Question

我有两个程序：

首先，它使用Console对象来读写数据
第二，应首先运行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程序开始时，System.console（）为空，并且在NPE时失败。

所以，问题是：有没有办法在System.console（）可用的情况下运行另一个进程？

Answer 1

答案很简单：如果您从Eclipse或IntelliJ IDEA等IDE运行启动程序，可能它首先没有设置System.console()，因此子进程无法继承。只是尝试从启动器向System.console()写入内容，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，则启动器和通过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"，它可以使用或不使用这些参数。

package de.scrum_master.stackoverflow;

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.stackoverflow.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.stackoverflow;

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

从IntelliJ IDEA启动时的控制台日志：

Exception in thread "main" java.lang.NullPointerException
    at de.scrum_master.stackoverflow.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!

随意提出任何后续问题。

更新：如果您不介意外部程序在其自己的交互式控制台中而不是在IDE控制台中运行，则可以使用Windows命令start来实现此目的，然后cmd /c（窗口在外部程序结束后立即关闭）或cmd /k（窗口保持打开状态以供检查结果）：

package de.scrum_master.stackoverflow;

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.stackoverflow.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();
  }
}

但如果那时你想从/向该控制台读/写，你就回到了＃1广场。您问为什么不能将System.console()继承到子进程。好吧，那是因为它是null，因为Eclipse和IntelliJ从IDE中启动Java程序的方式（参见[此处]获取背景信息）。但正如 Magnus 所说，你仍然有System.{out|in}，可以按如下方式使用它们：

package de.scrum_master.stackoverflow;

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.stackoverflow.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.stackoverflow;

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.

使用System.console启动另一个进程

1 个答案: