1 HTTP

    HTTP 協議可能是現在 Internet 上使用得最多、最重要的協議了，越來越多的 java 應用程序需要直接通過 HTTP 協議來訪問網絡資源。

    雖然在 JDK 的 java.net 包中已經提供了訪問 HTTP 協議的基本功能，但是對于大部分應用程序來說，JDK 庫本身提供的功能還不夠豐富和靈活。HttpClient 用來提供高效的、最新的、功能豐富的支持 HTTP 協議的客戶端編程工具包，并且它支持 HTTP 協議最新的版本和建議。

    一般的情況下我們都是使用Chrome或者其他瀏覽器來訪問一個WEB服務器，用來瀏覽頁面查看信息或者提交一些數據、文件上傳下載等等。所訪問的這些頁面有的僅僅是一些普通的頁面，有的需要用戶登錄后方可使用，或者需要認證以及是一些通過加密方式傳輸，例如HTTPS。目前我們使用的瀏覽器處理這些情況都不會構成問題。但是一旦我們有需求不通過瀏覽器來訪問服務器的資源呢？那該怎么辦呢？

    下面以本地客戶端發起文件的上傳、下載為例做個小Demo。HttpClient有兩種形式，一種是org.apache.http下的，一種是org.apache.commons.httpclient.HttpClient。

2 文件上傳

    文件上傳可以使用兩種方式實現，一種是PostMethod方式，一種是HttpPost方式。兩者的處理大同小異。PostMethod是使用FileBody將文件包裝流包裝起來，HttpPost是使用FilePart將文件流包裝起來。在傳遞文件流給服務端的時候，都可以同時傳遞其他的參數。

2.1 客戶端處理

2.1.1 PostMethod方式

     將文件封裝到FilePart中，放入Part數組，同時，其他參數可以放入StringPart中，這里沒有寫，只是單純的將參數以setParameter的方式進行設置。此處的HttpClient是org.apache.commons.httpclient.HttpClient。

public void upload(String localFile){        File file = new File(localFile);        PostMethod filePost = new PostMethod(URL_STR);        HttpClient client = new HttpClient();                try {            // 通過以下方法可以模擬頁面參數提交            filePost.setParameter("userName", userName);            filePost.setParameter("passwd", passwd);            Part[] parts = { new FilePart(file.getName(), file) };            filePost.setRequestEntity(new MultipartRequestEntity(parts, filePost.getParams()));                        client.getHttpConnectionManager().getParams().setConnectionTimeout(5000);                        int status = client.executeMethod(filePost);            if (status == HttpStatus.SC_OK) {                System.out.PRintln("上傳成功");            } else {                System.out.println("上傳失敗");            }        } catch (Exception ex) {            ex.printStackTrace();        } finally {            filePost.releaseConnection();        }    }記得搞完之后，要通過releaseConnection釋放連接。 
2.1.2 HttpPost方式
     這種方式，與上面類似，只不過變成了FileBody。上面的Part數組在這里對應HttpEntity。此處的HttpClient是org.apache.http.client.methods下的。
public void upload(String localFile){        CloseableHttpClient httpClient = null;        CloseableHttpResponse response = null;        try {            httpClient = HttpClients.createDefault();                        // 把一個普通參數和文件上傳給下面這個地址 是一個servlet            HttpPost httpPost = new HttpPost(URL_STR);                        // 把文件轉換成流對象FileBody            FileBody bin = new FileBody(new File(localFile));            StringBody userName = new StringBody("Scott", ContentType.create(                    "text/plain", Consts.UTF_8));            StringBody passWord = new StringBody("123456", ContentType.create(                    "text/plain", Consts.UTF_8));            HttpEntity reqEntity = MultipartEntityBuilder.create()                    // 相當于<input type="file" name="file"/>                    .addPart("file", bin)                                        // 相當于<input type="text" name="userName" value=userName>                    .addPart("userName", userName)                    .addPart("pass", password)                    .build();            httpPost.setEntity(reqEntity);            // 發起請求 并返回請求的響應            response = httpClient.execute(httpPost);                        System.out.println("The response value of token:" + response.getFirstHeader("token"));                            // 獲取響應對象            HttpEntity resEntity = response.getEntity();            if (resEntity != null) {                // 打印響應長度                System.out.println("Response content length: " + resEntity.getContentLength());                // 打印響應內容                System.out.println(EntityUtils.toString(resEntity, Charset.forName("UTF-8")));            }                        // 銷毀            EntityUtils.consume(resEntity);        }catch (Exception e){            e.printStackTrace();        }finally {            try {                if(response != null){                    response.close();                }            } catch (IOException e) {                e.printStackTrace();            }                        try {                if(httpClient != null){                    httpClient.close();                }            } catch (IOException e) {                e.printStackTrace();            }        }    }
2.2 服務端處理
     無論客戶端是哪種上傳方式，服務端的處理都是一樣的。在通過HttpServletRequest獲得參數之后，把得到的Item進行分類，分為普通的表單和File表單。    
     通過ServletFileUpload 可以設置上傳文件的大小及編碼格式等。
     總之，服務端的處理是把得到的參數當做HTML表單進行處理的。     
public void processUpload(HttpServletRequest request, HttpServletResponse response){        File uploadFile = new File(uploadPath);        if (!uploadFile.exists()) {            uploadFile.mkdirs();        }        System.out.println("Come on, baby .......");                request.setCharacterEncoding("utf-8");          response.setCharacterEncoding("utf-8");                    //檢測是不是存在上傳文件          boolean isMultipart = ServletFileUpload.isMultipartContent(request);                    if(isMultipart){              DiskFileItemFactory factory = new DiskFileItemFactory();                          //指定在內存中緩存數據大小,單位為byte,這里設為1Mb              factory.setSizeThreshold(1024*1024);                         //設置一旦文件大小超過getSizeThreshold()的值時數據存放在硬盤的目錄               factory.setRepository(new File("D://temp"));                          // Create a new file upload handler              ServletFileUpload upload = new ServletFileUpload(factory);                         // 指定單個上傳文件的最大尺寸,單位:字節，這里設為50Mb                upload.setFileSizeMax(50 * 1024 * 1024);                            //指定一次上傳多個文件的總尺寸,單位:字節，這里設為50Mb              upload.setSizeMax(50 * 1024 * 1024);                 upload.setHeaderEncoding("UTF-8");                          List<FileItem> items = null;                            try {                  // 解析request請求                  items = upload.parseRequest(request);              } catch (FileUploadException e) {                  e.printStackTrace();              }                          if(items!=null){                  //解析表單項目                  Iterator<FileItem> iter = items.iterator();                  while (iter.hasNext()) {                      FileItem item = iter.next();                                         //如果是普通表單屬性                      if (item.isFormField()) {                          //相當于input的name屬性   <input type="text" name="content">                          String name = item.getFieldName();                                                //input的value屬性                          String value = item.getString();                                                System.out.println("屬性:" + name + " 屬性值:" + value);                      }                      //如果是上傳文件                      else {                          //屬性名                          String fieldName = item.getFieldName();                                                  //上傳文件路徑                          String fileName = item.getName();                          fileName = fileName.substring(fileName.lastIndexOf("/") + 1);// 獲得上傳文件的文件名                                                  try {                              item.write(new File(uploadPath, fileName));                          } catch (Exception e) {                              e.printStackTrace();                          }                      }                 }              }          }                  response.addHeader("token", "hello");    }服務端在處理之后，可以在Header中設置返回給客戶端的簡單信息。如果返回客戶端是一個流的話，流的大小必須提前設置！
    response.setContentLength((int) file.length());
3 文件下載
     文件的下載可以使用HttpClient的GetMethod實現，還可以使用HttpGet方式、原始的HttpURLConnection方式。
3.1 客戶端處理
 3.1.1 GetMethod方式
    此處的HttpClient是org.apache.commons.httpclient.HttpClient。
public void downLoad(String remoteFileName, String localFileName) {        HttpClient client = new HttpClient();        GetMethod get = null;        FileOutputStream output = null;                try {            get = new GetMethod(URL_STR);            get.setRequestHeader("userName", userName);            get.setRequestHeader("passwd", passwd);            get.setRequestHeader("fileName", remoteFileName);            int i = client.executeMethod(get);            if (SUCCESS == i) {                System.out.println("The response value of token:" + get.getResponseHeader("token"));                File storeFile = new File(localFileName);                output = new FileOutputStream(storeFile);                                // 得到網絡資源的字節數組,并寫入文件                output.write(get.getResponseBody());            } else {                System.out.println("DownLoad file occurs exception, the error code is :" + i);            }        } catch (Exception e) {            e.printStackTrace();        } finally {            try {                if(output != null){                    output.close();                }            } catch (IOException e) {                e.printStackTrace();            }                        get.releaseConnection();            client.getHttpConnectionManager().closeIdleConnections(0);        }    }
3.1.2 HttpGet方式
    此處的HttpClient是org.apache.http.client.methods下的。
public void downLoad(String remoteFileName, String localFileName) {        DefaultHttpClient httpClient = new DefaultHttpClient();        OutputStream out = null;        InputStream in = null;                try {            HttpGet httpGet = new HttpGet(URL_STR);            httpGet.addHeader("userName", userName);            httpGet.addHeader("passwd", passwd);            httpGet.addHeader("fileName", remoteFileName);            HttpResponse httpResponse = httpClient.execute(httpGet);            HttpEntity entity = httpResponse.getEntity();            in = entity.getContent();            long length = entity.getContentLength();            if (length <= 0) {                System.out.println("下載文件不存在！");                return;            }            System.out.println("The response value of token:" + httpResponse.getFirstHeader("token"));            File file = new File(localFileName);            if(!file.exists()){                file.createNewFile();            }                        out = new FileOutputStream(file);              byte[] buffer = new byte[4096];            int readLength = 0;            while ((readLength=in.read(buffer)) > 0) {                byte[] bytes = new byte[readLength];                System.arraycopy(buffer, 0, bytes, 0, readLength);                out.write(bytes);            }                        out.flush();                    } catch (IOException e) {            e.printStackTrace();        } catch (Exception e) {            e.printStackTrace();        }finally{            try {                if(in != null){                    in.close();                }            } catch (IOException e) {                e.printStackTrace();            }                        try {                if(out != null){                    out.close();                }            } catch (IOException e) {                e.printStackTrace();            }        }    }
3.1.3 HttpURLConnection方式
public void download3(String remoteFileName, String localFileName) {        FileOutputStream out = null;        InputStream in = null;                try{            URL url = new URL(URL_STR);            URLConnection urlConnection = url.openConnection();            HttpURLConnection httpURLConnection = (HttpURLConnection) urlConnection;                        // true -- will setting parameters            httpURLConnection.setDoOutput(true);            // true--will allow read in from            httpURLConnection.setDoInput(true);            // will not use caches            httpURLConnection.setUseCaches(false);            // setting serialized            httpURLConnection.setRequestProperty("Content-type", "application/x-java-serialized-object");            // default is GET                                    httpURLConnection.setRequestMethod("POST");            httpURLConnection.setRequestProperty("connection", "Keep-Alive");            httpURLConnection.setRequestProperty("Charsert", "UTF-8");            // 1 min            httpURLConnection.setConnectTimeout(60000);            // 1 min            httpURLConnection.setReadTimeout(60000);            httpURLConnection.addRequestProperty("userName", userName);            httpURLConnection.addRequestProperty("passwd", passwd);            httpURLConnection.addRequestProperty("fileName", remoteFileName);            // connect to server (tcp)            httpURLConnection.connect();            in = httpURLConnection.getInputStream();// send request to                                                                // server            File file = new File(localFileName);            if(!file.exists()){                file.createNewFile();            }            out = new FileOutputStream(file);              byte[] buffer = new byte[4096];            int readLength = 0;            while ((readLength=in.read(buffer)) > 0) {                byte[] bytes = new byte[readLength];                System.arraycopy(buffer, 0, bytes, 0, readLength);                out.write(bytes);            }                        out.flush();        }catch(Exception e){            e.printStackTrace();        }finally{            try {                if(in != null){                    in.close();                }            } catch (IOException e) {                e.printStackTrace();            }                        try {                if(out != null){                    out.close();                }            } catch (IOException e) {                e.printStackTrace();            }        }    }
3.2 服務端處理
     盡管客戶端的處理方式不同，但是服務端是一樣的。
public void processDownload(HttpServletRequest request, HttpServletResponse response){        int BUFFER_SIZE = 4096;        InputStream in = null;        OutputStream out = null;                System.out.println("Come on, baby .......");                try{            request.setCharacterEncoding("utf-8");              response.setCharacterEncoding("utf-8");              response.setContentType("application/octet-stream");                        String userName = request.getHeader("userName");            String passwd = request.getHeader("passwd");            String fileName = request.getHeader("fileName");                        System.out.println("userName:" + userName);            System.out.println("passwd:" + passwd);            System.out.println("fileName:" + fileName);                        //可以根據傳遞來的userName和passwd做進一步處理，比如驗證請求是否合法等                         File file = new File(downloadPath + "//" + fileName);            response.setContentLength((int) file.length());            response.setHeader("Accept-Ranges", "bytes");                        int readLength = 0;                        in = new BufferedInputStream(new FileInputStream(file), BUFFER_SIZE);            out = new BufferedOutputStream(response.getOutputStream());                        byte[] buffer = new byte[BUFFER_SIZE];            while ((readLength=in.read(buffer)) > 0) {                byte[] bytes = new byte[readLength];                System.arraycopy(buffer, 0, bytes, 0, readLength);                out.write(bytes);            }                        out.flush();                        response.addHeader("token", "hello 1");                     }catch(Exception e){            e.printStackTrace();             response.addHeader("token", "hello 2");        }finally {            if (in != null) {                try {                    in.close();                } catch (IOException e) {                }            }            if (out != null) {                try {                    out.close();                } catch (IOException e) {                }            }        }    }
4 小結
    HttpClient最基本的功能就是執行Http方法。一個Http方法的執行涉及到一個或者多個Http請求/Http響應的交互，通常這個過程都會自動被HttpClient處理，對用戶透明。用戶只需要提供Http請求對象，HttpClient就會將http請求發送給目標服務器，并且接收服務器的響應，如果http請求執行不成功，httpclient就會拋出異常。所以在寫代碼的時候注意finally的處理。    
    所有的Http請求都有一個請求列（request line），包括方法名、請求的URI和Http版本號。HttpClient支持HTTP/1.1這個版本定義的所有Http方法：GET,HEAD,POST,PUT,DELETE,TRACE和OPTIONS。上面的上傳用到了Post，下載是Get。
    目前來說，使用org.apache.commons.httpclient.HttpClient多一些。看自己了~