synchronized Keyword

Syntax

synchronized (lockObject) {\n  // critical section\n}

Examples

class Counter {
    private int count = 0;

    public void increment() {  // NOT thread-safe
        count++;  // this is actually three steps: read, add, write
    }

    public int getCount() {
        return count;
    }
}

// Without synchronization, running this with multiple threads can produce
// a final count LESS than expected, because increments can be lost
// when two threads read the same value before either writes back.

class Counter {
    private int count = 0;

    public synchronized void increment() {  // now thread-safe
        count++;
    }

    public synchronized int getCount() {
        return count;
    }
}

Counter counter = new Counter();
Runnable task = () -> {
    for (int i = 0; i < 1000; i++) counter.increment();
};

Thread t1 = new Thread(task);
Thread t2 = new Thread(task);
t1.start();
t2.start();

class BankAccount {
    private double balance = 100;
    private final Object lock = new Object();

    public void withdraw(double amount) {
        synchronized (lock) {  // only this block is protected
            if (amount <= balance) {
                balance -= amount;
            }
        }
        // other, non-critical work can happen outside the lock
    }
}

Best practices

• Synchronize the smallest block of code necessary (a synchronized block) rather than an entire large method, to minimize how long threads wait on each other
• Always synchronize on a private, final lock object rather than "this" when possible, to prevent external code from accidentally locking on the same object
• Keep synchronized blocks free of slow operations (like network calls or file I/O) - holding a lock for a long time hurts overall concurrency
• Consider higher-level concurrency utilities from java.util.concurrent (like AtomicInteger or ConcurrentHashMap) instead of manual synchronized blocks when they fit your use case

At a glance

Purpose

General-purpose application development

File extension

.java

Runs in

Java Virtual Machine

Usually used with

JDK and Java libraries

Specifications & further reading

• Java Language Specification

Related Java documentation