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JDeps Maven Plugin 0.2 Released

With the second release the JDeps Maven Plugin allows the creation of flexible exemptions from build-breaking. This enables a self-paced migration away from dependencies of JDK-internal APIs that will be unavailable in Java 9.

It only took me six months after the initial release but last week I finally published version 0.2 of my JDeps Maven Plugin, now lovingly nicknamed JDeps Mvn.

Since Apache released theirs recently this begs the question "Why even bother?" Well, because mine actually understands jdeps and can do some stuff the official can't. This makes it much more useful for large projects.

Before we go on, let me point you to that old post again. You might want to read it if you need some background on how Java 9 may break your code and how jdeps can help. Many links lead to the documentation on GitHub, of which the walkthrough comes highly recommended.

What's So Special?

When including jdpes in a large project's build, two things are prohibitive:

  • Having to clean up all problematic dependencies before the build will pass.
  • Having to manually create unflexible exemptions from build-breaking.

JDeps Mvn helps with both:

  • Smartly interacting rules allow a detailed and flexible configuration.
  • Initial rules can be automatically created from identified dependencies.

If no rule matches, the plugin will apply a default value, which will usually be configured to FAIL. This way the rules can be used to create exemptions and have the build break on all unexpected dependencies.

Introducing Rules

Rules are of the form dependent -> dependee: severity, where both the dependent and the dependee are fully qualified class or package names.

The most basic rules are used to define severities for class-to-class dependencies:

org.food.fruits.Mango -> sun.misc.BASE64Decoder: INFORM
org.food.fruits.Mango -> sun.misc.BASE64Encoder: INFORM
org.food.fruits.Banana -> sun.misc.Unsafe: INFORM

So you will get get a log message on INFO level, when Mango uses either BASE64Decoder or BASE64Encoder or when Banana uses Unsafe.

But it is also possible to create broader rules. For example this will set all uses of Unsafe in org.food.fruits to INFORM:

org.food.fruits -> sun.misc.Unsafe: INFORM

It is up to you how the plugin will interpret package relations. By default it does it like the JVM, where the concept of "subpackages" does not exist and org.food and org.food.fruits are unrelated. But you can also configure a hierarchical mode where packages are interpreted like folders and org.food contains org.food.fruits.

So you have this package where the use of sun.misc is generally ok except for the one subpackage where everything but Unsafe could already be removed and must not creep back? No problem:

org.food -> sun.misc: WARN
org.food.fruit -> sun.misc: FAIL
org.food.fruit -> sun.misc.Unsafe: WARN

The interaction of conflicting rules is well-defined and follows general intuition. In short: Find the best match on the left side that still matches on the right. If there are several such matches, pick the one with the most specific right side.

Creating Rules

You can of course go through your code base or the output of a jdeps run and write rules to exempt the existing dependencies from breaking your build. Or you can have JDeps Mvn do that for you:

<configuration>
	<defaultSeverity>WARN</defaultSeverity>
	<outputRulesForViolations>true</outputRulesForViolations>
	<outputRuleFormat>ARROW</outputRuleFormat>
	<outputFilePath>path/to/dependency_rules.xml</outputFilePath>
</configuration>

With this configuration JDeps Mvn will write a rule for each dependency it finds. So if Mango uses BASE64Decoder and BASE64Encoder and Banana uses Unsafe, the result will be:

<arrowDependencyRules>
	<arrowRules>
		org.food.fruits.Mango -> sun.misc.BASE64Decoder: WARN
		org.food.fruits.Mango -> sun.misc.BASE64Encoder: WARN
		org.food.fruits.Banana -> sun.misc.Unsafe: WARN
	</arrowRules>
</arrowDependencyRules>

JDeps Mvn will always append to the file, so if you have a multi-module build and use an absolute path you can gather rules for all of them in one file.

You can now edit this block as you like and then move it into your pom. (As long as outputRulesForViolations is set to true JDeps Mvn will never break the build so make sure to turn it off when it's no longer needed.)

Changing the default severity to FAIL will then make sure the build breaks except for the defined exemptions. Your config might then look like this:

<configuration>
	<defaultSeverity>FAIL</defaultSeverity>
	<arrowDependencyRules>
		<arrowRules>
			org.food.fruits.Mango -> sun.misc.BASE64Decoder: WARN
			org.food.fruits.Mango -> sun.misc.BASE64Encoder: WARN
			org.food.fruits.Banana -> sun.misc.Unsafe: WARN
		</arrowRules>
	</arrowDependencyRules>
</configuration>

What Else Is There?

Nothing much feature-wise. As I said, there is proper documentation, so if you start using JDeps Mvn in anger, make sure to check it out. If you do, I'd be happy to hear your feedback!