Understanding of singleton mode

  Three private and one public

Private static variables （ Save an instance of the class ）

When the function is finished , Static variables do not disappear

Static variables are initialized only once

Private constructor （ Prevent out of class instantiation ）

Private clone function （ Prevent objects from being generated by cloning ）

Public static methods ( Usually named getInstance), Returns a reference to a unique instance （ Calling this class is equivalent to an interface ）

  The solution is how to create a unique object instance in the whole project

Make sure that a class has at most one instance , And provide a global access point

It does not create instance copies , Instead, a reference is returned to the instance stored inside the singleton class

Because every time you open and close the database, it is a great test and waste to the database

  advantage ：

Avoid excessive new Operation consumes a lot of resources , Solve memory overhead

Some configuration information can be controlled globally through a class

You can also look at the code

  There are two types ： Hungry and lazy

Hungry Chinese style ： The class was created when the instance was loaded , Waiting to be used by program

Slacker type ： Create the singleton class object only when you really need to use it

How does lazy style ensure that only one object is created

Double Check（ Double check ） + Lock（ Lock ）

If the object is not instantiated, lock to create , If you have instantiated , There is no need to lock , Get the instance directly

  The choice of hungry and lazy

If the memory requirements are not high during development , Use the hungry Chinese style , Because it's easy to make mistakes , And there are no concurrent security and performance issues

If the memory requirements are very high , So use the lazy way , You can create the object at a specific time