JAVA_單例模式懶漢式的線程安全問題

對於靜態方法而言，使用當前類本身充當鎖

package threadPage;

class Singleton {
    private Singleton() {

    }

    private static Singleton instance = null;

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

public class TestSingleton {
    public static void main(String[] args) {
        Singleton s1 = Singleton.getInstance();
        Singleton s2 = Singleton.getInstance();

        System.out.println(s1 == s2);
    }
}