我有两个程序:
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,并且在NPE上失败。
因此,问题是:有什么方法可以使用System.console()运行另一个进程?
最佳答案
答案很简单:如果从诸如Eclipse或IntelliJ IDEA之类的IDE运行启动器,则可能首先没有设置System.console()
,因此子进程无法继承任何内容。只需尝试从启动器向System.console()
写入内容,它也会因相同的错误而失败。但是,如果您从Cmd.exe或Git Bash之类的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"
,无论有没有这些参数,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!
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,则可以为此目的使用Windows命令
start
,然后使用cmd /c
(外部程序结束后立即关闭Windows) )或cmd /k
(窗口保持打开状态,您可以检查结果):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,如果您想从该控制台读/写该控制台,您将回到第一个正方形。您问为什么不能将
System.console()
继承到子进程。嗯,这是因为Eclipse和IntelliJ从IDE内部启动Java程序的方式,所以它是null
(有关背景信息,请参见[here])。但是正如Magnus所说,您仍然有System.{out|in}
并可以按以下方式使用它们: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.