单例模式Synchronizeed(Singleton.return SingletonHolder.

常见的单例模式

懒汉式(不安全)

public class Singleton{
    private Singletion（）{}
    private static Singleton Instance;
    public static Singeton getInstance(){
        if(Instance==null){
            Instance = new Singleton();
        }
        return Instance;
    }
}

懒汉式（安全低效率）

public class Singleton{
    private Singletion（）{}
    private static Singleton Instance;
    public static Synchronizeed Singeton getInstance(){
        if(Instance==null){
            Instance = new Singleton();
        }
        return Instance;
    }
}

恶汉式

public class Singleton{
    private Singletion（）{}
    private static Singleton Instance = new Singleton（）;
    public static Singeton getInstance(){
        return Instance;
    }
}

懒汉式（双校验）

public class Singleton{
    private Singletion（）{}
    private static Singleton Instance;
    public static Singeton getInstance(){
        if(Instance==null){
            Synchronizeed(Singleton.class){
                if(Instance==null){
                    Instance = new Singleton();
                }
            }

        }
        return Instance;
    }
}

懒汉式（静态式）

public class Singleton{
    private Singletion（）{}
    private static final class SingletonHolder {
        private final Singleton instance = new Singleton();
    }
    public static Singeton getInstance(){
        return SingletonHolder.instance;
    }
}

枚举

public enum Singleton{
    Instance;
    public void doMethod(){};
}