Sharing fixtures
A test fixture is composed of the objects and other artifacts (files, sockets, database
connections, etc.) tests use to do their work.
When multiple tests need to work with the same fixtures, it is important to try and avoid
duplicating the fixture code across those tests. The more code duplication you have in your
tests, the greater drag the tests will have on refactoring the actual production code.
ScalaTest recommends three techniques to eliminate such code duplication:
- Refactor using Scala
- Override
withFixture
- Mix in a before-and-after trait
Each technique is geared towards helping you reduce code duplication without introducing
instance var
s, shared mutable objects, or other dependencies between tests. Eliminating shared
mutable state across tests will make your test code easier to reason about and more amenable for parallel
test execution.
The following sections
describe these techniques, including explaining the recommended usage
for each. But first, here's a table summarizing the options:
Refactor using Scala when different tests need different fixtures.
|
get-fixture methods
|
The extract method refactor helps you create fresh instances of mutable fixture objects in each test
that needs them, but doesn't help you clean them up when you're done.
|
fixture-context objects
|
By placing fixture methods and fields into traits, you can easily give each test just the newly created
fixtures it needs by mixing together traits. Use this technique when you need different combinations
of mutable fixture objects in different tests, and don't need to clean up after.
|
loan-fixture methods
|
Factor out duplicate code with the loan pattern when different tests need different fixtures that must be cleaned up afterwards.
|
Override withFixture when most or all tests need the same fixture.
|
withFixture(NoArgTest)
|
The recommended default approach when most or all tests need the same fixture treatment. This general technique
allows you, for example, to perform side effects at the beginning and end of all or most tests,
transform the outcome of tests, retry tests, make decisions based on test names, tags, or other test data.
Use this technique unless:
- Different tests need different fixtures (refactor using Scala instead)
- An exception in fixture code should abort the suite, not fail the test (use a before-and-after trait instead)
- You have objects to pass into tests (override
withFixture(OneArgTest) instead)
|
withFixture(OneArgTest)
|
Use when you want to pass the same fixture object or objects as a parameter into all or most tests.
|
Mix in a before-and-after trait when you want an aborted suite, not a failed test, if the fixture code fails.
|
BeforeAndAfter
|
Use this boilerplate-buster when you need to perform the same side-effects before and/or after tests, rather than at the beginning or end of tests.
|
BeforeAndAfterEach
|
Use when you want to stack traits that perform the same side-effects before and/or after tests, rather than at the beginning or end of tests.
|
Calling get-fixture methods
If you need to create the same mutable fixture objects in multiple tests, and don't need to clean them up after using them, the simplest approach is to write one or
more get-fixture methods. A get-fixture method returns a new instance of a needed fixture object (or a holder object containing
multiple fixture objects) each time it is called. You can call a get-fixture method at the beginning of each
test that needs the fixture, storing the returned object or objects in local variables. Here's an example:
package org.scalatest.examples.flatspec.getfixture
import org.scalatest.flatspec.AnyFlatSpec
import collection.mutable.ListBuffer
class ExampleSpec extends AnyFlatSpec {
def fixture =
new {
val builder = new StringBuilder("ScalaTest is ")
val buffer = new ListBuffer[String]
}
"Testing" should "be easy" in {
val f = fixture
f.builder.append("easy!")
assert(f.builder.toString === "ScalaTest is easy!")
assert(f.buffer.isEmpty)
f.buffer += "sweet"
}
it should "be fun" in {
val f = fixture
f.builder.append("fun!")
assert(f.builder.toString === "ScalaTest is fun!")
assert(f.buffer.isEmpty)
}
}
The “f.
” in front of each use of a fixture object provides a visual indication of which objects
are part of the fixture, but if you prefer, you can import the the members with “import f._
” and use the names directly.
If you need to configure fixture objects differently in different tests, you can pass configuration into the get-fixture method. For example, if you could pass
in an initial value for a mutable fixture object as a parameter to the get-fixture method.
Instantiating fixture-context objects
An alternate technique that is especially useful when different tests need different combinations of fixture objects is to define the fixture objects as instance variables
of fixture-context objects whose instantiation forms the body of tests. Like get-fixture methods, fixture-context objects are only
appropriate if you don't need to clean up the fixtures after using them.
To use this technique, you define instance variables intialized with fixture objects in traits and/or classes, then in each test instantiate an object that
contains just the fixture objects needed by the test. Traits allow you to mix together just the fixture objects needed by each test, whereas classes
allow you to pass data in via a constructor to configure the fixture objects. Here's an example in which fixture objects are partitioned into two traits
and each test just mixes together the traits it needs:
package org.scalatest.examples.flatspec.fixturecontext
import collection.mutable.ListBuffer
import org.scalatest.flatspec.AnyFlatSpec
class ExampleSpec extends AnyFlatSpec {
trait Builder {
val builder = new StringBuilder("ScalaTest is ")
}
trait Buffer {
val buffer = ListBuffer("ScalaTest", "is")
}
"Testing" should "be productive" in new Builder {
builder.append("productive!")
assert(builder.toString === "ScalaTest is productive!")
}
"Test code" should "be readable" in new Buffer {
buffer += ("readable!")
assert(buffer === List("ScalaTest", "is", "readable!"))
}
it should "be clear and concise" in new Builder with Buffer {
builder.append("clear!")
buffer += ("concise!")
assert(builder.toString === "ScalaTest is clear!")
assert(buffer === List("ScalaTest", "is", "concise!"))
}
}
Overriding withFixture(NoArgTest)
Although the get-fixture method and fixture-context object approaches take care of setting up a fixture at the beginning of each
test, they don't address the problem of cleaning up a fixture at the end of the test. If you just need to perform a side-effect at the beginning or end of
a test, and don't need to actually pass any fixture objects into the test, you can override withFixture(NoArgTest)
, one of ScalaTest's
lifecycle methods defined in trait Suite
.
Trait Suite
's implementation of runTest
passes a no-arg test function to withFixture(NoArgTest)
. It is withFixture
's
responsibility to invoke that test function. Suite
's implementation of withFixture
simply
invokes the function, like this:
protected def withFixture(test: NoArgTest) = {
test()
}
You can, therefore, override withFixture
to perform setup before and/or cleanup after invoking the test function. If
you have cleanup to perform, you should invoke the test function inside a try
block and perform the cleanup in
a finally
clause, in case an exception propagates back through withFixture
. (If a test fails because of an exception,
the test function invoked by withFixture will result in a Failed
wrapping the exception. Nevertheless,
best practice is to perform cleanup in a finally clause just in case an exception occurs.)
The withFixture
method is designed to be stacked, and to enable this, you should always call the super
implementation
of withFixture
, and let it invoke the test function rather than invoking the test function directly. That is to say, instead of writing
“test()
”, you should write “super.withFixture(test)
”, like this:
override def withFixture(test: NoArgTest) = {
try super.withFixture(test)
finally {
}
}
Here's an example in which withFixture(NoArgTest)
is used to take a snapshot of the working directory if a test fails, and
send that information to the reporter:
package org.scalatest.examples.flatspec.noargtest
import java.io.File
import org.scalatest._
import org.scalatest.flatspec.AnyFlatSpec
class ExampleSpec extends AnyFlatSpec {
override def withFixture(test: NoArgTest) = {
super.withFixture(test) match {
case failed: Failed =>
val currDir = new File(".")
val fileNames = currDir.list()
info("Dir snapshot: " + fileNames.mkString(", "))
failed
case other => other
}
}
"This test" should "succeed" in {
assert(1 + 1 === 2)
}
it should "fail" in {
assert(1 + 1 === 3)
}
}
Running this version of ExampleSuite
in the interpreter in a directory with two files, hello.txt
and world.txt
would give the following output:
scala> new ExampleSuite execute
ExampleSuite:
This test
- should succeed
- should fail *** FAILED ***
2 did not equal 3 (<console>:33)
+ Dir snapshot: hello.txt, world.txt
Note that the NoArgTest
passed to withFixture
, in addition to
an apply
method that executes the test, also includes TestData
such as the test name and the config
map passed to runTest
. Thus you can also use the test name and configuration objects in your withFixture
implementation.
Calling loan-fixture methods
If you need to both pass a fixture object into a test and perform cleanup at the end of the test, you'll need to use the loan pattern.
If different tests need different fixtures that require cleanup, you can implement the loan pattern directly by writing loan-fixture methods.
A loan-fixture method takes a function whose body forms part or all of a test's code. It creates a fixture, passes it to the test code by invoking the
function, then cleans up the fixture after the function returns.
The following example shows three tests that use two fixtures, a database and a file. Both require cleanup after, so each is provided via a
loan-fixture method. (In this example, the database is simulated with a StringBuffer
.)
package org.scalatest.examples.flatspec.loanfixture
import java.util.concurrent.ConcurrentHashMap
object DbServer {
type Db = StringBuffer
private val databases = new ConcurrentHashMap[String, Db]
def createDb(name: String): Db = {
val db = new StringBuffer
databases.put(name, db)
db
}
def removeDb(name: String) {
databases.remove(name)
}
}
import org.scalatest.flatspec.AnyFlatSpec
import DbServer._
import java.util.UUID.randomUUID
import java.io._
class ExampleSpec extends AnyFlatSpec {
def withDatabase(testCode: Db => Any) {
val dbName = randomUUID.toString
val db = createDb(dbName)
try {
db.append("ScalaTest is ")
testCode(db)
}
finally removeDb(dbName)
}
def withFile(testCode: (File, FileWriter) => Any) {
val file = File.createTempFile("hello", "world")
val writer = new FileWriter(file)
try {
writer.write("ScalaTest is ")
testCode(file, writer)
}
finally writer.close()
}
"Testing" should "be productive" in withFile { (file, writer) =>
writer.write("productive!")
writer.flush()
assert(file.length === 24)
}
"Test code" should "be readable" in withDatabase { db =>
db.append("readable!")
assert(db.toString === "ScalaTest is readable!")
}
it should "be clear and concise" in withDatabase { db =>
withFile { (file, writer) =>
db.append("clear!")
writer.write("concise!")
writer.flush()
assert(db.toString === "ScalaTest is clear!")
assert(file.length === 21)
}
}
}
As demonstrated by the last test, loan-fixture methods compose. Not only do loan-fixture methods allow you to
give each test the fixture it needs, they allow you to give a test multiple fixtures and clean everything up afterwards.
Also demonstrated in this example is the technique of giving each test its own "fixture sandbox" to play in. When your fixtures
involve external side-effects, like creating files or databases, it is a good idea to give each file or database a unique name as is
done in this example. This keeps tests completely isolated, allowing you to run them in parallel if desired.
Overriding withFixture(OneArgTest)
If all or most tests need the same fixture, you can avoid some of the boilerplate of the loan-fixture method approach by using a FixtureAnyFlatSpec
and overriding withFixture(OneArgTest)
.
Each test in a FixtureAnyFlatSpec
takes a fixture as a parameter, allowing you to pass the fixture into
the test. You must indicate the type of the fixture parameter by specifying FixtureParam
, and implement a
withFixture
method that takes a OneArgTest
. This withFixture
method is responsible for
invoking the one-arg test function, so you can perform fixture set up before, and clean up after, invoking and passing
the fixture into the test function.
To enable the stacking of traits that define withFixture(NoArgTest)
, it is a good idea to let
withFixture(NoArgTest)
invoke the test function instead of invoking the test
function directly. To do so, you'll need to convert the OneArgTest
to a NoArgTest
. You can do that by passing
the fixture object to the toNoArgTest
method of OneArgTest
. In other words, instead of
writing “test(theFixture)
”, you'd delegate responsibility for
invoking the test function to the withFixture(NoArgTest)
method of the same instance by writing:
withFixture(test.toNoArgTest(theFixture))
Here's a complete example:
package org.scalatest.examples.flatspec.oneargtest
import org.scalatest.flatspec.FixtureAnyFlatSpec
import java.io._
class ExampleSpec extends FixtureAnyFlatSpec {
case class FixtureParam(file: File, writer: FileWriter)
def withFixture(test: OneArgTest) = {
val file = File.createTempFile("hello", "world")
val writer = new FileWriter(file)
val theFixture = FixtureParam(file, writer)
try {
writer.write("ScalaTest is ")
withFixture(test.toNoArgTest(theFixture))
}
finally writer.close()
}
"Testing" should "be easy" in { f =>
f.writer.write("easy!")
f.writer.flush()
assert(f.file.length === 18)
}
it should "be fun" in { f =>
f.writer.write("fun!")
f.writer.flush()
assert(f.file.length === 17)
}
}
In this example, the tests actually required two fixture objects, a File
and a FileWriter
. In such situations you can
simply define the FixtureParam
type to be a tuple containing the objects, or as is done in this example, a case class containing
the objects. For more information on the withFixture(OneArgTest)
technique, see the documentation for FixtureAnyFlatSpec
.
Mixing in BeforeAndAfter
In all the shared fixture examples shown so far, the activities of creating, setting up, and cleaning up the fixture objects have been
performed during the test. This means that if an exception occurs during any of these activities, it will be reported as a test failure.
Sometimes, however, you may want setup to happen before the test starts, and cleanup after the test has completed, so that if an
exception occurs during setup or cleanup, the entire suite aborts and no more tests are attempted. The simplest way to accomplish this in ScalaTest is
to mix in trait BeforeAndAfter
. With this trait you can denote a bit of code to run before each test
with before
and/or after each test each test with after
, like this:
package org.scalatest.examples.flatspec.beforeandafter
import org.scalatest._
import collection.mutable.ListBuffer
class ExampleSpec extends AnyFlatSpec with BeforeAndAfter {
val builder = new StringBuilder
val buffer = new ListBuffer[String]
before {
builder.append("ScalaTest is ")
}
after {
builder.clear()
buffer.clear()
}
"Testing" should "be easy" in {
builder.append("easy!")
assert(builder.toString === "ScalaTest is easy!")
assert(buffer.isEmpty)
buffer += "sweet"
}
it should "be fun" in {
builder.append("fun!")
assert(builder.toString === "ScalaTest is fun!")
assert(buffer.isEmpty)
}
}
Note that the only way before
and after
code can communicate with test code is via some side-effecting mechanism, commonly by
reassigning instance var
s or by changing the state of mutable objects held from instance val
s (as in this example). If using
instance var
s or mutable objects held from instance val
s you wouldn't be able to run tests in parallel in the same instance
of the test class unless you synchronized access to the shared, mutable state. This is why ScalaTest's ParallelTestExecution
trait extends
OneInstancePerTest
. By running each test in its own instance of the class, each test has its own copy of the instance variables, so you
don't need to synchronize. If you mixed ParallelTestExecution
into the ExampleSuite
above, the tests would run in parallel just fine
without any synchronization needed on the mutable StringBuilder
and ListBuffer[String]
objects.
Although BeforeAndAfter
provides a minimal-boilerplate way to execute code before and after tests, it isn't designed to enable stackable
traits, because the order of execution would be non-obvious. If you want to factor out before and after code that is common to multiple test suites, you
should use trait BeforeAndAfterEach
instead, as shown later in the next section,
composing fixtures by stacking traits.
Composing fixtures by stacking traits
In larger projects, teams often end up with several different fixtures that test classes need in different combinations,
and possibly initialized (and cleaned up) in different orders. A good way to accomplish this in ScalaTest is to factor the individual
fixtures into traits that can be composed using the stackable trait pattern. This can be done, for example, by placing
withFixture
methods in several traits, each of which call super.withFixture
. Here's an example in
which the StringBuilder
and ListBuffer[String]
fixtures used in the previous examples have been
factored out into two stackable fixture traits named Builder
and Buffer
:
package org.scalatest.examples.flatspec.composingwithfixture
import org.scalatest._
import collection.mutable.ListBuffer
trait Builder extends SuiteMixin { this: Suite =>
val builder = new StringBuilder
abstract override def withFixture(test: NoArgTest) = {
builder.append("ScalaTest is ")
try super.withFixture(test)
finally builder.clear()
}
}
trait Buffer extends SuiteMixin { this: Suite =>
val buffer = new ListBuffer[String]
abstract override def withFixture(test: NoArgTest) = {
try super.withFixture(test)
finally buffer.clear()
}
}
class ExampleSpec extends AnyFlatSpec with Builder with Buffer {
"Testing" should "be easy" in {
builder.append("easy!")
assert(builder.toString === "ScalaTest is easy!")
assert(buffer.isEmpty)
buffer += "sweet"
}
it should "be fun" in {
builder.append("fun!")
assert(builder.toString === "ScalaTest is fun!")
assert(buffer.isEmpty)
buffer += "clear"
}
}
By mixing in both the Builder
and Buffer
traits, ExampleSpec
gets both fixtures, which will be
initialized before each test and cleaned up after. The order the traits are mixed together determines the order of execution.
In this case, Builder
is “super” to Buffer
. If you wanted Buffer
to be “super”
to Builder
, you need only switch the order you mix them together, like this:
class Example2Spec extends Suite with Buffer with Builder
And if you only need one fixture you mix in only that trait:
class Example3Spec extends Suite with Builder
Another way to create stackable fixture traits is by extending the BeforeAndAfterEach
and/or BeforeAndAfterAll
traits.
BeforeAndAfterEach
has a beforeEach
method that will be run before each test (like JUnit's setUp
),
and an afterEach
method that will be run after (like JUnit's tearDown
).
Similarly, BeforeAndAfterAll
has a beforeAll
method that will be run before all tests,
and an afterAll
method that will be run after all tests. Here's what the previously shown example would look like if it
were rewritten to use the BeforeAndAfterEach
methods instead of withFixture
:
package org.scalatest.examples.flatspec.composingbeforeandaftereach
import org.scalatest._
import collection.mutable.ListBuffer
trait Builder extends BeforeAndAfterEach { this: Suite =>
val builder = new StringBuilder
override def beforeEach() {
builder.append("ScalaTest is ")
super.beforeEach()
}
override def afterEach() {
try super.afterEach()
finally builder.clear()
}
}
trait Buffer extends BeforeAndAfterEach { this: Suite =>
val buffer = new ListBuffer[String]
override def afterEach() {
try super.afterEach()
finally buffer.clear()
}
}
class ExampleSpec extends AnyFlatSpec with Builder with Buffer {
"Testing" should "be easy" in {
builder.append("easy!")
assert(builder.toString === "ScalaTest is easy!")
assert(buffer.isEmpty)
buffer += "sweet"
}
it should "be fun" in {
builder.append("fun!")
assert(builder.toString === "ScalaTest is fun!")
assert(buffer.isEmpty)
buffer += "clear"
}
}
To get the same ordering as withFixture
, place your super.beforeEach
call at the end of each
beforeEach
method, and the super.afterEach
call at the beginning of each afterEach
method, as shown in the previous example. It is a good idea to invoke super.afterEach
in a try
block and perform cleanup in a finally
clause, as shown in the previous example, because this ensures the
cleanup code is performed even if super.afterEach
throws an exception.
The difference between stacking traits that extend BeforeAndAfterEach
versus traits that implement withFixture
is
that setup and cleanup code happens before and after the test in BeforeAndAfterEach
, but at the beginning and
end of the test in withFixture
. Thus if a withFixture
method completes abruptly with an exception, it is
considered a failed test. By contrast, if any of the beforeEach
or afterEach
methods of BeforeAndAfterEach
complete abruptly, it is considered an aborted suite, which will result in a
SuiteAborted
event.