Java 进程执行外部程序造成阻塞的一种原因

/**
  * pdf转swf函数
  * @param path 输入输出文件路径
  * @param inputFileName 输入文件名
  * @param outputFileName 输出文件名
  * @return File 生成的swf文件
  */
 private static File toSwf(String sourceFile, String destFile, String command) {
  long beginTime = System.nanoTime();
  Runtime rt = Runtime.getRuntime();
  try {
   Process process = rt.exec(command);

   final InputStream isNormal = process.getInputStream();
   new Thread(new Runnable() {
       public void run() {
           BufferedReader br = new BufferedReader(new InputStreamReader(isNormal));
           StringBuilder buf = new StringBuilder();
     String line = null;
     try {
      while((line = br.readLine()) != null){
       buf.append(line + "\n");
      }
     } catch (IOException e) {
      e.printStackTrace();
     }
     System.out.println("输出结果为：" + buf);
       }
   }).start(); // 启动单独的线程来清空process.getInputStream()的缓冲区

   InputStream isError = process.getErrorStream();
   BufferedReader br2 = new BufferedReader(new InputStreamReader(isError));
   StringBuilder buf = new StringBuilder();
   String line = null;
   while((line = br2.readLine()) != null){
    buf.append(line + "\n");
   }
   System.out.println("错误输出结果为：" + buf);

   try {
    process.waitFor();
   } catch (InterruptedException e) {
    e.printStackTrace();
   }

  } catch (IOException e) {
   e.printStackTrace();
  }
  long endTime = System.nanoTime();
  System.out.println("转swf耗时: " + (endTime - beginTime) / 1000000000 + " 秒  " + sourceFile);
  return new File(destFile);
 }