java借助Runtime调用外部程序阻塞的代码有时候在java代码中会调用一些外部程序，比如SwfTools来转换swf、ffmpeg来转换视频等。如果你的代码这样写：Runtime.getRuntime().exec(command)，会发现程序一下就执行完毕，而在命令行里要执行一会，是因为java没有等待外部程序的执行完毕，此时就需要使用阻塞，来等待外部程序执行结果：
InputStream stderr = PRocess.getInputStream();InputStreamReader isr = new InputStreamReader(stderr, "GBK");BufferedReader br = new BufferedReader(isr);String line = null;while ((line = br.readLine()) != null) System.out.println(line);int exitValue = process.waitFor();
对于一般的外部程序使用上面的阻塞代码就可以，至少pdf2swf.exe是没有问题的。
但我发现对于ffmpeg来说，以上代码会让程序卡住不动，需要使用另一种方式，封装成了一个方法，如下：
@SuppressWarnings("static-access")public static int doWaitFor(Process process) {InputStream in = null;InputStream err = null;int exitValue = -1; // returned to caller when p is finishedtry {in = process.getInputStream();err = process.getErrorStream();boolean finished = false; // Set to true when p is finishedwhile (!finished) {try {while (in.available() > 0) {// Print the output of our system callCharacter c = new Character((char) in.read());System.out.print(c);}while (err.available() > 0) {// Print the output of our system callCharacter c = new Character((char) err.read());System.out.print(c);}// Ask the process for its exitValue. If the process// is not finished, an IllegalThreadStateException// is thrown. If it is finished, we fall through and// the variable finished is set to true.exitValue = process.exitValue();finished = true;} catch (IllegalThreadStateException e) {// Process is not finished yet;// Sleep a little to save on CPU cyclesThread.currentThread().sleep(500);}}} catch (Exception e) {e.printStackTrace();} finally {try {if (in != null) {in.close();}} catch (IOException e) {e.printStackTrace();}if (err != null) {try {err.close();} catch (IOException e) {e.printStackTrace();}}}return exitValue;}