The semantics of Java multithreading dictates all possible behaviors that a multithreaded Java program can exhibit on any platform. This is called the Java Memory Model (JMM) and describes the allowed reorderings among the memory operations in a thread. However, multiprocessor platforms traditionally have memory consistency models of their own. In this paper, we study the interaction between the JMM and the multiprocessor memory consistency models. In particular, memory barriers may have to be inserted to ensure that the multiprocessor execution of a multi-threaded Java program respects the JMM. We study the impact of these additional memory barriers on program performance. Our experimental results indicate that the performance gain achieved by relaxed hardware memory consistency models far exceeds the performance degradation due to the introduction of JMM.