Java Concurrency Skill

Master Java concurrency patterns for thread-safe applications.

Overview

This skill covers concurrency from basic threads to virtual threads (Java 21+), including thread pools, synchronization, and CompletableFuture.

When to Use This Skill

Use when you need to:

• Write thread-safe code
• Implement parallel processing
• Use async programming patterns
• Tune thread pools
• Debug concurrency issues

Topics Covered

Thread Management

• Thread lifecycle and states
• Daemon vs user threads
• Interrupt handling

Synchronization

• synchronized, volatile
• Lock interfaces (ReentrantLock)
• Atomic operations

Executors

• ThreadPoolExecutor configuration
• ForkJoinPool
• Virtual Threads (Java 21+)

CompletableFuture

• Async execution
• Chaining and composition
• Exception handling

Quick Reference

// Virtual Threads (Java 21+)
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    IntStream.range(0, 10_000).forEach(i ->
        executor.submit(() -> processRequest(i)));
}

// CompletableFuture composition
CompletableFuture<Result> result = fetchUser(id)
    .thenCompose(user -> fetchOrders(user.id()))
    .thenApply(orders -> processOrders(orders))
    .exceptionally(ex -> handleError(ex))
    .orTimeout(5, TimeUnit.SECONDS);

// Thread pool configuration
ThreadPoolExecutor executor = new ThreadPoolExecutor(
    10, 50, 60L, TimeUnit.SECONDS,
    new ArrayBlockingQueue<>(1000),
    new ThreadPoolExecutor.CallerRunsPolicy()
);

// Lock with timeout
ReentrantLock lock = new ReentrantLock();
if (lock.tryLock(1, TimeUnit.SECONDS)) {
    try {
        // critical section
    } finally {
        lock.unlock();
    }
}

Thread Pool Sizing

Workload	Formula	Example
CPU-bound	cores	8 threads
I/O-bound	cores * (1 + wait/compute)	80 threads

Troubleshooting

Problem	Cause	Solution
Deadlock	Circular lock	Lock ordering, tryLock
Race condition	Missing sync	Add locks/atomics
Thread starvation	Unfair scheduling	Fair locks

Debug Commands

jstack -l <pid> > threaddump.txt
jcmd <pid> Thread.print

Usage

Skill("java-concurrency")