About Singleton pattern in java, instead of assigning static variable to new local variable in method, why not use directly static variable?

Question

I haven't understood what the code's purpose => DataProvider instance = sInstance; is in below method. Anyone help me to explain in detail ? Why don't use directly sInstance ?

private static volatile DataProvider sInstance = null;
 public static DataProvider getInstance() {
     DataProvider instance = sInstance;
      if (instance == null) {
          synchronized (DataProvider.class) {
              instance = sInstance;
              if (instance == null) {
                  instance = sInstance = new DataProvider();
              }
          }
      }
      return instance;
  }

https://github.com/DrKLO/Telegram/blob/master/TMessagesProj/src/main/java/org/telegram/messenger/LocationController.java @Juan its code from telegram — Hien Nguyen, May 29 '18 at 03:40
@Radiodef may u explan in detail :( I don't know why need to assign to local variable before using => DataProvider instance = sInstance; — Hien Nguyen, May 29 '18 at 03:40

score 1 · Answer 1 · answered May 29 '18 at 04:13

1

It is used as a lazy initialization (e.i. only create the singleton instance when needed). The problem with this code is that it is broken. Apparently even when using the synchronize block, there is a posaibility that things goes wrong (due to raceconditions). So do not use this method if you want to be safe!

Alternatives: Using a direct assignment (like you sugessted);

private static volatile DataProvider sInstance = new DataProvider();

Or using a enum (as suggested by @MadProgrammer);

public enum DataProvider
{

    INSTANCE;

    // singleton content
}

answered May 29 '18 at 04:13

n247s

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I want to know why we should assign to local variable => DataProvider instance = sInstance; – Hien Nguyen May 29 '18 at 04:46
:( may you explain in detail for me :( – Hien Nguyen May 29 '18 at 08:58

score 1 · Accepted Answer · answered May 30 '18 at 02:04

According to the book Prentice.Hall.Effective.Java.2nd.Edition.May.2008 of Joshua Bloch,

In particular, the need for the local variable result may be unclear. What this variable does is to ensure that field is read only once in the common case where it’s already initialized. While not strictly necessary, this may improve performance and is more elegant by the standards applied to low-level concurrent programming. On my machine, the method above is about 25 percent faster than the obvious version without a local variable.

score 1 · Answer 3 · answered Jun 01 '18 at 09:52

The main reason is Volatile. As @Hien Nguyen's answer, it improve 25% performance. Cause Volatile is always get data from main memory instead of cache, so it's too slow. Declare instance = sInstance to avoid read data from main memory multiple time (slow). There're 3 time we read data from sInstance if we don't use temp variable, so we use temp variable will imporve performance.

See this topic to understand why access Volatile is slow: Why access volatile variable is about 100 slower than member?

Your answer maybe the same as this topic: Java: using a local variable in double check idiom

About Singleton pattern in java, instead of assigning static variable to new local variable in method, why not use directly static variable?

3 Answers3