useful -- though not required -- to include the schemas representing the data exchanged and other documentation with the endpoint Web ARchive (WAR) so that clients can inspect them and reference them as needed.

Code Sample 2

@javax.xml.ws.WebServiceProvider
@javax.xml.ws.ServiceMode(value=javax.xml.ws.Service.Mode.MESSAGE)
public class PurchaseOrderService implements Provider<Source>{
    private JAXBContext jc;
    @javax.annotation.Resource(type=Object.class)
    protected WebServiceContext wsContext;
    
    public PurchaseOrderService() {
        try {
            jc = JAXBContext.newInstance("com.sun.examples.rest");
           
        } catch(JAXBException je) {
        throw new WebServiceException("Cannot create JAXBContext", je);
        }
    }

public Source invoke(Source source) {
  try{
    MessageContext mc = wsContext.getMessageContext();
    String path = (String)mc.get(MessageContext.PATH_INFO);
    String method = (String)mc.get(MessageContext.HTTP_REQUEST_METHOD);
    if (method.equals("GET")) 
          return get(mc);
    if (method.equals("POST")) 
          return post(source, mc);
    if (method.equals("PUT")) 
          return put(source, mc); 
    if (method.equals("DELETE")) 
          return delete(source, mc); 
    throw new WebServiceException("Unsupported method:" +method); 
        } catch(JAXBException je) {
            throw new WebServiceException(je);
        }
    }
// Other methods not shown here
 

With RESTFul web services, there is a natural mapping between the HTTP methods and most CRUD-like business operations that many services expose. Though there are no hard and fast rules, the following general guidelines are applicable for most cases:

  • GET is used to retrieve data or perform a query on a resource. The data returned from the web service is a representation of the requested resource.

  • POST is used to create a new resource. The web service may respond with data or status indicating success or failure.

  • PUT is used to update existing resources or data.

  • DELETE is used to remove a resource or data.

In some cases, the update and delete actions may be performed with POST operations as well, for example, when the services are consumed by browsers that do not support PUT or DELETE. The GlassFish application server and the JAX-WS API support all four HTTP operations shown in Table 1.

Table 3 shows the HTTP request and HTTP response messages for the operation in the implementation of the purchase order service.

Table 3: HTTP Request and HTTP Response Messages for the Purchase Order Service
 
 
Table 3
(Click here for larger sample.)
 

Use cases in which the service needs to indicate an exception can do this by setting the appropriate HTTP status code and response message on the MessageContext. For example, a response to an order retrieval process with an invalid ID can be implemented by setting the HTTP 400 status code and including the XML from the POProcessingProblem.xsd schema. Code Fragment 1 illustrates this.

Code Fragment 1

Code Fragment 1
(Click here for larger sample.)
 
Strategy: Implement the verb as part of the URI.

To process a purchase order, make an HTTP request to the URL http://127.0.0.1:8080/restfulwebservice-war/poservice/acceptPO

The service or resource interprets the verb in the URI as the action that it must perform. The service retrieves the necessary data from the request, which physically can be an HTTP GET or POST, and it returns data based on the PurchaseOrder.xsd schema.

Strategy: Use the protocol method to describe the verb or operation.

To retrieve a purchase order in which the order ID is ABC123, make an HTTP request using a GET operation. In this case, the requested URL would be http://127.0.0.1:8080/restfulwebservice-war/poservice/ABC123

Following is a sample HTTP request:

GET /restfulwebservice-war/poservice/ABC123 HTTP/1.0
Accept-Language: en-us
Connection: close
Content-Type: text/xml
Host: 127.0.0.1:9090
 

To cancel a purchase order in which an order ID is ABC123, make an HTTP request using a DELETE operation. In this case, the URL would also be http://127.0.0.1:8080/restfulwebservice-war/poservice/ABC123

Following is a sample HTTP request:

DELETE /restfulwebservice-war/poservice/ABC123 HTTP/1.0
Accept-Language: en-us
Connection: close
Content-Type: text/xml
Host: 127.0.0.1:9090
Pragma: no-cache
 
Consuming RESTful Services

Applications can access RESTful services in one of two ways: programmatically or by using browsers.

Accessing Services Programmatically

JAX-WS enables a client to consume RESTful web services programmatically. The main API is the javax.xml.ws.Dispatch interface described in Code Sample 3.

Code Sample 3

// T is the message type.
public interface Dispatch<T> {
// synchronous request-response
   T invoke(T msg);
// async request-response
   Response<T> invokeAsync(T msg);
   Future<?> invokeAsync(T msg, AsyncHandler<T> h);

// one-way

   void invokeOneWay(T msg);
}
 

Unlike the Provider on the server side, developers don't actually implement this API. Instead, they obtain an instance from the Service object as shown here: 

service = Service.create(qname); 
service.addPort(qname, HTTPBinding.HTTP_BINDING, url);
Dispatch<Source> dispatcher = service.createDispatch(new QName("", ""),
                                        Source.class, Service.Mode.PAYLOAD);
 

The typed Dispatch<T> interface and the invoke method can accept and return four major datatypes:

  • Dispatch<javax.xml.transform.Source>. Useful for HTTP binding payload mode
  • Dispatch<javax.xml.soap.SOAPMessage>. Useful for SOAP message mode
  • Dispatch<javax.activation.DataSource>. Useful for handling MIME messages
  • Dispatch<Object>. Useful for payload mode with JAXB binding

Code Fragment 2 demonstrates how to make a POST request to http://127.0.0.1:8080/restfulwebservice-war/poservice/acceptPO with the XML as the body of the POST request read from a string.

Code Fragment 2

private void acceptPO() {
    Service service = Service.create(qname);
    service.addPort(qname, HTTPBinding.HTTP_BINDING, url + "acceptPO");
    Dispatch<Source> dispatcher = service.createDispatch(qname, 
                                           Source.class, Service.Mode.MESSAGE);
    Map<String, Object> requestContext = dispatcher.getRequestContext();
    requestContext.put(MessageContext.HTTP_REQUEST_METHOD, "POST");
    Source result = dispatcher.invoke(new StreamSource(new StringReader(poXML)));
    printSource(result);
}
 

Code Fragment 3 demonstrates how to make a similar POST request, but it differs from Code Fragment 2 in that it sends and returns JAXB-generated objects rather than handling strings directly.

Code Fragment 3

Code Fragment 3
(Click here for larger sample.)
 

Code Fragment 4 demonstrates how to make a similar PUT request using the Dispatch interface.

Code Fragment 4

Code Fragment 4
(Click here for larger sample.)
 

Accessing Services Using Browsers

Because RESTful web services deployed in JAX-WS are exposed using the standard HTTP protocol and methods, they can be easily accessed from browsers. In addition to using simple GET and POST requests directly from browsers, developers can leverage the capabilities of the JavaScript technology XMLHttpRequest object that most modern browsers support. This is the same object used for building Ajax user interfaces (UIs).

Code Sample 4 shows a simple script that is included with the downloadable sample code and test client that you can use to test RESTful web services from browsers.

Code Sample 4

Code Sample 4
(Click here for larger sample.)
 

Figure 1 shows the display resulting from this code.

Figure 1: Resulting Display
Figure 1: Resulting Display (Click image for larger view.)
 
Describing RESTful Endpoints

Unlike SOAP-based web services, which have a standard vocabulary to describe the web service interface through WSDL, RESTful web services currently have no such grammar. For a service consumer to understand the context and content of the data that must be sent to and received from the service, both the service consumer and service producer must have an out-of-band agreement. This takes the form of documentation, sample code, and an API that the service provider publishes for developers to use. For example, the many web-based services available from Google, Yahoo, Flickr, and so on have accompanying artifacts describing how to consume the services.

If you are developing RESTful web services, the following general guidelines provide a good starting point:

  • Make the XML schemas available to service consumers and package them with the WAR file.

  • Clearly document the expected input, output, and error conditions that may arise as result of invocation.
Web Application Description Language (WADL)

The style of documenting RESTful web services that this article has previously described is fine for use by developers, but it prevents tools from programmatically consuming such services and generating artifacts specific to programming languages. For example, a WSDL file can be consumed by various tools and proxies or by generated stubs that applications can use directly. A research effort from Sun Labs called Web Application Description Language (WADL) attempts to resolve some of these issues by providing a means to describe services in terms of schemas, HTTP methods, and the request or response structures exchanged. The schema in Code Sample 5 shows a sample WADL description for the example discussed previously.

Code Sample 5

Code Sample 5
(Click here for larger sample.)
 
Summary

JAX-WS provides comprehensive support for building web services. Developers can leverage the capabilities of this API to build and consume a variety of web services, whether those services are based on WSDL or are RESTful in behavior. The combination of the Provider and Dispatch interfaces allows web services to be built and consumed, and it provides developers with the flexibility to process the messages sent over the wire in a variety of ways. In addition, the future holds the possibility of describing RESTful web services for tools to consume, which will further simplify the developer's experience.
For More Information
About the Author

Sameer Tyagi is a senior staff engineer at Sun Microsystems with the Web Services group. He remains focused on architecture, design, and implementation of large-scale enterprise applications with Java technology. Among his publications are industry periodicals and books on Java and Java EE technologies, as well as a blog.

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1 楼 younker 2007-09-08  
这篇文章详细讲述了REST的实现，是一篇不可多得的好文章
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