Java to EXE - Why, When, When Not and How

"How do I make an .EXE file from my Java application?", "Need help converting jar to exe", "Is it possible to create a Windows executable using Java?" --- these and similar questions are among the most popular topics on Java developer forums. Should you start such a topic today, you are likely to encounter the following three types of replies:

• "You cannot"

• "You should not, because that would kill the very purpose of Java"

• "You can do that with third party software X and Y"
The truth is that there exist two completely different approaches to the creation of native executables from Java applications, addressing different sets of problems. Moreover, under certain conditions some of those problems may be solved without making an EXE. So the most correct way to reply to such a post would be a request for more information, namely, "What is the goal of conversion to EXE?" And the most frequent answer would be: Simplify Java Application Deployment Java compiles to platform-independent bytecode (.class files), which is not directly supported by PC hardware. So a Java program needs a Java Runtime Environment (JRE) to run, which would either interpret the bytecode instructions or compile them to native code on the fly. This in turn means that the author of that program has to ensure somehow that the proper version of the JRE is installed on an end user system. Generally, you cannot expect that your end users will know what a JRE is, how to check its version, and how to download and install it. This is especially true for consumer applications, such as games or multimedia. And those who already have a JRE installed may not like the idea of installing a different version, because it may break their existing Java applications and favorite applets. Then, even if you can make sure the right version of the JRE is properly installed on enduser systems, which is quite possible in a classroom or enterprise environment, the command line required to launch your Java application can be quite long: java -Xmx200m -cp whatever.jar -Dsome.property MyApp Yes, you may put that line into a batch file and call it runme.bat, but it looks so much easier to give your program to a friend, teacher or colleague as a single file that can be run by a double-click. Or, even better, enable it to be installed and uninstalled in a native manner without affecting other applications. So it comes as no surprise that the primary motivation for seeking a way to convert a Java application into an EXE file is to make its deployment and use simpler and safer for an average user, that is, a Windows user. What surprises newbie Java developers is that the JDK does not offer such functionality. Before J2SE 1.4, all you could make with JDK tools were: Executable Jars Pros - No need to use any third-party tools - Single distribution for all Java-enabled platforms Cons -Application will not start on systems that do not have a JRE (properly) installed - Application will not work if it uses APIs absent in the default JRE - Need to teach users that .JAR files are clickable Resources - One-JAR You can make your Java application runnable via a double-click by packaging it into a so called executable jar. You do that by specifying the main class of your application, any extra jar files it may require and so on in the jar's manifest file Main-Class: MyAppMain Class-Path: mylib.jar Then you use the jar utility from the Java SDK to package your classes and resource files, specifying the m option and the name of your manifest file: jar cvfm MyApp.jar MyApp.mf *.class *.gif This will result in the creation of MyApp.jar. Now, if you type java -jar MyApp.jar the Java launcher will read the manifest from MyApp.jar and invoke the main method from the class MyAppMain. Moreover, if you double-click that jar file on a system that has JRE installed, the java launcher will be invoked automatically. Note: As of J2SE 5.0, jar files are associated with the javaw launcher on Windows, which does not open a console on startup. If your application needs a console, write a batch file which would start it using the java launcher. If your application consists of more than one jar file, there is an open source tool called repackage multiple jars into one. The major problem with executable jars is compatibility. The default JRE may be of an older version than is required by your application or may not have the necessary Java Optional Packages (previously known as Standard Extensions) installed. For instance, if your app uses the java.nio package introduced in Java 2 version 1.4, it will not work on JRE 1.3.x. Similarly, if it uses JavaMail 1.3, and the default JRE has JavaMail 1.2 or JavaMail is not present at all, the double-clicked jar will not run. Fortunately, Sun has created a Java application deployment technology that eliminates this compatibility problem and adds some nice features. It is part of the Java 2 platform since version 1.4 and is called Java Web Start Pros - Available for all major desktop platforms - Single distribution for all JWS-enabled platforms - Code-signing and sandboxing - Versioning and incremental updates - Automatic installation of JREs and optional packages - Use of third-party tools is optional Cons - Internet connectivity required if JWS, JRE, and/or an Optional Package is not present on the system - Support for jnlp MIME type required on both Web server and browser - Limited desktop intergation capabilities Resources - Deploying Software with JNLP and Java Web Start - Java Web Start (JWS) and the underlying Java Network Launch Protocol (JNLP) enable Java application delivery from a standard Web server. The end user initiates application installation by clicking on an URL. If the Java Web Start engine is not present on the system, the user is prompted to download and install it. Once Java Web Start is in place, clicking on the same URL will initiate the application download and installation procedures. It may involve download and installation of the required version of the JRE and Optional Packages. Upon their successful completion, the application is launched. The application will be cached on the user's system so that the next time the user clicks on the same URL, the JWS engine will launch the local copy of the application from the cache, if it detects that the computer is offline or the application was not updated on the Web site. Another important feature of JWS is its ability to run your application in a sandbox - a restricted container based on Java security architecture. But, unlike an applet, your application can gain access to local system resources like the filesystem, printer and system clipboard using the JNLP API even if it comes from an untrusted environment, after prompting the user for confirmation. Java Web Start is available for Windows, Linux, and Solaris, and is part of MacOS X since v10.1. There are also third-party implementations of the JNLP protocol, some of them also include tools that assist you in the creation and maintenance of JNLP packages. That was the bright side. Now, what is not so good about JNLP? First off, for seamless operation both the browser and the Web server that hosts the JNLP-enabled application must support application/x-java-jnlp-file MIME type. Some hosting providers do not support it. Moreover, versioning and incremental updates require additional support from the Web server, which has to be implemented using servlets, cgi-bin scripts, etc. On the client side, a major browser would be configured to recognize the above MIME type during installation of the JWS engine, but users of less popular browsers, such as Opera, may have to do that manually. JNLP-enabling an application may involve minor changes in its code and (re)packaging it into a set of jar files. Before J2SE 5.0, JWS had very little to offer in terms of desktop integration - all it could do was create a desktop icon and/or a Start Menu entry for the application. On Windows, the application will not show up in Add/Remove Programs, so end users would have to run the Java Web Start application manager in order to remove your application. Finally, JWS user interface needs much polishing. As of J2SE 5.0, users still complain about ugly windows with incomprehensible messages. To sum it up, JWS can be a viable option in a controlled environment, such as corporate intranet, but it is not ready for the consumer market, where you may be better off using Custom Java Launchers And Wrappers Pros - JRE version check - JRE download or bundling - Unique process name and icon - No end-user training Cons - Platform specific - Desktop intergation capabilities absent or very limited Resources - JNI Tutorial Commercial: Java Native Interface (JNI), so it is standardized, and it is also very simple. This makes it relatively easy to write your own launcher with a unique name and icon. What it has to do is find a suitable JRE on the end user's system (unless you bundle the JRE with your application), load and initialize the JVM, and run your application on it. If you do not have the right tools, skills, or time to develop a custom launcher for your Java application, there are quite a few third-party Java launcher generators listed in the Tools section of the frame. Some of them provide additional features such as instant splash screen, stdout and stderr redirection, and so on, the most notable being wrapping. A Java wrapper in essentially a custom Java launcher that is also a self-extracting archive containing all the application's classes, jars and auxiliary files. The wrapper unpacks those files on startup and removes on termination. This way, your application is distributed as a single executable. A wrapper normally looks up the JRE upon startup. If the JRE is not present or its version does not match the application's compatibility requirements, some wrappers may install the JRE (if you have included it when wrapping your application) and/or download and install the required version of the JRE. The most sophisticated wrappers may also setup file associations and create shortcuts on first run. But if you need something more complex, such as support for automatic updates or uniform cross-platform deployment, have a look at Java-Aware Setup Authoring Tools Pros - Complete desktop integration - Can be platform-specific or cross-platform - Localization support - Flexibility Cons - Requires third-party tools which may be too pricey and/or complex Resources - native launchers
• User-editable JVM parameter files