7.4. Using Maven¶

If you haven’t already done so, change into the cs1302-mvn directory that you created earlier using Maven.

To use Maven, we need to talk about phases. Each Maven lifecycle phase is responsible for doing a particular sequence of actions. Phases are sometimes also referred to as goals. They help us automate some of the commands that we’re used to typing out manually. Here is a list of the basic phases that you may find helpful:

Phase	Description
`compile`	Compile the source code of the project.
`clean`	Remove compiled files from the project.
`exec:java`	Execute the class specified by `-Dexec.mainClass` with dependencies added to the classpath.
`site`	Generate a website for the project that includes the API documentation.
`package`	Take the compiled code and package it into a JAR file.

To execute a phase using Maven, simply type the phase name after the mvn command. Pay careful attention to the output of each command as it usually provides lots of useful information about what worked or didn’t work related to that command. More information about these phases can be found in the subsections below.

7.4.1. Phase: compile¶

Try compiling the code using the following command:

mvn compile

The first time that you do this for a particular project, Maven might need to download some of the dependencies defined in the POM – it should not need to download every time. If the compilation is successful, then you should see something similar to the following:

[INFO] Scanning for projects...
[INFO]p
[INFO] -----------------------< cs1302.mvn:cs1302-mvn >------------------------
[INFO] Building cs1302-mvn 1.0-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-resources-plugin:3.0.2:resources (default-resources) @ cs1302-mvn ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] skip non-existing resourceDirectory cs1302-mvn/src/main/resources
[INFO]
[INFO] --- maven-compiler-plugin:3.8.0:compile (default-compile) @ cs1302-mvn ---
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 1 source file to cs1302-mvn/target/classes
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  1.417 s
[INFO] Finished at: 2019-03-24T16:26:39-04:00
[INFO] ------------------------------------------------------------------------

If you pay careful attention to the output, you will see that Maven used the target/classes subdirectory as the default package for compiled code. With this in mind, we can run the cs1302.mvn.App class as follows:

java -cp target/classes cs1302.mvn.App

Which should print “Hello World!”.

7.4.2. Phase: clean¶

From time to time, you may need to delete the compiled files from your project (e.g., after moving a type from one package to another). This is commonly referred to as cleaning the project. To clean your project using Maven, use the following command:

mvn clean

If done correctly, you should see something similar to the following:

[INFO] Scanning for projects...
[INFO]
[INFO] -----------------------< cs1302.mvn:cs1302-mvn >------------------------
[INFO] Building cs1302-mvn 1.0-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-clean-plugin:3.1.0:clean (default-clean) @ cs1302-mvn ---
[INFO] Deleting cs1302-mvn/target
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  0.357 s
[INFO] Finished at: 2019-03-24T16:33:35-04:00
[INFO] ------------------------------------------------------------------------

7.4.3. Phase: exec:java¶

There is also a way to run a class using the Maven exec:java phase instead of running the java command directly:

mvn exec:java -Dexec.mainClass="cs1302.mvn.App"

This command will execute your class and result in output similar to the following:

[INFO] Scanning for projects...
[INFO]
[INFO] -----------------------< cs1302.mvn:cs1302-mvn >------------------------
[INFO] Building cs1302-mvn 1.0-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- exec-maven-plugin:1.6.0:java (default-cli) @ cs1302-mvn ---
Hello World!
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  0.747 s
[INFO] Finished at: 2019-03-24T17:03:22-04:00
[INFO] ------------------------------------------------------------------------

As you can see, this requires a little more typing (to include the -Dexec.mainClass option) and adds additional output to the output of the program. There are some more caveats involved when using the exec:java phase:

It does not automatically recompile code if changed – you will need to manually perform the compile phase, as needed.
It redirects standard output to an output stream that does not flush automatically. This can cause issues if your program reads and writes from standard input and standard output, respectively. This can usually be mitigated by performing System.out.flush() whenever you need to synchronize between a print and a subsequent read. For example:
```
System.out.print("enter a number: ");
System.out.flush();
int num = input.nextInt();
```

However, if your project has a runtime dependency, i.e., a library that is required during runtime, then the exec:java phase will automatically ensure that it is on the classpath when the driver class is executed! This is especially convenient when there are multiple such dependencies.

If you added some JavaFX code to your application (assume your file is called App.java), you could run the application using the following command to add the -Dprism.order=sw option:

mvn exec:java -Dprism.order=sw -Dexec.mainClass="cs1302.mvn.App"

7.4.4. Phase: site¶

Try using the site phase. The output will show you where the files for the website are created. You can copy or symlink these to a location under your ~/public_html directory to host the site on Odin just as you did for API documentation websites created using the Javadoc tool in the past.

The generated site is stored in target/site. You might use something like the following command to create a symbolic link named project-site (adjust as needed) under your ~/public_html directory:

Must be executed in the parent of the site directory¶

$ ln -s $(pwd)/target/site ~/public_html/project-site

Note

If Maven complains about missing classes when executing the site phase, then you may need to update the mvn-site-plugin to a newer version than the default. The dependency information that should be included/updated in your pom.xml file can be found here. Remember to backup your POM (i.e., make a backup copy) before making edits!

7.4.5. Phase: package¶

If you are interested in created your own JAR files, expand the dropdown box below: