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ScalaTest + JMock

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ScalaTest + JMock

The ScalaTest + JMock integration library makes it fun and easy to use JMock with ScalaTest. To use ScalaTest + JMock, please add the following to your SBT project dependency:

libraryDependencies += "org.scalatestplus" %% "jmock-2-8" % "3.2.9.0" % "test"

For maven you can use:

<dependency>
    <groupId>org.scalatestplus</groupId>
    <artifactId>jmock-2-8_2.13</artifactId>
    <version>3.2.9.0</version>
    <scope>test</scope>
</dependency>    

JMockCycle class, which wraps and manages the lifecycle of a single org.jmock.Mockery context object, provides some basic syntax sugar for using JMock in Scala.

Using the JMock API directly, you first need a Mockery context object:

val context = new Mockery

JMockCycle uses jMock's ClassImposterizer to support mocking of classes, so the following line would also be needed if you wanted that functionality as well:

context.setImposteriser(ClassImposteriser.INSTANCE)

When using this class, you would instead create an instance of this class (which will create and wrap a Mockery object) and import its members, like this:

val cycle = new JMockCycle
import cycle._

Using the JMock API directly, you would create a mock object like this:

val mockCollaborator = context.mock(classOf[Collaborator])

Having imported the members of an instance of this class, you can shorten that to:

val mockCollaborator = mock[Collaborator]

After creating mocks, you set expectations on them, using syntax like this:

context.checking(
  new Expectations() {
    oneOf (mockCollaborator).documentAdded("Document")
    exactly(3).of (mockCollaborator).documentChanged("Document")
  }
)

Having imported the members of an instance of this class, you can shorten this step to:

expecting { e => import e._
  oneOf (mockCollaborator).documentAdded("Document")
  exactly(3).of (mockCollaborator).documentChanged("Document")
}

The expecting method will create a new Expectations object, pass it into the function you provide, which sets the expectations. After the function returns, the expecting method will pass the Expectations object to the checking method of its internal Mockery context.

The expecting method passes an instance of class org.scalatestplus.jmock.JMockExpectations to the function you pass into expectations. JMockExpectations extends org.jmock.Expectations and adds several overloaded withArg methods. These withArg methods simply invoke corresponding with methods on themselves. Because with is a keyword in Scala, to invoke these directly you must surround them in back ticks, like this:

oneOf (mockCollaborator).documentAdded(`with`("Document"))

By importing the members of the passed JMockExpectations object, you can instead call withArg with no back ticks needed:

oneOf (mockCollaborator).documentAdded(withArg("Document"))

Once you've set expectations on the mock objects, when using the JMock API directly, you use the mock, then invoke assertIsSatisfied on the Mockery context to make sure the mock was used in accordance with the expectations you set on it. Here's how that looks:

classUnderTest.addDocument("Document", new Array[Byte](0))
classUnderTest.addDocument("Document", new Array[Byte](0))
classUnderTest.addDocument("Document", new Array[Byte](0))
classUnderTest.addDocument("Document", new Array[Byte](0))
context.assertIsSatisfied()

This class enables you to use the following, more declarative syntax instead:

whenExecuting {
  classUnderTest.addDocument("Document", new Array[Byte](0))
  classUnderTest.addDocument("Document", new Array[Byte](0))
  classUnderTest.addDocument("Document", new Array[Byte](0))
  classUnderTest.addDocument("Document", new Array[Byte](0))
}

The whenExecuting method will execute the passed function, then invoke assertIsSatisfied on its internal Mockery context object.

To summarize, here's what a typical test using JMockCycle looks like:

val cycle = new JMockCycle
import cycle._

val mockCollaborator = mock[Collaborator]
expecting { e => import e._ oneOf (mockCollaborator).documentAdded("Document") exactly(3).of (mockCollaborator).documentChanged("Document") }
whenExecuting { classUnderTest.addDocument("Document", new Array[Byte](0)) classUnderTest.addDocument("Document", new Array[Byte](0)) classUnderTest.addDocument("Document", new Array[Byte](0)) classUnderTest.addDocument("Document", new Array[Byte](0)) }

ScalaTest also provides a JMockCycleFixture trait, which will pass a new JMockCycle into each test that needs one.

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