FlatSpec

Facilitates a “behavior-driven” style of development (BDD), in which tests are combined with text that specifies the behavior the tests verify.

Trait FlatSpec is so named because your specification text and tests line up flat against the left-side indentation level, with no nesting needed. Here's an example FlatSpec:

package org.scalatest.examples.flatspec

import org.scalatest.FlatSpec

class SetSpec extends FlatSpec {

  behavior of "An empty Set"

  it should "have size 0" in {
    assert(Set.empty.size === 0)
  }

  it should "produce NoSuchElementException when head is invoked" in {
    assertThrows[NoSuchElementException] {
      Set.empty.head
    }
  }
}

Note: you can use must or can as well as should in a FlatSpec. For example, instead of it should "have..., you could write it must "have... or it can "have....

Instead of using a behavior of clause, you can alternatively use a shorthand syntax in which you replace the first it with the subject string, like this:

package org.scalatest.examples.flatspec

import org.scalatest.FlatSpec

class SetSpec extends FlatSpec {

  "An empty Set" should "have size 0" in {
    assert(Set.empty.size === 0)
  }

  it should "produce NoSuchElementException when head is invoked" in {
    assertThrows[NoSuchElementException] {
      Set.empty.head
    }
  }
}

Running either of the two previous versions of SetSpec in the Scala interpreter would yield:

An empty Set
- should have size 0
- should produce NoSuchElementException when head is invoked

In a FlatSpec you write a one (or more) sentence specification for each bit of behavior you wish to specify and test. Each specification sentence has a "subject," which is sometimes called the system under test (or SUT). The subject is the entity being specified and tested and also serves as the subject of the sentences you write for each test. Often you will want to write multiple tests for the same subject. In a FlatSpec, you name the subject once, with a behavior of clause or its shorthand, then write tests for that subject with it should/must/can "do something" phrases. Each it refers to the most recently declared subject. For example, the four tests shown in this snippet are all testing a stack that contains one item:

behavior of "A Stack (with one item)"

it should "be non-empty" in {}

it should "return the top item on peek" in {}

it should "not remove the top item on peek" in {}

it should "remove the top item on pop" in {}

The same is true if the tests are written using the shorthand notation:

"A Stack (with one item)" should "be non-empty" in {}

it should "return the top item on peek" in {}

it should "not remove the top item on peek" in {}

it should "remove the top item on pop" in {}

In a FlatSpec, therefore, to figure out what "it" means, you just scan vertically until you find the most recent use of behavior of or the shorthand notation.

Because sometimes the subject could be plural, you can alternatively use they instead of it:

"The combinators" should "be easy to learn" in {}

they should "be efficient" in {}

they should "do something cool" in {}

A FlatSpec's lifecycle has two phases: the registration phase and the ready phase. It starts in registration phase and enters ready phase the first time run is called on it. It then remains in ready phase for the remainder of its lifetime.

Tests can only be registered while the FlatSpec is in its registration phase. Any attempt to register a test after the FlatSpec has entered its ready phase, i.e., after run has been invoked on the FlatSpec, will be met with a thrown TestRegistrationClosedException. The recommended style of using FlatSpec is to register tests during object construction as is done in all the examples shown here. If you keep to the recommended style, you should never see a TestRegistrationClosedException.

Ignored tests

To support the common use case of temporarily disabling a test, with the good intention of resurrecting the test at a later time, FlatSpec provides a method ignore that can be used instead of it or they to register a test. For example, to temporarily disable the test with the name "An empty Set should produce NoSuchElementException when head is invoked", just change “it” into “ignore,” like this:

package org.scalatest.examples.flatspec.ignore

import org.scalatest.FlatSpec

class SetSpec extends FlatSpec {

  "An empty Set" should "have size 0" in {
    assert(Set.empty.size === 0)
  }

  ignore should "produce NoSuchElementException when head is invoked" in {
    assertThrows[NoSuchElementException] {
      Set.empty.head
    }
  }
}

If you run this version of SetSpec with:

scala> org.scalatest.run(new SetSpec)

It will run only the first test and report that the second test was ignored:

An empty Set
- should have size 0
- should produce NoSuchElementException when head is invoked !!! IGNORED !!!

When using shorthand notation, you won't have an it to change into ignore for the first test of each new subject. To ignore such tests, you must instead change in to ignore. For example, to temporarily disable the test with the name "An empty Set should have size 0", change “in” into “ignore” like this:

package org.scalatest.examples.flatspec.ignoreafter

import org.scalatest.FlatSpec

class SetSpec extends FlatSpec {

  "An empty Set" should "have size 0" ignore {
    assert(Set.empty.size === 0)
  }

  it should "produce NoSuchElementException when head is invoked" in {
    assertThrows[NoSuchElementException] {
      Set.empty.head
    }
  }
}

If you run this version of StackSpec with:

scala> org.scalatest.run(new SetSpec)

It will run only the second test and report that the first test was ignored:

An empty Set
- should have size 0 !!! IGNORED !!!
- should produce NoSuchElementException when head is invoked

If you wish to temporarily ignore an entire suite of tests, you can (on the JVM, not Scala.js) annotate the test class with @Ignore, like this:

package org.scalatest.examples.flatspec.ignoreall

import org.scalatest._

@Ignore
class SetSpec extends FlatSpec {

  "An empty Set" should "have size 0" in {
    assert(Set.empty.size === 0)
  }

  it should "produce NoSuchElementException when head is invoked" in {
    assertThrows[NoSuchElementException] {
      Set.empty.head
    }
  }
}

When you mark a test class with a tag annotation, ScalaTest will mark each test defined in that class with that tag. Thus, marking the SetSpec in the above example with the @Ignore tag annotation means that both tests in the class will be ignored. If you run the above SetSpec in the Scala interpreter, you'll see:

scala> org.scalatest.run(new SetSpec)
SetSpec:
An empty Set
- should have size 0 !!! IGNORED !!!
- should produce NoSuchElementException when head is invoked !!! IGNORED !!!

Note that marking a test class as ignored won't prevent it from being discovered by ScalaTest. Ignored classes will be discovered and run, and all their tests will be reported as ignored. This is intended to keep the ignored class visible, to encourage the developers to eventually fix and “un-ignore” it. If you want to prevent a class from being discovered at all (on the JVM, not Scala.js), use the DoNotDiscover annotation instead.

Informers

One of the parameters to FlatSpec's run method is a Reporter, which will collect and report information about the running suite of tests. Information about suites and tests that were run, whether tests succeeded or failed, and tests that were ignored will be passed to the Reporter as the suite runs. Most often the reporting done by default by FlatSpec's methods will be sufficient, but occasionally you may wish to provide custom information to the Reporter from a test. For this purpose, an Informer that will forward information to the current Reporter is provided via the info parameterless method. You can pass the extra information to the Informer via its apply method. The Informer will then pass the information to the Reporter via an InfoProvided event.

One use case for the Informer is to pass more information about a specification to the reporter. For example, the GivenWhenThen trait provides methods that use the implicit info provided by FlatSpec to pass such information to the reporter. Here's an example:

package org.scalatest.examples.flatspec.info

import collection.mutable
import org.scalatest._

class SetSpec extends FlatSpec with GivenWhenThen {

  "A mutable Set" should "allow an element to be added" in {
    Given("an empty mutable Set")
    val set = mutable.Set.empty[String]

    When("an element is added")
    set += "clarity"

    Then("the Set should have size 1")
    assert(set.size === 1)

    And("the Set should contain the added element")
    assert(set.contains("clarity"))

    info("That's all folks!")
  }
}

If you run this FlatSpec from the interpreter, you will see the following output:

scala> org.scalatest.run(new SetSpec)
SetSpec:
A mutable Set
- should allow an element to be added
  + Given an empty mutable Set
  + When an element is added
  + Then the Set should have size 1
  + And the Set should contain the added element
  + That's all folks! 

Documenters

FlatSpec also provides a markup method that returns a Documenter, which allows you to send to the Reporter text formatted in Markdown syntax. You can pass the extra information to the Documenter via its apply method. The Documenter will then pass the information to the Reporter via an MarkupProvided event.

Here's an example FlatSpec that uses markup:

package org.scalatest.examples.flatspec.markup

import collection.mutable
import org.scalatest._

class SetSpec extends FlatSpec with GivenWhenThen {

  markup { """

Mutable Set
———--

A set is a collection that contains no duplicate elements.

To implement a concrete mutable set, you need to provide implementations
of the following methods:

    def contains(elem: A): Boolean
    def iterator: Iterator[A]
    def += (elem: A): this.type
    def -= (elem: A): this.type

If you wish that methods like `take`,
`drop`, `filter` return the same kind of set,
you should also override:

    def empty: This

It is also good idea to override methods `foreach` and
`size` for efficiency.

  """ }

  "A mutable Set" should "allow an element to be added" in {
    Given("an empty mutable Set")
    val set = mutable.Set.empty[String]

    When("an element is added")
    set += "clarity"

    Then("the Set should have size 1")
    assert(set.size === 1)

    And("the Set should contain the added element")
    assert(set.contains("clarity"))

    markup("This test finished with a **bold** statement!")
  }
}

Although all of ScalaTest's built-in reporters will display the markup text in some form, the HTML reporter will format the markup information into HTML. Thus, the main purpose of markup is to add nicely formatted text to HTML reports. Here's what the above SetSpec would look like in the HTML reporter:

Notifiers and alerters

ScalaTest records text passed to info and markup during tests, and sends the recorded text in the recordedEvents field of test completion events like TestSucceeded and TestFailed. This allows string reporters (like the standard out reporter) to show info and markup text after the test name in a color determined by the outcome of the test. For example, if the test fails, string reporters will show the info and markup text in red. If a test succeeds, string reporters will show the info and markup text in green. While this approach helps the readability of reports, it means that you can't use info to get status updates from long running tests.

To get immediate (i.e., non-recorded) notifications from tests, you can use note (a Notifier) and alert (an Alerter). Here's an example showing the differences:

package org.scalatest.examples.flatspec.note

import collection.mutable
import org.scalatest._

class SetSpec extends FlatSpec {

  "A mutable Set" should "allow an element to be added" in {

    info("info is recorded")
    markup("markup is *also* recorded")
    note("notes are sent immediately")
    alert("alerts are also sent immediately")

    val set = mutable.Set.empty[String]
    set += "clarity"
    assert(set.size === 1)
    assert(set.contains("clarity"))
  }
}

Because note and alert information is sent immediately, it will appear before the test name in string reporters, and its color will be unrelated to the ultimate outcome of the test: note text will always appear in green, alert text will always appear in yellow. Here's an example:

scala> org.scalatest.run(new SetSpec)
SetSpec:
A mutable Set
  + notes are sent immediately
  + alerts are also sent immediately
- should allow an element to be added
  + info is recorded
  + markup is *also* recorded

Another example is slowpoke notifications. If you find a test is taking a long time to complete, but you're not sure which test, you can enable slowpoke notifications. ScalaTest will use an Alerter to fire an event whenever a test has been running longer than a specified amount of time.

In summary, use info and markup for text that should form part of the specification output. Use note and alert to send status notifications. (Because the HTML reporter is intended to produce a readable, printable specification, info and markup text will appear in the HTML report, but note and alert text will not.)

Pending tests

A pending test is one that has been given a name but is not yet implemented. The purpose of pending tests is to facilitate a style of testing in which documentation of behavior is sketched out before tests are written to verify that behavior (and often, before the behavior of the system being tested is itself implemented). Such sketches form a kind of specification of what tests and functionality to implement later.

To support this style of testing, a test can be given a name that specifies one bit of behavior required by the system being tested. The test can also include some code that sends more information about the behavior to the reporter when the tests run. At the end of the test, it can call method pending, which will cause it to complete abruptly with TestPendingException.

Because tests in ScalaTest can be designated as pending with TestPendingException, both the test name and any information sent to the reporter when running the test can appear in the report of a test run. (In other words, the code of a pending test is executed just like any other test.) However, because the test completes abruptly with TestPendingException, the test will be reported as pending, to indicate the actual test, and possibly the functionality it is intended to test, has not yet been implemented. You can mark tests as pending in FlatSpec like this:

package org.scalatest.examples.flatspec.pending

import org.scalatest._

class SetSpec extends FlatSpec {

  "An empty Set" should "have size 0" in (pending)

  it should "produce NoSuchElementException when head is invoked" in {
    assertThrows[NoSuchElementException] {
      Set.empty.head
    }
  }
}

If you run this version of FlatSpec with:

scala> org.scalatest.run(new SetSpec)

It will run both tests but report that An empty Set should have size 0 is pending. You'll see:

An empty Set
- should have size 0 (pending)
- should produce NoSuchElementException when head is invoked

One difference between an ignored test and a pending one is that an ignored test is intended to be used during a significant refactorings of the code under test, when tests break and you don't want to spend the time to fix all of them immediately. You can mark some of those broken tests as ignored temporarily, so that you can focus the red bar on just failing tests you actually want to fix immediately. Later you can go back and fix the ignored tests. In other words, by ignoring some failing tests temporarily, you can more easily notice failed tests that you actually want to fix. By contrast, a pending test is intended to be used before a test and/or the code under test is written. Pending indicates you've decided to write a test for a bit of behavior, but either you haven't written the test yet, or have only written part of it, or perhaps you've written the test but don't want to implement the behavior it tests until after you've implemented a different bit of behavior you realized you need first. Thus ignored tests are designed to facilitate refactoring of existing code whereas pending tests are designed to facilitate the creation of new code.

One other difference between ignored and pending tests is that ignored tests are implemented as a test tag that is excluded by default. Thus an ignored test is never executed. By contrast, a pending test is implemented as a test that throws TestPendingException (which is what calling the pending method does). Thus the body of pending tests are executed up until they throw TestPendingException. The reason for this difference is that it enables your unfinished test to send InfoProvided messages to the reporter before it completes abruptly with TestPendingException, as shown in the previous example on Informers that used the GivenWhenThen trait. For example, the following snippet in a FlatSpec:

"The Scala language" must "add correctly" in {
   Given("two integers")
   When("they are added")
   Then("the result is the sum of the two numbers")
   pending
 }
 // ...

Would yield the following output when run in the interpreter:

The Scala language
- must add correctly (pending)
  + Given two integers
  + When they are added
  + Then the result is the sum of the two numbers

Tagging tests

A FlatSpec's tests may be classified into groups by tagging them with string names. As with any suite, when executing a FlatSpec, groups of tests can optionally be included and/or excluded. To tag a FlatSpec's tests, you pass objects that extend class org.scalatest.Tag to methods that register tests. Class Tag takes one parameter, a string name. If you have created tag annotation interfaces as described in the Tag documentation, then you will probably want to use tag names on your test functions that match. To do so, simply pass the fully qualified names of the tag interfaces to the Tag constructor. For example, if you've defined a tag annotation interface with fully qualified name, com.mycompany.tags.DbTest, then you could create a matching tag for FlatSpecs like this:

package org.scalatest.examples.flatspec.tagging

import org.scalatest.Tag

object DbTest extends Tag("com.mycompany.tags.DbTest")

Given these definitions, you could place FlatSpec tests into groups with tags like this:

import org.scalatest.FlatSpec
import org.scalatest.tagobjects.Slow

class SetSpec extends FlatSpec {

  behavior of "An empty Set"

  it should "have size 0" taggedAs(Slow) in {
    assert(Set.empty.size === 0)
  }

  it should "produce NoSuchElementException when head is invoked" taggedAs(Slow, DbTest) in {
    assertThrows[NoSuchElementException] {
      Set.empty.head
    }
  }
}

This code marks both tests with the org.scalatest.tags.Slow tag, and the second test with the com.mycompany.tags.DbTest tag.

The run method takes a Filter, whose constructor takes an optional Set[String] called tagsToInclude and a Set[String] called tagsToExclude. If tagsToInclude is None, all tests will be run except those those belonging to tags listed in the tagsToExclude Set. If tagsToInclude is defined, only tests belonging to tags mentioned in the tagsToInclude set, and not mentioned in tagsToExclude, will be run.

It is recommended, though not required, that you create a corresponding tag annotation when you create a Tag object. A tag annotation (on the JVM, not Scala.js) allows you to tag all the tests of a FlatSpec in one stroke by annotating the class. For more information and examples, see the documentation for class Tag. On Scala.js, to tag all tests of a suite, you'll need to tag each test individually at the test site.

Shared 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 vars, 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:

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
import collection.mutable.ListBuffer

class ExampleSpec extends FlatSpec {

  class Fixture {
    val builder = new StringBuilder("ScalaTest is ")
    val buffer = new ListBuffer[String]
  }

  def fixture = new Fixture

  "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

class ExampleSpec extends FlatSpec {

  trait Builder {
    val builder = new StringBuilder("ScalaTest is ")
  }

  trait Buffer {
    val buffer = ListBuffer("ScalaTest", "is")
  }

  // This test needs the StringBuilder fixture
  "Testing" should "be productive" in new Builder {
    builder.append("productive!")
    assert(builder.toString === "ScalaTest is productive!")
  }

  // This test needs the ListBuffer[String] fixture
  "Test code" should "be readable" in new Buffer {
    buffer += ("readable!")
    assert(buffer === List("ScalaTest", "is", "readable!"))
  }

  // This test needs both the StringBuilder and ListBuffer
  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:

// Default implementation in trait Suite
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:

// Your implementation
override def withFixture(test: NoArgTest) = {
  // Perform setup
  try super.withFixture(test) // Invoke the test function
  finally {
    // Perform cleanup
  }
}

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._

class ExampleSpec extends FlatSpec {

  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> org.scalatest.run(new ExampleSuite)
ExampleSuite:
This test
- should succeed
- should fail *** FAILED ***
  2 did not equal 3 (: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 { // Simulating a database server
  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
import DbServer._
import java.util.UUID.randomUUID
import java.io._

class ExampleSpec extends FlatSpec {

  def withDatabase(testCode: Db => Any) {
    val dbName = randomUUID.toString
    val db = createDb(dbName) // create the fixture
    try {
      db.append("ScalaTest is ") // perform setup
      testCode(db) // "loan" the fixture to the test
    }
    finally removeDb(dbName) // clean up the fixture
  }

  def withFile(testCode: (File, FileWriter) => Any) {
    val file = File.createTempFile("hello", "world") // create the fixture
    val writer = new FileWriter(file)
    try {
      writer.write("ScalaTest is ") // set up the fixture
      testCode(file, writer) // "loan" the fixture to the test
    }
    finally writer.close() // clean up the fixture
  }

  // This test needs the file fixture
  "Testing" should "be productive" in withFile { (file, writer) =>
    writer.write("productive!")
    writer.flush()
    assert(file.length === 24)
  }

  // This test needs the database fixture
  "Test code" should "be readable" in withDatabase { db =>
    db.append("readable!")
    assert(db.toString === "ScalaTest is readable!")
  }

  // This test needs both the file and the database
  it should "be clear and concise" in withDatabase { db =>
    withFile { (file, writer) => // loan-fixture methods compose
      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 fixture.FlatSpec and overriding withFixture(OneArgTest). Each test in a fixture.FlatSpec 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.fixture
import java.io._

class ExampleSpec extends fixture.FlatSpec {

  case class FixtureParam(file: File, writer: FileWriter)

  def withFixture(test: OneArgTest) = {
    val file = File.createTempFile("hello", "world") // create the fixture
    val writer = new FileWriter(file)
    val theFixture = FixtureParam(file, writer)

    try {
      writer.write("ScalaTest is ") // set up the fixture
      withFixture(test.toNoArgTest(theFixture)) // "loan" the fixture to the test
    }
    finally writer.close() // clean up the fixture
  }

  "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 fixture.FlatSpec.

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 FlatSpec 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 vars or by changing the state of mutable objects held from instance vals (as in this example). If using instance vars or mutable objects held from instance vals you wouldn't be able to run tests in parallel in the same instance of the test class (on the JVM, not Scala.js) 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 TestSuiteMixin { this: TestSuite =>

  val builder = new StringBuilder

  abstract override def withFixture(test: NoArgTest) = {
    builder.append("ScalaTest is ")
    try super.withFixture(test) // To be stackable, must call super.withFixture
    finally builder.clear()
  }
}

trait Buffer extends TestSuiteMixin { this: TestSuite =>

  val buffer = new ListBuffer[String]

  abstract override def withFixture(test: NoArgTest) = {
    try super.withFixture(test) // To be stackable, must call super.withFixture
    finally buffer.clear()
  }
}

class ExampleSpec extends FlatSpec 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, ExampleSuite 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 FlatSpec with Buffer with Builder

And if you only need one fixture you mix in only that trait:

class Example3Spec extends FlatSpec 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() // To be stackable, must call super.beforeEach
  }

  override def afterEach() {
    try super.afterEach() // To be stackable, must call super.afterEach
    finally builder.clear()
  }
}

trait Buffer extends BeforeAndAfterEach { this: Suite =>

  val buffer = new ListBuffer[String]

  override def afterEach() {
    try super.afterEach() // To be stackable, must call super.afterEach
    finally buffer.clear()
  }
}

class ExampleSpec extends FlatSpec 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.

Shared tests

Sometimes you may want to run the same test code on different fixture objects. In other words, you may want to write tests that are "shared" by different fixture objects. To accomplish this in a FlatSpec, you first place shared tests in behavior functions. These behavior functions will be invoked during the construction phase of any FlatSpec that uses them, so that the tests they contain will be registered as tests in that FlatSpec. For example, given this stack class:

import scala.collection.mutable.ListBuffer

class Stack[T] {

  val MAX = 10
  private val buf = new ListBuffer[T]

  def push(o: T) {
    if (!full)
      buf.prepend(o)
    else
      throw new IllegalStateException("can't push onto a full stack")
  }

  def pop(): T = {
    if (!empty)
      buf.remove(0)
    else
      throw new IllegalStateException("can't pop an empty stack")
  }

  def peek: T = {
    if (!empty)
      buf(0)
    else
      throw new IllegalStateException("can't pop an empty stack")
  }

  def full: Boolean = buf.size == MAX
  def empty: Boolean = buf.size == 0
  def size = buf.size

  override def toString = buf.mkString("Stack(", ", ", ")")
}

You may want to test the Stack class in different states: empty, full, with one item, with one item less than capacity, etc. You may find you have several tests that make sense any time the stack is non-empty. Thus you'd ideally want to run those same tests for three stack fixture objects: a full stack, a stack with a one item, and a stack with one item less than capacity. With shared tests, you can factor these tests out into a behavior function, into which you pass the stack fixture to use when running the tests. So in your FlatSpec for stack, you'd invoke the behavior function three times, passing in each of the three stack fixtures so that the shared tests are run for all three fixtures. You can define a behavior function that encapsulates these shared tests inside the FlatSpec that uses them. If they are shared between different FlatSpecs, however, you could also define them in a separate trait that is mixed into each FlatSpec that uses them.

For example, here the nonEmptyStack behavior function (in this case, a behavior method) is defined in a trait along with another method containing shared tests for non-full stacks:

trait StackBehaviors { this: FlatSpec =>

  def nonEmptyStack(newStack: => Stack[Int], lastItemAdded: Int) {

    it should "be non-empty" in {
      assert(!newStack.empty)
    }

    it should "return the top item on peek" in {
      assert(newStack.peek === lastItemAdded)
    }

    it should "not remove the top item on peek" in {
      val stack = newStack
      val size = stack.size
      assert(stack.peek === lastItemAdded)
      assert(stack.size === size)
    }

    it should "remove the top item on pop" in {
      val stack = newStack
      val size = stack.size
      assert(stack.pop === lastItemAdded)
      assert(stack.size === size - 1)
    }
  }

  def nonFullStack(newStack: => Stack[Int]) {

    it should "not be full" in {
      assert(!newStack.full)
    }

    it should "add to the top on push" in {
      val stack = newStack
      val size = stack.size
      stack.push(7)
      assert(stack.size === size + 1)
      assert(stack.peek === 7)
    }
  }
}

Given these behavior functions, you could invoke them directly, but FlatSpec offers a DSL for the purpose, which looks like this:

it should behave like nonEmptyStack(stackWithOneItem, lastValuePushed)
it should behave like nonFullStack(stackWithOneItem)

If you prefer to use an imperative style to change fixtures, for example by mixing in BeforeAndAfterEach and reassigning a stack var in beforeEach, you could write your behavior functions in the context of that var, which means you wouldn't need to pass in the stack fixture because it would be in scope already inside the behavior function. In that case, your code would look like this:

it should behave like nonEmptyStack // assuming lastValuePushed is also in scope inside nonEmptyStack
it should behave like nonFullStack

The recommended style, however, is the functional, pass-all-the-needed-values-in style. Here's an example:

class SharedTestExampleSpec extends FlatSpec with StackBehaviors {

  // Stack fixture creation methods
  def emptyStack = new Stack[Int]

  def fullStack = {
    val stack = new Stack[Int]
    for (i <- 0 until stack.MAX)
      stack.push(i)
    stack
  }

  def stackWithOneItem = {
    val stack = new Stack[Int]
    stack.push(9)
    stack
  }

  def stackWithOneItemLessThanCapacity = {
    val stack = new Stack[Int]
    for (i <- 1 to 9)
      stack.push(i)
    stack
  }

  val lastValuePushed = 9

  "A Stack (when empty)" should "be empty" in {
    assert(emptyStack.empty)
  }

  it should "complain on peek" in {
    assertThrows[IllegalStateException] {
      emptyStack.peek
    }
  }

  it should "complain on pop" in {
    assertThrows[IllegalStateException] {
      emptyStack.pop
    }
  }

  "A Stack (with one item)" should behave like nonEmptyStack(stackWithOneItem, lastValuePushed)

  it should behave like nonFullStack(stackWithOneItem)

  "A Stack (with one item less than capacity)" should behave like nonEmptyStack(stackWithOneItemLessThanCapacity, lastValuePushed)

  it should behave like nonFullStack(stackWithOneItemLessThanCapacity)

  "A Stack (full)" should "be full" in {
    assert(fullStack.full)
  }

  it should behave like nonEmptyStack(fullStack, lastValuePushed)

  it should "complain on a push" in {
    assertThrows[IllegalStateException] {
      fullStack.push(10)
    }
  }
}

If you load these classes into the Scala interpreter (with scalatest's JAR file on the class path), and execute it, you'll see:

scala> org.scalatest.run(new SharedTestExampleSpec)
A Stack (when empty)
- should be empty
- should complain on peek
- should complain on pop
A Stack (with one item)
- should be non-empty
- should return the top item on peek
- should not remove the top item on peek
- should remove the top item on pop
- should not be full
- should add to the top on push
A Stack (with one item less than capacity)
- should be non-empty
- should return the top item on peek
- should not remove the top item on peek
- should remove the top item on pop
- should not be full
- should add to the top on push
A Stack (full)
- should be full
- should be non-empty
- should return the top item on peek
- should not remove the top item on peek
- should remove the top item on pop
- should complain on a push

One thing to keep in mind when using shared tests is that in ScalaTest, each test in a suite must have a unique name. If you register the same tests repeatedly in the same suite, one problem you may encounter is an exception at runtime complaining that multiple tests are being registered with the same test name. A good way to solve this problem in a FlatSpec is to make sure each invocation of a behavior function is in the context of a different set of when, verb (should, must, or can), and that clauses, which will prepend a string to each test name. For example, the following code in a FlatSpec would register a test with the name "A Stack (when empty) should be empty":

behavior of "A Stack (when empty)"

it should "be empty" in {
  assert(emptyStack.empty)
}
// ...

Or, using the shorthand notation:

"A Stack" when {
  "empty" should {
    "be empty" in {
      assert(emptyStack.empty)
    }
  }
}
// ...

If the "should be empty" test was factored out into a behavior function, it could be called repeatedly so long as each invocation of the behavior function is in the context of a different combination of when, verb, and that clauses.