Detailed explanation of HttpURLConnection attachment upload in Java

/** 
 * 以Http协议传输文件 
 * 
 * @author mingxue.zhang@163.com 
 * 
 */ 
public class HttpPostUtil { 
 
  private final static char[] MULTIPART_CHARS = "-_1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ" 
      .toCharArray(); 
 
  private URL url; 
  private HttpURLConnection conn; 
  private String boundary = null; 
  private Map<String, String> textParams = new HashMap<String, String>(); 
  private Map<String, File> fileparams = new HashMap<String, File>(); 
 
  public HttpPostUtil(String url) throws Exception { 
    this.url = new URL(url); 
  } 
 
  // 重新设置要请求的服务器地址，即上传文件的地址。 
  public void setUrl(String url) throws Exception { 
    this.url = new URL(url); 
  } 
 
  // 增加一个普通字符串数据到form表单数据中 
  public void addTextParameter(String name, String value) { 
    textParams.put(name, value); 
  } 
 
  // 增加一个文件到form表单数据中 
  public void addFileParameter(String name, File value) { 
    fileparams.put(name, value); 
  } 
 
  // 清空所有已添加的form表单数据 
  public void clearAllParameters() { 
    textParams.clear(); 
    fileparams.clear(); 
  } 
 
  /** 
   * 发送数据到服务器 
   * 
   * @return 一个字节包含服务器的返回结果的数组 
   * @throws Exception 
   */ 
  public byte[] send() throws Exception { 
    initConnection(); 
    try { 
      conn.connect(); 
    } catch (SocketTimeoutException e) { 
      throw new Exception(e); 
    } 
 
    OutputStream connOutStream = new DataOutputStream( 
        conn.getOutputStream()); 
 
    writeFileParams(connOutStream); 
    writeStringParams(connOutStream); 
    writesEnd(connOutStream); 
 
    InputStream responseInStream = conn.getInputStream(); 
    ByteArrayOutputStream responseOutStream = new ByteArrayOutputStream(); 
    int len; 
    byte[] bufferByte = new byte[1024]; 
    while ((len = responseInStream.read(bufferByte)) != -1) { 
      responseOutStream.write(bufferByte, 0, len); 
    } 
    responseInStream.close(); 
    connOutStream.close(); 
 
    conn.disconnect(); 
    byte[] resultByte = responseOutStream.toByteArray(); 
    responseOutStream.close(); 
    return resultByte; 
  } 
 
  // 文件上传的connection的一些必须设置 
  private void initConnection() throws Exception { 
    StringBuffer buf = new StringBuffer("----"); 
    Random rand = new Random(); 
    for (int i = 0; i < 15; i++) { 
      buf.append(MULTIPART_CHARS[rand.nextInt(MULTIPART_CHARS.length)]); 
    } 
    this.boundary = buf.toString(); 
 
    conn = (HttpURLConnection) this.url.openConnection(); 
    conn.setDoOutput(true); 
    conn.setUseCaches(false); 
    conn.setConnectTimeout(3 * 60 * 1000); // 连接超时为10秒 
    conn.setRequestMethod("POST"); 
    conn.setRequestProperty("Content-Type", 
        "multipart/form-data; boundary=" + boundary); 
  } 
 
  // 普通字符串数据 
  private void writeStringParams(OutputStream out) throws Exception { 
    Set<String> keySet = textParams.keySet(); 
    for (Iterator<String> it = keySet.iterator(); it.hasNext();) { 
      String name = it.next(); 
      String value = textParams.get(name); 
 
      out.write(("--" + boundary + "\r\n").getBytes()); 
      out.write(("Content-Disposition: form-data; name=\"" + name + "\"\r\n") 
          .getBytes()); 
      out.write(("\r\n").getBytes()); 
      out.write((encode(value) + "\r\n").getBytes()); 
    } 
  } 
 
  // 文件数据 
  private void writeFileParams(OutputStream out) throws Exception { 
    Set<String> keySet = fileparams.keySet(); 
    for (Iterator<String> it = keySet.iterator(); it.hasNext();) { 
      String name = it.next(); 
      File value = fileparams.get(name); 
 
      out.write(("--" + boundary + "\r\n").getBytes()); 
      out.write(("Content-Disposition: form-data; name=\"" + name 
          + "\"; filename=\"" + encode(value.getName()) + "\"\r\n") 
          .getBytes()); 
      out.write(("Content-Type: " + getContentType(value) + "\r\n") 
          .getBytes()); 
      out.write(("Content-Transfer-Encoding: " + "binary" + "\r\n") 
          .getBytes()); 
 
      out.write(("\r\n").getBytes()); 
 
      FileInputStream inStream = new FileInputStream(value); 
      int bytes = 0; 
      byte[] bufferByte = new byte[1024]; 
      while ((bytes = inStream.read(bufferByte)) != -1) { 
        out.write(bufferByte, 0, bytes); 
      } 
      inStream.close(); 
 
      out.write(("\r\n").getBytes()); 
    } 
  } 
 
  // 添加结尾数据 
  private void writesEnd(OutputStream out) throws Exception { 
    out.write(("--" + boundary + "--" + "\r\n").getBytes()); 
    out.write(("\r\n").getBytes()); 
  } 
 
  // 获取文件的上传类型，图片格式为image/png,image/jpg等。非图片为application/octet-stream 
  private String getContentType(File f) throws Exception { 
    String fileName = f.getName(); 
    if (fileName.endsWith(".jpg")) { 
      return "image/jpeg"; 
    } else if (fileName.endsWith(".png")) { 
      return "image/png"; 
    } 
    return "application/octet-stream"; 
  } 
 
  // 对包含中文的字符串进行转码，此为UTF-8。服务器那边要进行一次解码 
  private String encode(String value) throws Exception { 
    return URLEncoder.encode(value, "UTF-8"); 
  } 
}