[Java] Java文件上传与文件下载实现方法详解

• 文件上传要求客户端提交特殊的请求——multipart请求，即包含多部分数据的请求。必须将<form/>标签的enctype属性值设为“multipart/form-data”,enctype表示encodingtype，及编码类型
  
• 由于客户端上传文件的大小是不确定的，所以http协议规定，文件上传的数据要存放于请求正文中，不能出现在url地址栏内。也就是说，想要上传文件必须提交post请求。
  
• 表单中要有<input type="file" />标签
  
• 注意：multipart/form-data请求与普通请求不同
  

public class registerservlet extends httpservlet {
  private static final long serialversionuid = 1l;
  public registerservlet() {
  super();
  }
  protected void doget(httpservletrequest request, httpservletresponse response)
  throws servletexception, ioexception {
  response.getwriter().append("served at: ").append(request.getcontextpath());
  }
  protected void dopost(httpservletrequest request, httpservletresponse response)
  throws servletexception, ioexception {
  //第一步、判断请求是否为multipart请求
  if(!servletfileupload.ismultipartcontent(request)) {
    throw new runtimeexception("当前请求只支持文件上传");
  }
  try {
    //第二步、创建fileitem工厂
    diskfileitemfactory factory = new diskfileitemfactory();
    //设置临时文件存储目录
    string path = this.getservletcontext().getrealpath("/temp");
    file temp = new file(path);
    factory.setrepository(temp);
    //单位：字节。本例设置边界值为2mb，超过该值会创建临时文件
    factory.setsizethreshold(1024*1024*2);
    //第三步、创建文件上传核心组件
    servletfileupload upload = new servletfileupload(factory);
    //设置item的头部字符编码，解决中文乱码问题
    upload.setheaderencoding("utf-8");
    //设置单个上传文件的最大值为5mb
    upload.setfilesizemax(1024*1024*5);
    //设置一次上传所有文件总和的最大值为10mb（上传多个文件时起作用）
    upload.setfilesizemax(1024*1024*10);
    //第四步、解析请求获取所有的item
    list<fileitem> items = upload.parserequest(request);
    //第五步、遍历item
    for(fileitem item:items) {
    if (item.isformfield()) {
      processformfield(item);
      } else {
      processuploadedfile(item);
      }
    }
  } catch (fileuploadexception e) {
    e.printstacktrace();
  }
  }
  private void processformfield(fileitem item) {
  try {
    string name = item.getfieldname();
    //解决中文乱码问题
    string value = item.getstring("utf-8");
    system.out.println(name+"="+value);
  } catch (unsupportedencodingexception e) {
    e.printstacktrace();
  }
  }
  private void processuploadedfile(fileitem item) {
  try {
    inputstream inputstream = item.getinputstream();
    string fieldname = item.getfieldname();
    //获取上传文件原始名称
    string filename = item.getname();
    //解决文件同名问题
    filename = system.currenttimemillis()+filename;
    string contenttype = item.getcontenttype();
    boolean isinmemory = item.isinmemory();
    long sizeinbytes = item.getsize();
    string path = this.getservletcontext().getrealpath("/uploadcontent");
    //date now = new date();
    calendar now = calendar.getinstance();
    //对上传的文件进行分类管理
    path += "/"+now.get(calendar.year)+"/"+(now.get(calendar.month)+1)+"/"+now.get(calendar.day_of_month);
    //若目录不存在，则创建该目录
    file directory = new file(path);
    if(!directory.exists()) {
    directory.mkdirs();
    }
    file descfile = new file(path,filename);
    outputstream outputstream = new fileoutputstream(descfile);
    int length = -1;
    byte[] buffer = new byte[1024];
    while((length=inputstream.read(buffer))!=-1) {
    outputstream.write(buffer, 0, length);
    }
    outputstream.close();
    inputstream.close();
    //删除临时文件
    item.delete();
  } catch (ioexception e) {
    e.printstacktrace();
  }
  }
}

• 浏览器能解析的文件会直接显示，如：pdf、jpg......
  
• 浏览器解析不了的文件会被另存为，如：rar、exe......
  
• 浏览器版本不一样，对文件的解析能力也不同
  
• 缺点：下载内容的形式（直接显示/另存为）由浏览器决定，跟服务器无关
  

• 设置响应头部属性content-disposition值为attachment
  
• 获取连接服务器源文件的输入流
  
• 获取输出流
  
• 将输入流中的数据写到输出流中
  

public class downloadservlet extends httpservlet {
  private static final long serialversionuid = 1l;
  public downloadservlet() {
  super();
  }
  protected void doget(httpservletrequest request, httpservletresponse response)
  throws servletexception, ioexception {
  //设置响应的头部属性content-disposition值为attachment
  //使用filename来指定文件名
  string filename = "超跑.png";
  byte[] bytes = filename.getbytes("utf-8");
  //http协议规定浏览器只能接受iso8859-1类型的字节数据
  filename = new string(bytes,"iso8859-1");
  response.setheader("content-disposition", "attachment;filename="+filename);
  //获取连接服务器资源文件的输入流
  inputstream is = request.getservletcontext().getresourceasstream("/resources/bs架构原理图1.png");
  //获取输出流
  servletoutputstream os = response.getoutputstream();
  //将输入流中的数据写到输出流中
  int length = -1;
  byte[] buffer = new byte[1024];
  while((length=is.read(buffer))!=-1) {
    os.write(buffer,0,length);
  }
  os.close();
  is.close();
  }
  protected void dopost(httpservletrequest request, httpservletresponse response)
  throws servletexception, ioexception {
  doget(request, response);
  }
}