静态方法内synchronized及基于session的同步块是否为类级别锁？

Let's break down these two common Java sync questions clearly—no overly technical jargon, just straight, practical explanations.

1. Does using the synchronized keyword inside a static method count as a class-level lock? #

Short answer: Yes, but let's make sure you understand the "why" behind it.

When you mark a static method as synchronized, the lock is acquired on the Class object of the method's containing class (think YourClassName.class). This is exactly what we mean by a "class-level lock"—it’s tied to the class itself, not any specific instance of that class.

For example:

public class SyncDemo {
    // Locks on SyncDemo.class (class-level lock)
    public static synchronized void staticSyncMethod() {
        // Only one thread can run this method across ALL instances of SyncDemo
    }

    // Locks on the specific SyncDemo instance (instance-level lock)
    public synchronized void instanceSyncMethod() {
        // Threads can run this on different instances at the same time
    }
}

So two threads calling staticSyncMethod() will block each other, but a thread calling staticSyncMethod() and another calling instanceSyncMethod() won’t—they’re locking on completely separate objects.

2. In the code public static void getResult(HttpSession session) { synchronized(session) { .... } }, even though the method is static, does this create a class-level lock? #

Absolutely not. Here’s the key point:

The synchronized block only cares about the object inside its parentheses—in this case, the session instance. The method being static doesn’t change what’s being locked at all.

• If two threads call getResult() with the same HttpSession object, they’ll block each other (they’re fighting for the same session lock).
• If they call it with different HttpSession objects, they won’t block at all—each thread locks its own unique session instance.

This has nothing to do with a class-level lock. A class-level lock would only apply if you locked on YourClassName.class directly, or used a static synchronized method.

内容的提问来源于stack exchange，提问作者programmer