Singleton mode (Singleton) : Ensure that a class has only one instance, and provide a global access point to access it.
Features :
- A singleton class can only have one instance.
- The singleton class must create its own unique instance.
- The singleton class must provide this instance to all other objects.
The difference between a lazy man and a hungry man :
-
The hungry man means that once the class is loaded, the singleton is initialized to ensure that the singleton already exists when getInstance;
-
Lazy people are lazy, and only go back to initialize this singleton when getInstance is called.
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The hungry man is inherently thread-safe and can be directly used for multi-threading without problems;
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The lazy style itself is not thread-safe. In order to achieve thread safety, there are double locks and,
Lazy singleton
Method 1: Synchronized() double lock
class Singleton
{
private static Singleton instance;
private static readonly object syncRoot = new object();//只读
//构造方法private，规避了外界利用new创建此类的可能
private Singleton()
{
}
//此方法是获得本类实例的唯一全局访问点
public static Singleton GetInstance()
{
if (instance == null) //第一次判断是否已创建实例
{
//没有就让一条线程进入并加锁，其它线程等待，等第一条出去以后解锁，然后下一条线程再进入并加锁，直到所有线程处理完毕。
lock (syncRoot)
{
if (instance == null) //二次判断是否已创建实例
{
instance = new Singleton(); //若没有就创建一个
}
}
}
return instance;//否则返回已有的实例
}
}
Method 2: Synchronized() method
//Synchronized在指定的对象周围创建线程安全（同步）包装,每次执行都要同步
//无论是读和写，都要一个进行完在进行下一个
public static synchronized Singleton getInstance()
{
if (instance == null)
{
instance = new Singleton();
}
return instance;
}
Method 3: Static class : I don’t understand this yet, so I’ll keep it here for now
public class Singleton {
private static class LazyHolder {
private static final Singleton INSTANCE = new Singleton();
}
private Singleton (){
}
public static final Singleton getInstance() {
return LazyHolder.INSTANCE;
}
}
Hungry singleton
Instantiate objects as soon as the class is loaded, occupying system resources in advance
//密封类阻止了继承，杜绝继承会增加实例的可能
public sealed class Singleton
{
//在第一次引用类的任何成员时创建实例
private static readonly Singleton instance = new Singleton();
private Singleton()//构造方法私有
{
}
public static Singleton GetInstance()
{
return instance;
}
}