单例模式的八种写法比较

1、饿汉式（静态常量）[可用]

public class Singleton {

    private final static Singleton INSTANCE = new Singleton();

    private Singleton(){}

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

2、饿汉式（静态代码块）[可用]

public class Singleton {

    private static Singleton instance;

    static {
        instance = new Singleton();
    }

    private Singleton() {}

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

3、懒汉式(线程不安全)[不可用]

public class Singleton {

    private static Singleton singleton;

    private Singleton() {}

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

4、懒汉式(线程安全，同步方法)[不推荐用]

public class Singleton {

    private static Singleton singleton;

    private Singleton() {}

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

5、懒汉式(线程安全，同步代码块)[不可用]

public class Singleton {

    private static Singleton singleton;

    private Singleton() {}

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

6、双重检查[推荐用]

public class Singleton {

    private static volatile Singleton singleton;

    private Singleton() {}

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

7、静态内部类[推荐用]

public class Singleton {

    private Singleton() {}

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

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

8、枚举[推荐用]

    INSTANCE;
    public void whateverMethod() {

    }
}