In general, synchronized
methods are used to protect access to resources that are accessed concurrently. When a resource that is being accessed concurrently belongs to each instance of your class, you use a synchronized
instance method; when the resource belongs to all instances (i.e. when it is in a static
variable) then you use a synchronized static
method to access it.
For example, you could make a static
factory method that keeps a "registry" of all objects that it has produced. A natural place for such registry would be a static
collection. If your factory is used from multiple threads, you need to make the factory method synchronized
(or have a synchronized
block inside the method) to protect access to the shared static
collection.
Note that using synchronized
without a specific lock object is generally not the safest choice when you are building a library to be used in code written by others. This is because malicious code could synchronize on your object or a class to block your own methods from executing. To protect your code against this, create a private "lock" object, instance or static, and synchronize on that object instead.