the synchronized keyword already supports the happen-before rules , so why do we still need to use volatile on the filed .

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

https://docs.oracle.com/javase/specs/jls/se21/html/jls-17.html#jls-17.4.5
enter image description here

there are several ways to support happen-before rules ,such as using volatile , synchronized keyword . in the double-checked locking pattern for Singletons , we don’t need to
use both methods to ensure the happen-before rule is followed .