我有两个程序:

• 首先，使用Console对象读写数据
• 其次，应该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启动时，System.console() 为 null 并且它因 NPE 而失败。

因此，问题是:是否有任何方法可以使用可用的 System.console() 运行另一个进程？

最佳答案

答案很简单:如果您从像 Eclipse 或 IntelliJ IDEA 这样的 IDE 运行您的启动器，它可能一开始就没有设置 System.console()，因此没有任何子进程可以继承自。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，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"，无论是否使用这些参数，它都可以工作。

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!

请随时提出任何后续问题。

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，您可以为此使用 Windows 命令 start目的，然后 cmd/c(外部程序结束后立即关闭窗口)或 cmd/k(窗口保持打开状态供您检查结果):

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();
  }
}

但是如果你想从那个控制台读/写，你又回到了第一个方 block 。您问为什么不能将 System.console() 继承给子进程。嗯，这是因为它是 null，这是由于 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)。但正如 Magnus 所说，您仍然拥有 System.{out|in} 并且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.

关于java - 使用可用的 System.console 启动另一个进程，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/42042166/