Introduction to singleton pattern of Java design pattern

Definition: the main function of singleton pattern is to ensure that there is only one instance of a class in Java applications.

In many operations, such as establishing a directory database connection, such a single thread operation is required.

Also, singleton can be stateful; In this way, multiple singleton classes can be used as a state warehouse to provide services. For example, you need to count the post counter in the forum every time you browse. Can singleton classes maintain this count and automatically add 1 to synchronize security? If you want to permanently save this number to the database, You can easily do this without modifying the singleton interface.

On the other hand, singleton can also be stateless. Provide functions of tool nature,

Singleton pattern provides us with the possibility of such implementation. The advantage of using singleton is that it can save memory, because it limits the number of instances and is conducive to Java garbage collection.

We often see that the class loader in factory mode is also implemented in singleton mode, because the loaded class actually belongs to resources.

How to use singleton mode

The general singleton mode usually has several forms: