The JarRunner application is launched with a command of this form:
\njava JarRunner url [arguments]<\/I>
\n<\/PRE><\/BLOCKQUOTE>In the previous section, we’ve seen how JarClassLoader<\/TT> is able to identify and load the main class of a JAR-bundled application from a given URL. To complete the JarRunner application, therefore, we need to be able to take a URL and any arguments from the command line, and pass them to an instance of JarClassLoader<\/TT>. These tasks belong to the JarRunner<\/TT> class, the entry point of the JarRunner application.
\nIt begins by creating a java.net.URL<\/TT> object from the URL specified on the command line: <\/P>
\npublic static void main(String[] args) {
\n if (args.length < 1) {
\n usage();
\n }
\n URL url = null;
\n try {
\n url = new URL(args[0]);
\n } catch (MalformedURLException e) {
\n fatal(“Invalid URL: ” + args[0]);
\n }
\n …
\n<\/PRE><\/BLOCKQUOTE>If args.length < 1<\/TT>, that means no URL was specified on the command line, so a usage message is printed. If the first command-line argument is a good URL, a new URL<\/TT> object is created to represent it.
\nNext, JarRunner creates a new instance of JarClassLoader<\/TT>, passing to the constructor the URL that was specified on the command-line: <\/P>
\nJarClassLoader cl = new JarClassLoader(url);
\n<\/PRE><\/BLOCKQUOTE>As we saw in the previous section, it’s through JarClassLoader<\/TT> that JarRunner taps into the JAR-handling APIs.
\nThe URL that’s passed to the JarClassLoader<\/TT> constructor is the URL of the JAR-bundled application that you want to run. JarRunner next calls the class loader’s getMainClassName<\/TT> method to identify the entry-point class for the application: <\/P>
\nString name = null;
\ntry {
\n name = cl.getMainClassName();<\/B>
\n} catch (IOException e) {
\n System.err.println(“I\/O error while loading JAR file:”);
\n e.printStackTrace();
\n System.exit(1);
\n}
\nif (name == null) {
\n fatal(“Specified jar file does not contain a ‘Main-Class'” +
\n ” manifest attribute”);
\n}
\n<\/PRE><\/BLOCKQUOTE>The key statement is highlighted in bold. The other statements are for error handling.
\nOnce JarRunner<\/TT> has identified the application’s entry-point class, only two steps remain: passing any arguments to the application and actually launching the application. JarRunner<\/TT> performs these steps with this code: <\/P>
\n\/\/ Get arguments for the application
\nString[] newArgs = new String[args.length – 1];
\nSystem.arraycopy(args, 1, newArgs, 0, newArgs.length);<\/B>
\n\/\/ Invoke application’s main class
\ntry {
\n cl.invokeClass(name, newArgs);
\n} catch (ClassNotFoundException e) {
\n fatal(“Class not found: ” + name);
\n} catch (NoSuchMethodException e) {
\n fatal(“Class does not define a ‘main’ method: ” + name);
\n} catch (InvocationTargetException e) {
\n e.getTargetException().printStackTrace();
\n System.exit(1);
\n}
\n<\/PRE><\/BLOCKQUOTE>Recall that the first command-line argument was the URL of the JAR-bundled application. Any arguments to be passed to that application are therefore in element 1<\/TT> and beyond in the args<\/TT> array. JarRunner<\/TT> takes those elements, and creates a new array called newArgs<\/TT> to pass to the application (bold line above). JarRunner<\/TT> then passes the entry-point’s class name and the new argument list to the invokeClass<\/TT> method of JarClassLoader<\/TT>. As we saw in the previous section, invokeClass<\/TT> will load the application’s entry-point class, pass it any arguments, and launch the application.<\/BLOCKQUOTE><\/DIV><\/p>\n","protected":false},"excerpt":{"rendered":"The JarRunner Class The JarRunner application is launched with a command of this form: java JarRunner url [arguments] In the previous section, we’ve seen how JarClassLoader is able to identify and load the main class of a JAR-bundled application from a given URL. To complete the JarRunner application, therefore, we need to be able to … Continue reading The JarRunner Class<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[],"tags":[],"class_list":["post-372","post","type-post","status-publish","format-standard","hentry"],"_links":{"self":[{"href":"https:\/\/www.strongd.net\/index.php?rest_route=\/wp\/v2\/posts\/372","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.strongd.net\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.strongd.net\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.strongd.net\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.strongd.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=372"}],"version-history":[{"count":0,"href":"https:\/\/www.strongd.net\/index.php?rest_route=\/wp\/v2\/posts\/372\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.strongd.net\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=372"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.strongd.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=372"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.strongd.net\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=372"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}