You need to declare the field volatile because that will force the write to the field to be "flushed" to main memory. Otherwise the JVM specification allows each thread to keep its local version of the field and never communicate its writes to other threads. This is generally nice because it allows aggressive optimizations in the JVM.
Hope that helps! Else I can recommend getting a really strong cup of coffee, a very quiet room and then read the Java Memory Model which explains how it works and the interaction between threads. I think you will be surprised in how few situations a thread is required to communicate its writes to (shared) memory to other threads and the reordering of reads and writes that the JVM can perform!
An exciting read!
