Synchronization and concurrency before Java 5

Prior to Java 5, there were basically five means to correctly synchronize data between threads ("shared" here means "accessed by multiple threads"):

For most programmers, the first two of these are probably the bread-and-butter means of synchronizing data. I suspect that the last two or three on this list are unknown to many otherwise experienced Java programmers. (The last is actually rare and generally unrecommended, but we consider it for completeness.) We'll review each of these methods briefly here, starting with the synchronized keyword.