Concjunit unit testing for concurrent programs
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ConcJUnit: Unit Testing for Concurrent Programs. PPPJ 2009 Mathias Ricken and Robert Cartwright Rice University August 28, 2009. Concurrency in Practice. Brian Goetz, Java Concurrency in Practice , Addison-Wesley, 2006. Concurrency Practiced Badly. *1. Unit Tests…. Occur early

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ConcJUnit:Unit Testing for Concurrent Programs

PPPJ 2009

Mathias Ricken and Robert Cartwright

Rice University

August 28, 2009


Concurrency in Practice

Brian Goetz, Java Concurrency in Practice, Addison-Wesley, 2006


Concurrency Practiced Badly

*1


Unit Tests…

  • Occur early

  • Automate testing

  • Keep the shared repository clean

  • Serve as documentation

  • Prevent bugs from reoccurring

  • Allow safe refactoring


Existing Unit Testing Frameworks

  • JUnit, TestNG

  • Don’t detect test failures in child threads

  • Don’t ensure that child threads terminate

  • Tests that should fail may succeed


ConcJUnit

  • Replacement for JUnit

    • Backward compatible, just replace junit.jar file

    • Available at www.concutest.org

  • Detects failures in all threads

  • Warns if child threads outlive main thread

  • Warns if child threads are not joined


Sample JUnit Tests

publicclass Test extends TestCase {

public void testException() {

thrownew RuntimeException("booh!");

}

public void testAssertion() {

assertEquals(0, 1);

}

}

Both tests fail.

Both tests fail.

}

if (0!=1)

throw new AssertionFailedError();


JUnit Test with Child Thread

Main thread

publicclass Test extends TestCase {

public void testException() {

new Thread() {

public void run() {

thrownew RuntimeException("booh!");

}

}.start();

}

}

new Thread() {

public void run() {

thrownew RuntimeException("booh!");

}

}.start();

thrownew RuntimeException("booh!");

Child thread

end of test

spawns

Main thread

success!

uncaught!

Child thread


JUnit Test with Child Thread

publicclass Test extends TestCase {

public void testException() {

new Thread() {

public void run() {

thrownew RuntimeException("booh!");

}

}.start();

}

}

new Thread() {

public void run() {

thrownew RuntimeException("booh!");

}

}.start();

thrownew RuntimeException("booh!");

Uncaught exception, test should fail but does not!

  • By default, no uncaught exception handler installed for child threads


Changes to JUnit (1 of 3)

  • Thread group with exception handler

    • JUnit test runs in a separate thread, not main thread

    • Child threads are created in same thread group

    • When test ends, check if handler was invoked

      Reasoning:

  • Uncaught exceptions in all threads must cause failure


JUnit Test with Child Thread

publicclass Test extends TestCase {

public void testException() {

new Thread() {

public void run() {

thrownew RuntimeException("booh!");

}

}.start();

}

}

new Thread() {

public void run() {

thrownew RuntimeException("booh!");

}

}.start();

thrownew RuntimeException("booh!");

spawns and joins

resumes

Main thread

failure!

check group’s handler

end of test

Test thread

uncaught!

invokes group’s handler

Child thread


Child Thread Outlives Parent

publicclass Test extends TestCase {

public void testException() {

new Thread() {

public void run() {

thrownew RuntimeException("booh!");

}

}.start();

}

}

new Thread() {

public void run() {

thrownew RuntimeException("booh!");

}

}.start();

thrownew RuntimeException("booh!");

check group’s handler

Main thread

success!

Test thread

Too late!

end of test

uncaught!

invokes group’s handler

Child thread


Changes to JUnit (2 of 3)

  • Check for living child threads after test ends

    Reasoning:

  • Uncaught exceptions in all threads must cause failure

  • If the test is declared a success before all child threads have ended, failures may go unnoticed

  • Therefore, all child threads must terminate before test ends


Check for Living Child Threads

publicclass Test extends TestCase {

public void testException() {

new Thread() {

public void run() {

thrownew RuntimeException("booh!");

}

}.start();

}

}

new Thread() {

public void run() {

thrownew RuntimeException("booh!");

}

}.start();

thrownew RuntimeException("booh!");

check for living

child threads

check group’s handler

Main thread

failure!

Test thread

end of test

uncaught!

invokes group’s handler

Child thread


Correctly Written Test

publicclass Test extends TestCase {

public void testException() {

Thread t = new Thread() {

public void run() { /* child thread */ }

};

t.start();

t.join();

}

}

Thread t = new Thread() {

public void run() { /* child thread */ }

};

t.start();

t.join(); // wait until child thread has ended

/* child thread */

check for living

child threads

check group’s handler

Main thread

success!

Test thread

end of test

Child thread

*4


Changes to JUnit (3 of 3)

  • Check if any child threads were not joined

    Reasoning:

  • All child threads must terminate before test ends

  • Without join() operation, a test may get “lucky”

  • Require all child threads to be joined


Fork/Join Model

  • Parent thread joins with each of its child threads

  • May be too limited for a general-purpose programming language

Main thread

Child thread 1

Child thread 2


Example of Other Join Models

  • Chain of child threads guaranteed to outlive parent

  • Main thread joins with last thread of chain

Main thread

Child thread 1

Child thread 2

Child thread 3


Generalize to Join Graph

  • Threads as nodes; edges to joined thread

  • Test is well-formed as long as all threads are reachable from main thread

Main thread

MT

Child thread 1

CT1

Child thread 2

CT2

Child thread 3

CT3


Unreachable Nodes

  • An unreachable node has not been joined

    • Child thread may outlive the test

Main thread

MT

Child thread 1

CT1

Child thread 2

CT2


Constructing the Graph: start()

// in mainThreadchildThread.start();

  • Add node for childThread

main Thread

MT

childThread

CT


Constructing the Graph : join()

// in mainThreadchildThread.join();

  • When leaving join(), add edge from mainThread to childThread

main Thread

MT

child

Thread

CT

*3


Modifying the Java Runtime

  • Changing Thread.start()and join()

    • Need to modify Java Runtime Library

    • Utility to process user’s rt.jar file

    • Put new jar file on boot classpath:-Xbootclasspath/p:newrt.jar

  • Still works without modified Thread class

    • Just does not emit “lucky” warnings


Evaluation

  • JFreeChart

    • All tests passed; tests are not concurrent

  • DrJava: 900 unit tests

    • Passed:880

    • No join: 1

    • Lucky: 18

    • Timeout: 1

    • Runtime overhead: ~1 percent


Limitations

  • Only checks chosen schedule

    • A different schedule may still fail

  • Example:

    Thread t = new Thread(…);if (nondeterministic()) t.join();

*2


Future Work

  • Insert sleeps or yields in critical code locations

    • Example:If a notify() is delayed, a wait() may time out.

    • Can detect a number of sample problems

  • Run tests several times on build server

  • Record schedule, replay if test fails

    • Makes failures reproducible if found

*5


ConcJUnit Demo


Thank you!

www.concutest.org


Notes


Notes

  • Probably not caused by concurrency problems; just a metaphor. ←

  • Also cannot detect uncaught exceptions in a program’s uncaught exception handler (JLS limitation) ←

  • Only add edge if joined thread is really dead; do not add if join ended spuriously. ←


Spurious Wakeup

publicclass Test extends TestCase {

public void testException() {

Thread t = new Thread(new Runnable() {

public void run() {

thrownew RuntimeException("booh!");

}

});

t.start();

while(t.isAlive()) {

try { t.join(); }

catch(InterruptedException ie) { }

}

}

}

Thread t = new Thread(new Runnable() {

public void run() {

thrownew RuntimeException("booh!");

}

});

t.start();

while(t.isAlive()) {

try { t.join(); }

catch(InterruptedException ie) { }

}

thrownew RuntimeException("booh!");

Loop since join() may end spuriously


Notes

  • Have not studied probabilities or durations for sleeps/yields:One inserted delay may negatively impact a second inserted delayExample: If both notify() and wait() are delayed. ←


Extra Slides


JUnit Test with Child Thread

publicclass Test extends TestCase {

public void testException() {

new Thread(new Runnable() {

public void run() {

thrownew RuntimeException("booh!");

}

}).start();

}

}

new Thread(new Runnable() {

public void run() {

thrownew RuntimeException("booh!");

}

}).start();

thrownew RuntimeException("booh!");

invokes

checks

TestGroup’s Uncaught Exception Handler


Join with All Offspring Threads

  • Main thread joins with all offspring threads, regardless of what thread spawned them

Main thread

Child thread 1

Child thread 2


Join Graph Examples

Main thread

MT

Child thread 1

CT1

Child thread 2

CT2

Main thread

MT

Child thread 1

CT1

Child thread 2

CT2


Thread Creation Context

  • In Thread.start()– also record stack trace of Thread.currentThread(

    • Easy to find source code of a child thread that is not joined

    • Also available for uncaught exception stack traces


Creation Context Example

class Helper extends Thread {

void m() { Assert.fail(); }

public void run() { m(); }

}

class Main {

void foo() {

// which one?

new Helper().start();

new Helper().start();

// ...

}

}

AssertionError:

at Helper.m(Helper.java:2)

at Helper.run(Helper.java:3)

Started at:

at Main.foo(Main.java:4)

at Main.bar(Main.java:15)

at Main.main(Main.java:25)


Many Thanks To…

  • My advisor

    • Corky Cartwright

  • My committee members

    • Walid Taha

    • David Scott

    • Bill Scherer

  • NSF and Texas ATP

    • For providing partial funding


Concurrency Problems NotFound During Testing

*1


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