Java gets system properties and environment variables

When the program needs to use operating system-related variables (for example: file delimiters, newline characters), Java provides the static methods getenv() and getProperty() of the System class to return system-related variables and properties, getenv Most of the variables returned by the method are related to the system, and most of the variables returned by the getProperty method are related to the Java program.

System properties and environment variables are mappings between names and values. Both mechanisms can be used to pass user-defined information to the Java process. Environment variables have more of a global effect, since they are visible not only to Java child processes, but to all children of the process in which they are defined. On different operating systems, their semantics are slightly different, for example, they are not case sensitive. Environment variables are therefore more likely to have unintended side effects. Use system properties whenever possible in your program. Environment variables should be used when global effects are required, or when an external system interface requires the use of environment variables (such as PATH).

1. Environment variable getenv() : used to obtain the string mapping view of the current system environment; the test code and running results are as follows:

publicclass Test {  
    publicstaticvoid main(String[] args) {  
        Map<String, String> map = System.getenv();  
        for(Iterator<String> itr = map.keySet().iterator();itr.hasNext();){  
            String key = itr.next();  
            System.out.println(key + "=" + map.get(key));  
        }     
    }  
}

USERPROFILE        ：用户目录  
USERDNSDOMAIN      ：用户域  
PATHEXT            ：可执行后缀  
JAVA_HOME          ：Java安装目录  
TEMP               ：用户临时文件目录  
SystemDrive        ：系统盘符  
ProgramFiles       ：默认程序目录  
USERDOMAIN         ：帐户的域的名称  
ALLUSERSPROFILE    ：用户公共目录  
SESSIONNAME        ：Session名称  
TMP                ：临时目录  
Path               ：path环境变量  
CLASSPATH          ：classpath环境变量  
PROCESSOR_ARCHITECTURE ：处理器体系结构  
OS                     ：操作系统类型  
PROCESSOR_LEVEL    ：处理级别  
COMPUTERNAME       ：计算机名  
Windir             ：系统安装目录  
SystemRoot         ：系统启动目录  
USERNAME           ：用户名  
ComSpec            ：命令行解释器可执行程序的准确路径  
APPDATA            ：应用程序数据目录

2. System property getProperty() : the current system property. The test code and running results are as follows:

publicclass Test {  
    publicstaticvoid main(String[] args) {  
        Properties props = System.getProperties();  
        props.list(System.out);  
    }  
}

java.version Java            ：运行时环境版本  
java.vendor Java             ：运行时环境供应商  
java.vendor.url              ：Java供应商的 URL  
java.home                    ：Java安装目录  
java.vm.specification.version： Java虚拟机规范版本  
java.vm.specification.vendor ：Java虚拟机规范供应商  
java.vm.specification.name   ：Java虚拟机规范名称  
java.vm.version         ：Java虚拟机实现版本  
java.vm.vendor          ：Java虚拟机实现供应商  
java.vm.name            ：Java虚拟机实现名称  
java.specification.version   ：Java运行时环境规范版本  
java.specification.vendor    ：Java运行时环境规范供应商  
java.specification.name      ：Java运行时环境规范名称  
java.class.version      ：Java类格式版本号  
java.class.path         ：Java类路径  
java.library.path       ：加载库时搜索的路径列表  
java.io.tmpdir          ：默认的临时文件路径  
java.compiler           ：要使用的 JIT编译器的名称  
java.ext.dirs           ：一个或多个扩展目录的路径  
os.name                 ：操作系统的名称  
os.arch                 ：操作系统的架构  
os.version              ：操作系统的版本  
file.separator      ：文件分隔符  
path.separator      ：路径分隔符  
line.separator      ：行分隔符  
user.name           ：用户的账户名称  
user.home           ：用户的主目录  
user.dir            ：用户的当前工作目录

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