Since Java 1.5 in 2004. Twenty years this year. Before that it existed as a separate library developed by Doug Lee. I remember using that library before Java 1.5 was released. There is quite a bit of Java code out there that predates that of course. Also, lots of people continued to not grasp the essentials of that library and stuck with the primitives they knew. So there's a lot of code there with synchronized blocks post Java 1.5 instead of the more robust concurrency primitives that came with the java.util.concurrent package.