Singleton in Multithreaded - Various Scenarios

How below 2 codes are different in terms of multithreaded environment?

Code 1:

public class Singleton {
private Singleton() {}

private static class SingletonHolder { 
    private static final Singleton INSTANCE = new Singleton();
}

pub开发者_JS百科lic static Singleton getInstance() {
    return SingletonHolder.INSTANCE;
}
}

Code 2:

class Singleton {
private static Singleton uniqueInstance;

private Singleton() { ... }

public static Singleton getInstance() {
    if (uniqueInstance == null) {
        uniqueInstance = new Singleton();
    }

    return uniqueInstance;
}
}

Why Code 2 will not work in multi threaded environment, when it has also static variable declared which will get loaded once class is loaded & thus it'll have only one instance?

Thanks!

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Remember that more than one thread can call getInstance at a given time.

In Example 1, the initializer for the uniqueInstance member is guaranteed to be only run once - at class load time.

In Example 2, since initialization takes place inline in getInstance, more than one thread can independently and concurrently find the uniqueInstance member variable to be null. Each thread will then call new Singleton(), with indeterminate results that depend on the timing of the two (or more) threads.

For Example 2 to work, you could (for example) add synchronized on the getInstance method.

Your comment about the variable being fully initialized at class load time is true for Example 1, but not for 2 - in example 2 the member variable is set to null at class load time, but populated with an object instance later, during the first call(s) to getInstance.

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Multiple threads may be inside of the:

if (uniqueInstance == null) {
...
}

condition simultaneously.