Why can a static synchronized method and a non-static synchronized method run at the same time?
Answer:
In Java, synchronization is a means to control access to resources by multiple threads. When a method is declared as `synchronized`, the method is locked by the thread executing it, which prevents other threads from entering any other `synchronized` method that requires the same lock.
A static synchronized method and a non-static synchronized method can run simultaneously because they lock on different objects. A static synchronized method locks on the class object associated with the class (`Class` object). In contrast, a non-static synchronized method locks on the instance of the object the method belongs to (the `this` reference).
Since the class-level lock is distinct from the object-level lock, a thread accessing a static synchronized method cannot interfere with threads accessing non-static synchronized methods of an instance of that class, and vice versa. This separation allows for concurrent execution of static and instance synchronized methods, which is by design to allow for finer-grained control over concurrency in Java.
This behavior is particularly beneficial when you have to synchronize access to static data (class-level data) and instance data (object-level data) separately. It allows threads to concurrently access class-level data and instance-level data as long as they are not trying to access the same type of synchronized method (static or non-static) on the same class or instance.
In a nutshell, because they will not block each other.