Using volatile variables in java

Question

I am going through book Java concurrency in practice

I came across the statement for volatile should be used in following criteria

Writes to the variable do not depend on its current value, or you can ensure that only a single thread ever updates/writes the value.
The variable does not participate in invariants with other state variables
Locking is not required for any other reason while the variable is being accessed.

Can anyone explain me these three scenerios with example.

To my understanding, volatile variables are loaded from actual memory every time it is called instead of loading from previous cache... hence the variable should not used for other purposes. In multi threaded env, there is possible for a thread can read the volatile data before another thread completes writing on it. — PrabaharanKathiresan
– PrabaharanKathiresan, Commented Dec 22, 2016 at 11:38
To understand the word volatile u will have to go through the memory model of java. An access to volatile variable introduces a memory barrier. And also compiler cannot optimize instruction around the access of the volatile variable. For reference this is pretty good read slideshare.net/michalwarecki/java-memory-model-23207253 — pannu
– pannu, Commented Dec 22, 2016 at 11:53

Bohemian · Accepted Answer · 2016-12-22 11:42:24Z

1

Answering your points:

volatile is about reading the value; it ensures that writes are visible to all threads. It doesn't prevent write race conditions
Atomicity of writes is on the field only; volatile does not provide coordination of writes to other fields
No other actions are protected or included with the write

answered Dec 22, 2016 at 11:42

Bohemian♦

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3 Comments

coder25 Over a year ago

please give examples.In interviews they ask for examples

Bohemian Over a year ago

@coder25 Examples would be situations as per my answer. I'm sure you can think of some. Also, if you can't think of some, you shouldn't apply for the job.

ddarellis Over a year ago

I would like to add that only with volatile it is visible to all threads but you dont know the exact time. @coder25 If you want to know more please read about it docs.oracle.com/javase/specs/jls/se7/html/jls-17.html

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