JavaVersionChecker.java

import javax.swing.JOptionPane;

/*
 * Copyright (C) 2009 Rob Manning
 * manningr@users.sourceforge.net
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */

/**
 * The purpose of this class is to provide a Java2 (1.2.x) compatible class that can simply check the
 * JVM version and put up a dialog message if the version isn't sufficiently recent.  This class should be
 * compiled and jar'd with JDK 1.2.2 so that any JVM 1.2.2 and higher can execute it.  This is so that the
 * user sees a nice informative message telling them their JVM is old, instead of the dreaded :
 * <p>
 * Cannot find main class. Program will exit.
 */
public class JavaVersionChecker
{
	/**
	 * The system property which gives the install location of the java that this small app runs in
	 */
	private static final String JAVA_HOME_PROPERTY = "java.home";

	/**
	 * The system property which gives the version of java that this small application runs in
	 */
	private static final String JAVA_VERSION_PROPERTY = "java.version";

	/**
	 * @param args should contain at least one version of Java which is the acceptable minimum version.  If
	 *             that version is not the latest version of Java available, then newer versions (which are also
	 *             acceptable) should also be specified after the minimum acceptable version.
	 */
	public static void main(String[] args)
	{
		if (args.length == 0)
		{
			System.err.println("JavaVersionChecker: Must specify one or more JVM versions");
			System.exit(1);
		}

		String jvmVersion = System.getProperty(JAVA_VERSION_PROPERTY);
		if (!checkVersion(jvmVersion, args))
		{
			String javaHome = System.getProperty(JAVA_HOME_PROPERTY);

			String msg = "Your Java Virtual Machine version must be ";

			if(1 == args.length)
			{
				msg += args[0] + " ";
			}
			else
			{
				msg += "one of ";

				for (int i = 0; i < args.length; i++)
				{
					msg += args[i] + " ";
				}
			}

			msg += "to run SQuirreL\n" +
					"  JVM Version used: " + jvmVersion + "\n" +
					"  JVM Location: " + javaHome;

			JOptionPane.showMessageDialog(null,msg);
			System.exit(1);
		}
		System.exit(0);
	}

	/**
	 * Check that the specified JVM version matches one of the jvm versions specified in minimumJavaVersions.
	 *
	 * @param jvmVersion          the version of the JVM that this app runs in
	 * @param minimumJavaVersions one or more acceptable versions of the JVM
	 * @return true if the JVM matches one or more of the minimum JVM versions; false otherwise.
	 */
	private static boolean checkVersion(String jvmVersion, String[] minimumJavaVersions)
	{

		if (jvmVersion == null)
		{
			System.err.println("jvm version could not be determined. The " + JAVA_VERSION_PROPERTY +
					" system property is null");
		}

		boolean result = false;
		for (int i = 0; i < minimumJavaVersions.length; i++)
		{
			if (jvmVersion.startsWith(minimumJavaVersions[i]))
			{
				result = true;
			}
		}
		return result;
	}

}