org.scalatest.concurrent.Conductor

Class that facilitates the testing of classes, traits, and libraries designed to be used by multiple threads concurrently.

A Conductor conducts a multi-threaded scenario by maintaining a clock of "beats." Beats are numbered starting with 0. You can ask aConductor to run threads that interact with the class, trait, or library (the subject) you want to test. A thread can call the Conductor'swaitForBeat method, which will cause the thread to block until that beat has been reached. The Conductor will advance the beat only when all threads participating in the test are blocked. By tying the timing of thread activities to specific beats, you can write tests for concurrent systems that have deterministic interleavings of threads.

A Conductor object has a three-phase lifecycle. It begins its life in the setup phase. During this phase, you can start threads by invoking the thread method on the Conductor. When conduct is invoked on a Conductor, it enters the conducting phase. During this phase it conducts the one multi-threaded scenario it was designed to conduct. After all participating threads have exited, either by returning normally or throwing an exception, the conduct method will complete, either by returning normally or throwing an exception. As soon as the conduct method completes, the Conductorenters its defunct phase. Once the Conductor has conducted a multi-threaded scenario, it is defunct and can't be reused. To run the same test again, you'll need to create a new instance of Conductor.

Here's an example of the use of Conductor to test the ArrayBlockingQueueclass from java.util.concurrent:

import org.scalatest.fixture.FunSuite
import org.scalatest.matchers.ShouldMatchers
import java.util.concurrent.ArrayBlockingQueue
class ArrayBlockingQueueSuite extends FunSuite with ShouldMatchers {
  test("calling put on a full queue blocks the producer thread") {
    val conductor = new Conductor
    import conductor._
    val buf = new ArrayBlockingQueue[Int](1)
    thread("producer") {
      buf put 42
      buf put 17
      beat should be (1)
    }
    thread("consumer") {
      waitForBeat(1)
      buf.take should be (42)
      buf.take should be (17)
    }
    whenFinished {
      buf should be ('empty)
    }
  }
}

When the test shown is run, it will create one thread named producer and another namedconsumer. The producer thread will eventually execute the code passed as a by-name parameter to thread("producer"):

buf put 42
buf put 17
beat should be (1)

Similarly, the consumer thread will eventually execute the code passed as a by-name parameter to thread("consumer"):

waitForBeat(1)
buf.take should be (42)
buf.take should be (17)

The thread calls create the threads and starts them, but they will not immediately execute the by-name parameter passed to them. They will first block, waiting for the Conductorto give them a green light to proceed.

The next call in the test is whenFinished. This method will first call conduct on the Conductor, which will wait until all threads that were created (in this case, producer and consumer) are at the "starting line", i.e., they have all started and are blocked, waiting on the green light. The conduct method will then give these threads the green light and they will all start executing their blocks concurrently.

When the threads are given the green light, the beat is 0. The first thing the producer thread does is put 42 in into the queue. As the queue is empty at this point, this succeeds. The producer thread next attempts to put a 17 into the queue, but because the queue has size 1, this can't succeed until the consumer thread has read the 42 from the queue. This hasn't happened yet, so producer blocks. Meanwhile, the consumer thread's first act is to call waitForBeat(1). Because the beat starts out at 0, this call will block the consumer thread. As a result, once the producer thread has executed buf put 17 and the consumer thread has executedwaitForBeat(1), both threads will be blocked.

The Conductor maintains a clock that wakes up periodically and checks to see if all threads participating in the multi-threaded scenario (in this case, producer and consumer) are blocked. If so, it increments the beat. Thus sometime later the beat will be incremented, from 0 to 1. Because consumer was waiting for beat 1, it will wake up (i.e., the waitForBeat(1) call will return) and execute the next line of code in its block, buf.take should be (42). This will succeed, because the producer thread had previously (during beat 0) put 42 into the queue. This act will also make producer runnable again, because it was blocked on the second put, which was waiting for another thread to read that 42.

Now both threads are unblocked and able to execute their next statement. The order is non-deterministic, and can even be simultaneous if running on multiple cores. If the consumer thread happens to execute buf.take should be (17) first, it will block (buf.take will not return), because the queue is at that point empty. At some point later, the producer thread will execute buf put 17, which will unblock the consumer thread. Again both threads will be runnable and the order non-deterministic and possibly simulataneous. The producer thread may charge ahead and run its next statement, beat should be (1). This will succeed because the beat is indeed 1 at this point. As this is the last statement in the producer's block, the producer thread will exit normally (it won't throw an exception). At some point later the consumer thread will be allowed to complete its last statement, the buf.take call will return 17. The consumer thread will execute 17 should be (17). This will succeed and as this was the last statement in its block, the consumer will return normally.

If either the producer or consumer thread had completed abruptbly with an exception, the conduct method (which was called by whenFinished) would have completed abruptly with an exception to indicate the test failed. However, since both threads returned normally, conduct will return. Because conduct doesn't throw an exception, whenFinished will execute the block of code passed as a by-name parameter to it: buf should be ('empty). This will succeed, because the queue is indeed empty at this point. The whenFinished method will then return, and because the whenFinished call was the last statement in the test and it didn't throw an exception, the test completes successfully.

This test tests ArrayBlockingQueue, to make sure it works as expected. If there were a bug in ArrayBlockingQueuesuch as a put called on a full queue didn't block, but instead overwrote the previous value, this test would detect it. However, if there were a bug in ArrayBlockingQueue such that a call to take called on an empty queue never blocked and always returned 0, this test might not detect it. The reason is that whether the consumer thread will ever calltake on an empty queue during this test is non-deterministic. It depends on how the threads get scheduled during beat 1. What is deterministic in this test, because the consumer thread blocks during beat 0, is that the producer thread will definitely attempt to write to a full queue. To make sure the other scenario is tested, you'd need a different test:

test("calling take on an empty queue blocks the consumer thread") {
  val conductor = new Conductor
  import conductor._
  val buf = new ArrayBlockingQueue[Int](1)
  thread("producer") {
    waitForBeat(1)
    buf put 42
    buf put 17
  }
  thread("consumer") {
    buf.take should be (42)
    buf.take should be (17)
    beat should be (1)
  }
  whenFinished {
    buf should be ('empty)
  }
}

In this test, the producer thread will block, waiting for beat 1. The consumer thread will invoke buf.takeas its first act. This will block, because the queue is empty. Because both threads are blocked, the Conductorwill at some point later increment the beat to 1. This will awaken the producer thread. It will return from itswaitForBeat(1) call and execute buf put 42. This will unblock the consumer thread, which will take the 42, and so on.

The problem that Conductor is designed to address is the difficulty, caused by the non-deterministic nature of thread scheduling, of testing classes, traits, and libraries that are intended to be used by multiple threads. If you just create a test in which one thread reads from an ArrayBlockingQueue and another writes to it, you can't be sure that you have tested all possible interleavings of threads, no matter how many times you run the test. The purpose of Conductoris to enable you to write tests with deterministic interleavings of threads. If you write one test for each possible interleaving of threads, then you can be sure you have all the scenarios tested. The two tests shown here, for example, ensure that both the scenario in which a producer thread tries to write to a full queue and the scenario in which a consumer thread tries to take from an empty queue are tested.

Class Conductor was inspired by theMultithreadedTC project, created by Bill Pugh and Nat Ayewah of the University of Maryland.

Although useful, bear in mind that a Conductor's results are not guaranteed to be accurate 100% of the time. The reason is that it uses java.lang.Thread's getState method to decide when to advance the beat. This kind of use is advised against in the Javadoc documentation forgetState, which says, "This method is designed for use in monitoring of the system state, not for synchronization." In short, sometimes the return value of <code>getState</code may be inacurrate, which in turn means that sometimes a Conductor may decide to advance the beat too early.

attributes: final

linear super types: AnyRef, Any

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Instance constructors

new Conductor ()

Value Members

def != (arg0: AnyRef) : Boolean

attributes: final
definition classes: AnyRef
def != (arg0: Any) : Boolean

o != arg0 is the same as !(o == (arg0)).

o != arg0 is the same as !(o == (arg0)).
arg0
the object to compare against this object for dis-equality.
returns
false if the receiver object is equivalent to the argument; true otherwise.
attributes: final
definition classes: Any
def ## () : Int

attributes: final
definition classes: AnyRef → Any
def $asInstanceOf [T0] () : T0

attributes: final
definition classes: AnyRef
def $isInstanceOf [T0] () : Boolean

attributes: final
definition classes: AnyRef
def == (arg0: AnyRef) : Boolean

o == arg0 is the same as if (o eq null) arg0 eq null else o.equals(arg0).

o == arg0 is the same as if (o eq null) arg0 eq null else o.equals(arg0).
arg0
the object to compare against this object for equality.
returns
true if the receiver object is equivalent to the argument; false otherwise.
attributes: final
definition classes: AnyRef
def == (arg0: Any) : Boolean

o == arg0 is the same as o.equals(arg0).

o == arg0 is the same as o.equals(arg0).
arg0
the object to compare against this object for equality.
returns
true if the receiver object is equivalent to the argument; false otherwise.
attributes: final
definition classes: Any
def asInstanceOf [T0] : T0

This method is used to cast the receiver object to be of type T0.

This method is used to cast the receiver object to be of type T0.
Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression1.asInstanceOf[String] will throw a ClassCastException at runtime, while the expressionList(1).asInstanceOf[List[String]] will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested typed.
returns
the receiver object.
attributes: final
definition classes: Any
def beat : Int

The current value of the thread clock.

The current value of the thread clock.
returns
the current beat value
def clone () : AnyRef

This method creates and returns a copy of the receiver object.

This method creates and returns a copy of the receiver object.
The default implementation of the clone method is platform dependent.
returns
a copy of the receiver object.
attributes: protected
definition classes: AnyRef
def conduct (clockPeriod: Int, timeout: Int) : Unit

Conducts a multithreaded test with the specified clock period (in milliseconds) and timeout (in seconds).

Conducts a multithreaded test with the specified clock period (in milliseconds) and timeout (in seconds).
A Conductor instance maintains an internal clock, which will wake up periodically and check to see if it should advance the beat, abort the test, or go back to sleep. It sleeps clockPeriod milliseconds each time. It will abort the test if either deadlock is suspected or the beat has not advanced for the number of seconds specified as timeout. Suspected deadlock will be declared if for some number of consecutive clock cycles, all test threads are in the BLOCKED orWAITING states and none of them are waiting for a beat.
clockPeriod
The period (in ms) the clock will sleep each time it sleeps
timeout
The maximum allowed time between successive advances of the beat. If this time is exceeded, the Conductor will abort the test.
def conduct () : Unit

Conducts a multithreaded test with a default clock period of 10 milliseconds and default run limit of 5 seconds.

Conducts a multithreaded test with a default clock period of 10 milliseconds and default run limit of 5 seconds.
def conductingHasBegun : Boolean

Indicates whether either of the two overloaded conduct methods have been invoked.

Indicates whether either of the two overloaded conduct methods have been invoked.
This method returns true if either conduct method has been invoked. Theconduct method may have returned or not. (In other words, a trueresult from this method does not mean the conduct method has returned, just that it was already been invoked and,therefore, the multi-threaded scenario it conducts has definitely begun.)
def eq (arg0: AnyRef) : Boolean

This method is used to test whether the argument (arg0) is a reference to the receiver object (this).

This method is used to test whether the argument (arg0) is a reference to the receiver object (this).
The eq method implements an [http://en.wikipedia.org/wiki/Equivalence_relation equivalence relation] on non-null instances of AnyRef: * It is reflexive: for any non-null instance x of type AnyRef, x.eq(x) returns true. * It is symmetric: for any non-null instances x and y of type AnyRef, x.eq(y) returns true if and only if y.eq(x) returns true. * It is transitive: for any non-null instances x, y, and z of type AnyRef if x.eq(y) returns true and y.eq(z) returns true, then x.eq(z) returns true.
Additionally, the eq method has three other properties. * It is consistent: for any non-null instances x and y of type AnyRef, multiple invocations of x.eq(y) consistently returns true or consistently returns false. * For any non-null instance x of type AnyRef, x.eq(null) and null.eq(x) returns false. * null.eq(null) returns true.
When overriding the equals or hashCode methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2), they should be equal to each other (o1 == o2) and they should hash to the same value (o1.hashCode == o2.hashCode).
arg0
the object to compare against this object for reference equality.
returns
true if the argument is a reference to the receiver object; false otherwise.
attributes: final
definition classes: AnyRef
def equals (arg0: Any) : Boolean

This method is used to compare the receiver object (this) with the argument object (arg0) for equivalence.

This method is used to compare the receiver object (this) with the argument object (arg0) for equivalence.
The default implementations of this method is an [http://en.wikipedia.org/wiki/Equivalence_relation equivalence relation]: * It is reflexive: for any instance x of type Any, x.equals(x) should return true. * It is symmetric: for any instances x and y of type Any, x.equals(y) should return true if and only if y.equals(x) returns true. * It is transitive: for any instances x, y, and z of type AnyRef if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
If you override this method, you should verify that your implementation remains an equivalence relation. Additionally, when overriding this method it is often necessary to override hashCode to ensure that objects that are "equal" (o1.equals(o2) returns true) hash to the same scala.Int (o1.hashCode.equals(o2.hashCode)).
arg0
the object to compare against this object for equality.
returns
true if the receiver object is equivalent to the argument; false otherwise.
definition classes: AnyRef → Any
def finalize () : Unit

This method is called by the garbage collector on the receiver object when garbage collection determines that there are no more references to the object.

This method is called by the garbage collector on the receiver object when garbage collection determines that there are no more references to the object.
The details of when and if the finalize method are invoked, as well as the interaction between finalizeand non-local returns and exceptions, are all platform dependent.
attributes: protected
definition classes: AnyRef
def getClass () : java.lang.Class[_]

Returns a representation that corresponds to the dynamic class of the receiver object.

Returns a representation that corresponds to the dynamic class of the receiver object.
The nature of the representation is platform dependent.
returns
a representation that corresponds to the dynamic class of the receiver object.
attributes: final
definition classes: AnyRef
def hashCode () : Int

Returns a hash code value for the object.

Returns a hash code value for the object.
The default hashing algorithm is platform dependent.
Note that it is allowed for two objects to have identical hash codes (o1.hashCode.equals(o2.hashCode)) yet not be equal (o1.equals(o2) returns false). A degenerate implementation could always return 0. However, it is required that if two objects are equal (o1.equals(o2) returns true) that they have identical hash codes (o1.hashCode.equals(o2.hashCode)). Therefore, when overriding this method, be sure to verify that the behavior is consistent with the equals method.
returns
the hash code value for the object.
definition classes: AnyRef → Any
def isConductorFrozen : Boolean

Indicates whether the conductor has been frozen.

Indicates whether the conductor has been frozen.
Note: The only way a thread can freeze the conductor is by calling withConductorFrozen.
def isInstanceOf [T0] : Boolean

This method is used to test whether the dynamic type of the receiver object is T0.

This method is used to test whether the dynamic type of the receiver object is T0.
Note that the test result of the test is modulo Scala's erasure semantics. Therefore the expression1.isInstanceOf[String] will return false, while the expression List(1).isInstanceOf[List[String]] will return true. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested typed.
returns
true if the receiver object is an instance of erasure of type T0; false otherwise.
attributes: final
definition classes: Any
def ne (arg0: AnyRef) : Boolean

o.ne(arg0) is the same as !(o.eq(arg0)).

o.ne(arg0) is the same as !(o.eq(arg0)).
arg0
the object to compare against this object for reference dis-equality.
returns
false if the argument is not a reference to the receiver object; true otherwise.
attributes: final
definition classes: AnyRef
def notify () : Unit

Wakes up a single thread that is waiting on the receiver object's monitor.

Wakes up a single thread that is waiting on the receiver object's monitor.
attributes: final
definition classes: AnyRef
def notifyAll () : Unit

Wakes up all threads that are waiting on the receiver object's monitor.

Wakes up all threads that are waiting on the receiver object's monitor.
attributes: final
definition classes: AnyRef
def synchronized [T0] (arg0: T0) : T0

attributes: final
definition classes: AnyRef
def thread (name: String)(fun: ⇒ Unit) : Thread

Creates a new thread with the specified name that will execute the specified function.

Creates a new thread with the specified name that will execute the specified function.
This method may be safely called by any thread.
name
the name of the newly created thread
fun
the function to be executed by the newly created thread
returns
the newly created thread
def thread (fun: ⇒ Unit) : Thread

Creates a new thread that will execute the specified function.

Creates a new thread that will execute the specified function.
The name of the thread will be of the form Conductor-Thread-N, where N is some integer.
This method may be safely called by any thread.
fun
the function to be executed by the newly created thread
returns
the newly created thread
def toString () : String

Returns a string representation of the object.

Returns a string representation of the object.
The default representation is platform dependent.
returns
a string representation of the object.
definition classes: AnyRef → Any
def wait () : Unit

attributes: final
definition classes: AnyRef
def wait (arg0: Long, arg1: Int) : Unit

attributes: final
definition classes: AnyRef
def wait (arg0: Long) : Unit

attributes: final
definition classes: AnyRef
def waitForBeat (beat: Int) : Unit

Blocks the current thread until the thread beat reaches the specified value, at which point the current thread will be unblocked.

Blocks the current thread until the thread beat reaches the specified value, at which point the current thread will be unblocked.
beat
the tick value to wait for
def whenFinished (fun: ⇒ Unit) : Unit

Invokes conduct and after conduct method returns, if conduct returns normally (i.

Invokes conduct and after conduct method returns, if conduct returns normally (i.e., without throwing an exception), invokes the passed function.
If conduct completes abruptly with an exception, this method will complete abruptly with the same exception and not execute the passed function.
This method must be called by the thread that instantiated this Conductor, and that same thread will invoke conduct and, if it returns noramlly, execute the passed function.
Because whenFinished invokes conduct, it can only be invoked once on a Conductor instance. As a result, if you need to pass a block of code to whenFinished it should be the last statement of your test. If you don't have a block of code that needs to be run once all the threads have finished successfully, then you can simply invoke conduct and never invokewhenFinished.
fun
the function to execute after conduct call returns
def withConductorFrozen [T] (fun: ⇒ T) : Unit

Executes the passed function with the Conductor frozen so that it won't advance the clock.

Executes the passed function with the Conductor frozen so that it won't advance the clock.
While the Conductor is frozen, the beat will not advance. Once the passed function has completed executing, the Conductor will be unfrozen so that the beat will advance when all threads are blocked, as normal.
fun
the function to execute while the Conductor is frozen.

Inherited from AnyRef

Inherited from Any

Conductor

class Conductor extends AnyRef

Instance constructors

new Conductor ()

Value Members

def != (arg0: AnyRef) : Boolean

def != (arg0: Any) : Boolean

def ## () : Int

def $asInstanceOf [T0] () : T0

def $isInstanceOf [T0] () : Boolean

def == (arg0: AnyRef) : Boolean

def == (arg0: Any) : Boolean

def asInstanceOf [T0] : T0

def beat : Int

def clone () : AnyRef

def conduct (clockPeriod: Int, timeout: Int) : Unit

def conduct () : Unit

def conductingHasBegun : Boolean

def eq (arg0: AnyRef) : Boolean

def equals (arg0: Any) : Boolean

def finalize () : Unit

def getClass () : java.lang.Class[_]

def hashCode () : Int

def isConductorFrozen : Boolean

def isInstanceOf [T0] : Boolean

def ne (arg0: AnyRef) : Boolean

def notify () : Unit

def notifyAll () : Unit

def synchronized [T0] (arg0: T0) : T0

def thread (name: String)(fun: ⇒ Unit) : Thread

def thread (fun: ⇒ Unit) : Thread

def toString () : String

def wait () : Unit

def wait (arg0: Long, arg1: Int) : Unit

def wait (arg0: Long) : Unit

def waitForBeat (beat: Int) : Unit

def whenFinished (fun: ⇒ Unit) : Unit

def withConductorFrozen [T] (fun: ⇒ T) : Unit

Inherited from AnyRef

Inherited from Any