启动另一个可用 System.console 的进程

Question

我有两个程序：

首先，使用Console对象读写数据
其次，应该运行首先使用一些动态计算的参数

第二个程序代码如下所示：

String[] arguments = {
    "cmd", "/c",
    "java", "-cp", classPath
    lauchClass,
    // Arguments for first program
}
ProcessBuilder pb = new ProcessBuilder(arguments);
pb.environment().putAll(System.getenv());
pb.directory(workDir);
pb.inheritIO();

Process process = pb.start();
process.waitFor();

当第一个程序从第二个开始时，System.console() 为 null 并且失败并出现 NPE。

所以，问题是：有什么方法可以运行另一个可用 System.console() 的进程吗？

Answer 1

答案很简单：如果您运行您的启动器来自 IDE，如 Eclipse 或 IntelliJ IDEA，可能它没有在 System.console() 中设置首先，因此没有任何子进程可以继承。只需尝试从启动器向 System.console() 写入一些内容，它也会因同样的错误而失败。但是，如果您从 Cmd.exe 或 Git Bash 等交互式控制台启动启动器，启动器和通过 ProcessBuilder 启动的进程都可以写入 System.console()。您的启动器甚至不需要 "cmd", "/c"，无论是否使用这些参数，它都可以工作。

package de.scrum_master.Whosebug;

import java.io.File;
import java.io.IOException;

public class Launcher {
  public static void main(String[] args) throws IOException, InterruptedException {
    String classPath = "out/production/SO_ExternalProcessSystemConsole";
    String launchClass = "de.scrum_master.Whosebug.MyApp";
    File workDir = new File(".");
    System.console().printf("Hi, I am the launcher app!%n");

    String[] arguments = new String[] {
//      "cmd", "/c",
      "java", "-cp", classPath,
      launchClass
    };
    ProcessBuilder pb = new ProcessBuilder(arguments);
    pb.environment().putAll(System.getenv());
    pb.directory(workDir);
    pb.inheritIO();

    Process process = pb.start();
    process.waitFor();
  }
}

package de.scrum_master.Whosebug;

public class MyApp {
  public static void main(String[] args) {
    System.console().printf("Hi, I am an externally started app!%n");
  }
}

从 IntelliJ 启动时的控制台日志 IDEA:

Exception in thread "main" java.lang.NullPointerException
    at de.scrum_master.Whosebug.Launcher.main(Launcher.java:11)
    (...)

从 cmd.exe:

启动时的控制台日志

Hi, I am the launcher app!
Hi, I am an externally started app!

如有任何 follow-up 问题，请随时提出。

更新： 如果您不介意外部程序运行在其自己的交互式控制台中而不是在 IDE 控制台中，您可以为此目的使用 Windows 命令 start 然后 cmd /c （windows 在外部程序结束后立即关闭）或 cmd /k （window 保持开放供您检查结果):

package de.scrum_master.Whosebug;

import java.io.File;
import java.io.IOException;

public class Launcher {
  public static void main(String[] args) throws IOException, InterruptedException {
    String classPath = "out/production/SO_ExternalProcessSystemConsole";
    String launchClass = "de.scrum_master.Whosebug.MyApp";
    String[] arguments = new String[] {
      "cmd", "/c", "start",
      "cmd", "/k", "java", "-cp", classPath, launchClass
    };
    ProcessBuilder pb = new ProcessBuilder(arguments);
    Process process = pb.start();
    process.waitFor();
  }
}

但是如果你想 read/write from/to 那个控制台，你又回到了第 1 个方块。您问为什么不能将 System.console() 继承给子进程。好吧，那是因为它是 null 由于 Eclipse 和 IntelliJ 从 IDE 中启动 Java 程序的方式（请参阅 [此处] 了解背景信息）。但是正如 Magnus 所说，您仍然有 System.{out|in} 并且可以按如下方式使用它们：

package de.scrum_master.Whosebug;

import java.io.File;
import java.io.IOException;
import java.util.Scanner;

public class Launcher {
  public static void main(String[] args) throws IOException, InterruptedException {
    String classPath = "out/production/SO_ExternalProcessSystemConsole";
    String launchClass = "de.scrum_master.Whosebug.MyApp";
    File workDir = new File(".");
    System.out.println("Hi, I am the launcher app!");

    String[] arguments = new String[] { "cmd", "/c", "java", "-cp", classPath, launchClass };
    ProcessBuilder pb = new ProcessBuilder(arguments);
    pb.environment().putAll(System.getenv());
    pb.directory(workDir);
    pb.inheritIO();
    Process process = pb.start();
    process.waitFor();

    System.out.print("What is your favourite city? ");
    Scanner scanner = new Scanner(System.in);
    String city = scanner.nextLine();
    System.out.println("I guess that " + city + " is a nice place.");
  }
}

package de.scrum_master.Whosebug;

import java.util.Scanner;

public class MyApp {
  public static void main(String[] args) {
    System.out.println("----------------------------------------");
    System.out.println("Hi, I am an externally started app.");
    System.out.print("Please enter your name: ");
    Scanner scanner = new Scanner(System.in);
    String name = scanner.nextLine();
    System.out.println("Hello " + name + "!");
    System.out.println("----------------------------------------");
  }
}

Hi, I am the launcher app!
----------------------------------------
Hi, I am an externally started app.
Please enter your name: Alexander
Hello Alexander!
----------------------------------------
What is your favourite city? Berlin
I guess that Berlin is a nice place.

启动另一个可用 System.console 的进程

Start another Process with System.console available

java

process

processbuilder