通过 ProcessBuilder 运行 java.exe 导致应用程序挂起？

Question

I have the following code:

    ProcessBuilder pb = new ProcessBuilder("C:\\Program Files\\Java\\jdk1.8.0_111\\bin\\java", "-cp", "project_folder\\target\\classes", "package.ExternalProcess"); 
    Process p = pb.start();
    OutputStream processOutputStream = p.getOutputStream();
    IOUtils.write("1" + System.lineSeparator(), processOutputStream);
    InputStream processInputStream = p.getInputStream();
    System.out.println("--1--");
    System.out.println(process.isAlive()); // outputs true
    String result = IOUtils.toString(processInputStream, "UTF-8"); //<-- hangs here
    System.out.println("--2--");
    p.waitFor();
    System.out.println(result);

ExternalProcess source:

public class ExternalProcess {
    public static void main(String[] args) {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String input = null;
        try {
            input = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        System.out.println("processed[" + input + "]");
    }
}

The first code produces
--1-- true

and hangs on

How to correct program and why does it hangs on?

PS

When I just try to write

java -cp project_folder\target\classes package.ExternalProcess

from cmd it waiting when I write something to the console and returns expected result

Answer 1

The problem is buffering, in particular, buffering of Process.getOutputStream() . If I add:

processOutputStream.flush();

after:

IOUtils.write("1" + System.lineSeparator(), processOutputStream);

it works as expected. IOUtils.write writes to processOutputStream , which just saves the data in a buffer in the parent process, but the data isn't actually sent across the connection to the child process, so the child process hangs in br.readLine() , waiting for data that's never coming. The flush() call sends the data from the buffer in the parent to the child where it can be read.

(You can see in the javadocs for Process.getOutputStream() it says:

Implementation note: It is a good idea for the returned output stream to be buffered.

While that doesn't specifically say the standard implementation is buffered, it's reasonable to assume Oracle will follow their own implementation recommendations.)

If you don't want to have to call flush() after every write, you can wrap the OutputStream in a PrintStream with autoFlush on:

PrintStream processPrintStream = new PrintStream(processOutputStream, true);

Then instead of using IOUtils.write , just use the PrintStream methods:

processPrintStream.println("1");

Because autoFlush is on in the PrintStream , whenever one of the println methods is called (or a byte array or newline is written), the PrintStream calls flush() on the underlying OutputStream for you.