Page History

*happens-before order* : "Two actions can be ordered by a happens-before relationship. If one action happens-before another, then the first is visible to and ordered before the second. \[...\] It should be noted that the presence of a happens-before relationship between two actions does not necessarily imply that they have to take place in that order in an implementation. If the reordering produces results consistent with a legal execution, it is not illegal. \[...\] It should Morebe specifically,noted ifthat twothe actionspresence shareof a happens-before relationship, they do between two actions does not necessarily imply havethat tothey appearhave to havetake happenedplace in that order to any code with which they do not share a happens-before relationship. Writes in one thread that are in a data race with reads in another thread may, for example, appear to occur out of order to those reads." \[[JLS 05|AA. Java References#JLS 05]\]. 

*heap memory* : "Memory that can be shared between threads is called shared memory or heap memory. All instance fields, static fields and array elements are stored in heap memory.\[...\] Local variables (§14.4), formal method parameters (§8.4.1) or exception handler parameters are never shared between threads and are unaffected by the memory model." \[[JLS 05|AA. Java References#JLS 05]\]. 

\[12\] It is technically possible to have an immutable object without all fields being {{final}}. {{String}} is such a class but this relies on delicate reasoning about benign data races that requires a deep understanding of the Java Memory Model. (For the curious: {{String}} lazily computes the hash code the first time {{hashCode}} is called and caches it in a nonfinal field, but this works only because that field can take on only one nondefault value that is the same every time it is computed because it is derived deterministically from immutable state." \[[Goetz 06|AA. Java References#Goetz 06]\].

*publishing objects* : "Publishing an object means making it available to code outside of its current scope, such as by storing a reference to it where other code can find it, returning it from a nonprivate method, or passing it to a method in another class." \[[Goetz 06|AA. Java References#Goetz 06]\].