企业应用中会经常用到基于Http的文件上传，其中Multipart是多文件上传所使用的，在用JAVA开发时有时会用第三方类库来拼HTTP报文，有时则手动组装，
然后手动组装前先要了解一下报文的格式，如下是HTTP的报文头：
POST /test/upload HTTP/1.1 Content-Type: multipart/form-data;boundary=0xKhTmLbOuNdArY-35123DE7-B577-4495-AA1E-092BB0CCFC64 charset: utf-8 Content-Length: 19520216 Host: 10.19.220.234:8080 Connection: Keep-Alive User-Agent: Apache-HttpClient/4.2.5 (java 1.5)
重点是：
Content-Type: multipart/form-data;boundary=0xKhTmLbOuNdArY-35123DE7-B577-4495-AA1E-092BB0CCFC64其中Content-Type决定了报文的类型，boundary则用于分隔各条目项。
示例如下：
--0xKhTmLbOuNdArY-35123DE7-B577-4495-AA1E-092BB0CCFC64 Content-Disposition: form-data; name="messageId" Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8bit 1395271935 --0xKhTmLbOuNdArY-35123DE7-B577-4495-AA1E-092BB0CCFC64 Content-Disposition: form-data; name="name" Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8bit image --0xKhTmLbOuNdArY-35123DE7-B577-4495-AA1E-092BB0CCFC64 Content-Disposition: form-data; name="type" Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8bit 3 --0xKhTmLbOuNdArY-35123DE7-B577-4495-AA1E-092BB0CCFC64 Content-Disposition: form-data; name="content"; filename="11.mp4" Content-Type: application/octet-stream Content-Transfer-Encoding: binary ....ftypisom....isomavc1..4.moov...lmvhd.....1...1.....X........(省略文件) --0xKhTmLbOuNdArY-35123DE7-B577-4495-AA1E-092BB0CCFC64 Content-Disposition: form-data; name="vimg"; filename="test.jpg" Content-Type: application/octet-stream Content-Transfer-Encoding: binary ......JFIF.............C.............. .. ................. $.' ",#..(7),01444.'9=82<.342...C........(省略文件) --0xKhTmLbOuNdArY-35123DE7-B577-4495-AA1E-092BB0CCFC64--
对于multipart的解析我用的是apache的commons-fileupload-1.3.jar，demo如下：
boolean isHaveData = ServletFileUpload.isMultipartContent(request);
if (isHaveData) {
FileItemFactory factory = new DiskFileItemFactory();
ServletFileUpload upload = new ServletFileUpload(factory);
try {
List<?> items = upload.parseRequest(request);
Iterator<?> iter = items.iterator();
while (iter.hasNext()) {
FileItem item = (FileItem) iter.next();
if (item.isFormField()) {
// 普通文本信息处理
String paramName = item.getFieldName();
String paramValue = item.getString();
map.put(paramName, paramValue);
} else {
// 上传文件信息处理
if("vimage".equalsIgnoreCase(item.getName()) || "vimg".equalsIgnoreCase(item.getFieldName())){
vimg = item.get();
} else {
data = item.get();
}
}
}
} catch (FileUploadException e) {
logger.error("FileUploadException : ", e);
}
}其中判断类型的代码如下： public static final String MULTIPART = "multipart/";
public static final boolean isMultipartContent(RequestContext ctx) {
String contentType = ctx.getContentType();
if (contentType == null) {
return false;
}
if (contentType.toLowerCase(Locale.ENGLISH).startsWith(MULTIPART)) {
return true;
}
return false;
}有一点需要注意的是，upload.parseRequest(request)方法里的源码是这样的：
boolean nextPart;
if (skipPreamble) {
nextPart = multi.skipPreamble();
} else {
nextPart = multi.readBoundary();
}
if (!nextPart) {
if (currentFieldName == null) {
// Outer multipart terminated -> No more data
eof = true;
return false;
}
// Inner multipart terminated -> Return to parsing the outer
multi.setBoundary(boundary);
currentFieldName = null;
continue;
}也就是说如果两个boundary之间是空的，就认为到eof了，也就是结束了往下的遍历，因为在实际的使用当中，我在拼报文时不小心在两个条项目中多加了一个boundary，这时后面的条项目就取不到了，这个问题让我纠结了半天，觉得很诡异，后来读源码才发现，原来如此。如果使用第三方的类库就不会有此烦恼了，我用的是apache的httpmime-4.2.2.jar，测试的代码如下：
MultipartEntity entity = new MultipartEntity(null, "0xKhTmLbOuNdArY-35123DE7-B577-4495-AA1E-092BB0CCFC64", null);
File file = new File("E:\\ceshi\\11.flv");
File file2 = new File("E:\\ceshi\\test.jpg");
entity.addPart("messageId", new StringBody("1395271935"));
entity.addPart("name", new StringBody("image"));
entity.addPart("type", new StringBody("3"));
entity.addPart("to", new StringBody("6000028403"));
entity.addPart("from", new StringBody("6000608066"));
entity.addPart("content", new FileBody(file));
entity.addPart("vimage", new FileBody(file2));
httpost.setEntity(entity);
测试的过程中我用的抓包工具是Wireshark，界面如下：
图1--Wireshark界面
查看到的TCP流如下：
图2--TCP流
最后总结一下：multipart/form-data对于多文件上传还是很好用的