如何使用 Java ProcessBuilder 自动化命令行脚本

Question

我是 java 的新手，正在尝试使用 java 自动化命令行。我试图在这里搜索解决方案，但找不到。

我创建了一个简单的测试 shell 脚本，如下所示，用于测试我的程序：

#!/bin/bash
echo "What is your name?";
read name;
echo "Hello, $name"
echo "What is your contact number?";
read num;
echo "Saved contact number $num for $name"

Java代码如下：

import java.io.*;
import java.util.*;

public class CmdLineMain {
    public static void main(String args[]) throws InterruptedException, IOException {
        List<String> command = new ArrayList<String>();
        command.add("./test.sh");

        ProcessBuilder builder = new ProcessBuilder(command);

        final Process process = builder.start();
        InputStream is = process.getInputStream();
        InputStreamReader isr = new InputStreamReader(is);
        BufferedReader br = new BufferedReader(isr);
        String line = "";
        BufferedWriter bw = null;

        while (process.isAlive()) {
            line = br.readLine();
            // since stream may be closed earlier, re-open it
            bw = new BufferedWriter(new OutputStreamWriter(process.getOutputStream()));
            System.out.println(line);

            if (line != null) {
                switch (line) {
                case "What is your name?":
                    bw.write("John Doe");
                    bw.close();
                    break;
                case "What is your contact number?":
                    bw.write("123456789");
                    bw.close();
                    break;

                }
            }

        }

        System.out.println("Program terminated!");
    }
}

问题：进程的第二个输入失败并出现错误：

    What is your name?
    Hello, John Doe
    What is your contact number?
    Exception in thread "main" java.io.IOException: Stream Closed
        at java.io.FileOutputStream.writeBytes(Native Method)
        at java.io.FileOutputStream.write(FileOutputStream.java:326)
        at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:82)
        at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:140)
        at java.io.FilterOutputStream.close(FilterOutputStream.java:158)
        at sun.nio.cs.StreamEncoder.implClose(StreamEncoder.java:320)
        at sun.nio.cs.StreamEncoder.close(StreamEncoder.java:149)
        at java.io.OutputStreamWriter.close(OutputStreamWriter.java:233)
        at java.io.BufferedWriter.close(BufferedWriter.java:266)
        at nkh.app.CmdLineMain.main(CmdLineMain.java:34)

Answer 1

关闭 BufferedWriter 将关闭其基础流，这包括您通过 process.getOutputStream() 获得的进程 OutputStream。因此，一旦它在一个循环中关闭，在下一个循环中，您的 BufferedWriter 将包装一个关闭的流。而是只包装输出流一次并重复使用它。

像这样：

BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(process.getOutputStream()));
// while the stream is open and there is something to read
// probably a better condition than `process.isAlive()`
while ((line = br.readLine()) != null) {
    switch (line) {
        case "What is your name?":
            bw.write("John Doe");
            bw.newLine();
            bw.flush();
            break;
        case "What is your contact number?":
            bw.write("123456789");
            bw.newLine();
            bw.flush();
            break;
    }
}

当你的新 BufferedWriter 包装封闭的底层流时，你的原始代码死于 close() 而不是死于它之前的 write() 的原因是 BufferedOutputStream.write() 可能还没有真正写入底层流，因为它被缓冲了。调用 flush() 应该告诉流实际写入，正如您在堆栈跟踪中看到的那样 close() 调用 flush() 最终将缓冲字节写入底层 FileOutputStream，然后意识到 FileOutputStream 已经关闭。

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