1. 第一种（懒汉，线程不安全）

public class Singleton {

    private static Singleton instance;

    private Singleton() {
    }

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

}

2. 第二种（懒汉，线程安全）

public class Singleton {

    private static Singleton instance;

    private Singleton() {
    }

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

}

3. （饿汉）

public class Singleton {

    private static Singleton instance = new Singleton();

    private Singleton() {
    }

    public static Singleton getInstance() {
        return instance;
    }

}

4. 第四种（饿汉，变种）

public class Singleton {

    private static Singleton instance = null;

    static {
        instance = new Singleton();
    }

    private Singleton() {
    }

    public static Singleton getInstance() {
        return instance;
    }

}

5. 第五种（静态内部类）

public class Singleton {

    private static class SingletonHolder {
        private static final Singleton INSTANCE = new Singleton();
    }

    private Singleton() {
    }

    public static final Singleton getInstance() {
        return SingletonHolder.INSTANCE;
    }

}

6. （枚举）

public enum Singleton {
    INSTANCE;

    public void whateverMethod() {
    }
}

7. 第七种（双重校验锁）

public class Singleton {
    private volatile static Singleton singleton;

    private Singleton() {
    }

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