What does 'synchronized' mean?

The synchronized keyword prevents concurrent access to a block of code or object by multiple threads. All the methods of Hashtable are synchronized, so only one thread can execute any of them at a time.

When using non-synchronized constructs like HashMap, you must build thread-safety features in your code to prevent consistency errors.

4 years ago

To my understanding synchronized basically means that the compiler write a monitor.enter and monitor.exit around your method. As such it may be thread safe depending on how it is used (what I mean is you can write an object with synchronized methods that isn't threadsafe depending on what your class does).

4 years ago

The synchronized keyword is all about different threads reading and writing to the same variables, objects and resources. This is not a trivial topic in Java, but here is a quote from Sun:

synchronized methods enable a simple strategy for preventing thread interference and memory consistency errors: if an object is visible to more than one thread, all reads or writes to that object's variables are done through synchronized methods.

In a very, very small nutshell: When you have two threads that are reading and writing to the same 'resource', say a variable named foo, you need to ensure that these threads access the variable in an atomic way. Without the synchronized keyword, your thread 1 may not see the change thread 2 made to foo, or worse, it may only be half changed. This would not be what you logically expect.

4 years ago