Java System Class in the java.lang Package

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Java

The System class has properties and methods related to the runtime environment. It provides class variables for standard input and output using out and in, error output using err, methods for copying objects, and useful methods that can be used when writing programs.

System Variables

Variable	Description
final static InputStream in	Used for standard input.
final static PrintStream out	Used for standard output. It receives print() and println() parameters and outputs them to the monitor.
final static PrintStream err	Used for error output. It receives print() and println() parameters and outputs errors to the monitor.

Main System Methods

Method	Description
static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length)	Copies an array. src and dest are the array variables to copy from and to, srcPos and destPos are the positions where copying starts, and length is the size of the array to copy.
static long currentTimeMillis()	Returns the time from midnight on January 1, 1970 to the present in milliseconds.
static void exit(int status)	Terminates the currently running application. Returns the termination status as status. Usually, a negative value means abnormal termination.
static void gc()	Runs garbage collection.
static Properties getProperties()	Gets the list of system environment variables.
static String getProperty(String key)	Gets a system environment variable.
static String getProperty(String key, String def)	Gets a system environment variable with a default value if it does not exist.

System Examples

Example) System.currentTimeMillis()

The following is an example of the System.currentTimeMillis() method.

package com.devkuma.tutorial.java.lang;

public class SystemCurrentTimeMillis {

    public static void main(String[] args) throws InterruptedException {

        long start = System.currentTimeMillis();
        System.out.println("start time : " + System.currentTimeMillis());

        Thread.sleep(1000); // Stop for 1 second

        long end = System.currentTimeMillis();
        System.out.println("end time : " + System.currentTimeMillis());

        System.out.println("duration : " + (end - start));
    }
}

The execution result is as follows.

start time : 1497970891769
end time : 1497970892771
duration : 1002

Example) System.arraycopy()

The following is an example of the System.arraycopy() method.

package com.devkuma.tutorial.java.lang;

public class SystemArraycopy {

    public static void main(String[] args) {
        char[] src = { 'a', 'b', 'c', 'd', 'e' };
        char[] dest = { 'x', 'x', 'x', 'x', 'x' };

        System.out.println("src=" + new String(src));
        System.out.println("dest=" + new String(dest));
        System.arraycopy(src, 0, dest, 0, src.length);
        System.out.println();
        System.out.println("src=" + new String(src));
        System.out.println("dest=" + new String(dest));

    }
}

The execution result is as follows.

src=abcde
dest=xxxxx

src=abcde
dest=abcde

Example) System.getProperties(), System.getProperty()

The following is an example of the System.getProperties() and System.getProperty() methods related to system environment variables.

package com.devkuma.tutorial.java.lang;

import java.util.Enumeration;
import java.util.Properties;

public class SystemProperties {

    public static void main(String[] args) {

        // Java Runtime Environment version
        System.out.println("java.version=" + System.getProperty("java.version"));
        // Java Runtime Environment vendor
        System.out.println("java.vendor=" + System.getProperty("java.vendor"));
        // Java Runtime Environment vendor URL
        System.out.println("java.vendor.url=" + System.getProperty("java.vendor.url"));
        // Java Runtime Environment installation directory
        System.out.println("java.home=" + System.getProperty("java.home"));

        // Java virtual machine specification
        System.out.println("java.vm.specification.version=" + System.getProperty("java.vm.specification.version"));
        // Java virtual machine specification vendor
        System.out.println("java.vm.specification.vendor=" + System.getProperty("java.vm.specification.vendor"));
        // Java virtual machine specification name
        System.out.println("java.vm.specification.name=" + System.getProperty("java.vm.specification.name"));
        // Java virtual machine implementation version
        System.out.println("java.vm.version=" + System.getProperty("java.vm.version")); //
        // Java virtual machine implementation vendor
        System.out.println("java.vm.vendor=" + System.getProperty("java.vm.vendor")); //
        // Java virtual machine implementation name
        System.out.println("java.vm.name=" + System.getProperty("java.vm.name"));

        // Java Runtime Environment specification version
        System.out.println("java.specification.version=" + System.getProperty("java.specification.version"));
        // Java Runtime Environment specification vendor
        System.out.println("java.specification.vendor=" + System.getProperty("java.specification.vendor"));
        // Java Runtime Environment specification name
        System.out.println("java.specification.name=" + System.getProperty("java.specification.name"));
        // Java class format version number
        System.out.println("java.class.version=" + System.getProperty("java.class.version"));
        // Java class path
        System.out.println("java.class.path=" + System.getProperty("java.class.path"));
        // List of paths searched when loading libraries
        System.out.println("java.library.path=" + System.getProperty("java.library.path"));
        // Default temporary file path
        System.out.println("java.io.tmpdir=" + System.getProperty("java.io.tmpdir"));
        // Name of the JIT compiler in use
        System.out.println("java.compiler=" + System.getProperty("java.compiler"));
        // Extension directory path
        System.out.println("java.ext.dirs=" + System.getProperty("java.ext.dirs"));

        // operating system name
        System.out.println("os.name=" + System.getProperty("os.name"));
        // operating system architecture
        System.out.println("os.arch=" + System.getProperty("os.arch"));
        // operating system version
        System.out.println("os.version=" + System.getProperty("os.version"));

        // file separator ("/" on UNIX)
        System.out.println("file.separator=" + System.getProperty("file.separator"));
        // path separator (":" on UNIX)
        System.out.println("path.separator=" + System.getProperty("path.separator"));
        // line separator ("\n" on UNIX)
        System.out.println("line.separator=" + System.getProperty("line.separator"));

        // user name
        System.out.println("user.name=" + System.getProperty("user.name"));
        // user home directory
        System.out.println("user.home=" + System.getProperty("user.home"));
        // user's current working directory
        System.out.println("user.dir=" + System.getProperty("user.dir"));
        System.out.println();

        // Query all information
        Properties prop = System.getProperties();
        Enumeration<?> enu = prop.keys();
        while (enu.hasMoreElements()) {
            String key = (String) enu.nextElement();
            String value = (String) prop.get(key);
            System.out.println(key + "=" + value);
        }
    }
}

The execution result is as follows.

java.version=1.8.0_161
java.vendor=Oracle Corporation
java.vendor.url=http://java.oracle.com/
java.home=/Library/Java/JavaVirtualMachines/jdk1.8.0_161.jdk/Contents/Home/jre
java.vm.specification.version=1.8
java.vm.specification.vendor=Oracle Corporation
java.vm.specification.name=Java Virtual Machine Specification
java.vm.version=25.161-b12
java.vm.vendor=Oracle Corporation
java.vm.name=Java HotSpot(TM) 64-Bit Server VM
java.specification.version=1.8
java.specification.vendor=Oracle Corporation
java.specification.name=Java Platform API Specification
java.class.version=52.0
java.class.path=/Library/Java/JavaVirtualMachines...(omitted)...
java.library.path=/Users/kimkc/Library/Java/Extensions:...(omitted)...
java.io.tmpdir=/var/folders/f2/s8f8bt7j2vdf4538y4lydp480000gn/T/
java.compiler=null
java.ext.dirs=/Users/kimkc/Library/Java/Extensions:...(omitted)...
os.name=Mac OS X
os.arch=x86_64
os.version=10.13.6
file.separator=/
path.separator=:
line.separator=

user.name=kimkc
user.home=/Users/kimkc
user.dir=/Users/kimkc/dev/workspace/java-tutorial

java.runtime.name=Java(TM) SE Runtime Environment
...
sun.cpu.isalist=