单例模式保证一个类只有一个实例，并提供唯一一个访问实例的方法。

1.饿汉式(线程安全)：

public class Singleton {

    private final static Singleton instance = new Singleton();

    private Singleton(){}

    public static Singleton getInstance(){
        return instance;
    }
}

2.懒汉式(线程安全，但是使用了同步锁)：

public class Singleton {

    private static Singleton instance;

    private Singleton() {}

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

3.静态内部类

public class Singleton {

    private Singleton() {}

    private static class InnerSingleton{
        private static final Singleton instance = new Singleton();
    }

    public static Singleton getInstance() {
        return InnerSingleton.instance;
    }
}