16.2. Publication

Chapter 3 explored how an object could be safely or improperly published. The safe publication techniques described there derive their safety from guarantees provided by the JMM; the risks of improper publication are consequences of the absence of a happens-before ordering between publishing a shared object and accessing it from another thread.

16.2.1. Unsafe Publication

The possibility of reordering in the absence of a happens-before relationship explains why publishing an object without adequate synchronization can allow another thread to see a partially constructed object (see Section 3.5). Initializing a new object involves writing to variables—the new object’s fields. Similarly, publishing a reference involves writing to another variable—the reference to the new object. If you do not ensure that publishing the shared reference happens-before another thread loads that shared reference, then the write of the reference to the new object can be reordered (from the perspective of the thread consuming the object) with the writes to its fields. In that case, another thread could see an up-to-date value for the object reference but out-of-date values for some or all of that object’s state—a partially constructed object.