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


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

这二者有什么区别和联系？

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


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

这二者有什么区别和联系？