To Obtain Explanation On Singleton Implementation

I've found following code for Singleton Implementation from this forum, I am having a difficulty of understanding following,

1. When Singleton instance will be created, ( From which call ) ?

2. What feature of Static Inner class allow object to be singleton ?

3. How Inner Class implementation is thread safe ?

4. What will happen if variable "instance" mark as non-static, Will it still grantee singleton implementation ?

Java Code:

public class Singleton {
// Note private constructor
private Singleton() {}
private static class SingletonInner {
private final static Singleton instance = new Singleton();
}
public static Singleton getInstance() {
return SingletonInner.instance;
}
} mh_sh_highlight_all('java');