Synchronizing on a method locks the object's entire method for the duration of the call, blocking any other thread from entering any synchronized method on that same object, whereas synchronizing on a block scopes the lock to just the enclosed lines, allowing everything else in the method to run unsynchronized.
Key Points: • Method-level synchronization is simpler to write — just add the synchronized modifier — but it locks more code than may actually be necessary. • Block-level synchronization requires explicitly choosing a lock object, which can even be a private, dedicated lock rather than "this", avoiding accidental interference from external code locking on the same instance. • Narrower locking scope from blocks generally means shorter lock hold times and less contention among threads, improving throughput under load. • A common anti-pattern is synchronizing an entire large method when only a few lines actually touch shared mutable state — blocks fix that by isolating just those lines. • Both ultimately rely on the same intrinsic (monitor) locking mechanism; the difference is purely about how much code, and which lock object, is covered.
Example: A method that does expensive but thread-safe computation, followed by a quick update to a shared cache, is better served by wrapping only the cache update in a synchronized block rather than marking the whole method synchronized, so the expensive computation doesn't unnecessarily block other threads.
Code Example:
private final Object cacheLock = new Object();
public void process(Data data) {
Result result = expensiveComputation(data); // no lock needed here
synchronized (cacheLock) {
cache.put(data.getId(), result); // only this needs protection
}
}Interview Tip: A concise interview answer is:
"The difference is scope: synchronizing a whole method locks every line of it and blocks other synchronized methods on that object, while synchronizing just a block narrows the lock to only the lines that touch shared state, often using a dedicated lock object. Narrower locking generally means less contention and better throughput."