Factory method pattern of Java design pattern

A calf carrying a load 2020-11-08 21:03:40
factory method pattern java design

Definition of factory method pattern :

The factory approach pattern is also known as the polymorphic factory pattern , Defines an interface for creating objects , But it's up to the class that implements the interface to decide which class to instantiate , The factory method defers the instantiation of a class to a subclass .

In the factory method pattern , There is no longer a single factory that produces products , Instead, subclasses of factory classes implement the creation of specific products . therefore , When adding a product , Just add a subclass of the corresponding factory class , Realization

Product creation , It can solve the problem that the simple factory produces too many products, resulting in its internal code bloated (switch...case There are too many branches ) The problem of , It also conforms to the opening and closing principle . Although in the simple factory model , We use reflection to solve

That's the question , But there are limitations to this approach , Because we can't guarantee the construction method of every object we need to create , The parameters are uniform .

Application scenarios of the factory method pattern :

  • Creating objects requires a lot of repetitive code .
  • client ( application layer ) Does not depend on how product class instances are created 、 Implementation details, etc .
  • A class uses its subclasses to specify which object to create .

Factory method mode UML Class diagram :

As can be seen from the picture above , The factory approach pattern mainly includes 4 A character .

  1. Abstract factory (IFactory): It's the core of the factory approach pattern , It's not about the application . Any factory class that creates objects in a pattern must implement this interface .
  2. Specific factory (FactoryA,B,C): Is a concrete factory class that implements the abstract factory interface , Contains logic that is closely related to the application , And is called directly by the application to create the object of the product .
  3. Abstract product (IProduct): Is the supertype of the object created by the factory method pattern , That is, the public parent class of the product object or the public owned interface , In order to achieve the decoupling effect .
  4. Specific products :(ProductA,B,C): This role implements the interface defined by the abstract product role . Each specific product corresponds to a specific factory .

  Factory method model case :

package com.liuyi.designmode.creational.factory;
* E-book interface
public interface EBook {
* Reading methods
public void read();
package com.liuyi.designmode.creational.factory;
* java Language books
public class JavaBook implements EBook {
public void read() {
System.out.println(" read java Language books ");
package com.liuyi.designmode.creational.factory;
* C Language books
public class CBook implements EBook {
public void read() {
System.out.println(" read c Language books ");
package com.liuyi.designmode.creational.factory;
* Abstract factory interface
public interface EBookFactory {
public EBook createEBook();
package com.liuyi.designmode.creational.factory;
* establish java The specific factory class of language books
public class EJavaBookFactory implements EBookFactory{
public EBook createEBook(){
EBook eBook = new JavaBook();
return eBook;
package com.liuyi.designmode.creational.factory;
* establish C The specific factory class of language books
public class ECBookFactory implements EBookFactory{
public EBook createEBook(){
EBook eBook = new CBook();
return eBook;
package com.liuyi.designmode.creational.factory;
* Simple factory mode test class
public class FactoryTest {
public static void main(String[] args) {
// For example, I want to read java Books
// use java Factory class creation 
EBookFactory javaFactory = new EJavaBookFactory();
EBook eBook = javaFactory.createEBook();

The factory approach pattern is in slf4j Source code in the use of :

  We all specify , Use slf4j You can switch between different log frameworks , But users themselves don't feel that the underlying logging framework has changed , Because the way users use logs is always like this

public static final Logger logger = LoggerFactory.getLogger(FactoryTest.class); Let's see slf4j How to realize the source code of :

  Get into getLogger Method , You can see an abstract factory role



Enter the factory interface , You can see that there is only one way



  See the subclass of the interface :


  Get into getILoggerFactory Method , You can see that getting the specific factory class is practical to the simple factory method pattern , Here it is slf4j The mechanism through which to choose which logging framework to use will not be discussed , Students who are interested can do it by themselves

Go to see .


  Here we get the specific set-up factory , And then create the corresponding logger object , If you need to add a logging implementation , First create an implementation ILoggerFactory Our factory , Then create an implementation Logger

Log product class of , We can extend the function without changing the usage .

Advantages of factory method mode :

  • Increased flexibility , For the creation of new products , Just write one more factory class .
  • A typical decoupling framework . High level modules only need to know the abstract class of the product , Don't care about other implementation classes , Satisfy the Dimitar rule 、 Rely on the inversion principle and the Ritz substitution principle .

Disadvantages of factory method pattern :

  • The number of classes is easy to increase , Add complexity .
  • Increases the abstractness of the system and the difficulty of understanding .
  • Abstract products can only be one product of Holy Communion , This drawback can be solved by using the abstract factory pattern .
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