Static type checking








Delivered in Groovy 2.0 and enhanced in later versions


C├ędric Champeau



Last modification 


Abstract: Static Type Checking

This GEP introduces a new feature in the language known as static type checking. It is often disturbing for developers coming from a statically typed language (say Java) to discover that the Groovy compiler will not complain at compile time:

  • when assignments are made on different types

  • when a method doesn’t exist

  • when a property or variable doesn’t exist

  • when returned object type doesn’t match the method signature

  • …​

All those are silent because the dynamic nature of the Groovy language makes such code perfectly valid. However, in some situations, a developer may want Groovy to behave like a statically typed language and have the compiler give hints about such "errors". To do this, Groovy must introduce static type checking.

Rationale: Static Type Checking vs Static compilation

It is important to make the difference between static type checking and static compilation. The goal of this GEP is to have an option to turn static type checking (STC) on. If STC is activated, the compiler will be more verbose (you will also see the term "grumpy"), but in the end, the generated bytecode and runtime behaviour will be exactly the same as if you did not activate this mode. This is a major difference from an alternate compiler like Groovy++ which will perform STC then produce a different bytecode and therefore produce different runtime semantics. The scope of this GEP is only a static type checker, and therefore should only be considered as a feature which allows developers to write statically checked code, so is an elegant way for example to leverage the Groovy syntax to reduce verbosity of Java code while still getting strongly checked code. Eventually, IDE could support the STC mode and provide information to the developer.

Implementation details

Development branch

Since Groovy 2.0-beta-2, code has been merged into master branch. However, if heavy developments are done on the type checker, it is advisable to work on the grumpy branch. It adds an AST transformation named TypeChecked. If set, then the AST transformation will perform type inference and store type information in AST nodes metadata. Eventually, if errors are found, it will add errors to the compiler through a dedicated addStaticTypeError method which basically does the same as the traditional addError method but prefixes the messages with a "Static type checking" message. This is done to help the developer determine whether the error they are seeing is a "plain Groovy" error, or an error thrown by the STC mode.

The StaticTypeCheckingTestCase class

Static type checking behaviour must be tested. As there are tons of possible checks to be done, a base test class provides a framework for testing this mode. Unit tests for static type checking should override this class.

Decisions made

About this section

The goal of this section is to provide code samples which demonstrates in what case the STC transformation will actually complain and what is the expected error message, and serves as a basis to future STC documentation. This section may not be up-to-date, and one should always take a look at the STC unit tests found in the src/test/groovy/transform/stc directory.

Feature Example Behavior Status

Method does not exist

def method() {
methode() // typo

Complains about undefined method


Property does not exist

class A {
  int x
A obj = new A()
a.y = 2

Complains about undefined property "y"


Assignment type checking

int x = 2
x = 'String'

Assigning a String to an int is forbidden


Incompatible binary expressions

1 + 'string'

Checks that arguments of a binary expression are compatible (here, no 'plus' method is available


Possible loss of precision (1/2)

long myLong = ...
int myInt = myLong

Complains about possible loss of precision


Possible loss of precision (2/2)

int myInt = 2L

Will not complain because '2' can be represented as an int


Arrays components

String[] arr = { '1', '2', '3' }
arr[2] = 200

Cannot assign an int value in an array of type String[]


Method return type check

String method() { 'Hello' }
int x = method() // return types don't match

Ensures that assignments are compatible with method return type


Explicit return type checking

int method() {
  return 'String' // return type is not compatible

Ensures that returned value is compatible with declared return type


Implicit return type checking

int method() {
  'String' // return type is not compatible

Ensures that returned value is compatible with declared return type


Implicit toString()

String method(String name) {
  StringBuilder sb = new StringBuilder()
  sb 'Hi ' << name << '!'

Implicit call to toString()


Basic type inference

def str = 'My string'
str.toUpperCase() // type of 'str' is inferred

Method calls as well as property access are checked against inferred type


Basic flow analysis

def o
if (o instanceof String) {
  o.toUpperCase() // no explicit cast required

Casts should not be necessary when type can be inferred from a previous instanceof check


DefaultGroovyMethods support

'123'.toInteger() // toInteger() is a Groovy extension method

Method calls can be resolved against Groovy extension methods



class A {
  int x
def a = new A()
a.with {
  x = 1

Method calls can be resolved against Groovy extension methods



use (MyStringCategory) {

Compiler should be aware that extension method is found in a category

N/A (support will be limited as category support is inherently dynamic)

Groovy list constructor

Dimension d = [100, 200]

Type checks the arguments and the number of arguments


Groovy map constructor

Bean myBean = [x: 100, y: 200]

Type checks the properties and checks for incorrect property names


Closure parameter types

def closure = { int x, int y -> x + y }
closure(1, 2)
closure('1', '2') // complains

Type checking the arguments when calling a closure


Closure return type inference

def closure = { int x, int y -> x + y }
int sum = closure(1, 2)

Closure return type can be inferred from block


Method return type inference

def method(int x, int y) { x + y }
int sum = method(1, 2)

Return type can be inferred from a method if the method is itself annotated with @TypeChecked (or class is annotated with @TypeChecked)


Multiple assignments

def (x, y) = [1, 2]

In case of inline declaration, type check arguments


Multiple assignments from a variable

def (x, y) = list

In case of inline declaration, type check arguments



List<String> list = []
List<String> list = ['a', 'b', 'c']
List<String> list = [1, 2, 3] // should throw error

Type checking of generic parameters


Spread operator

def list = ['a', 'b', 'c']

Type checking against component type


Closure shared variables

def x = new Date()
def cl = { x = 'hello' }
x.toUpperCase() // should throw an error because the toUpperCase() method doesn't belong to both Date and String classes

Type check assignments of closure shared variables. The type checker is required to perform a two-pass verification, in order to check that method calls on a closure shared variables belong to the lowest upper bound of all assignment types.


Open discussions

Closure parameter type inference

With the current version of the checker, idiomatic constructs like :

['a','b','c'].collect { it.toUpperCase() }

Are not properly recognized. You have to explicitly set the type of the "it" parameter inside the closure. It is because the expected parameter types of closures are unknown at compile time. There is a discussion about how to add this type information to source code so that the inference engine can deal with them properly. The implementation of closure parameter type inference requires a change to the method signatures. It will probably not belong to the initial release of the type checker.

Unification Types

In cases of for example x instanceof A || x instanceof B with A and B being unrelated we could still make an artificial union kind of type, that contains everything present in A and B, to allow those kinds of method calls. The alternative to this is to allow only methods from Object here, which is less interesting. This typing can also be used for multicatch, ensuring that a method call is only valid if it exists on each of the exceptions for the multicatch. In the current implementation (2011-10-14) the multicatch is already expanded at the point @TypeChecked will check. Meaning effectively this already represents a kind of union type, as the same code is in each catch block and thus the method call would fail, if the method is not available on each type. The proposed behaviour is therefore to align the instanceof case with multicatch.

Mailing-list discussions

  • groovy-user: What to do on assignment? Discussion about the expected behaviour when STC detects a potential error on assignment (for example, possible loose of precision on implicit number casts)

Update history

8 (2012-02-21)

Version as extracted from Codehaus wiki

9 (2018-10-16)

Numerous minor tweaks