Groovy: 创建一个单例类

package com.mrhaki.blog

// 旧式单例类
public class StringUtil {
    private static final StringUtil instance = new StringUtil();

    private StringUtil() {
    }

    public static StringUtil getInstance() { 
        return instance;
    }
    
    int count(text) { 
        text.size() 
    }
}

assert 6 == StringUtil.instance.count('mrhaki')

//使用@Singleton创建有效的单例类。
//我们也可以使用@Singleton(lazy=true)进行延迟加载
//单例类
@Singleton 
class Util {
    int count(text) {
        text.size()
    }
}

assert 6 == Util.instance.count("mrhaki")

try {
    new Util()
} catch (e) {
    assert e instanceof RuntimeException
    assert "Can't instantiate singleton com.mrhaki.blog.Util. Use com.mrhaki.blog.Util.instance" == e.message
}