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Apache Commons Exec is a library to reliably execute external processes from within the JVM.
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
*/
package org.apache.commons.exec;
import java.util.Enumeration;
import java.util.Vector;
/**
* Destroys all registered {@code Process}es when the VM exits.
*
* @version $Id: ShutdownHookProcessDestroyer.java 1636056 2014-11-01 21:12:52Z ggregory $
*/
public class ShutdownHookProcessDestroyer implements ProcessDestroyer, Runnable {
/** the list of currently running processes */
private final Vector processes = new Vector();
/** The thread registered at the JVM to execute the shutdown handler */
private ProcessDestroyerImpl destroyProcessThread = null;
/** Whether or not this ProcessDestroyer has been registered as a shutdown hook */
private boolean added = false;
/**
* Whether or not this ProcessDestroyer is currently running as shutdown hook
*/
private volatile boolean running = false;
private class ProcessDestroyerImpl extends Thread {
private boolean shouldDestroy = true;
public ProcessDestroyerImpl() {
super("ProcessDestroyer Shutdown Hook");
}
@Override
public void run() {
if (shouldDestroy) {
ShutdownHookProcessDestroyer.this.run();
}
}
public void setShouldDestroy(final boolean shouldDestroy) {
this.shouldDestroy = shouldDestroy;
}
}
/**
* Constructs a {@code ProcessDestroyer} and obtains
* {@code Runtime.addShutdownHook()} and
* {@code Runtime.removeShutdownHook()} through reflection. The
* ProcessDestroyer manages a list of processes to be destroyed when the VM
* exits. If a process is added when the list is empty, this
* {@code ProcessDestroyer} is registered as a shutdown hook. If
* removing a process results in an empty list, the
* {@code ProcessDestroyer} is removed as a shutdown hook.
*/
public ShutdownHookProcessDestroyer() {
}
/**
* Registers this {@code ProcessDestroyer} as a shutdown hook, uses
* reflection to ensure pre-JDK 1.3 compatibility.
*/
private void addShutdownHook() {
if (!running) {
destroyProcessThread = new ProcessDestroyerImpl();
Runtime.getRuntime().addShutdownHook(destroyProcessThread);
added = true;
}
}
/**
* Removes this {@code ProcessDestroyer} as a shutdown hook, uses
* reflection to ensure pre-JDK 1.3 compatibility
*/
private void removeShutdownHook() {
if (added && !running) {
final boolean removed = Runtime.getRuntime().removeShutdownHook(
destroyProcessThread);
if (!removed) {
System.err.println("Could not remove shutdown hook");
}
/*
* start the hook thread, a unstarted thread may not be eligible for
* garbage collection Cf.: http://developer.java.sun.com/developer/
* bugParade/bugs/4533087.html
*/
destroyProcessThread.setShouldDestroy(false);
destroyProcessThread.start();
// this should return quickly, since it basically is a NO-OP.
try {
destroyProcessThread.join(20000);
} catch (final InterruptedException ie) {
// the thread didn't die in time
// it should not kill any processes unexpectedly
}
destroyProcessThread = null;
added = false;
}
}
/**
* Returns whether or not the ProcessDestroyer is registered as as shutdown
* hook
*
* @return true if this is currently added as shutdown hook
*/
public boolean isAddedAsShutdownHook() {
return added;
}
/**
* Returns {@code true} if the specified {@code Process} was
* successfully added to the list of processes to destroy upon VM exit.
*
* @param process
* the process to add
* @return {@code true} if the specified {@code Process} was
* successfully added
*/
public boolean add(final Process process) {
synchronized (processes) {
// if this list is empty, register the shutdown hook
if (processes.size() == 0) {
addShutdownHook();
}
processes.addElement(process);
return processes.contains(process);
}
}
/**
* Returns {@code true} if the specified {@code Process} was
* successfully removed from the list of processes to destroy upon VM exit.
*
* @param process
* the process to remove
* @return {@code true} if the specified {@code Process} was
* successfully removed
*/
public boolean remove(final Process process) {
synchronized (processes) {
final boolean processRemoved = processes.removeElement(process);
if (processRemoved && processes.size() == 0) {
removeShutdownHook();
}
return processRemoved;
}
}
/**
* Returns the number of registered processes.
*
* @return the number of register process
*/
public int size() {
return processes.size();
}
/**
* Invoked by the VM when it is exiting.
*/
public void run() {
synchronized (processes) {
running = true;
final Enumeration e = processes.elements();
while (e.hasMoreElements()) {
final Process process = e.nextElement();
try {
process.destroy();
}
catch (final Throwable t) {
System.err.println("Unable to terminate process during process shutdown");
}
}
}
}
}