1. Double check lock

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

public class Singleton { private static class SingletonHolder{ public static Singleton instance = new Singleton(); } private Singleton(){} public static Singleton getInstance(){ return SingletonHolder.instance; }}Copy the code

3. The enumeration

public enum Singleton {
    INSTANCE;
}
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