如何在Java中获取系统属性和环境变量的值？

如何获取环境变量的值？

Java中的 System 类提供了一个名为System.getenv()的方法，可用于获取当前系统中设置的环境变量的值。

句法：

public static String getenv(String key);

where key is the Environment variable
whose values we want

下面的例子说明了如何使用 System.getenv() 来获取系统环境变量：

示例 1：获取特定环境变量的值

// Java program to get the value
// of a specific environment variable
// using System.getenv() method
  
public class GFG {
    public static void main(String[] args)
    {
  
        // Get the value of
        // the TEMP environment variable
        System.out.println(System.getenv("TEMP"));
  
        // Get the value of
        // the OS environment variable
        System.out.println(System.getenv("OS"));
  
        // Get the value of
        // the JAVA_HOME environment variable
        System.out.println(System.getenv("JAVA_HOME"));
    }
}

输出：

示例 2：一次获取所有环境变量的值

// Java program to get the value
// of all environment variables at once
// using System.getenv() method
  
import java.util.Map;
  
public class GFG {
    public static void main(String[] args)
    {
  
        // Get the value of
        // all environment variables at once
        // and store it in Map
        Map env
            = System.getenv();
  
        for (String envName : env.keySet()) {
            System.out.format("%s=%s%n",
                              envName,
                              env.get(envName));
        }
    }
}

输出：

注意：输出将取决于您运行上述代码的系统。上面给出了一个示例输出

如何获取系统属性的值？

Java中的 System 类有两种用于读取系统属性的方法：

• Java.lang.System.getProperty(String key)：仅获取那些属性——您将使用键指定的值（与您想要的特定值相关联）。

  例子：

  // Java Program illustrating the working
  // of getProperty(String key) method
    
  import java.lang.*;
  import java.util.Properties;
    
  public class NewClass {
      public static void main(String[] args)
      {
          // Printing Name of the system property
          System.out.println("user.dir: "
                             + System.getProperty(
                                   "user.dir"));
    
          // Fetches the property set with 'home' key
          System.out.println("home: "
                             + System.getProperty(
                                   "home"));
          // Resulting in Null as no property is present
    
          // Printing 'name of Operating System'
          System.out.println("os.name: "
                             + System.getProperty(
                                   "os.name"));
    
          // Printing 'JAVA Runtime version'
          System.out.println("version: "
                             + System.getProperty(
                                   "java.runtime.version"));
    
          // Printing 'name' property
          System.out.println("name: "
                             + System.getProperty(
                                   "name"));
          // Resulting in Null as no property is present
      }
  }
  

  输出：

  user.dir: /tmp/hsperfdata_bot
  home: null
  os.name: Linux
  version: 1.8.0_101-b13
  name: null
  

• Java.lang.System.getProperty(String key, String definition)：帮助您创建自己想要的键值集。

  例子：

  // Java Program illustrating the working of
  // getProperty(String key, String definition) method
    
  import java.lang.*;
  import java.util.Properties;
    
  public class NewClass {
      public static void main(String[] args)
      {
    
          // Here key = "Hello" and
          // System Property = "Geeks"
          System.out.println("Hello property : "
                             + System.getProperty(
                                   "Hello", "Geeks"));
    
          // Here key = "Geek" and
          // System Property = "For Geeks"
          System.out.println("System-property :"
                             + System.getProperty(
                                   "System", "For Geeks"));
    
          // Here key = "Property" and
          // System Property = null
          System.out.println("Property-property :"
                             + System.getProperty(
                                   "Property"));
      }
  }
  

  输出：

  Hello key property : Geeks
  System key property :For Geeks
  Property key property :null
  

• Java.lang.System.getProperties()：获取所有属性——系统上的 JVM 从操作系统获取的值。

  例子：

  // Java Program illustrating the working of
  // getProperties() method
    
  public class GFG {
      public static void main(String[] args)
      {
    
          System.out.println(System.getProperties())
      }
  }
  

  输出：