使用 ProcessBuilder 发送多个命令？

Question

I want to spawn a child process in Java and send different commands inside the app. My child process has authentication and each user can have a variety of internal commands.

For example:

> login myuser passowrd
OK
> list certs
cert1 abc
cert2 efg
> logout
> exit

Well, to simulate that I will make my example with "node" as IO CLI.

public class JAVAMain {

    public static void main(String[] args) throws Exception {
        action();
    }

    public static String action() throws Exception {
        ProcessBuilder pb = new ProcessBuilder("node");
        pb.redirectErrorStream(true);
        Process process = pb.start();

        // streams
        InputStream stdout = process.getInputStream();
        OutputStream stdin = process.getOutputStream();
        BufferedReader reader = new BufferedReader(new InputStreamReader(stdout));
        BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(stdin));
        String buff = "";
        String res = "";

        System.out.println("1");
        writer.write("console.log('OK');\n"); // simulate "login answer"
        writer.flush();

        System.out.println("2");
        res = "";
        while ((buff = reader.readLine()) != null) {
            res += buff;
        }

        if (!res.equals("OK")) {
            reader.close();
            writer.close();
            throw new Exception("Invalid auth");
        }

        System.out.println("3");
        writer.write("console.log('any text...');\n");
        writer.flush();

        System.out.println("4");
        res = "";
        while ((buff = reader.readLine()) != null) {
            res += buff;
        }

        reader.close();
        writer.close();
        return res;
    }
}

I expect to print 1,2,3,4 and get the res any text... for this example. But the program never stops and stay in 1,2 . If I close the writer after the flush in 1 I get this output:

1
2
3
Exception in thread "main" java.io.IOException: Stream closed
        at java.io.BufferedWriter.ensureOpen(BufferedWriter.java:116)
        at java.io.BufferedWriter.write(BufferedWriter.java:221)
        at java.io.Writer.write(Writer.java:157)
        at com.keynua.kades.JAVAMain2.action(JAVAMain2.java:48)
        at com.keynua.kades.JAVAMain2.main(JAVAMain2.java:14)

That's why I close the writer and the reader works but I can't write again. So, how I can make to send multiples commands to the child app and read the output to follow the flow with other commands?

Answer 1

One problem is that your while loops

while ((buff = reader.readLine()) != null) { ... }

only terminate when the reader has reached the end of the input stream.

The end of the input stream is only reached when the subprocess terminates.

---

The second problem is that you seem to be using NodeJS as sample command executor.

• If NodeJS is started from a console, you can enter JavaScript statements and they are executed one by one.
• You are however starting NodeJS not from a console, but from some other application. In this case, NodeJS wants to read a complete script from stdin and executes the complete script at once
• You could start NodeJS with the -i parameter (force interactive mode), at the expense of some additional output.

To achieve this, you would create the ProcessBuilder with

ProcessBuilder pb = new ProcessBuilder("node", "-i");

---

Communicating with a subprocess in this way only works when you know how many lines to read from the reader before sending the next command.

Knowing how many lines to read can mean:

• knowing how many lines of output a command produces (login: 1 line of output, logout: no output)
• knowing that a command produces a distinct last line (for example an empty line or a line with only "END" in it)
• that a command produces a line count as the first result
• executing another command first that returns the result line count of the subsequent command

---

The list certs command could either:

• produce

   cert1 abc cert2 def END

• produce (where the last line would be empty instead of containing a dot)

   cert1 abc cert2 def.

• produce

   2 certs cert1 abc cert2 def

• or you could execute a count certs command before executing list certs