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 finished
try {
in = process.getInputStream();
err = process.getErrorStream();
boolean finished = false; // Set to true when p is finished
while (!finished) {
try {
while (in.available() > 0) {
// Print the output of our system call
Character c = new Character((char) in.read());
System.out.print(c);
}
while (err.available() > 0) {
// Print the output of our system call
Character 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 cycles
Thread.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;
}