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Extending detekt

The following page describes how to extend detekt and how to customize it to your domain-specific needs. The associated code samples to this guide can be found in the package detekt/detekt-sample-extensions.

Custom RuleSets​

detekt uses the ServiceLoader pattern to collect all instances of RuleSetProvider interfaces. So it is possible to define rules/rule sets and enhance detekt with your own flavor.

Attention

You need a resources/META-INF/services/io.gitlab.arturbosch.detekt.api.RuleSetProvider file which has as content the fully qualified name of your RuleSetProvider e.g. io.gitlab.arturbosch.detekt.sample.extensions.SampleProvider.

You can use our GitHub template to have a basic scaffolding to develop your own custom rules. Another option is to clone the provided detekt/detekt-sample-extensions project.

Own rules have to extend the abstract Rule class and override the visitXXX()-functions from the AST.
A RuleSetProvider must be implemented, which declares a RuleSet in the instance()-function. To leverage the configuration mechanism of detekt you must pass the Config object from your rule set provider to your rule. An Issue property defines what ID and message should be printed on the console or on any other output format.

Example of a custom rule:

class TooManyFunctions(config: Config) : Rule(
    config,
    "This rule reports a file with an excessive function count.",
) {
    private val threshold = 10
    private var amount: Int = 0

    override fun visitKtFile(file: KtFile) {
        super.visitKtFile(file)
        if (amount > threshold) {
            report(CodeSmell(Entity.from(file), 
                "Too many functions can make the maintainability of a file costlier"))
        }
        amount = 0
    }

    override fun visitNamedFunction(function: KtNamedFunction) {
        super.visitNamedFunction(function)
        amount++
    }
}

Example of a much preciser rule in terms of more specific CodeSmell constructor and Rule attributes:

class TooManyFunctions2(config: Config) : Rule(
    config,
    "This rule reports a file with an excessive function count.",
) {
    private val threshold: Int by config(defaultValue = 10)
    private var amount: Int = 0

    override fun visitKtFile(file: KtFile) {
        super.visitKtFile(file)
        if (amount > threshold) {
            report(ThresholdedCodeSmell(issue,
                entity = Entity.from(file),
                metric = Metric(type = "SIZE", value = amount, threshold = threshold),
                message = "The file ${file.name} has $amount function declarations. " +
                        "Threshold is specified with $threshold.",
                references = emptyList())
            )
        }
        amount = 0
    }

    override fun visitNamedFunction(function: KtNamedFunction) {
        super.visitNamedFunction(function)
        amount++
    }
}

If you want your rule to be configurable, write down your properties inside the detekt.yml file. Please note that this will only take effect, if the Config object is passed on by the RuleSetProvider to the rule itself.

MyRuleSet:
  TooManyFunctions2:
    active: true
    threshold: 5
  OtherRule:
    active: false

By specifying the rule set and rule ids, detekt will use the sub configuration of TooManyFunctions2:

val threshold = valueOrDefault("threshold", THRESHOLD)

note

As of version 1.2.0 detekt now verifies if all configured properties actually exist in a configuration created by --generate-config. This means that by default detekt does not know about your new properties. Therefore we need to mention them in the configuration under config>excludes.

config:
  validation: true
  # 1. exclude rule set 'sample' and all its nested members
  # 2. exclude every property in every rule under the rule set 'sample'
  excludes: "sample.*,sample>.*>.*"
Testing your rules​

To test your rules, add the dependency on detekt-test to your project: testCompile "io.gitlab.arturbosch.detekt:detekt-test:$version".

The easiest way to detect issues with your newly created rule is to use the lint extension function:

  • Rule.lint(StringContent/Path/KtFile): List<Finding>

If you need to reuse the Kotlin file for performance reasons within similar test cases, please use one of these functions:

  • compileContentForTest(content: String): KtFile
  • compileForTest(path: Path): KtFile

Custom Processors​

Custom processors can be used for example to implement additional project metrics.

When for whatever reason you want to count all loop statements inside your code base, you could write something like:

class NumberOfLoopsProcessor : FileProcessListener {

	override fun onProcess(file: KtFile) {
		val visitor = LoopVisitor()
		file.accept(visitor)
		file.putUserData(numberOfLoopsKey, visitor.numberOfLoops)
	}

	companion object {
		val numberOfLoopsKey = Key<Int>("number of loops")
	}

	class LoopVisitor : DetektVisitor() {

		internal var numberOfLoops = 0
		override fun visitLoopExpression(loopExpression: KtLoopExpression) {
			super.visitLoopExpression(loopExpression)
			numberOfLoops++
		}
	}
}

To let detekt know about the new processor, we specify a resources/META-INF/services/io.gitlab.arturbosch.detekt.api.FileProcessListener file with the full qualify name of our processor as the content: io.gitlab.arturbosch.detekt.sample.extensions.processors.NumberOfLoopsProcessor.

To test the code we use the detekt-test module and write a JUnit 5 testcase.

class NumberOfLoopsProcessorTest {

    @Test
    fun `should expect two loops`() {
        val code = """
            fun main() {
                for (i in 0..10) {
                    while (i < 5) {
                        println(i)
                    }
                }
            }
        """

        val ktFile = compileContentForTest(code)
        NumberOfLoopsProcessor().onProcess(ktFile)

        assertThat(ktFile.getUserData(NumberOfLoopsProcessor.numberOfLoopsKey)).isEqualTo(2)
    }
}

Custom Reports​

detekt allows you to extend the console output and to create custom output formats. If you want to customize the output, take a look at the ConsoleReport and OutputReport classes.

All they need are an implementation of the render()-function which takes an object with all findings and returns a string to be printed out.

abstract fun render(detektion: Detektion): String?

Let detekt know about your extensions​

So you have implemented your own rules or other extensions and want to integrate them into your detekt run? Great, make sure to have a jar with all your needed dependencies minus the ones detekt brings itself.

Take a look at our sample project on how to achieve this with gradle.

Integrate your extension with the detekt CLI​

Mention your jar with the --plugins flag when calling the cli fatjar:

detekt --input ... --plugins /path/to/my/jar
Integrate your extension with the Detekt Gradle Plugin​

For example detekt itself provides a wrapper over ktlint as a custom formatting rule set. To enable it, we add the published dependency to detekt via the detektPlugins configuration:

Gradle (Kotlin/Groovy DSL)​
dependencies {
    detektPlugins("io.gitlab.arturbosch.detekt:detekt-formatting:1.23.7")
}
Pitfalls​
  • All rules are disabled by default and have to be explicitly enabled in the detekt yaml configuration file.
  • If you do not pass the Config object from the RuleSetProvider to the rule, the rule is active, but you will not be able to use any configuration options or disable the rule via config file.
  • If your extension is part of your project and you integrate it like detektPlugins project(":my-rules") make sure that this subproject is build before gradle detekt is run. In the kotlin-dsl you could add something like tasks.withType<Detekt> { dependsOn(":my-rules:assemble") } to explicitly run detekt only after your extension sub project is built.
  • If you use detekt for your Android project, and if you want to integrate all your custom rules in a new module, please make sure that you created a pure kotlin module which has no Android dependencies. apply plugin: "kotlin" is enough to make it work.
  • Sometimes when you run detekt task, you may not see the violations detected by your custom rules. In this case open a terminal and run ./gradlew --stop to stop gradle daemons and run the task again.