How to create executable jar file?

Method 1: I used to create a executable jar file by eclipse export “Runnable JAR file”

It’s very handy, and it has 3 options to handle libraries:

• Extract required libraries into generated JAR
• Package required libraries into generated JAR
• Copy required libraries into sub folder next to the generated JAR.

I really like it. You don’t need to configure anything but just a few clicks. I think it’s the best way if it’s just for test.It works well except when I use the 1st option. I run into this problem :

org.xml.sax.SAXParseException: schema_reference.4: Failed to read schema document 'http://www.springframework.org/schema/beans/spring-beans-4.0.xsd', because 1) could not find the document; 2) the document could not be read; 3) the root element of the document is not <xsd:schema>.

This is because the xsd file is missed/overwroten when eclipse extract the libs, and my runtime host cannot connect to Internet that makes it impossible to get it. While this is  not a real problem for eclipse, it can be overcome by using the 2nd option. It didn’t extract the lib, but includes them as they’re.

I wonder why eclipse takes a lot of efforts to extract libs (it’s obviously slower than other 2 options in the list). At first, I thought it may want to build small jar packages(it’s obviously smaller ). Later, I found out the real reason is: java can NOT load classes from a Jar inside a Jar! (In real world, how can you put an even larger jar inside a jar? You are kidding!) Java(class loader) is designed to like this.

Now the question is, how eclipse overcome this jar inside jar problem? I unzipped the generated jar file, and found there’s a jarinjarloader class which did a trick behind. See more below.

Method 2: Using onejar-maven-plugin
Because the network between my laptop and the target machine is slow. It takes times to transfer the runnable jar file, even it’s not big. So I need to build it remotely without eclipse, use maven instead. This plugin did the similar trick jarinjarloder did, the 2nd option in eclipse. It basically let java call its main function, and it help you load all dependency jar files including yourselves. Here is the configuration clip:

		  	<plugin>
		  	    <groupId>com.jolira</groupId>
		  	    <artifactId>onejar-maven-plugin</artifactId>
		  	    <version>1.4.4</version>
		  	    <executions>
		  	        <execution>
		  	            <configuration>
		  	                <attachToBuild>true</attachToBuild>
		  	                <classifier>onejar</classifier>
		  	            </configuration>
		  	            <goals>
		  	                <goal>one-jar</goal>
		  	            </goals>
		  	        </execution>
		  	    </executions>
		  	</plugin>

The associated plugins may also needed:

			<plugin>
				<groupId>org.apache.maven.plugins</groupId>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>2.3.2</version>
				<configuration>
					<source>1.6</source>
					<target>1.6</target>
					<encoding>UTF-8</encoding>
				</configuration>
			</plugin>
		   	<plugin>
		  	    <groupId>org.apache.maven.plugins</groupId>
		  	    <artifactId>maven-jar-plugin</artifactId>
		  	    <configuration>
		  	        <archive>
		  	            <manifest>
		  	                <mainClass>com.wordpress.alexzeng.automation</mainClass>
		  	            </manifest>
		  	        </archive>
		  	    </configuration>
		  	</plugin>

It works well as long as you get the dependency configured well in pom.xml. I missed an ojdbc6.jar at first.

Method 3: Using maven-dependency-plugin, corresponding to the 3rd option in eclipse.
I tried this in my project, but failed somehow. I have 23 jar files in final lib directory. But somehow it only includes 19 jar files in the Class-Path of META-INF/MANIFEST.MF. It refused to work even after I added the jar files to classpath variable in environment. I think this method should work, and this is the best one for big projects (you don’t want to have a very big single jar file). You can reference http://www.mkyong.com/maven/how-to-create-a-jar-file-with-maven/ for a simple case.

Last but not least, their’s also a corresponding method in maven to match the eclipse 1st option, use maven-assembly-plugin pre-defined descriptor jar-with-dependencies. It’s really simple and handy if extract dependency jars didn’t cause troubles. 

		    <plugin>
                      <groupId>org.apache.maven.plugins</groupId>
                      <artifactId>maven-assembly-plugin</artifactId>
                      <version>2.4</version>
		      <configuration>
		          <descriptorRefs>
		             <descriptorRef>jar-with-dependencies</descriptorRef>
		          </descriptorRefs>
		        <archive>
		          <manifest>
		            <mainClass>com.wordpress.alexzeng.automation</mainClass>
		          </manifest>
		        </archive>
		      </configuration>
                <executions>
                    <execution>
                        <id>make-assembly</id>
                        <phase>package</phase>
                        <goals>
                            <goal>single</goal>
                        </goals>
                    </execution>
                </executions>
		    </plugin>

As a newbie, I spent a lot of time to get it clear. I hope it can save you sometime to if you need to do the same.