Learn Understanding System Properties | System Properties and Environment Variables

System properties in Java are key-value pairs maintained by the Java Virtual Machine (JVM) that provide information about the runtime environment and allow you to configure application behavior. These properties are available from the time the JVM starts until it shuts down, and they can be set before the application launches, modified during runtime, and accessed from anywhere in your code. Commonly, system properties are used for specifying file paths, enabling debug modes, or passing configuration values that should not be hardcoded.

To set a system property, you can use the -D flag on the command line when starting your Java application. For example, running java -DmyProperty=value MyApp sets a property called myProperty with the value value. You can also set or change system properties within your code using System.setProperty(String key, String value). To access a system property, use System.getProperty(String key), which returns the value as a string or null if the property is not set. If you want to provide a default value, you can use the overloaded method System.getProperty(String key, String defaultValue).

System properties can be overridden in several ways. If a property is specified both at the command line and set within the code, the most recent assignment takes precedence. This means you can provide sensible defaults in your code but allow users or administrators to override them at deployment or launch time.

Main.java


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package com.example;

public class Main {
    public static void main(String[] args) {
        String javaVersion = System.getProperty("java.version");
        String userHome = System.getProperty("user.home");
        String osName = System.getProperty("os.name");

        System.out.println("Java Version: " + javaVersion);
        System.out.println("User Home: " + userHome);
        System.out.println("Operating System: " + osName);
    }
}

System properties and environment variables are both used to configure Java applications, but they serve different roles and are managed differently. System properties are specific to the JVM and exist only while the Java process is running. They can be set at JVM startup or during runtime from within Java code. Environment variables, on the other hand, are managed by the operating system and are available to any process running on the system, not just Java. Environment variables are typically used for broader system configuration, such as specifying the location of Java installations or database hosts, while system properties are more focused on Java-specific or application-specific settings.

You should use system properties when you want configuration values to be tightly scoped to the JVM instance and possibly modifiable at runtime. Use environment variables when the configuration should be shared across multiple processes or languages, or when you want to externalize secrets and credentials away from application code entirely.

Main.java


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package com.example;

public class Main {
    public static void main(String[] args) {
        String customProp = System.getProperty("app.mode");
        if (customProp == null) {
            System.out.println("Custom property 'app.mode' is not set.");
        } else {
            System.out.println("Custom property 'app.mode': " + customProp);
        }
    }
}

1. How can you set a system property when launching a Java application?

2. Which method is used to retrieve a system property in Java?

Everything was clear?

Thanks for your feedback!

Section 2. Chapter 1

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Main.java


              1234567891011121314
            
package com.example;

public class Main {
    public static void main(String[] args) {
        String javaVersion = System.getProperty("java.version");
        String userHome = System.getProperty("user.home");
        String osName = System.getProperty("os.name");

        System.out.println("Java Version: " + javaVersion);
        System.out.println("User Home: " + userHome);
        System.out.println("Operating System: " + osName);
    }
}

Main.java


              123456789101112
            
package com.example;

public class Main {
    public static void main(String[] args) {
        String customProp = System.getProperty("app.mode");
        if (customProp == null) {
            System.out.println("Custom property 'app.mode' is not set.");
        } else {
            System.out.println("Custom property 'app.mode': " + customProp);
        }
    }
}

1. How can you set a system property when launching a Java application?

2. Which method is used to retrieve a system property in Java?

Everything was clear?

Thanks for your feedback!

Section 2. Chapter 1