How to Create Linux-Cron Job from a Java Program

Question

Hello,
Can anybody help me to CREATE/EDIT/DELETE Linux Cron job from Java Program. Its Very Urgent.Thanks in advance..
from
Chakri

Answer 1

I decided this didn't sound too tough so I played around with it a little. Basically because I'd never tried executing external processes out of java before, so I wanted to see how it was done.
Just change whatever you like in the jobs ArrayList and call writeJobs.
If you call writeJobs without putting anything in the list first you'll wipe out all of your crontab entries.
import java.lang.*;
import java.io.*;
import java.util.*;
public class cron {
    ArrayList jobs;
    Runtime rt;
    cron() {
     rt = Runtime.getRuntime();
     jobs = new ArrayList();
    void readCron() {
     String[] list = { "crontab", "-l" };
     jobs = new ArrayList();
     try {
         // Stick a job into crontab
         Process child = rt.exec(list);
         BufferedReader cronout = new BufferedReader(new InputStreamReader(child.getInputStream()));
         String cronjob = cronout.readLine();
         while (cronjob != null) {
          jobs.add(cronjob);
          cronjob = cronout.readLine();
         child.waitFor();
     catch(IOException e) {
         System.err.println("IOException starting process!");
     catch(InterruptedException e) {
         System.err.println("Interrupted waiting for process!");
    void listJobs() {
     Iterator iter = jobs.iterator();
     while (iter.hasNext()) {
         System.out.println((String)iter.next());
    void writeJobs() {
     String[] edit = { "crontab"};
     try {
         // Stick a job into crontab
         Process child = rt.exec(edit);
         PrintWriter cronIn = new PrintWriter(child.getOutputStream());
         Iterator iter = jobs.iterator();
         while (iter.hasNext()) {
          cronIn.println((String)iter.next());
         cronIn.close();
         child.waitFor();
     catch(IOException e) {
         System.err.println("IOException starting process!");
     catch(InterruptedException e) {
         System.err.println("Interrupted waiting for process!");
    void doStuff() {
     readCron();
     listJobs();
     jobs.add("* * * * 4 cronjob");
     writeJobs();
     readCron();
     listJobs();
    public static void main(String[] args) {
     cron c = new cron();
     c.doStuff();
}