Java Concurrency Skill Skill

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")

Agent Skills: Java Concurrency Skill

Skill Files