C#以post方式调用struts rest-plugin service的问题

struts2: 玩转 rest-plugin 一文中，学习了用struts2开发restful service的方法，发现用c#以post方式调用时各种报错，但java、ajax，包括firefox 的rest client插件测试也无问题。

先给出rest service中的这个方法：

    // POST /orders
    public HttpHeaders create() throws IOException, ServletException {
        ordersService.doSave(model);
        HttpServletResponse response = ServletActionContext.getResponse();
        HttpServletRequest request = ServletActionContext.getRequest();
        String ContentType = request.getHeader("Content-Type").toLowerCase();
        if (ContentType.startsWith("application/xml")) { // 返回xml视图
            response.sendRedirect("orders/" + model.getId() + ".xml");
        } else if (ContentType.startsWith("application/json")) { // 返回json视图
            response.sendRedirect("orders/" + model.getId() + ".json");
        } else {// 返回xhtml页面视图
            response.sendRedirect("orders/");
        }
        return null;
    }

代码不复杂，post一段String过来(xml/json/html格式均可)，自动映射成Order对象的实例model，然后根据请求HttpHeader中的Content-Type，如果是xml(application/xml)，则返回model对应的xml，如果是json(application/json)，则返回model对应的json，其它则返回页面

c#的调用代码：

static string PostDataByWebClient(String postUrl, String paramData, String mediaType)
{
    String result = String.Empty;
    try
    {
        byte[] postData = Encoding.UTF8.GetBytes(paramData);
        WebClient webClient = new WebClient();
        webClient.Headers.Add("Content-Type", mediaType);
        byte[] responseData = webClient.UploadData(new Uri(postUrl), "POST", postData);
        result = Encoding.UTF8.GetString(responseData);
    }
    catch (Exception e)
    {
        Console.WriteLine(e);
        result = e.Message;
    }
    return result;
}

static string PostDataByWebRequest(string postUrl, string paramData, String mediaType)
{
    string result = string.Empty;
    Stream newStream = null;
    StreamReader sr = null;
    HttpWebResponse response = null;
    try
    {
        byte[] byteArray = Encoding.UTF8.GetBytes(paramData);
        HttpWebRequest webReq = (HttpWebRequest)WebRequest.Create(new Uri(postUrl));
        webReq.Method = "POST";
        webReq.ContentType = mediaType;
        webReq.ContentLength = byteArray.Length;
        newStream = webReq.GetRequestStream();
        newStream.Write(byteArray, 0, byteArray.Length);
        response = (HttpWebResponse)webReq.GetResponse();
        sr = new StreamReader(response.GetResponseStream(), Encoding.UTF8);
        result = sr.ReadToEnd();
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex);
        result = ex.Message;
    }
    finally
    {
        if (sr != null)
        {
            sr.Close();
        }
        if (response != null)
        {
            response.Close();
        }
        if (newStream != null)
        {
            newStream.Close();
        }
    }
    return result;
}

这二种常用的调用方式，居然全跪了，返回的结果是一堆java异常：
 java.lang.NullPointerException
        at org.apache.struts2.convention.ConventionUnknownHandler.handleUnknownActionMethod(ConventionUnknownHandler.java:423)
        at com.opensymphony.xwork2.DefaultUnknownHandlerManager.handleUnknownMethod(DefaultUnknownHandlerManager.java:96)

...

无奈百度了一圈，发现还有另一种方法，利用TcpClient调用

static string PostDataByTcpClient(string postUrl, string paramData, String mediaType)
{
    String result = String.Empty;
    TcpClient clientSocket = null;
    Stream readStream = null;
    try
    {
        clientSocket = new TcpClient();
        Uri URI = new Uri(postUrl);
        clientSocket.Connect(URI.Host, URI.Port);
        StringBuilder RequestHeaders = new StringBuilder();//用来保存HTML协议头部信息
        RequestHeaders.AppendFormat("{0} {1} HTTP/1.1
", "POST", URI.PathAndQuery);
        RequestHeaders.AppendFormat("Connection:close
");
        RequestHeaders.AppendFormat("Host:{0}:{1}
", URI.Host,URI.Port);
        RequestHeaders.AppendFormat("Content-Type:{0}
", mediaType);
        RequestHeaders.AppendFormat("
");
        RequestHeaders.Append(paramData + "
");
        Encoding encoding = Encoding.UTF8;
        byte[] request = encoding.GetBytes(RequestHeaders.ToString());
        clientSocket.Client.Send(request);
        readStream = clientSocket.GetStream();
        StreamReader sr = new StreamReader(readStream, Encoding.UTF8);
        result = sr.ReadToEnd();
    }
    catch (Exception e)
    {
        Console.WriteLine(e);
        result = e.Message;
    }
    finally
    {
        if (readStream != null)
        {
            readStream.Close();
        }
        if (clientSocket != null)
        {
            clientSocket.Close();
        }
    }
    return result;
}

总算调用成功了，但是由于java端是用SendRedirect在客户端重定向的，所以该方法得到的返回结果如下：

HTTP/1.1 302 Found
Server: Apache-Coyote/1.1
Location: http://localhost:8080/struts2-rest-ex/rest/orders/230.xml
Content-Length: 0
Date: Mon, 27 Oct 2014 03:18:56 GMT
Connection: close

是一堆http头的原文，只能曲线救国，将其中的Location:后的部分（即重定向的url），取出来再次get请求。

这样的解决方案显然有点笨拙，继续深挖：

org.apache.struts2.rest.RestActionMapper这个类的getMapping()方法，看下源码：

    public ActionMapping getMapping(HttpServletRequest request,
            ConfigurationManager configManager) {
        ActionMapping mapping = new ActionMapping();
        String uri = RequestUtils.getUri(request);

        uri = dropExtension(uri, mapping);
        if (uri == null) {
            return null;
        }

        parseNameAndNamespace(uri, mapping, configManager);

        handleSpecialParameters(request, mapping);

        if (mapping.getName() == null) {
            return null;
        }

        // handle "name!method" convention.
        handleDynamicMethodInvocation(mapping, mapping.getName());

        String fullName = mapping.getName();
        // Only try something if the action name is specified
        if (fullName != null && fullName.length() > 0) {

            // cut off any ;jsessionid= type appendix but allow the rails-like ;edit
            int scPos = fullName.indexOf(';');
            if (scPos > -1 && !"edit".equals(fullName.substring(scPos + 1))) {
                fullName = fullName.substring(0, scPos);
            }

            int lastSlashPos = fullName.lastIndexOf('/');
            String id = null;
            if (lastSlashPos > -1) {

                // fun trickery to parse 'actionName/id/methodName' in the case of 'animals/dog/edit'
                int prevSlashPos = fullName.lastIndexOf('/', lastSlashPos - 1);
                if (prevSlashPos > -1) {
                    mapping.setMethod(fullName.substring(lastSlashPos + 1));
                    fullName = fullName.substring(0, lastSlashPos);
                    lastSlashPos = prevSlashPos;
                }
                id = fullName.substring(lastSlashPos + 1);
            }



            // If a method hasn't been explicitly named, try to guess using ReST-style patterns
            if (mapping.getMethod() == null) {

                if (isOptions(request)) {
                    mapping.setMethod(optionsMethodName);
                
                // Handle uris with no id, possibly ending in '/'
                } else if (lastSlashPos == -1 || lastSlashPos == fullName.length() -1) {

                    // Index e.g. foo
                    if (isGet(request)) {
                        mapping.setMethod(indexMethodName);
                        
                    // Creating a new entry on POST e.g. foo
                    } else if (isPost(request)) {
                        if (isExpectContinue(request)) {
                            mapping.setMethod(postContinueMethodName);
                        } else {
                            mapping.setMethod(postMethodName);
                        }
                    }

                // Handle uris with an id at the end
                } else if (id != null) {
                    
                    // Viewing the form to edit an item e.g. foo/1;edit
                    if (isGet(request) && id.endsWith(";edit")) {
                        id = id.substring(0, id.length() - ";edit".length());
                        mapping.setMethod(editMethodName);
                        
                    // Viewing the form to create a new item e.g. foo/new
                    } else if (isGet(request) && "new".equals(id)) {
                        mapping.setMethod(newMethodName);

                    // Removing an item e.g. foo/1
                    } else if (isDelete(request)) {
                        mapping.setMethod(deleteMethodName);
                        
                    // Viewing an item e.g. foo/1
                    } else if (isGet(request)) {
                        mapping.setMethod(getMethodName);
                    
                    // Updating an item e.g. foo/1    
                    }  else if (isPut(request)) {
                        if (isExpectContinue(request)) {
                            mapping.setMethod(putContinueMethodName);
                        } else {
                            mapping.setMethod(putMethodName);
                        }
                    }
                }
            }

            // cut off the id parameter, even if a method is specified
            if (id != null) {
                if (!"new".equals(id)) {
                    if (mapping.getParams() == null) {
                        mapping.setParams(new HashMap());
                    }
                    mapping.getParams().put(idParameterName, new String[]{id});
                }
                fullName = fullName.substring(0, lastSlashPos);
            }

            mapping.setName(fullName);
            return mapping;
        }
        // if action name isn't specified, it can be a normal request, to static resource, return null to allow handle that case
        return null;
    }

注意91-96行，这里有一个判断：

                    }  else if (isPut(request)) {
                        if (isExpectContinue(request)) {
                            mapping.setMethod(putContinueMethodName);
                        } else {
                            mapping.setMethod(putMethodName);
                        }
                    }

再来细看下：isExpectContinue

    protected boolean isExpectContinue(HttpServletRequest request) {
        String expect = request.getHeader("Expect");
        return (expect != null && expect.toLowerCase().contains("100-continue")); 
    }

这段代码的意思是如果请求Http头里有Except信息，且等于100-continue，则返回true。如果返回true，刚才那段判断，会返回putContinueMethodName这个变量所指的方法：

 private String postContinueMethodName = "createContinue";

但是Controller里只有create方法，并没有createContinue方法，所以找不到方法，当然报错。

而c#中如果以post方法请求url时，不论是HttpWebRequest还是WebClient，默认都会添加expect = 100-continue的头信息，因此c#调用时会报错，而firefox的RestClient插件、java调用、ajax调用，因为没有拼except信息，不会出错。

那么except = 100-continue是什么东西呢？为何c#要自动拼这上这行头信息？可以参见园友的文章：http之100-continue，大意是说：

如果客户端向服务端post数据，考虑到post的数据可能很大，搞不好能把服务器玩坏（或者超时），所以，有一个贴心的约定，客户端先发一个except头信息给服务器，问下：我要post数据了，可能很大，你想想要不要收，采用什么措施收？如果服务器很聪明，可能会对这种情况做出特殊响应，就比如刚才的java代码，遇到这种头信息，不是调用create方法，而是createContinue方法。

这本是一个不错的约定，但是偏偏本文中的Controller方法，又没有提供createContinue方法，所以辜负了客户端的美意，好心当成驴肝肺了。

终极解决方案：

方案A：HttpWebRequest请求时，把默认的except行为去掉

webReq.ServicePoint.Expect100Continue = false;//禁止自动添加Except:100-continue到http头信息

这样，最终发出去的头信息，就不会有except行

方案B: Controller中把createContinue方法补上

    public HttpHeaders createContinue() throws IOException, ServletException{
        return create();
    }

直接调用create方法，安抚下双方，不让调用出错即可。