Java tutorial
/* * The MIT License * * Copyright (c) 2004-2009, Sun Microsystems, Inc., Kohsuke Kawaguchi * * Permission is hereby granted, free of charge, to any person obtaining a copy * of this software and associated documentation files (the "Software"), to deal * in the Software without restriction, including without limitation the rights * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell * copies of the Software, and to permit persons to whom the Software is * furnished to do so, subject to the following conditions: * * The above copyright notice and this permission notice shall be included in * all copies or substantial portions of the Software. * * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN * THE SOFTWARE. */ package hudson.util; import hudson.model.Hudson; import hudson.triggers.SafeTimerTask; import hudson.triggers.Trigger; import org.apache.commons.io.FileUtils; import org.kohsuke.stapler.StaplerRequest; import org.kohsuke.stapler.StaplerResponse; import javax.servlet.ServletException; import javax.servlet.ServletContext; import static javax.servlet.http.HttpServletResponse.SC_INTERNAL_SERVER_ERROR; import java.io.File; import java.io.IOException; import java.util.Random; import java.util.logging.Logger; import java.util.logging.Level; import java.lang.management.ManagementFactory; import java.lang.reflect.Method; /** * Makes sure that no other Hudson uses our <tt>HUDSON_HOME</tt> directory, * to forestall the problem of running multiple instances of Hudson that point to the same data directory. * * <p> * This set up error occasionally happens especialy when the user is trying to reassign the context path of the app, * and it results in a hard-to-diagnose error, so we actively check this. * * <p> * The mechanism is simple. This class occasionally updates a known file inside the hudson home directory, * and whenever it does so, it monitors the timestamp of the file to make sure no one else is updating * this file. In this way, while we cannot detect the problem right away, within a reasonable time frame * we can detect the collision. * * <p> * More traditional way of doing this is to use a lock file with PID in it, but unfortunately in Java, * there's no reliabe way to obtain PID. * * @author Kohsuke Kawaguchi * @since 1.178 */ public class DoubleLaunchChecker { /** * The timestamp of the owner file when we updated it for the last time. * 0 to indicate that there was no update before. */ private long lastWriteTime = 0L; /** * Once the error is reported, the user can choose to ignore and proceed anyway, * in which case the flag is set to true. */ private boolean ignore = false; private final Random random = new Random(); public final File home; /** * ID string of the other Hudson that we are colliding with. * Can be null. */ private String collidingId; public DoubleLaunchChecker() { home = Hudson.getInstance().getRootDir(); } protected void execute() { File timestampFile = new File(home, ".owner"); long t = timestampFile.lastModified(); if (t != 0 && lastWriteTime != 0 && t != lastWriteTime && !ignore) { try { collidingId = FileUtils.readFileToString(timestampFile); } catch (IOException e) { LOGGER.log(Level.SEVERE, "Failed to read collision file", e); } // we noticed that someone else have updated this file. // switch GUI to display this error. Hudson.getInstance().servletContext.setAttribute("app", this); LOGGER.severe("Collision detected. timestamp=" + t + ", expected=" + lastWriteTime); // we need to continue updating this file, so that the other Hudson would notice the problem, too. } try { FileUtils.writeStringToFile(timestampFile, getId()); lastWriteTime = timestampFile.lastModified(); } catch (IOException e) { // if failed to write, err on the safe side and assume things are OK. lastWriteTime = 0; } schedule(); } /** * Figures out a string that identifies this instance of Hudson. */ public String getId() { Hudson h = Hudson.getInstance(); // in servlet 2.5, we can get the context path String contextPath = ""; try { Method m = ServletContext.class.getMethod("getContextPath"); contextPath = (String) m.invoke(h.servletContext); } catch (Exception e) { // maybe running with Servlet 2.4 } return h.hashCode() + contextPath + " at " + ManagementFactory.getRuntimeMXBean().getName(); } public String getCollidingId() { return collidingId; } /** * Schedules the next execution. */ public void schedule() { // randomize the scheduling so that multiple Hudson instances will write at the file at different time long MINUTE = 1000 * 60; Trigger.timer.schedule(new SafeTimerTask() { protected void doRun() { execute(); } }, (random.nextInt(30) + 60) * MINUTE); } /** * Serve all URLs with the index view. */ public void doDynamic(StaplerRequest req, StaplerResponse rsp) throws IOException, ServletException { rsp.setStatus(SC_INTERNAL_SERVER_ERROR); req.getView(this, "index.jelly").forward(req, rsp); } /** * Ignore the problem and go back to using Hudson. */ public void doIgnore(StaplerRequest req, StaplerResponse rsp) throws IOException { ignore = true; Hudson.getInstance().servletContext.setAttribute("app", Hudson.getInstance()); rsp.sendRedirect2(req.getContextPath() + '/'); } private static final Logger LOGGER = Logger.getLogger(DoubleLaunchChecker.class.getName()); }