#pom.xml

Don’t code when you’re tired, guys.

SED - AWK

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <!-- ..... --> <!-- For revision number into MANIFEST.MF --> <scm> <connection>scm:svn:http://127.0.0.1/dummy</connection> <developerConnection>scm:svn:https://127.0.0.1/dummy</developerConnection> <tag>HEAD</tag> <url>http://127.0.0.1/dummy</url> </scm> <!-- For revision number into MANIFEST.MF --> <build> <plugins> <!-- For revision number into MANIFEST.MF --> <plugin> <groupId>org.codehaus.mojo</groupId> <artifactId>buildnumber-maven-plugin</artifactId> <executions> <execution> <phase>validate</phase> <goals> <goal>create</goal> <goal>hgchangeset</goal> </goals> </execution> </executions> <configuration> <doCheck>false</doCheck> <doUpdate>false</doUpdate> </configuration> </plugin> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-jar-plugin</artifactId> <configuration> <archive> <manifestEntries> <SCM-Revision>${changeSet}</SCM-Revision> </manifestEntries> </archive> </configuration> </plugin> <!-- For revision number into MANIFEST.MF --> </plugins> </build> </project>

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The task we're going to walk through is the process of creating a Java application using IntelliJ, packaging that as a JAR file, deploying that JAR file to [Clojars](https://clojars.org), adding it as a dependency in a Clojure application, and finally using [Java Interop](http://clojure.org/java_interop) to make use of your Java code in your Clojure application. *Remember, Clojure is built on top of Java and compiles to JVM bytecode.* We start off by creating a new project in IntelliJ. We'll make a Command Line App.  It will automatically create the default project structure including a `src` directory (for our source code) containing just a `Main` class. The `Main` class has a `main` function that serves as the entry point for our application.  We can add packages to store related classes. A Java package is a mechanism for organizing Java classes into namespaces, a way to logically group them.  Okay, so now we have a very simple Java application. Let's do some reverse engineering to figure out how we're going to use this code in the Clojure application we'll build later. In Clojure we have a `project.clj` file that specifies dependencies (like JAR files we want to use). (defproject sample_clojure_app "1.0.0-SNAPSHOT" :description "A Sample Clojure App" :dependencies [[org.clojure/clojure "1.3.0"] [sample-java-app "1.0"]] :main sample_clojure_app.core) Where does it look for the dependencies (such as sample-java-app "1.0")? It looks in Clojars. So we need to deploy our sample-java-app JAR file to Clojars. To do that, we can run this command: scp pom.xml myjar-1.0.jar [email protected]: Two issues there. We don't have a pom.xml file and we don't have a JAR file. If we try running this command, it won't even matter that the JAR file specified doesn't exist, it'll fail because it can't find a pom.xml file, giving us this message: pom.xml: No such file or directory So let's create that. In IntelliJ we can use Maven as our build tool. What's Maven? Its website describes it as a tool that can be used for building and managing any Java-based project. Clicking on our project module, we're presented with a menu of options. If we select Add Framework Support we'll see this:  We can select Maven to add that. This will generate our pom.xml file, with code like this: <?xml version="1.0" encoding="UTF-8"?> 4.0.0 sample-java-app sample-java-app 1.0-SNAPSHOT So now one of our issues has been taken care of. Next up is packaging this Java application as a JAR file. Right now this is our project's root directory:  If we select File --> Project Structure --> Artifacts and click the + we'll see a list of options. We want to add a JAR. We will choose the option *From modules with dependencies* and select the appropriate `Main` class. Now let's go up to the Build menu and click *Build Artifacts*. Now look at our directory:  In our `out` directory we have another directory called `artifacts` and in that directory we see yet another directory, this one containing our JAR file, called `sample-java-app.jar`. Let's look back to the command we need to run to deploy to Clojars. scp pom.xml myjar-1.0.jar [email protected]: So let's enter this command (using the relative path to our JAR file): scp pom.xml out/artifacts/sample_java_app_jar/sample-java-app.jar [email protected]: And now we've successfully packaged our Java application in a JAR and deployed it to Clojars. If we create a new Clojure app and take a look at our `project.clj` file, we'll see a list of dependencies. We can add our JAR file as a dependency and it will check Clojars to find the specified file. :dependencies [[org.clojure/clojure "1.3.0"] [sample-java-app "1.0-SNAPSHOT"] In our code we can import one of the Java classes that was compressed in our JAR file like this: (ns clojure_app.core (:import (animals Dog))) (defn -main [] (let [dog (Dog. )] (doto dog (.bark)))) We're importing the `Dog` class from our `animals` package, then we're creating a new instance of the `Dog` class by using Java interop, which defines this as the proper syntax for creating a new instance of a class: (new Classname args*) (Classname. args*) We conclude by calling a non-static method on our instance of `Dog`. Our `bark` method simply sent "whoof!" to `STDOUT`. That quickly (or not so quickly) covers the entire process. At the end we're all ready to incorporate code we wrote in our Java classes in our new Clojure app to take advantage of the best both languages have to offer. *Some Helpful Stuff* [Java Programming Tutorial: Java Archive (Jar)](http://www3.ntu.edu.sg/home/ehchua/programming/java/J9d_Jar.html) [Learning Clojure/Calling Java](http://en.wikibooks.org/wiki/Learning_Clojure/Calling_Java) [Stack Overflow: Why maven ? What are the benefits?](http://stackoverflow.com/questions/3589562/why-maven-what-are-the-benefits) [Clojure: Challenge your Java assumptions](http://www.javaworld.com/javaworld/jw-05-2009/jw-05-clojure.html) [Clojure & Java Interop](http://blog.jayfields.com/2011/12/clojure-java-interop.html) [Stack Overflow: Understanding JARs and Packages in Java](http://stackoverflow.com/questions/5319694/understanding-jars-and-packages-in-java) [Clojure: Integrating With Java](http://blog.m.artins.net/clojure-integrating-with-java)

See the rest in this gist: https://gist.github.com/1226189