Suite

A suite of tests. A Suite instance encapsulates a conceptual suite (i.e., a collection) of tests.

This trait provides an interface that allows suites of tests to be run. Its implementation enables a default way of writing and executing tests. Subtraits and subclasses can override Suite's methods to enable other ways of writing and executing tests. This trait's default approach allows tests to be defined as methods whose name starts with "test." This approach is easy to understand, and a good way for Scala beginners to start writing tests. More advanced Scala programmers may prefer to mix together other Suite subtraits defined in ScalaTest, or create their own, to write tests in the way they feel makes them most productive. Here's a quick overview of some of the options to help you get started:

For JUnit 3 users

If you are using JUnit 3 (version 3.8 or earlier releases) and you want to write JUnit 3 tests in Scala, look at AssertionsForJUnit, ShouldMatchersForJUnit, and JUnit3Suite.

For JUnit 4 users

If you are using JUnit 4 and you want to write JUnit 4 tests in Scala, look at JUnitSuite, and JUnitRunner. With JUnitRunner, you can use any of the traits described here and still run your tests with JUnit 4.

For TestNG users

If you are using TestNG and you want to write TestNG tests in Scala, look at TestNGSuite.

For high-level testing

If you want to write tests at a higher level than unit tests, such as integration tests, acceptance tests, or functional tests, check out FeatureSpec.

For unit testing

If you prefer a behavior-driven development (BDD) style, in which tests are combined with text that specifies the behavior being tested, look at FunSpec, FlatSpec, FreeSpec, and WordSpec. Otherwise, if you just want to write tests and don't want to combine testing with specifying, look at FunSuite or read on to learn how to write tests using this base trait, Suite.

To use this trait's approach to writing tests, simply create classes that extend Suite and define test methods. Test methods have names of the form testX, where X is some unique, hopefully meaningful, string. A test method must be public and can have any result type, but the most common result type is Unit. Here's an example:

import org.scalatest.Suite

class ExampleSuite extends Suite {

  def testAddition {
    val sum = 1 + 1
    assert(sum === 2)
  }

  def testSubtraction {
    val diff = 4 - 1
    assert(diff === 3)
  }
}

You can run a Suite by invoking execute on it. This method, which prints test results to the standard output, is intended to serve as a convenient way to run tests from within the Scala interpreter. For example, to run ExampleSuite from within the Scala interpreter, you could write:

scala> (new ExampleSuite).execute()

And you would see:

ExampleSuite:
- testAddition
- testSubtraction

Or, to run just the testAddition method, you could write:

scala> (new ExampleSuite).execute("testAddition")

And you would see:

ExampleSuite:
- testAddition

You can also pass to execute a config map of key-value pairs, which will be passed down into suites and tests, as well as other parameters that configure the run itself. For more information on running in the Scala interpreter, see the documentation for execute (below) and the ScalaTest shell.

The execute method invokes a run method takes seven parameters. This run method, which actually executes the suite, will usually be invoked by a test runner, such as org.scalatest.tools.Runner or an IDE. See the documentation for Runner for more details.

Assertions and ===

Inside test methods in a Suite, you can write assertions by invoking assert and passing in a Boolean expression, such as:

val left = 2
val right = 1
assert(left == right)

If the passed expression is true, assert will return normally. If false, assert will complete abruptly with a TestFailedException. This exception is usually not caught by the test method, which means the test method itself will complete abruptly by throwing the TestFailedException. Any test method that completes abruptly with an exception is considered a failed test. A test method that returns normally is considered a successful test.

If you pass a Boolean expression to assert, a failed assertion will be reported, but without reporting the left and right values. You can alternatively encode these values in a String passed as a second argument to assert, as in:

val left = 2
val right = 1
assert(left == right, left + " did not equal " + right)

Using this form of assert, the failure report will include the left and right values, helping you debug the problem. However, ScalaTest provides the === operator to make this easier. (The === operator is defined in trait Assertions which trait Suite extends.) You use it like this:

val left = 2
val right = 1
assert(left === right)

Because you use === here instead of ==, the failure report will include the left and right values. For example, the detail message in the thrown TestFailedException from the assert shown previously will include, "2 did not equal 1". From this message you will know that the operand on the left had the value 2, and the operand on the right had the value 1.

If you're familiar with JUnit, you would use === in a ScalaTest Suite where you'd use assertEquals in a JUnit TestCase. The === operator is made possible by an implicit conversion from Any to Equalizer. If you're curious to understand the mechanics, see the documentation for Equalizer and the convertToEqualizer method.

Expected results

Although === provides a natural, readable extension to Scala's assert mechanism, as the operands become lengthy, the code becomes less readable. In addition, the === comparison doesn't distinguish between actual and expected values. The operands are just called left and right, because if one were named expected and the other actual, it would be difficult for people to remember which was which. To help with these limitations of assertions, Suite includes a method called expect that can be used as an alternative to assert with ===. To use expect, you place the expected value in parentheses after expect, followed by curly braces containing code that should result in the expected value. For example:

val a = 5
val b = 2
expect(2) {
  a - b
}

In this case, the expected value is 2, and the code being tested is a - b. This expectation will fail, and the detail message in the TestFailedException will read, "Expected 2, but got 3."

Intercepted exceptions

Sometimes you need to test whether a method throws an expected exception under certain circumstances, such as when invalid arguments are passed to the method. You can do this in the JUnit style, like this:

val s = "hi"
try {
  s.charAt(-1)
  fail()
}
catch {
  case _: IndexOutOfBoundsException => // Expected, so continue
}

If charAt throws IndexOutOfBoundsException as expected, control will transfer to the catch case, which does nothing. If, however, charAt fails to throw an exception, the next statement, fail(), will be executed. The fail method always completes abruptly with a TestFailedException, thereby signaling a failed test.

To make this common use case easier to express and read, ScalaTest provides an intercept method. You use it like this:

val s = "hi"
intercept[IndexOutOfBoundsException] {
  s.charAt(-1)
}

This code behaves much like the previous example. If charAt throws an instance of IndexOutOfBoundsException, intercept will return that exception. But if charAt completes normally, or throws a different exception, intercept will complete abruptly with a TestFailedException. The intercept method returns the caught exception so that you can inspect it further if you wish, for example, to ensure that data contained inside the exception has the expected values. Here's an example:

val s = "hi"
val caught =
  intercept[IndexOutOfBoundsException] {
    s.charAt(-1)
  }
assert(caught.getMessage === "String index out of range: -1")

Using other assertions

ScalaTest also supports another style of assertions via its matchers DSL. By mixing in trait ShouldMatchers, you can write suites that look like:

import org.scalatest.Suite
import org.scalatest.matchers.ShouldMatchers

class ExampleSuite extends Suite with ShouldMatchers {

  def testAddition {
    val sum = 1 + 1
    sum should equal (2)
  }

  def testSubtraction {
    val diff = 4 - 1
    diff should equal (3)
  }
}

If you prefer the word "must" to the word "should," you can alternatively mix in trait MustMatchers.

If you are comfortable with assertion mechanisms from other test frameworks, chances are you can use them with ScalaTest. Any assertion mechanism that indicates a failure with an exception can be used as is with ScalaTest. For example, to use the assertEquals methods provided by JUnit or TestNG, simply import them and use them. (You will of course need to include the relevant JAR file for the framework whose assertions you want to use on either the classpath or runpath when you run your tests.) Here's an example in which JUnit's assertions are imported, then used within a ScalaTest suite:

import org.scalatest.Suite
import org.junit.Assert._

class ExampleSuite extends Suite {

  def testAddition {
    val sum = 1 + 1
    assertEquals(2, sum)
  }

  def testSubtraction {
    val diff = 4 - 1
    assertEquals(3, diff)
  }
}

Nested suites

A Suite can refer to a collection of other Suites, which are called nested Suites. Those nested Suites can in turn have their own nested Suites, and so on. Large test suites can be organized, therefore, as a tree of nested Suites. This trait's run method, in addition to invoking its test methods, invokes run on each of its nested Suites.

A List of a Suite's nested Suites can be obtained by invoking its nestedSuites method. If you wish to create a Suite that serves as a container for nested Suites, whether or not it has test methods of its own, simply override nestedSuites to return a List of the nested Suites. Because this is a common use case, ScalaTest provides a convenience Suites class, which takes a variable number of nested Suites as constructor parameters. Here's an example:

import org.scalatest.Suite
import org.scalatest.Suites

class ASuite extends Suite {
  def testA {}
}
class BSuite extends Suite {
  def testB {}
}
class CSuite extends Suite {
  def testC {}
}

class AlphabetSuite extends Suites(
  new ASuite,
  new BSuite,
  new CSuite
)

If you now run AlphabetSuite:

scala> (new AlphabetSuite).execute()

You will see reports printed to the standard output that indicate the nested suites—ASuite, BSuite, and CSuite—were run:

AlphabetSuite:
ASuite:
- testA
BSuite:
- testB
CSuite:
- testC

Note that Runner can discover Suites automatically, so you need not necessarily define nested Suites explicitly. See the documentation for Runner for more information.

The config map

In some cases you may need to pass information to a suite of tests. For example, perhaps a suite of tests needs to grab information from a file, and you want to be able to specify a different filename during different runs. You can accomplish this in ScalaTest by passing the filename in a config map of key-value pairs, which is passed to run as a Map[String, Any]. The values in the config map are called "config objects," because they can be used to configure suites, reporters, and tests.

You can specify a string config object is via the ScalaTest Runner, either via the command line or ScalaTest's ant task. (See the documentation for Runner for information on how to specify config objects on the command line.) The config map is passed to run, runNestedSuites, runTests, and runTest, so one way to access it in your suite is to override one of those methods. If you need to use the config map inside your tests, you can access it from the NoArgTest passed to withFixture, or the OneArgTest passed to withFixture in the traits in the org.scalatest.fixture package. (See the documentation for fixture.Suite for instructions on how to access the config map in tests.)

Ignored tests

Another common use case is that tests must be “temporarily” disabled, with the good intention of resurrecting the test at a later time. ScalaTest provides an Ignore annotation for this purpose. You use it like this:

import org.scalatest.Suite
import org.scalatest.Ignore

class ExampleSuite extends Suite {

  def testAddition {
    val sum = 1 + 1
    assert(sum === 2)
  }

  @Ignore
  def testSubtraction {
    val diff = 4 - 1
    assert(diff === 3)
  }
}

If you run this version of ExampleSuite with:

scala> (new ExampleSuite).run()

It will run only testAddition and report that testSubtraction was ignored. You'll see:

ExampleSuite:
- testAddition
- testSubtraction !!! IGNORED !!!

Ignore is implemented as a tag. The Filter class effectively adds org.scalatest.Ignore to the tagsToExclude Set if it not already in the tagsToExclude set passed to its primary constructor. The only difference between org.scalatest.Ignore and the tags you may define and exclude is that ScalaTest reports ignored tests to the Reporter. The reason ScalaTest reports ignored tests is to encourage ignored tests to be eventually fixed and added back into the active suite of tests.

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.

Although pending tests may be used more often in specification-style suites, such as org.scalatest.FunSpec, you can also use it in Suite, like this:

import org.scalatest.Suite

class ExampleSuite extends Suite {

  def testAddition {
    val sum = 1 + 1
    assert(sum === 2)
  }

  def testSubtraction { pending }
}

If you run this version of ExampleSuite with:

scala> (new ExampleSuite).run()

It will run both tests but report that testSubtraction is pending. You'll see:

ExampleSuite:
- testAddition
- testSubtraction (pending)

Informers

One of the parameters to run 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 Suite's methods will be sufficient, but occasionally you may wish to provide custom information to the Reporter from a test method. For this purpose, you can optionally include an Informer parameter in a test method, and then pass the extra information to the Informer via its apply method. The Informer will then pass the information to the Reporter by sending an InfoProvided event. Here's an example:

import org.scalatest._

class ExampleSuite extends Suite {

  def testAddition(info: Informer) {
    assert(1 + 1 === 2)
    info("Addition seems to work")
  }
}

If you run this Suite from the interpreter, you will see the message included in the printed report:

scala> (new ExampleSuite).run()
ExampleSuite:
- testAddition(Informer)
  + Addition seems to work 

Executing suites in parallel

The run method takes as one of its parameters an optional Distributor. If a Distributor is passed in, this trait's implementation of run puts its nested Suites into the distributor rather than executing them directly. The caller of run is responsible for ensuring that some entity runs the Suites placed into the distributor. The -c command line parameter to Runner, for example, will cause Suites put into the Distributor to be run in parallel via a pool of threads.

Tagging tests

A Suite's tests may be classified into groups by tagging them with string names. When executing a Suite, groups of tests can optionally be included and/or excluded. In this trait's implementation, tags are indicated by annotations attached to the test method. To create a new tag type to use in Suites, simply define a new Java annotation that itself is annotated with the org.scalatest.TagAnnotation annotation. (Currently, for annotations to be visible in Scala programs via Java reflection, the annotations themselves must be written in Java.) For example, to create a tag named SlowAsMolasses, to use to mark slow tests, you would write in Java:

Because of a Scaladoc bug in Scala 2.8, I had to put a space after the at sign in one the target annotation example below. If you want to copy and paste from this example, you'll need to remove the space by hand. - Bill Venners

import java.lang.annotation.*;
import org.scalatest.TagAnnotation

@TagAnnotation
@Retention(RetentionPolicy.RUNTIME)
@ Target({ElementType.METHOD, ElementType.TYPE})
public @interface SlowAsMolasses {}

Given this new annotation, you could place a Suite test method into the SlowAsMolasses group (i.e., tag it as being SlowAsMolasses) like this:

@SlowAsMolasses
def testSleeping { sleep(1000000) }

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.

Shared fixtures

A test fixture is objects or other artifacts (such as files, sockets, database connections, etc.) used by tests to do their work. If a fixture is used by only one test method, then the definitions of the fixture objects can be local to the method, such as the objects assigned to sum and diff in the previous ExampleSuite examples. If multiple methods need to share an immutable fixture, one approach is to assign them to instance variables.

In some cases, however, shared mutable fixture objects may be changed by test methods such that they need to be recreated or reinitialized before each test. Shared resources such as files or database connections may also need to be created and initialized before, and cleaned up after, each test. JUnit 3 offered methods setUp and tearDown for this purpose. In ScalaTest, you can use the BeforeAndAfterEach trait, which will be described later, to implement an approach similar to JUnit's setUp and tearDown, however, this approach usually involves reassigning vars or mutating objects between tests. Before going that route, you may wish to consider some more functional approaches that avoid side effects.

Calling create-fixture methods

One approach is to write one or more create-fixture methods that return a new instance of a needed fixture object (or an holder object containing multiple needed fixture objects) each time it is called. You can then call a create-fixture method at the beginning of each test method that needs the fixture, storing the returned object or objects in local variables. Here's an example:

import org.scalatest.Suite
import collection.mutable.ListBuffer

class ExampleSuite extends Suite {

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

  def testEasy {
    val f = fixture
    f.builder.append("easy!")
    assert(f.builder.toString === "ScalaTest is easy!")
    assert(f.buffer.isEmpty)
    f.buffer += "sweet"
  }

  def testFun {
    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.

Instantiating fixture traits

A related technique is to place the fixture objects in a fixture trait and run your test code in the context of a new anonymous class instance that mixes in the fixture trait, like this:

import org.scalatest.Suite
import collection.mutable.ListBuffer

class ExampleSuite extends Suite {

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

  def testEasy {
    new Fixture {
      builder.append("easy!")
      assert(builder.toString === "ScalaTest is easy!")
      assert(buffer.isEmpty)
      buffer += "sweet"
    }
  }

  def testFun {
    new Fixture {
      builder.append("fun!")
      assert(builder.toString === "ScalaTest is fun!")
      assert(buffer.isEmpty)
    }
  }
}

Mixing in OneInstancePerTest

If every test method requires the same set of mutable fixture objects, one other approach you can take is make them simply vals and mix in trait OneInstancePerTest. If you mix in OneInstancePerTest, each test will be run in its own instance of the Suite, similar to the way JUnit tests are executed. Here's an example:

import org.scalatest.Suite
import org.scalatest.OneInstancePerTest
import collection.mutable.ListBuffer

class ExampleSuite extends Suite with OneInstancePerTest {

  val builder = new StringBuilder("ScalaTest is ")
  val buffer = new ListBuffer[String]

  def testEasy {
    builder.append("easy!")
    assert(builder.toString === "ScalaTest is easy!")
    assert(buffer.isEmpty)
    buffer += "sweet"
  }

  def testFun {
    builder.append("fun!")
    assert(builder.toString === "ScalaTest is fun!")
    assert(buffer.isEmpty)
  }
}

Although the create-fixture, fixture-trait, and OneInstancePerTest approaches take care of setting up a fixture before each test, they don't address the problem of cleaning up a fixture after the test completes. In this situation, you'll need to either use side effects or the loan pattern.

Mixing in BeforeAndAfter

One way to use side effects is to mix in the BeforeAndAfter trait. 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:

import org.scalatest.Suite
import org.scalatest.BeforeAndAfter
import collection.mutable.ListBuffer

class ExampleSuite extends Suite with BeforeAndAfter {

  val builder = new StringBuilder
  val buffer = new ListBuffer[String]

  before {
    builder.append("ScalaTest is ")
  }

  after {
    builder.clear()
    buffer.clear()
  }

  def testEasy {
    builder.append("easy!")
    assert(builder.toString === "ScalaTest is easy!")
    assert(buffer.isEmpty)
    buffer += "sweet"
  }

  def testFun {
    builder.append("fun!")
    assert(builder.toString === "ScalaTest is fun!")
    assert(buffer.isEmpty)
  }
}

Overriding withFixture(NoArgTest)

An alternate way to take care of setup and cleanup via side effects is to override withFixture. Trait Suite's implementation of runTest passes a no-arg test function to withFixture. It is withFixture's responsibility to invoke that test function. Suite's implementation of withFixture simply invokes the function, like this:

// Default implementation
protected def withFixture(test: NoArgTest) {
  test()
}

You can, therefore, override withFixture to perform setup before, and 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. Here's an example:

import org.scalatest.Suite
import collection.mutable.ListBuffer

class ExampleSuite extends Suite {

  val builder = new StringBuilder
  val buffer = new ListBuffer[String]

  override def withFixture(test: NoArgTest) {
    builder.append("ScalaTest is ") // perform setup
    try {
      test() // invoke the test function
    }
    finally {
      builder.clear() // perform cleanup
      buffer.clear()
    }
  }

  def testEasy {
    builder.append("easy!")
    assert(builder.toString === "ScalaTest is easy!")
    assert(buffer.isEmpty)
    buffer += "sweet"
  }

  def testFun {
    builder.append("fun!")
    assert(builder.toString === "ScalaTest is fun!")
    assert(buffer.isEmpty)
    buffer += "clear"
  }
}

Note that the NoArgTest passed to withFixture, in addition to an apply method that executes the test, also includes the test name as well as the config map passed to runTest. Thus you can also use the test name and configuration objects in withFixture.

The reason you should perform cleanup in a finally clause is that withFixture is called by runTest, which expects an exception to be thrown to indicate a failed test. Thus when you invoke the test function inside withFixture, it may complete abruptly with an exception. The finally clause will ensure the fixture cleanup happens as that exception propagates back up the call stack to runTest.

Overriding withFixture(OneArgTest)

To use the loan pattern, you can extend fixture.Suite (from the org.scalatest.fixture package) instead of Suite. Each test in a fixture.Suite 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. Here's an example:

import org.scalatest.fixture
import java.io.FileWriter
import java.io.File

class ExampleSuite extends fixture.Suite {

  final val tmpFile = "temp.txt"

  type FixtureParam = FileWriter

  def withFixture(test: OneArgTest) {

    val writer = new FileWriter(tmpFile) // set up the fixture
    try {
      test(writer) // "loan" the fixture to the test
    }
    finally {
      writer.close() // clean up the fixture
    }
  }

  def testEasy(writer: FileWriter) {
    writer.write("Hello, test!")
    writer.flush()
    assert(new File(tmpFile).length === 12)
  }

  def testFun(writer: FileWriter) {
    writer.write("Hi, test!")
    writer.flush()
    assert(new File(tmpFile).length === 9)
  }
}

For more information, see the documentation for fixture.Suite.

Providing different fixtures to different tests

If different tests in the same Suite require different fixtures, you can combine the previous techniques and provide each test with just the fixture or fixtures it needs. Here's an example in which a StringBuilder and a ListBuffer are provided via fixture traits, and file writer (that requires cleanup) is provided via the loan pattern:

import java.io.FileWriter
import java.io.File
import collection.mutable.ListBuffer
import org.scalatest.Suite

class ExampleSuite extends Suite {

  final val tmpFile = "temp.txt"

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

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

  def withWriter(testCode: FileWriter => Any) {
    val writer = new FileWriter(tmpFile) // set up the fixture
    try {
      testCode(writer) // "loan" the fixture to the test
    }
    finally {
      writer.close() // clean up the fixture
    }
  }

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

  def testReadable { // This test needs the ListBuffer[String] fixture
    new Buffer {
      buffer += ("readable!")
      assert(buffer === List("ScalaTest", "is", "readable!"))
    }
  }

  def testFriendly { // This test needs the FileWriter fixture
    withWriter { writer =>
      writer.write("Hello, user!")
      writer.flush()
      assert(new File(tmpFile).length === 12)
    }
  }

  def testClearAndConcise { // This test needs the StringBuilder and ListBuffer
    new Builder with Buffer {
      builder.append("clear!")
      buffer += ("concise!")
      assert(builder.toString === "ScalaTest is clear!")
      assert(buffer === List("ScalaTest", "is", "concise!"))
    }
  }

  def testComposable { // This test needs all three fixtures
    new Builder with Buffer {
      builder.append("clear!")
      buffer += ("concise!")
      assert(builder.toString === "ScalaTest is clear!")
      assert(buffer === List("ScalaTest", "is", "concise!"))
      withWriter { writer =>
        writer.write(builder.toString)
        writer.flush()
        assert(new File(tmpFile).length === 19)
      }
    }
  }
}

In the previous example, testProductive uses only the StringBuilder fixture, so it just instantiates a new Builder, whereas testReadable uses only the ListBuffer fixture, so it just intantiates a new Buffer. testFriendly needs just the FileWriter fixture, so it invokes withWriter, which prepares and passes a FileWriter to the test (and takes care of closing it afterwords).

Two tests need multiple fixtures: testClearAndConcise needs both the StringBuilder and the ListBuffer, so it instantiates a class that mixes in both fixture traits with new Builder with Buffer. testComposable needs all three fixtures, so in addition to new Builder with Buffer it also invokes withWriter, wrapping just the of the test code that needs the fixture.

Note that in this case, the loan pattern is being implemented via the withWriter method that takes a function, not by overriding fixture.Suite's withFixture(OneArgTest) method. fixture.Suite makes the most sense if all (or at least most) tests need the same fixture, whereas in this Suite only two tests need the FileWriter.

Note also that two test methods, testFriendly and testComposable, are declared as parameterless methods even though they have a side effect. In production code you would normally declare these as empty-paren methods, and call them with empty parentheses, to make it more obvious to readers of the code that they have a side effect. Whether or not a test method has a side effect, however, is a less important distinction than it is for methods in production code. Moreover, test methods are not normally invoked directly by client code, but rather through reflection by running the Suite that contains them, so a lack of parentheses on an invocation of a side-effecting test method would not normally appear in any client code. Given the empty parentheses do not add much value in the test methods case, the recommended style is to simply always leave them off.

In the previous example, the withWriter method passed an object into the tests. Passing fixture objects into tests is generally a good idea when possible, but sometimes a side affect is unavoidable. For example, if you need to initialize a database running on a server across a network, your with-fixture method will likely have nothing to pass. In such cases, simply create a with-fixture method that takes a by-name parameter and performs setup and cleanup via side effects, like this:

def withDataInDatabase(test: => Any) {
  // initialize the database across the network
  try {
    test // "loan" the initialized database to the test
  }
  finally {
    // clean up the database
  }
}

You can then use it like:

def testUserLogsIn {
  withDataInDatabase {
    // test user logging in scenario
  }
}

Composing stackable fixture 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:

import org.scalatest.Suite
import org.scalatest.AbstractSuite
import collection.mutable.ListBuffer

trait Builder extends AbstractSuite { this: Suite =>

  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 AbstractSuite { this: Suite =>

  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 ExampleSuite extends Suite with Builder with Buffer {

  def testEasy {
    builder.append("easy!")
    assert(builder.toString === "ScalaTest is easy!")
    assert(buffer.isEmpty)
    buffer += "sweet"
  }

  def testFun {
    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 Example2Suite extends Suite with Buffer with Builder

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

class Example3Suite 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:

import org.scalatest.Suite
import org.scalatest.BeforeAndAfterEach
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 ExampleSuite extends Suite with Builder with Buffer {

  def testEasy {
    builder.append("easy!")
    assert(builder.toString === "ScalaTest is easy!")
    assert(buffer.isEmpty)
    buffer += "sweet"
  }

  def testFun {
    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.afterAll throws an exception.

One difference to bear in mind between the before-and-after traits and the withFixture methods, is that if a withFixture method completes abruptly with an exception, it is considered a failed test. By contrast, if any of the methods on the before-and-after traits (i.e., before and after of BeforeAndAfter, beforeEach and afterEach of BeforeAndAfterEach, and beforeAll and afterAll of BeforeAndAfterAll) complete abruptly, it is considered a failed suite, which will result in a SuiteAborted event.

Treatment of java.lang.Errors

The Javadoc documentation for java.lang.Error states:

An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch. Most such errors are abnormal conditions.

Because Errors are used to denote serious errors, trait Suite and its subtypes in the ScalaTest API do not always treat a test that completes abruptly with an Error as a test failure, but sometimes as an indication that serious problems have arisen that should cause the run to abort. For example, if a test completes abruptly with an OutOfMemoryError, it will not be reported as a test failure, but will instead cause the run to abort. Because not everyone uses Errors only to represent serious problems, however, ScalaTest only behaves this way for the following exception types (and their subclasses):

• java.lang.annotation.AnnotationFormatError
• java.awt.AWTError
• java.nio.charset.CoderMalfunctionError
• javax.xml.parsers.FactoryConfigurationError
• java.lang.LinkageError
• java.lang.ThreadDeath
• javax.xml.transform.TransformerFactoryConfigurationError
• java.lang.VirtualMachineError

The previous list includes all Errors that exist as part of Java 1.5 API, excluding java.lang.AssertionError. ScalaTest does treat a thrown AssertionError as an indication of a test failure. In addition, any other Error that is not an instance of a type mentioned in the previous list will be caught by the Suite traits in the ScalaTest API and reported as the cause of a test failure.

Although trait Suite and all its subtypes in the ScalaTest API consistently behave this way with regard to Errors, this behavior is not required by the contract of Suite. Subclasses and subtraits that you define, for example, may treat all Errors as test failures, or indicate errors in some other way that has nothing to do with exceptions.

Extensibility

Trait Suite provides default implementations of its methods that should be sufficient for most applications, but many methods can be overridden when desired. Here's a summary of the methods that are intended to be overridden:

• run - override this method to define custom ways to run suites of tests.
• runNestedSuites - override this method to define custom ways to run nested suites.
• runTests - override this method to define custom ways to run a suite's tests.
• runTest - override this method to define custom ways to run a single named test.
• testNames - override this method to specify the Suite's test names in a custom way.
• tags - override this method to specify the Suite's test tags in a custom way.
• nestedSuites - override this method to specify the Suite's nested Suites in a custom way.
• suiteName - override this method to specify the Suite's name in a custom way.
• expectedTestCount - override this method to count this Suite's expected tests in a custom way.

For example, this trait's implementation of testNames performs reflection to discover methods starting with test, and places these in a Set whose iterator returns the names in alphabetical order. If you wish to run tests in a different order in a particular Suite, perhaps because a test named testAlpha can only succeed after a test named testBeta has run, you can override testNames so that it returns a Set whose iterator returns testBeta before testAlpha. (This trait's implementation of run will invoke tests in the order they come out of the testNames Set iterator.)

Alternatively, you may not like starting your test methods with test, and prefer using @Test annotations in the style of Java's JUnit 4 or TestNG. If so, you can override testNames to discover tests using either of these two APIs @Test annotations, or one of your own invention. (This is in fact how org.scalatest.junit.JUnitSuite and org.scalatest.testng.TestNGSuite work.)

Moreover, test in ScalaTest does not necessarily mean test method. A test can be anything that can be given a name, that starts and either succeeds or fails, and can be ignored. In org.scalatest.FunSuite, for example, tests are represented as function values. This approach might look foreign to JUnit users, but may feel more natural to programmers with a functional programming background. To facilitate this style of writing tests, FunSuite overrides testNames, runTest, and run such that you can define tests as function values.

You can also model existing JUnit 3, JUnit 4, or TestNG tests as suites of tests, thereby incorporating tests written in Java into a ScalaTest suite. The "wrapper" classes in packages org.scalatest.junit and org.scalatest.testng exist to make this easy. No matter what legacy tests you may have, it is likely you can create or use an existing Suite subclass that allows you to model those tests as ScalaTest suites and tests and incorporate them into a ScalaTest suite. You can then write new tests in Scala and continue supporting older tests in Java.