Test-First Java Concurrency for the Classroom

SIGCSE 2010

Mathias Ricken and Robert Cartwright

Rice University

March 12, 2009

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Two Trends

Test-driven development Concurrent programming

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

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Unit Testing Benefits

• Occurs early

• Automates testing

• Keeps the shared repository clean

• Prevents bugs from reoccurring

• Allows safe refactoring

• Serves as documentation

4

Unit Testing in Assignments

• Hand out test cases to students– Improves confidence and understanding

• Instill good practices– Require students to extend test suites

• Automated grading– Part graded automatically, part by hand

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Moore’s Law Requires Concurrency

Adopted fromSutter 2009

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Concurrency Is Difficult

Unit testing not effective in multi-threaded programs

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Existing Unit Testing Frameworks

• JUnit, TestNG

• Don’t detect test failures in threads other than main thread– Failures in event thread not detected either

• Don’t ensure that other threads terminate

• Tests that should fail may succeed

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Sample JUnit Tests

public class SimpleTest extends TestCase { public void testException() { throw new RuntimeException("booh!"); } public void testAssertion() { assertEquals(0, 1); }}

if (0!=1) throw new AssertionFailedError();

}} Both tests fail.

Both tests fail.

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JUnit Test with Child Thread

public class SimpleTest extends TestCase { public void testException() { new Thread() { public void run() { throw new RuntimeException("booh!"); } }.start(); }}

new Thread() { public void run() { throw new RuntimeException("booh!"); }}.start();

throw new RuntimeException("booh!");

Main thread

Child thread

Main thread

Child thread

spawns

uncaught!

end of test

success!

Uncaught exception, test should fail but

does not!

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ConcJUnit

• Backward compatible replacement for JUnit

• Detects exceptions in all threads– Exception handler for all child threads and the

event thread

• Ensures that child threads have terminated and event thread is done– Enumerate live threads after test– Inspect event queue

• Requires all child threads to be joined– Analyze join graph

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Thread Creation Coordinates

• In Thread.start() record stack trace of Thread.currentThread()– Easy to find where a thread that caused a

failure was started

– Also shows where threads that outlived the test were started

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Creation Coordinates Example

class Main { void foo() { // which one? new Helper(true).start(); new Helper(false).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)

class Helper extends Thread { void m() { if (b) Assert.fail(); } public void run() { m(); } private boolean b; // …}

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ConcJUnit Demo

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Concurrency Examples

• In-class discussion– Multi-threaded counter: data races– Multi-threaded bank: deadlock

• Homework– Bounded buffer– Readers-writer lock– Test suite handed out to help students

• Multi-threaded Breakout

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Example: Counter

• Class that can increment an integer variable N times

• Write test first

public class CounterTest extends TestCase { final long PER_THREAD = 1000000; public void testSingle() { Counter c = new Counter(); c.incrementNTimes(PER_THREAD); assertEquals(PER_THREAD, c.getCount()); }}

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Counter: Implementation

• Write implementation

public class Counter { private long count = 0;

public long getCount() { return count; } public void incrementNTimes(long n) { for(long i=0; i<n; ++i) { ++count; } }}

Test passes!

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Counter: Multi-threaded Test

• Write multi-threaded test

public void testMulti() { final Counter c = new Counter(); for(int i=0; i<NUM_THREADS; ++i) { new Thread() { public void run() { c.incrementNTimes(PER_THREAD); } }.start(); } TestUtils.waitForOtherThreads(); assertEquals(NUM_THREADS*PER_THREAD,c.getCount()); }

Test fails (most likely)!

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Shared Data• Why does the multi-threaded counter test fail?

– The count field is shared among threads– The ++count operation is not atomic– Thread may be interrupted after reading count, but

before writing back to count

count=0 regA=? regB=?A1 regA = count; 0 0 ? B1 regB = count; 0 0 0A2 regA = regA + 1; 0 1 0A3 count = regA; 1 1 0 B2 regB = regB + 1; 1 1 1 B3 count = regB; 1 1 1

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Data Races

• Definition– Two threads access the same data– At least one access is a write– Nothing prevents the order from changing

• Would like code to execute atomically (without interruption)– Java does not support atomicity

(for general code)

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Java Locks & Synchronized

• Java provides “lock objects” and synchronized blocks

synchronized(lock) { ++count; }

– Thread must compete for ownership of lock object before entering synchronized block

– Synchronized block is not atomic– But once a thread has a lock object, no other

thread can execute code protected by the same lock object

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Counter: Re-Write

• Rewrite implementation

// ... private Object lock = new Object(); public void incrementNTimes(long n) { for(long i=0; i<n; ++i) { synchronized(lock) { ++count; } } }

Test passes!

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Concurrency Still Difficult

• Even race-free, deadlock-free programs are not deterministic– Thread scheduling is essentially non-

deterministic

• Different schedules may compute different results– May or may not be acceptable, depending

on the task

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Multi-threaded Breakout• Uses ACM Java Task Force material

– Based on “Breakout - Nifty Assignment” by Eric Roberts, SIGCSE 2006

• Multiple balls, each in its own thread– Atomicity assumption when removing bricks– Ends game before all bricks are removed

• Other problems– X,Y coordinate changes not atomic– X,Y coordinates not volatile or synchronized, event

thread may never see the updates

• Correctly synchronized version still not deterministic

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Future Work• Testing all schedules is intractable

• Insert random delays/yields before synchronization operations– Must consider volatile variable accesses to

comply with Java Memory Model– Re-run program several times– Can detect a number of sample problems

• Record schedule, replay if test fails– Makes failures reproducible if found

3

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Conclusion

• Unit testing has important benefits in industry and in the classroom

• Concurrent programming is becoming more important, and it’s difficult

• ConcJUnit helps…

www.concutest.orgwww.drjava.org

Notes

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Notes1. Also cannot detect uncaught exceptions in a

program’s uncaught exception handler (JLS limitation) ←

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

3. 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. ←

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public class Test extends TestCase { public void testException() { Thread t = new Thread(new Runnable() { public void run() { throw new RuntimeException("booh!"); } }); t.start(); while(t.isAlive()) { try { t.join(); } catch(InterruptedException ie) { } } }}

Thread t = new Thread(new Runnable() { public void run() { throw new RuntimeException("booh!"); }});t.start();while(t.isAlive()) { try { t.join(); } catch(InterruptedException ie) { }}

throw new RuntimeException("booh!");

Loop since join() may end

spuriously

4. ←

Spurious Wakeup

Image Attribution

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Image Attribution1. Left image on Two Trends:

Test Driven Development, Damian Cugley.

2. Right image on Two Trends: adapted from Brian Goetz et al. 2006, Addison Wesley.

3. Graph on Moore’s Law:Adapted from Herb Sutter 2009

4. Image on Concurrency Is Difficult:Caption Fridays

Extra Slides

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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

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JUnit Test with Child Thread

public class Test extends TestCase { public void testException() { new Thread(new Runnable() { public void run() { throw new RuntimeException("booh!"); } }).start(); }}

new Thread(new Runnable() { public void run() { throw new RuntimeException("booh!"); }}).start();

throw new RuntimeException("booh!");

invokeschecks

TestGroup’s Uncaught Exception Handler

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JUnit Test with Child Thread

public class Test extends TestCase { public void testException() { new Thread() { public void run() { throw new RuntimeException("booh!"); } }.start(); }}

new Thread() { public void run() { throw new RuntimeException("booh!"); }}.start();

throw new RuntimeException("booh!");

Test thread

Child thread

uncaught!

end of test

Main thread

spawns and joins resumes

check exception handler

invokes exception handler

failure!

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Child Thread Outlives Parent

public class Test extends TestCase { public void testException() { new Thread() { public void run() { throw new RuntimeException("booh!"); } }.start(); }}

new Thread() { public void run() { throw new RuntimeException("booh!"); }}.start();

throw new RuntimeException("booh!");

Test thread

Child thread

uncaught!end of test

success!

invokes exception handler

Main thread

check exception handler

Too late!

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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

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Check for Living Child Threads

public class Test extends TestCase { public void testException() { new Thread() { public void run() { throw new RuntimeException("booh!"); } }.start(); }}

new Thread() { public void run() { throw new RuntimeException("booh!"); }}.start();

throw new RuntimeException("booh!");

Test thread

Child thread

uncaught!end of test

failure!

invokes group’s handler

Main thread

check for livingchild threads

check group’s handler

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Correctly Written Test

public class 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 */

Test thread

Child thread

end of test

success! Main thread

check for livingchild threads

check group’s handler

4

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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

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Fork/Join Model

• Parent thread joins with each of its child threads

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

Child thread 1

Child thread 2

Main thread

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Example of Other Join Models

• Chain of child threads guaranteed to outlive parent

• Main thread joins with last thread of chain

Child thread 1

Child thread 2

Main thread

Child thread 3

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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

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Join with All Offspring Threads

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

Child thread 1

Child thread 2

Main thread

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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

Child thread 1

Child thread 2

Main thread

Child thread 3

MT

CT1

CT2

CT3

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Child thread 1

Child thread 2

Main thread MT

CT1

CT2

Child thread 1

Child thread 2

Main thread

MT

CT1

CT2

Join Graph Examples

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Child thread 1

Child thread 2

Main thread

MT

CT1

CT2

Unreachable Nodes

• An unreachable node has not been joined– Child thread may outlive the test

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childThread

main Thread

MT

CT

Constructing the Graph// in mainThreadchildThread.start();

• Add node for childThread

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// in mainThreadchildThread.join();

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

childThread

main Thread

MT

CT

Constructing the Graph

2

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Example: Multi-threaded Bank

• Program simulating checking accounts• Account balances are shared data

– To avoid data races, use synchronized– Need access to two accounts for transfers

synchronized(locks[from]) { synchronized(locks[to]) { accounts[from] -= amount; accounts[to] += amount; }} Test hangs!

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Deadlock• Thread A transfers from account 0 to 1• Thread B transfers from account 1 to 0• Thread A gets interrupted after acquiring locks[0]

// thread A // thread Bsynchronized(locks[0]) {

synchronized(locks[1]) { synchronized(locks[0]) // can’t continue, locks[0] // is owned by thread A */

synchronized(locks[1]) // can’t continue, locks[1] // is owned by thread B */

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Lock Acquisition Order• No deadlock if both threads had attempted to

acquire lock 0 first

• When acquiring more than one lock object, always acquire them in the same order– e.g. acquire lower account’s lock object first

synchronized(locks[Math.min(from,to)]) { synchronized(locks[Math.max(from,to)]) { accounts[from] -= amount; accounts[to] += amount; }}

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Homework Assignment

• Common structures students will see time and again– Bounded buffer– Readers-writer lock

• Grade correctness and efficiency, e.g.– Maximize concurrency– Only wake up as few threads as possible

• Provide students with test suites

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Many Thanks To…• My advisor

– Corky Cartwright

• My committee members– Walid Taha– David Scott– Bill Scherer

• NSF and Texas ATP– For providing partial funding