Restlet第一个Resource（翻译）

猫咪继续翻译Restlet教程。看了这篇文章，应该对Restlet框架有所了解。

Restlet第一个Resource
目录
这篇文章说明Resource如何处理GET，POST，PUT和DELETE方法。
1.引言
2.例子应用
3.实现Items Resource
4.实现Item Resource
5.实现Base Resource
6.运行应用
7.客户端应用
8.结尾

引言
在开始开发前，我们需要简单介绍一下Restlet框架的Resource概念。REST告诉我们，Resource根据URL进行辨认，同时能够有一种或多种表现（也被称作变异体）呈现（Oaxaca的天气），能够响应处理方法。
在Restlet框架中，服务器连接的标准响应由Resource的实例对象最终处理。一个Resource负责声明支持的表现方式列表（Variant对象的实例）和实现你想要支持的REST方法。
 GET依赖可更改的“variants”列表和“getRepresentation(Variant)”方法。
 POST依赖“allowPost”方法和“post(Representation)”方法。
 DELETE依赖“allowPut”方法和“put(Representation)”方法。
 DELETE依赖“allowDelete”方法和“delete()”方法。
还有，每一个到达的响应由一个专门的Resource实例处理，你不需要操心这些方法的线程安全。
我们假设你已经读过“第一步”（http://www.restlet.org/documentation/1.0/firstSteps），并且有了一些元件和应用的概念。

例子应用
让我们开始描述我们的例子应用。一个Item列表用来管理创建，读取，更新，和删除活动像一个简单的CRUD应用。一个Item是以名字和描述的简单特征。在简短的分析后，我们定义了两个资源：
 Items Resource等于所有可用Item的集合。
 Item Resource等于一个单独的item。
现在，让我们定义Resource URIs。假设我们的应用运行在本机“localhost”并且监听8182端口：
 http://localhost:8182/firstResource/items：“items”Resource URI。
 http://localhost:8182/firstResource/items/{itemName}：“item”Resource URI，每个{itemName}代表一个Item的名字。
下一步，该定义允许访问每个Resource的方法列表。
 “items”Resource响应GET请求并以一个XML文档展示当前注册的Item列表。另外，Resource支持通过POST请求创建新的Item。提交的实体包含新Item的名字和描述并接收格式化的Web表单。如果Resource成功创建新Item，它返回一个“Success - resource created”状态（HTTP 201状态代码）并且告诉客户端新Resource在哪里可以找到（HTTP "Location" header）。否则，它返回一个“Client error”状态（HTTP 404状态代码）和一个简单的错误信息。
 “item”Resource响应GET请求并以一个XML文档来展示该Resource的名字和描述。通过PUT和DELETE请求也能更新和删除Resource。
在描述两个Resource对象前，先编写你的应用。为了简化，注册的Item列表做为应用的一个属性简单地保存到内存里，并不保存到一个真实的数据库。不管怎样，我们假设你想邀请你的朋友们同时测试这个应用。因为我们只有一个“FirstResourceApplication”实例在运行，所以不得不考虑线程安全。这也就是为什么你会发现Map对象Items是不变的，是一个ConcurrentHashMap对象的实例。

package firstResource;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import org.restlet.Application;
import org.restlet.Context;
import org.restlet.Restlet;
import org.restlet.Router;
public class FirstResourceApplication extends Application {
   /** The list of items is persisted in memory. */
   private final Map items;
   public FirstResourceApplication(Context parentContext) {
      super(parentContext);
      // We make sure that this attribute will support concurrent access.
      items = new ConcurrentHashMap();
   }
   /**
    * Creates a root Restlet that will receive all incoming calls.
    */
   @Override
   public synchronized Restlet createRoot() {
      // Create a router Restlet that defines routes.
      Router router = new Router(getContext());
      // Defines a route for the resource "list of items"
      router.attach("/items", ItemsResource.class);
      // Defines a route for the resource "item"
      router.attach("/items/{itemName}", ItemResource.class);
      return router;
   }
   /**
    * Returns the list of registered items.
    * @return the list of registered items.
    */
   public Map getItems() {
      return items;
   }
}

实现Items Resource
让我们开始编写Items Resource。如上文所示，允许GET和POST请求。POST请求支持通过“post(Representation)”方法确定你如何处理消息实体。此外，资源通过“allowPost”方法支持POST。缺省情况下，资源是不可更改的，拒绝POST、PUT和DELETE方法并返回“Method not allowed”状态（HTTP 405状态代码）。同样，GET响应支持通过“represent(Variant)”方法确定你如何依照特定的Variant生成实体。假设，我们只生成“text/xml”形式。

import java.io.IOException;
import java.util.Collection;
import org.restlet.Context;
import org.restlet.data.Form;
import org.restlet.data.MediaType;
import org.restlet.data.Request;
import org.restlet.data.Response;
import org.restlet.data.Status;
import org.restlet.resource.DomRepresentation;
import org.restlet.resource.Representation;
import org.restlet.resource.StringRepresentation;
import org.restlet.resource.Variant;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
/**
 * Resource that manages a list of items.
 * 
 */
public class ItemsResource extends BaseResource {
   /** List of items. */
   Collection items;
   public ItemsResource(Context context, Request request, Response response) {
      super(context, request, response);
      // Get the items directly from the "persistence layer".
      items = getItems().values();
      // Declare the kind of representations supported by this resource.
      getVariants().add(new Variant(MediaType.TEXT_XML));
   }
   @Override
   public boolean allowPost() {
      return true;
   }
   /**
    * Returns a listing of all registered items.
    */
   @Override
   public Representation getRepresentation(Variant variant) {
      // Generate the right representation according to its media type.
      if (MediaType.TEXT_XML.equals(variant.getMediaType())) {
         try {
            DomRepresentation representation = new DomRepresentation(
                  MediaType.TEXT_XML);
            // Generate a DOM document representing the list of
            // items.
            Document d = representation.getDocument();
            Element r = d.createElement("items");
            d.appendChild(r);
            for (Item item : items) {
               Element eltItem = d.createElement("item");
               Element eltName = d.createElement("name");
               eltName.appendChild(d.createTextNode(item.getName()));
               eltItem.appendChild(eltName);
               Element eltDescription = d.createElement("description");
               eltDescription.appendChild(d.createTextNode(item
                     .getDescription()));
               eltItem.appendChild(eltDescription);
               r.appendChild(eltItem);
            }
            d.normalizeDocument();
            // Returns the XML representation of this document.
            return representation;
         } catch (IOException e) {
            e.printStackTrace();
         }
      }
      return null;
   }
   /**
    * Handle POST requests: create a new item.
    */
   @Override
   public void post(Representation entity) {
      // Parse the given representation and retrieve pairs of
      // "name=value" tokens.
      Form form = new Form(entity);
      String itemName = form.getFirstValue("name");
      String itemDescription = form.getFirstValue("description");
      // Check that the item is not already registered.
      if (getItems().containsKey(itemName)) {
         generateErrorRepresentation(
               "Item " + itemName + " already exists.", "1", getResponse());
      } else {
         // Register the new item
         getItems().put(itemName, new Item(itemName, itemDescription));
         // Set the response's status and entity
         getResponse().setStatus(Status.SUCCESS_CREATED);
         Representation rep = new StringRepresentation("Item created",
               MediaType.TEXT_PLAIN);
         // Indicates where is located the new resource.
         rep.setIdentifier(getRequest().getResourceRef().getIdentifier()
               + "/" + itemName);
         getResponse().setEntity(rep);
      }
   }
   /**
    * Generate an XML representation of an error response.
    * 
    * @param errorMessage
    *            the error message.
    * @param errorCode
    *            the error code.
    */
   private void generateErrorRepresentation(String errorMessage,
         String errorCode, Response response) {
      // This is an error
      response.setStatus(Status.CLIENT_ERROR_NOT_FOUND);
      try {
         // Generate the output representation
         DomRepresentation representation = new DomRepresentation(
               MediaType.TEXT_XML);
         Document d = representation.getDocument();
         Element eltError = d.createElement("error");
         Element eltCode = d.createElement("code");
         eltCode.appendChild(d.createTextNode(errorCode));
         eltError.appendChild(eltCode);
         Element eltMessage = d.createElement("message");
         eltMessage.appendChild(d.createTextNode(errorMessage));
         eltError.appendChild(eltMessage);
         response.setEntity(new DomRepresentation(MediaType.TEXT_XML, d));
      } catch (IOException e) {
         e.printStackTrace();
      }
   }
}

实现Item Resource
让我们继续编写Item Resource。如上文所示，允许GET，PUT和DELETE请求。PUT请求支持通过“put(Representation)”方法确定你如何处理消息实体。
同样，DELETE请求支持执行“delete()”方法。

import java.io.IOException;
import org.restlet.Context;
import org.restlet.data.Form;
import org.restlet.data.MediaType;
import org.restlet.data.Request;
import org.restlet.data.Response;
import org.restlet.data.Status;
import org.restlet.resource.DomRepresentation;
import org.restlet.resource.Representation;
import org.restlet.resource.Variant;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
public class ItemResource extends BaseResource {
   /** The underlying Item object. */
   Item item;
   /** The sequence of characters that identifies the resource. */
   String itemName;
   public ItemResource(Context context, Request request, Response response) {
      super(context, request, response);
      // Get the "itemName" attribute value taken from the URI template
      // /items/{itemName}.
      this.itemName = (String) getRequest().getAttributes().get("itemName");
      // Get the item directly from the "persistence layer".
      this.item = getItems().get(itemName);
      if (this.item != null) {
         // Define the supported variant.
         getVariants().add(new Variant(MediaType.TEXT_XML));
      }
   }
   /**
    * This resource supports DELETE requests.
    */
   @Override
   public boolean allowDelete() {
      return true;
   }
   /**
    * This resource supports PUT requests.
    */
   @Override
   public boolean allowPut() {
      return true;
   }
   /**
    * Handle DELETE requests.
    */
   @Override
   public void delete() {
      if (item != null) {
         // Remove the item from the list.
         getItems().remove(item.getName());
      }
      // Tells the client that the request has been successfully fulfilled.
      getResponse().setStatus(Status.SUCCESS_NO_CONTENT);
   }
   @Override
   public Representation getRepresentation(Variant variant) {
      if (MediaType.TEXT_XML.equals(variant.getMediaType())) {
         // Generate the XML representation of this resource.
         try {
            // Generate a DOM document representing the item.
            DomRepresentation representation = new DomRepresentation(
                  MediaType.TEXT_XML);
            Document d = representation.getDocument();
            Element eltItem = d.createElement("item");
            d.appendChild(eltItem);
            Element eltName = d.createElement("name");
            eltName.appendChild(d.createTextNode(item.getName()));
            eltItem.appendChild(eltName);
            Element eltDescription = d.createElement("description");
            eltDescription.appendChild(d.createTextNode(item
                  .getDescription()));
            eltItem.appendChild(eltDescription);
            d.normalizeDocument();
            // Returns the XML representation of this document.
            return representation;
         } catch (IOException e) {
            e.printStackTrace();
         }
      }
      return null;
   }
   /**
    * Handle PUT requests.
    */
   @Override
   public void put(Representation entity) {
      // Tells if the item is to be created of not.
      boolean creation = (item == null);
      // The PUT request updates or creates the resource.
      if (item == null) {
         item = new Item(itemName);
      }
      // Update the description.
      Form form = new Form(entity);
      item.setDescription(form.getFirstValue("description"));
      // Update the item in the list.
      getItems().put(item.getName(), item);
      if (creation) {
         getResponse().setStatus(Status.SUCCESS_CREATED);
      } else {
         getResponse().setStatus(Status.SUCCESS_OK);
      }
   }
}

实现Base Resource
由于我们的Resource都需要访问“FirstResourceApplication”实例中保存Item的Map，这些方法因此被放入父类“BaseResource”。

import java.util.Map;
import org.restlet.Application;
import org.restlet.Context;
import org.restlet.data.Request;
import org.restlet.data.Response;
import org.restlet.resource.Resource;
/**
 * Base resource class that supports common behaviours or attributes shared by
 * all resources.
 * 
 */
public abstract class BaseResource extends Resource {
   public BaseResource(Context context, Request request, Response response) {
      super(context, request, response);
   }
   /**
    * Returns the map of items managed by this application.
    * 
    * @return the map of items managed by this application.
    */
   protected Map getItems() {
      return ((FirstResourceApplication) getContext().getAttributes().get(
            Application.KEY)).getItems();
   }
}

运行应用
请查阅“第一步”（http://www.restlet.org/documentation/1.0/firstSteps）的相关章节学习如何在一个Servlet容器中运行应用或作为一个单独应用执行。

客户端应用
一旦我们的应用在一个Servlet容器或作为一个单独应用开始运行，我们建议你通过一个简单的客户端应用测试我们的Resource。它简单地创建、读取、更新和删除一个Item Resource并在每一个操作执行时打印标准输出结果。
客户端应用作为单独Java应用运行的时候需要如下的Jar包：
 org.restlet.jar
 com.noelios.restlet.jar
 com.noelios.restlet.ext.httpclient.jar
 org.apache.commons.httpclient.jar

import java.io.IOException;
import org.restlet.Client;
import org.restlet.data.Form;
import org.restlet.data.Protocol;
import org.restlet.data.Reference;
import org.restlet.data.Response;
import org.restlet.resource.Representation;
public class FirstResourceClientMain {
   public static void main(String[] args) throws IOException {
      // Define our Restlet HTTP client.
      Client client = new Client(Protocol.HTTP);
      // The URI of the resource "list of items".
      Reference itemsUri = new Reference(
            "http://localhost:8182/firstResource/items");
      // Create a new item
      Item item = new Item("item1", "this is an item.");
      Reference itemUri = createItem(item, client, itemsUri);
      if (itemUri != null) {
         // Prints the representation of the newly created resource.
         get(client, itemUri);
      }
      // Prints the list of registered items.
      get(client, itemsUri);
      // Update the item
      item.setDescription("This is an other description");
      updateItem(item, client, itemUri);
      // Prints the list of registered items.
      get(client, itemsUri);
      // delete the item
      deleteItem(client, itemUri);
      // Print the list of registered items.
      get(client, itemsUri);
   }
   /**
    * Try to create a new item.
    *
    * @param item
    *            the new item.
    * @param client
    *            the Restlet HTTP client.
    * @param itemsUri
    *            where to POST the data.
    * @return the Reference of the new resource if the creation succeeds,
    *         null otherwise.
    */
   public static Reference createItem(Item item, Client client,
         Reference itemsUri) {
      // Gathering informations into a Web form.
      Form form = new Form();
      form.add("name", item.getName());
      form.add("description", item.getDescription());
      Representation rep = form.getWebRepresentation();
      // Launch the request
      Response response = client.post(itemsUri, rep);
      if (response.getStatus().isSuccess()) {
         if (response.isEntityAvailable()) {
            try {
               // Always consume the response's entity, if available.
               response.getEntity().write(System.out);
            } catch (IOException e) {
               e.printStackTrace();
            }
         }
         return response.getEntity().getIdentifier();
      }
      return null;
   }
   /**
    * Prints the resource's representation.
    *
    * @param client
    *            client Restlet.
    * @param reference
    *            the resource's URI
    * @throws IOException
    */
   public static void get(Client client, Reference reference)
         throws IOException {
      Response response = client.get(reference);
      if (response.getStatus().isSuccess()) {
         if (response.isEntityAvailable()) {
            response.getEntity().write(System.out);
         }
      }
   }
   /**
    * Try to update an item.
    *
    * @param item
    *            the item.
    * @param client
    *            the Restlet HTTP client.
    * @param itemUri
    *            the resource's URI.
    */
   public static boolean updateItem(Item item, Client client,
            Reference itemUri) {
      // Gathering informations into a Web form.
      Form form = new Form();
      form.add("name", item.getName());
      form.add("description", item.getDescription());
      Representation rep = form.getWebRepresentation();
      // Launch the request
      Response response = client.put(itemUri, rep);
      if (response.isEntityAvailable()) {
         try {
            // Always consume the response's entity, if available.
            response.getEntity().write(System.out);
         } catch (IOException e) {
            e.printStackTrace();
         }
      }
      return response.getStatus().isSuccess();
   }
   /**
    * Try to delete an item.
    *
    * @param client
    *            the Restlet HTTP client.
    * @param itemUri
    *            the resource's URI.
    */
   public static boolean deleteItem(Client client, Reference itemUri) {
      // Launch the request
      Response response = client.delete(itemUri);
      if (response.isEntityAvailable()) {
         try {
            // Always consume the response's entity, if available.
            response.getEntity().write(System.out);
         } catch (IOException e) {
            e.printStackTrace();
         }
      }
      return response.getStatus().isSuccess();
   }
}

你可以从http://www.restlet.org/documentation/1.0/examples/firstResource/sources.zip下载这个例子应用的源文件。

结尾
这篇文章说明了使用Restlet框架的重要方法。当思考你的Resource的时候，记住这些重要问题：
 我应该如何识别我的Resource?
 它们能够生成哪些表现？
 它们有哪些处理方法？
 我需要把同级Resource的哪些行为和属性共享？
我们希望你喜欢这些简单步骤并且我们现在鼓励你开始深入Restlet Tutorial。

备注：
感谢Tim Peierls反馈线程安全方面的考虑。