Java run time system

Содержание

Java Tutorial/Development/Runtime System
Display the amount of free memory in the Java Virtual Machine.
Display the maximum amount of memory
Display the total amount of memory in the Java virtual machine.
Execute a command from code
Execute a command with more than one argument
Execute system command
From Runtime.exec() to ProcessBuilder
Get Number of Available Processors
Launch a Unix script with Java
Milliseconds elapsed since January 1, 1970
Read output from a Command execution
Registering Shutdown Hooks for Virtual Machine
Send an Input to a Command
System.getProperty
Timing program execution.
Using arraycopy().
Java Runtime class
Methods of Java Runtime class
1. Runtime getRuntime()
2. exit(int status)
3. addShutdownHook(Thread Hook)
4. Process exec(String command) throws IOException
5. availableProcesors()
6. freeMemory()
7. totalMemory()
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Java Tutorial/Development/Runtime System

Display the amount of free memory in the Java Virtual Machine.

import java.text.DecimalFormat; public class Main

public static void main(String[] args) < DecimalFormat df = new DecimalFormat("0.00"); long freeMem = Runtime.getRuntime().freeMemory(); System.out.println(df.format(freeMem / 1000000F) + " MB"); >

Display the maximum amount of memory

import java.text.DecimalFormat; public class Main

public static void main(String[] args) < DecimalFormat df = new DecimalFormat("0.00"); long maxMem = Runtime.getRuntime().maxMemory(); System.out.println(df.format(maxMem / 1000000F) + " MB"); >

Display the total amount of memory in the Java virtual machine.

import java.text.DecimalFormat; public class Main

public static void main(String[] args) < DecimalFormat df = new DecimalFormat("0.00"); long totalMem = Runtime.getRuntime().totalMemory(); System.out.println(df.format(totalMem / 1000000F) + " MB"); >

Execute a command from code

public static void main(String[] argv) throws Exception < // Execute a command without arguments String command = "ls"; Process child = Runtime.getRuntime().exec(command); // Execute a command with an argument command = "ls /tmp"; child = Runtime.getRuntime().exec(command); >

Execute a command with more than one argument

public static void main(String[] argv) throws Exception < // Execute a command with an argument that contains a space String[] commands = new String[] < "grep", "hello world", "/tmp/f.txt" >; commands = new String[] < "grep", "hello world", "c:\\f.txt" >; Process child = Runtime.getRuntime().exec(commands); >

Execute system command

import java.io.IOException; public class Main

public static void main(String[] args) throws IOException < String cmd = "cmd.exe /c start "; // String file = "c:\\version.txt"; // String file nofollow" href="http://www.google.ru">http://www.google.ru"; // String file = "c:\\"; // String file nofollow" href="mailto:author@my.ru">mailto:author@my.ru"; String file = "mailto:"; Runtime.getRuntime().exec(cmd + file); >

From Runtime.exec() to ProcessBuilder

import java.io.BufferedReader; import java.io.IOException; import java.io.InputStream; import java.io.InputStreamReader; import java.util.Arrays; public class Main

public static void main(String args[]) throws IOException < Runtime runtime = Runtime.getRuntime(); Process process = runtime.exec(args); InputStream is = process.getInputStream(); InputStreamReader isr = new InputStreamReader(is); BufferedReader br = new BufferedReader(isr); String line; System.out.printf("Output of running %s is:", Arrays.toString(args)); while ((line = br.readLine()) != null) < System.out.println(line); >>

Get Number of Available Processors

public static void main(String[] args)

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> // Number of processors available to the Java Virtual Machine: 2

Launch a Unix script with Java

public static void main(String[] argv) throws Exception < String[] cmd = < "/bin/sh", "-c", "ls >hello" >; Runtime.getRuntime().exec(cmd); >

Milliseconds elapsed since January 1, 1970

import java.util.Properties; public class SystemApp
public static void main(String args[])

Read output from a Command execution

import java.io.InputStream; public class Main

public static void main(String[] argv) throws Exception < String command = "ls"; Process child = Runtime.getRuntime().exec(command); InputStream in = child.getInputStream(); int c; while ((c = in.read()) != -1) < System.out.println((char) c); >in.close(); >

Registering Shutdown Hooks for Virtual Machine

public class Main implements Runnable
public void run() < System.out.println("Shutting down"); >public static void main(String[] arg)

Send an Input to a Command

import java.io.OutputStream; public class Main
public static void main(String[] argv) throws Exception

System.getProperty

public static void main(String args[])

> The following properties are available: file.separator java.specification.version java.vm.version java.class.path java.vendor line.separator java.class.version java.vendor.url os.arch java.rupiler java.version os.name java.ext.dirs java.vm.name os.version java.home java.vm.specification.name path.separator java.io.tmpdir java.vm.specification.vendor user.dir java.library.path java.vm.specification.version user.home java.specification.name java.vm.vendor user.name java.specification.vendor

Timing program execution.

public static void main(String args[]) < long start, end; System.out.println("Timing a for loop from 0 to 1,000,000"); start = System.currentTimeMillis(); // get starting time for (int i = 0; i < 1000000; i++) ; end = System.currentTimeMillis(); // get ending time System.out.println("Elapsed time: " + (end - start)); >

Using arraycopy().

public static void main(String args[]) < Runtime r = Runtime.getRuntime(); Process p = null; try < p = r.exec("notepad"); p.waitFor(); >catch (Exception e) < System.out.println("Error executing notepad."); >System.out.println("Notepad returned " + p.exitValue()); >

Источник

Java Runtime class

The class used for the interaction with the run time environment of Java is called run time class in Java which provides several methods such as Runtime.getRuntime(), exit(int status), addShutdownHook(Thread hook), Process exec(String command) throws an input-output exception, availableProcessors(), freeMemory(), totalMemory(), etc. for process execution, for invoking GC, to obtain the total memory, to obtain the free memory, etc. whose declaration is public class Runtime extends object and only one application of Java can use only one instance of java.lang.Runtime class and a singleton instance of Runtime class is returned using Runtime.getRuntime() method in Java.

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Methods of Java Runtime class

There are several methods of Java Runtime Class. They are:

1. Runtime getRuntime()

An instance of the Run time class is returned using the Runtime getRuntime() method. Consider the below Java program to demonstrate the Runtime getRuntime() method of the Java Run time class.

//a class called program is defined public class program < //main method is called public static void main(String[] args) < // The current runtime in association with process is obtained using getRuntime method. Runtime roll = Runtime.getRuntime(); // The current free memory for the current runtime is obtained using freeMemory() method. System.out.println("The current free memory for the current runtime is" + roll.freeMemory()); >>

Explanation: In the above program, a class called program is defined. Then the main method is called. Then The current runtime in association with the process is obtained using getRuntime method. Then The current free memory for the current runtime is obtained using freeMemory() method.

2. exit(int status)

The current virtual machine is terminated using the Runtime exit(int status)method. Consider the below Java program to demonstrate the exit(int status) method of the Java Run time class.

//a class called program is defined public class program < //main method is called public static void main(String[] args) < //the current runtime or program is caused to exit using exit(int status) method Runtime.getRuntime().exit(0); //This statement is not executed because the program is terminated by the usage of exit(int status) method above System.out.println("Checking if the program executes this statement depsite the use of exit(int status) method"); >>

The output of the above program is blank.

Explanation: In the above program, a class called program is defined. Then the main method is called. Then the current runtime or program is caused to exit using the exit(int status) method. Then the statement is not executed because the program is terminated by the usage of the exit(int status) method in the above statement.

3. addShutdownHook(Thread Hook)

A new hook thread is registered usingaddShutdownHook(Thread Hook)method. Consider the below Java program to demonstrate the addShutdownHook(Thread Hook)method of the Java Run time class.

//a class called program is defined public class program < // when the program is about to exit, this class extending the thread is called static class Mess extends Thread < public void run() < System.out.println("The program is going to exit"); >> //main method is called public static void main(String[] args) < try < //the class mess is registered as shut down hook Runtime.getRuntime().addShutdownHook(new Mess()); //The thread is made to sleep for certain seconds System.out.println("Five seconds of waiting time for the program to exit"); Thread.sleep(5); >catch (Exception ex) < ex.printStackTrace(); >> >

Explanation: In the above program, a class called program is defined. When the program is about to exit, this class extending the thread is called. Then the main method is called. Then the class mess is registered as shut down the hook. Then the thread is made to sleep for certain seconds.

4. Process exec(String command) throws IOException

The given command is executed in a separate process using Process exec(String command) throws IOExceptionmethod. Consider the below Java program to demonstrate Process exec(String command) throws IOExceptionmethod of Java Run time class

//a class called program is defined public class program < //main method is called public static void main(String[] args) < try < // a process is created to execute google chrome Process proc = Runtime.getRuntime().exec("google-chrome"); System.out.println("Successfully started google chrome"); >catch (Exception e) < e.printStackTrace(); >> >

Explanation: In the above program, a class called program is defined. Then the main method is called. Then a process is created to execute google chrome.

5. availableProcesors()

The number of available processors are returned using availableProcesors()method. Consider the below Java program to demonstrate the availableProcessors() method of the Java Run time class.

//a class called program is defined public class program < //main method is called public static void main(String[] args) < //checking for the number of available processors using availableprocessors() method. System.out.println("The number of available processors are " + Runtime.getRuntime().availableProcessors()); >>

Explanation: In the above program, a class called program is defined. Then the main method is called. Then a check for the number of available processors is done using availableprocessors() method.

6. freeMemory()

The amount of free memory is returned in JVM using freeMemory()method.

Consider the below Java program to demonstrate freeMemory() method of the Java Run time class.

//a class called program is defined public class program < //main method is called public static void main(String[] args) < //The amount of free memory available is given by using freememory() method System.out.println("The amount of free memory available is " + Runtime.getRuntime().freeMemory()); >>

Explanation: In the above program, a class called program is defined. Then the main method is called. Then the amount of free memory available is given by using freememory() method.

7. totalMemory()

The amount of total memory is returned in JVM using totalMemory()method. Consider the below Java program to demonstrate totalMemory() method of the Java Run time class.

//a class called program is defined public class program < //main method is called public static void main(String[] args) < //The amount of total memory available is given by using totalmemory() method System.out.println("The amount of free memory available is " + Runtime.getRuntime().totalMemory()); >>

Explanation: In the above program, a class called program is defined. Then the main method is called. Then the amount of total memory available is given by using totalmemory() method.