Java单例模式的6种实现方式详解

/**
 * 饿汉式实现单例模式
 */
public class Singleton {
    private static Singleton instance = new Singleton();
    private Singleton() {
    }
    public static Singleton getInstance() {
        return instance;
    }
}

/**
 * 懒汉式实现单例模式
 */
public class Singleton {
    private static Singleton instance;
    private Singleton() {
    }
    // synchronized方法,多线程情况下保证单例对象唯一
    public static synchronized Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

/**
 * DCL实现单例模式
 */
public class Singleton {
    private static Singleton instance = null;
    private Singleton() {
    }
    public static Singleton getInstance() {
        // 两层判空，第一层是为了避免不必要的同步
        // 第二层是为了在null的情况下创建实例
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

/**
 * 静态内部类实现单例模式
 */
public class Singleton {
    private Singleton() {
    }
    public static Singleton getInstance() {
        return SingletonHolder.instance;
    }
    /**
     * 静态内部类
     */
    private static class SingletonHolder {
        private static Singleton instance = new Singleton();
    }
}

/**
 * 枚举实现单例模式
 */
public enum SingletonEnum {
    INSTANCE;
    public void doSomething() {
        System.out.println("do something");
    }
}

import java.util.HashMap;
import java.util.Map;
/**
 * 容器类实现单例模式
 */
public class SingletonManager {
    private static Map<String, Object> objMap = new HashMap<String, Object>();
    public static void regsiterService(String key, Object instance) {
        if (!objMap.containsKey(key)) {
            objMap.put(key, instance);
        }
    }
    public static Object getService(String key) {
        return objMap.get(key);
    }
}