What exactly is user.dir in Java? System.getProperty method to get properties

String workDirectory = System.getProperty("user.dir");

Many students must have written or seen the above code. So have you ever thought about what does "user.dir" mean? Is it a system environment variable? Or user folder?

As the saying goes, "Ask the source code for everything", let us go to the source code to find out:

 public static String getProperty(String key) {
    
    
        checkKey(key);
        SecurityManager sm = getSecurityManager();
        if (sm != null) {
    
    
            sm.checkPropertyAccess(key);
        }

        return props.getProperty(key);
    }

// 本地方法，用于初始化props属性
  private static native Properties initProperties(Properties props);  

From the source code above, we can see that the getProperty method obtains the corresponding value from a static property props of the System class, and the props property is given an initial value through the local method initProperties. That is, this system property is automatically initialized when the JVM starts by executing the native method. Fortunately, jdk's documentation notes have explained to us which properties the JVM ensures, and we list them in the following table:

attribute name	illustrate	example value
java.version	Java version number	11.0.5
java.version.date	Java version date	2019-10-15
java.vendor	Java vendor specific string	Oracle Corporation
java.vendor.url	Java provider URL	http://java.oracle.com/
java.vendor.version	Java vendor version	18.9
java.home	Java installation root directory	/usr/lib/jvm/jdk-11.0.5
java.class.version	Java class file version number	55.0
java.class.path	Java classpath	Too long, omitted here
os.name	operating system name	Linux
os.arch	operating system architecture	amd64
os.version	operating system version	5.0.0-37-generic
file.separator	file separator	/
path.separator	path separator	:
line.separator	line break	\n
user.name	user account	lhing17
user.home	user root directory	/home/lhing17
user.dir	user's current working directory	/home/lhing17/IdeaProjects/rocketDownloader

The example values ​​in the above table are the results output by the test class on my personal computer. We can clearly see that "user.dir" refers to the user's current working directory. If you are running the project in the IDE, this directory is the root directory of your current project. In addition to "user.dir", some other useful properties are provided here, such as "java.home", "user.home" and so on. The JVM (referring to the java virtual machine) guarantees that the properties in the above table are valuable. In the project, we can safely obtain them through System.getProperty(propertyName) without worrying about null pointer exceptions.

We can also modify these properties by means of System.setProperty(propertyName, propertyValue), or customize some new properties. However, the official java documentation clearly reminds us that modifying these system properties may cause unexpected side effects, and we should not usually modify these properties.

Summarize

The properties obtained by the System.getProperty method are properties initialized when the JVM is loaded, such as "user.dir" representing the user's current working directory. The JVM ensures that some specified property values ​​exist, and we can safely call them.