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Last week, we used ANTLR to generate a library to be able to analyze Kotlin code. It’s time to use the generated API to check for specific patterns.

API overview

Let’s start by having a look at the generated API:

  • KotlinLexer: Executes lexical analysis.
  • KotlinParser: Wraps classes representing all Kotlin tokens, and handles parsing errors.
  • KotlinParserVisitor: Contract for implementing the Visitor pattern on Kotlin code. KotlinParserBaseVisitor is its empty implementation, to ease the creation of subclasses.
  • KotlinParserListener: Contract for callback-related code when visiting Kotlin code, with KotlinParserBaseListener its empty implementation.
parser lexer class diagram

Class diagrams are not the greatest diagrams to ease the writing of code. The following snippet is a very crude analysis implementation. I’ll be using Kotlin, but any JVM language interoperable with Java could be used:

val stream = CharStreams.fromString("fun main(args : Array<String>) {}")                      (1)
val lexer = KotlinLexer(stream)                                                               (2)
val tokens = CommonTokenStream(lexer)                                                         (3)
val parser = KotlinParser(tokens)                                                             (4)
val context = parser.kotlinFile()                                                             (5)
ParseTreeWalker().apply {                                                                     (6)
    walk(object : KotlinParserBaseListener() {                                                (7)
        override fun enterFunctionDeclaration(ctx: KotlinParser.FunctionDeclarationContext) { (8)
            println(ctx.SimpleName().text)                                                    (9)
    }, context)

Here’s the explanation:

1 Create a CharStream to feed the lexer on the next line. The CharStreams offers plenty of static fromXXX() methods, each accepting a different type (String, InputStream, etc.)
2 Instantiate the lexer, with the stream
3 Instantiate a token stream over the lexer. The class provides streaming capabilities over the lexer.
4 Instantiate the parser, with the token stream
5 Define the entry point into the code. In that case, it’s a Kotlin file - and probably will be for the plugin.
6 Create the overall walker that will visit each node in turn
7 Start the visiting process by calling walk and passing the desired behavior as an object
8 Override the desired function. Here, it will be invoked every time a function node is entered
9 Do whatever is desired e.g. print the function name

Obviously, lines 1 to 7 are just boilerplate to wire all components together. The behavior that need to be implemented should replace lines 8 and 9.

First simple check

In Kotlin, if a function returns Unit - nothing, then explicitly declaring its return type is optional. It would be a great rule to check that there’s no such explicit return. The following snippets, both valid Kotlin code, are equivalent - one with an explicit return type and the other without:

fun hello1(): Unit {

fun hello2() {

Let’s use grun to graphically display the parse tree (grun was explained in the previous post). It yields the following:

Parse tree returns Unit

As can be seen, the snippet with an explicit return type has a type branch under functionDeclaration. This is confirmed by the snippet from the KotlinParser ANTLR grammar file:

  : modifiers 'fun' typeParameters?
      (type '.' | annotations)?
      typeParameters? valueParameters (':' type)?

The rule should check that if such a return type exists, then it shouldn’t be Unit. Let’s update the above code with the desired effect:

ParseTreeWalker().apply {
    walk(object : KotlinParserBaseListener() {
        override fun enterFunctionDeclaration(ctx: KotlinParser.FunctionDeclarationContext) {
            if (ctx.type().isNotEmpty()) {                                       (1)
                val typeContext = ctx.type(0)                                    (2)
                with(typeContext.typeDescriptor().userType().simpleUserType()) { (3)
                    val typeName = this[0].SimpleName()
                    if (typeName.symbol.text == "Unit") {                        (4)
                        println("Found Unit as explicit return type " +          (5)
                                "in function ${ctx.SimpleName()} at line ${typeName.symbol.line}")
    }, context)

Here’s the explanation:

1 Check there’s an explicit return type, whatever it is
2 Strangely enough, the grammar allows for a multi-valued return type. Just take the first one.
3 Follow the parse tree up to the final type name - refer to the above parse tree screenshot for a graphical representation of the path.
4 Check that the return type is Unit
5 Prints a message in the console. In the next step, we will call the SonarQube API there.

Running the above code correctly yields the following output:

Found Unit as explicit return type in function hello1 at line 1

A more advanced check

In Kotlin, the following snippets are all equivalent:

fun hello1(name: String): String {
    return "Hello $name"

fun hello2(name: String): String = "Hello $name"

fun hello3(name: String) = "Hello $name"

Note that in the last case, the return type can be inferred by the compiler and omitted by the developer. That would make a good check: in the case of a expression body, the return type should be omitted. The same technique as above can be used:

  1. Display the parse tree from the snippet using grun:
  2. Check for differences. Obviously:
    • Functions that do not have an explicit return type miss a type node in the functionDeclaration tree, as above
    • Functions with an expression body have a functionBody whose first child is = and whose second child is an expression
  3. Refer to the initial grammar, to make sure all cases are covered.
      : block
      | '=' expression
  4. Code!
    ParseTreeWalker().apply {
        walk(object : KotlinParserBaseListener() {
            override fun enterFunctionDeclaration(ctx: KotlinParser.FunctionDeclarationContext) {
                val bodyChildren = ctx.functionBody().children
                if (bodyChildren.size > 1
                        && bodyChildren[0] is TerminalNode && bodyChildren[0].text == "="
                        && ctx.type().isNotEmpty()) {
                    val firstChild = bodyChildren[0] as TerminalNode
                    println("Found explicit return type for expression body " +
                            "in function ${ctx.SimpleName()} at line ${firstChild.symbol.line}")
        }, context)

The code is pretty self-explanatory and yields the following:

Found explicit return type for expression body in function hello2 at line 5