创建型模式-单件模式(1)
单件模式特点:
1 :单件类只可有一个实例。
2 :单件类必须自己创建自己这惟一的实例。
3 :单件类必须给所有其他对象提供这一实例。
结构:
如何实现:
其实我们只需要把构造函数私有化,把new对象的过程进行控制就行了,具体代码如下:
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public class Singleton
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{
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private static Singleton uniqueInstance; // 用一个静态变量来记录Singleton类的唯一实例
4![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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private Singleton() // 构造器必须私有
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{
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}
8![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public static Singleton Instance
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{
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get
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{
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if (null == uniqueInstance) // 如果uniqueInstance为空,说明对象没有被创建
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{
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uniqueInstance = new Singleton();
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}
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return uniqueInstance;
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}
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}
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}
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但是上面的代码存在一个问题。当使用多线程时,产生的对象可能不只一个,分析见下图:
如何解决这个问题:
a)、使用加锁机制,具体代码如下:
1public class Singleton
2{
3private static Singleton uniqueInstance; // 用一个静态变量来记录Singleton类的唯一实例
4private static Object obj = new object();
5![]()
6private Singleton() // 构造器必须私有
7{
8}
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10public static Singleton Instance
11{
12get
13{
14lock (obj) // 使用加锁,避免两个线程同时进入
15{
16if (null == uniqueInstance) // 如果uniqueInstance为空,说明对象没有被创建
17{
18uniqueInstance = new Singleton();
19}
20return uniqueInstance;
21}
22}
23}
24}
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b)、使用“急切”创建实例,不用延迟加载。具体代码如下:
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public class Singleton
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{
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static Singleton uniqueInstance = new Singleton(); // 用一个静态变量来记录Singleton类的唯一实例
4![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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private Singleton() // 构造器必须私有
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{
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}
8![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public static Singleton Instance
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{
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get
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{
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return uniqueInstance;
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}
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}
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}
17![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
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c)、使用“双检锁”机制。具体代码如下:
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public class Singleton
2
{
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static Singleton uniqueInstance = null; // 用一个静态变量来记录Singleton类的唯一实例
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private static Object obj = new object();
5![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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private Singleton() // 构造器必须私有
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{
8
}
9![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
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public static Singleton Instance
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{
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get
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{
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if (null == uniqueInstance) // 先判断uniqueInstance是否为空,如果为空,再进行加锁
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{
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lock (obj) // 使用加锁,避免两个线程同时进入
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{
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if (null == uniqueInstance) // 如果uniqueInstance为空,说明对象没有被创建
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{
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uniqueInstance = new Singleton();
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}
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}
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}
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return uniqueInstance;
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}
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}
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}
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对上面三种方法的评价:
1)、a方法采用了加锁机制,每次实例化都必须加锁,而加锁耗费的系统资源比较多,因此执行效率比较低,不推荐使用。
2)、b方法书写比较简单,但没有采用延迟加载,所以可能浪费部分资源。当使用频率高且运行时负担不重时推荐使用。
3)、c方法解决了多线程问题,并且只在第一次创建对象时加锁,执行效率比a方法高,推荐使用。
在什么情形下使用单例模式:
使用Singleton模式有一个必要条件:在一个系统要求一个类只有一个实例时才应当使用单例模式。反过来,如果一个类可以有几个实例共存,就不要使用单例模式。
注意:
a)、不要使用单例模式存取全局变量。这违背了单例模式的用意,最好放到对应类的静态成员中。
b)、不要将数据库连接做成单例,因为一个系统可能会与数据库有多个连接,并且在有连接池的情况下,应当尽可能及时释放连接。Singleton模式由于使用静态成员存储类实例,所以可能会造成资源无法及时释放,带来问题。