What is volatile

Volatile keyword is used to mark a variable as “being stored in main memory”. More precisely that means, that every read of a volatile variable will be read from the computer’s main memory, and not from the CPU cache, and that every write to a volatile variable will be written to main memory, and not just to the CPU cache.

Features

• guarantees visibility of changes to variables across threads.
• gives a “happens-before” guarantee
• not guarantee atomic

Create a Singleton class without volatile

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1 public class Singleton {
2    private static Singleton instance;
3
4    private Singleton() {
5    }
6
7    public static Singleton getUniqueInstance() {
8        if (instance == null) {
9            synchronized (Singleton.class) {
10                if (instance == null) {
11                    instance = new Singleton();
12                }
13            }
14        }
15        return instance;
16    }
17 }

The problem is that new operator is not atomic, it will cause some problem.If Thread A execute in 11 line, it try to assign a memory space to instance before it is finished all construct. Thread B execute in 8 line, the instance is not null and B try to use the instance, which has not be initialized. This results in Thread B failed to use instance because it try to use a partially constructed instance.