java-interview-lab/multithreading

multithreading

Classic multithreading interview programs in plain Java 25, each runnable and each pinned by a JUnit test with a hard timeout (no sleep() is used to synchronise anything). Companion to Java Multithreading Programs for Interviews.

Versions

Component	Version	Verified against
JDK	25.0.5+8 (Temurin LTS), --release 25	java -version
JUnit Jupiter	6.1.3	same version as the other modules in this repo
Maven	3.9.11	mvn -version

Run it

cd multithreading
./scripts/run.sh odd-even      # also: ping-pong | ordered | deadlock | virtual
mvn test                       # 25 tests
./scripts/run-all.sh           # regenerates everything in output/

Programs

Question	Class	Technique
Odd-even printing	OddEvenWaitNotify, OddEvenSemaphore, OddEvenLockCondition	synchronized+wait/notifyAll; two Semaphores; ReentrantLock+2 Conditions
Ping-pong	PingPong	ReentrantLock+2 Conditions, boolean turn flag
Ordered threads	OrderedThreads	CountDownLatch chain; join() chain; Semaphore ring (3 or 1,000 threads)
Deadlock	Transfers, DeadlockScenario, DeadlockDetector	naive vs lock ordering vs tryLock(timeout); detection by ThreadMXBean.findDeadlockedThreads
Virtual threads	every class takes a ThreadKind (PLATFORM / VIRTUAL)	same code, both thread types

Support: Group starts named threads and waits with a deadline (a hang becomes a failing test).

Captured output (output/)

File	What it shows
01-odd-even.txt	the three odd-even implementations, 1..10
02-ping-pong.txt	4 rounds of ping/pong
03-ordered-threads.txt	T3,T2,T1 started in that order, run T1,T2,T3; round-robin of 3 threads
04-deadlock.txt	real findDeadlockedThreads report, both fixes, and virtual threads being invisible to it
05-virtual-single-carrier.txt	wait/notify and 1,000 virtual threads on ONE carrier thread
06-test-results.txt	every test name, pass/fail

License

MIT, see the repository root.