1 HTTP
HTTP 协议可能是现在 Internet 上使用得最多、最重要的协议了，越来越多的 Java 应用程序需要直接通过 HTTP 协议来访问网络资源。
虽然在 JDK 的 java.net 包中已经提供了访问 HTTP 协议的基本功能，但是对于大部分应用程序来说，JDK 库本身提供的功能还不够丰富和灵活。HttpClient 用来提供高效的、最新的、功能丰富的支持 HTTP 协议的客户端编程工具包，并且它支持 HTTP 协议最新的版本和建议。
一般的情况下我们都是使用Chrome或者其他浏览器来访问一个WEB服务器，用来浏览页面查看信息或者提交一些数据、文件上传下载等等。所访问的这些页面有的仅仅是一些普通的页面，有的需要用户登录后方可使用，或者需要认证以及是一些通过加密方式传输，例如HTTPS。目前我们使用的浏览器处理这些情况都不会构成问题。但是一旦我们有需求不通过浏览器来访问服务器的资源呢？那该怎么办呢？
下面以本地客户端发起文件的上传、下载为例做个小Demo。HttpClient有两种形式，一种是org.apache.http下的，一种是org.apache.commons.httpclient.HttpClient。
2 文件上传
文件上传可以使用两种方式实现，一种是PostMethod方式，一种是HttpPost方式。两者的处理大同小异。PostMethod是使用FileBody将文件包装流包装起来，HttpPost是使用FilePart将文件流包装起来。在传递文件流给服务端的时候，都可以同时传递其他的参数。
2.1 客户端处理
2.1.1 PostMethod方式
将文件封装到FilePart中，放入Part数组，同时，其他参数可以放入StringPart中，这里没有写，只是单纯的将参数以setParameter的方式进行设置。此处的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 {
// 通过以下方法可以模拟页面参数提交
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("上传成功");
} else {
System.out.println("上传失败");
}
} catch (Exception ex) {
ex.printStackTrace();
} finally {
filePost.releaseConnection();
}
}
记得搞完之后，要通过releaseConnection释放连接。
2.1.2 HttpPost方式
这种方式，与上面类似，只不过变成了FileBody。上面的Part数组在这里对应HttpEntity。此处的HttpClient是org.apache.http.client.methods下的。
public void upload(String localFile){
CloseableHttpClient httpClient = null;
CloseableHttpResponse response = null;
try {
httpClient = HttpClients.createDefault();
// 把一个普通参数和文件上传给下面这个地址 是一个servlet
HttpPost httpPost = new HttpPost(URL_STR);
// 把文件转换成流对象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()
// 相当于
.addPart("file", bin)
// 相当于
.addPart("userName", userName)
.addPart("pass", password)
.build();
httpPost.setEntity(reqEntity);
// 发起请求 并返回请求的响应
response = httpClient.execute(httpPost);
System.out.println("The response value of token:" + response.getFirstHeader("token"));
// 获取响应对象
HttpEntity resEntity = response.getEntity();
if (resEntity != null) {
// 打印响应长度
System.out.println("Response content length: " + resEntity.getContentLength());
// 打印响应内容
System.out.println(EntityUtils.toString(resEntity, Charset.forName("UTF-8")));
}
// 销毁
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 服务端处理
无论客户端是哪种上传方式，服务端的处理都是一样的。在通过HttpServletRequest获得参数之后，把得到的Item进行分类，分为普通的表单和File表单。
通过ServletFileUpload 可以设置上传文件的大小及编码格式等。
总之，服务端的处理是把得到的参数当做HTML表单进行处理的。
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");
//检测是不是存在上传文件
boolean isMultipart = ServletFileUpload.isMultipartContent(request);
if(isMultipart){
DiskFileItemFactory factory = new DiskFileItemFactory();
//指定在内存中缓存数据大小,单位为byte,这里设为1Mb
factory.setSizeThreshold(1024*1024);
//设置一旦文件大小超过getSizeThreshold()的值时数据存放在硬盘的目录
factory.setRepository(new File("D:\\temp"));
// Create a new file upload handler
ServletFileUpload upload = new ServletFileUpload(factory);
// 指定单个上传文件的最大尺寸,单位:字节，这里设为50Mb
upload.setFileSizeMax(50 * 1024 * 1024);
//指定一次上传多个文件的总尺寸,单位:字节，这里设为50Mb
upload.setSizeMax(50 * 1024 * 1024);
upload.setHeaderEncoding("UTF-8");
List items = null;
try {
// 解析request请求
items = upload.parseRequest(request);
} catch (FileUploadException e) {
e.printStackTrace();
}
if(items!=null){
//解析表单项目
Iterator iter = items.iterator();
while (iter.hasNext()) {
FileItem item = iter.next();
//如果是普通表单属性
if (item.isFormField()) {
//相当于input的name属性
String name = item.getFieldName();
//input的value属性
String value = item.getString();
System.out.println("属性:" + name + " 属性值:" + value);
}
//如果是上传文件
else {
//属性名
String fieldName = item.getFieldName();
//上传文件路径
String fileName = item.getName();
fileName = fileName.substring(fileName.lastIndexOf("/") + 1);// 获得上传文件的文件名
try {
item.write(new File(uploadPath, fileName));
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
}
response.addHeader("token", "hello");
}
服务端在处理之后，可以在Header中设置返回给客户端的简单信息。如果返回客户端是一个流的话，流的大小必须提前设置！
response.setContentLength((int) file.length());
3 文件下载
文件的下载可以使用HttpClient的GetMethod实现，还可以使用HttpGet方式、原始的HttpURLConnection方式。
3.1 客户端处理
3.1.1 GetMethod方式
此处的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);
// 得到网络资源的字节数组,并写入文件
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
此处的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("下载文件不存在！");
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 服务端处理
尽管客户端的处理方式不同，但是服务端是一样的。
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);
//可以根据传递来的userName和passwd做进一步处理，比如验证请求是否合法等
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 小结
HttpClient最基本的功能就是执行Http方法。一个Http方法的执行涉及到一个或者多个Http请求/Http响应的交互，通常这个过程都会自动被HttpClient处理，对用户透明。用户只需要提供Http请求对象，HttpClient就会将http请求发送给目标服务器，并且接收服务器的响应，如果http请求执行不成功，httpclient就会抛出异常。所以在写代码的时候注意finally的处理。
所有的Http请求都有一个请求列(request line)，包括方法名、请求的URI和Http版本号。HttpClient支持HTTP/1.1这个版本定义的所有Http方法：GET,HEAD,POST,PUT,DELETE,TRACE和OPTIONS。上面的上传用到了Post，下载是Get。
目前来说，使用org.apache.commons.httpclient.HttpClient多一些。看自己了~
以上就是小编为大家带来的使用HttpClient实现文件的上传下载方法全部内容了
