Java Memory model is set of rules and guidelines which allows Java programs to behave deterministically across multiple memory architecture, CPU, and operating system. It's particularly important in case of multi-threading. Java Memory Model provides some guarantee on which changes made by one thread should be visible to others, one of them is happens-before relationship. This relationship defines several rules which allows programmers to anticipate and reason behaviour of concurrent Java programs. For example, happens-before relationship guarantees :
Each action in a thread happens-before every action in that thread that comes later in the program order, this is known as program order rule.
An unlock on a monitor lock happens-before every subsequent lock on that same monitor lock, also known as Monitor lock rule.
A write to a volatile field happens-before every subsequent read of that same field, known as Volatile variable rule.
A call to Thread.start on a thread happens-before any other thread detects that thread has terminated, either by successfully return from Thread.join() or by Thread.isAlive() returning false, also known as Thread start rule.
A thread calling interrupt on another thread happens-before the interrupted thread detects the interrupt( either by having InterruptedException thrown, or invoking isInterrupted or interrupted), popularly known as Thread Interruption rule.
The end of a constructor for an object happens-before the start of the finalizer for that object, known as Finalizer rule.
If A happens-before B, and B happens-before C, then A happens-before C, which means happens-before guarantees Transitivity.
I strongly suggest to read Chapter 16 of Java Concurrency in Practice to understand Java Memory model in more detail.
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