fix add file to jar using context default toolchain

Author: fvarrui

src/main/java/io/github/fvarrui/javapackager/packagers/Context.java

@@ -88,7 +88,10 @@ public static GradleContext getGradleContext() {
 	}
 
 	public File getDefaultToolchain() {
-		return new File(System.getProperty("java.home")); // Use java.home as fallback
+		if (System.getenv("JAVA_HOME") != null) {
+			return new File(System.getenv("JAVA_HOME")); // Use JAVA_HOME as fallback	
+		}
+		return new File(System.getProperty("java.home"));
 	}
 
 }

src/main/java/io/github/fvarrui/javapackager/utils/JarUtils.java

@@ -5,11 +5,20 @@
 
 import org.codehaus.plexus.util.cli.CommandLineException;
 
+import io.github.fvarrui.javapackager.packagers.Context;
+
 public class JarUtils {
 
-	public static void addFileToJar(File jarFile, File newFile) throws IOException, CommandLineException {
-		File jar = new File(System.getProperty("java.home"), "/bin/jar");
+	/**
+	 * Runs "jar uf <jarfile> <newfile>" to add newfile into jarfile. 
+	 * @param jarFile JAR file
+	 * @param newFile File to add to jar file
+	 * @throws IOException If something related to IO went wrong
+	 * @throws CommandLineException If something related to command execution went wrong 
+	 */
+	public static void addFileToJar(File jarFile, File newFile) throws IOException, CommandLineException {		
+		File jar = new File(Context.getContext().getDefaultToolchain(), "/bin/jar");
 		CommandUtils.executeOnDirectory(newFile.getParentFile(), jar.getAbsolutePath(), "uf", jarFile, newFile.getName());
 	}
 
-}
+}