如何编写单例模型？（饿汉和饱汉模式）

package cn.kgc.tools;

import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

//读取配置文件的工具类-单例模式
public class ConfigManager {
	private static ConfigManager configManager = new ConfigManager();
	private static Properties properties;
	//私有构造器-读取数据库配置文件
	private ConfigManager(){
		String configFile = "database.properties";
		properties = new Properties();
		InputStream is = 
				ConfigManager.class.getClassLoader().getResourceAsStream(configFile);
		try {
			properties.load(is);
			is.close();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
	/*//全局访问点-(懒汉模式)
	public static synchronized ConfigManager getInstance(){
		if(configManager == null){
			configManager = new ConfigManager();
		}
		return configManager;
	}*/
	
	//饿汉模式
	public static ConfigManager getInstance(){
		return configManager;
	}
	
	public String getValue(String key){
		return properties.getProperty(key);
	}
}