Common Mistakes in Java Multithreading and How to Avoid Them

Ramesh Fadatare

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4 min read

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Mar 17, 2025

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Multithreading in Java is powerful but also tricky. A small mistake can lead to race conditions, deadlocks, performance issues, and unpredictable behavior. Many developers struggle with multithreading, and common mistakes can make debugging extremely difficult.

In this article, we’ll go through 10 common mistakes in Java multithreading and how to avoid them with best practices. Let’s dive in! 🔥

🔴 1️⃣ Incorrect Use of synchronized Keyword

❌ Mistake: Synchronizing on the Wrong Object

class SharedResource {
    public void doSomething() {
        synchronized (new Object()) {  // ❌ Each thread locks a different object
            System.out.println("Critical section");
        }
    }
}

👉 Issue: Synchronizing on a new object each time makes the lock useless.

✅ Best Practice: Synchronize on a Shared Object

class SharedResource {
    private final Object lock = new Object();  // ✅ Shared lock object
    
    public void doSomething() {
        synchronized (lock) {  // ✅ Proper synchronization
            System.out.println("Critical section");
        }
    }
}

🔹 Fix: Always synchronize on a shared, final object to ensure thread safety.

🔴 2️⃣ Using Non-Thread-Safe Collections