饿汉式

• 优点：简单
• 缺点：不支持延迟加载

public class Sington {
    private static final Sington instance = new Sington();
    private Sington() {}
    public static  Sington getInstance() {
        return instance;
    }
}

懒汉式

• 优点：支持延迟加载
• 缺点：加锁，并发度基本为1，频繁调用效率太低

public class Sington {
    private static Sington instance;
    private Sington() {}
    public static  synchronized Sington getInstance() {
        if (instance==null) {
            return new Sington();
        }
        return instance;
    }
}

双重检测

• 优点：延迟加载，线程安全
• 缺点：

public class Sington {
    private static Sington instance;
    private Sington() {}
    public static Sington getInstance() {
        if (instance==null) {
            synchronized (Sington.class) {
                if (instance==null) {
                    return new Sington();
                }
            }
        }
        return instance;
    }
}

静态内部类

• 优点：静态内部类只有在调用的时候加载，由JVM保证唯一性和安全性
• 缺点：

public class Sington {
    private Sington() {}
    private static class SingtonInner {
        private static final Sington instance = new Sington();
    }
    public static Sington getInstance() {
        return SingtonInner.instance;
    }
}

单例模式特点

• 私有化构造器，防止外部创建
• 私有化静态变量
• 公有的获取实例方法