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javasingleton
Jaanus
Jaanus

Reputation: 16541

Singleton in Java example is illogical

This is the example:

http://www.tutorialspoint.com/java/java_using_singleton.htm

When looking at the 1st example, when user writes : Singleton.getInstance() , then it calls out :

new Singleton()

I don't get it, how it is singleton, when everytime it creates a new singleton object?

I understand the 2nd example. If singleton is null, then create new object, but in first example, it always creates new object??

Whats up with that?

Upvotes: 1

Views: 1556

Answers (5)

Matt
Matt

Reputation: 11805

In the second example, as someone mentioned, it's not thread safe. Here's two options for making the getInstance() method thread safe:

 public static synchronized Singleton getInstance() {
     if (singleton == null) {
         singleton = new Singleton();
     }
     return singleton;
 }

 public static Singleton getInstance() {
     if (singleton == null) {
         synchronized(Singleton.class) {
             if (singleton == null) {
                 singleton = new Singleton();
             }
         }
     }
     return singleton;
 }

The second version of the method allows you to get an already constructed singleton instance without any synchronization overhead. However, when it is null, you then enter a synchronized block. You check the null again just in case there was a thread context switch and a second thread actually gets into the synchronized block before you do. That ensures only a single instances is ever constructed.

Either approach should be entirely thread safe.

Upvotes: 1

Jim Flood
Jim Flood

Reputation: 8467

The line:

private static Singleton singleton = new Singleton( );

only executes once when the class is loaded -- it's a class field initializer.

Look at this question: Best Practice: Initialize class fields in constructor or at declaration? -- it's the same idea with the new Random() in that question.

Edit: I popped in to Stack Overflow for a moment to see if I could answer an unanswered question, and before I could submit my response, it looks like three others already have answered it. I like Stack Overflow!

Upvotes: 0

dckrooney
dckrooney

Reputation: 3121

The member singleton is declared as static, so new Singleton() is only ever called once.

Upvotes: 0

Jon Lin
Jon Lin

Reputation: 143886

The new Singleton() is only getting called when the class is loaded.

Upvotes: 0

Jon Skeet
Jon Skeet

Reputation: 1500525

No, in the first example the only call to new Singleton() is here (within Singleton):

private static Singleton singleton = new Singleton( );

That's a static variable initializer. It gets executed once, and only when needed. (If you never touch the Singleton class, the initializer won't get executed.)

Upvotes: 10

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