1, What is the software design pattern?

Software Design Pattern, also known as design pattern, is a summary of code design experience that is repeatedly used, known by most people, classified and catalogued. It describes some recurring problems in the software design process and the solutions to the problems.

It is a series of routines to solve specific problems and a summary of the code design experience of predecessors. It is universal and can be used repeatedly.

Its purpose is to improve the reusability, readability and reliability of the code.

2, Singleton mode

1. What is singleton mode?

Definition of Singleton pattern: a pattern in which a class has only one instance and the class can create the instance itself.

2. Advantages and disadvantages of singleton mode

advantage:

• Singleton mode can ensure that there is only one instance in memory and reduce the memory overhead.
• Multiple occupation of resources can be avoided.
• Set global access points in singleton mode to optimize and share resource access.

Disadvantages:

• The singleton mode generally has no interface and is difficult to expand. If you want to expand, there is no second way except to modify the original code, which violates the opening and closing principle.
• In concurrent testing, singleton mode is not conducive to code debugging. During debugging, if the code in the singleton is not executed, a new object cannot be simulated.
• The function code of singleton mode is usually written in a class. If the function design is unreasonable, it is easy to violate the principle of single responsibility.

3. Implementation of singleton mode

Common implementation methods include lazy mode, hungry mode, double check lock, static internal class, enumeration, etc.

1. Lazy mode

public class Singleton {
    private static Singleton instance = null;
    private Singleton(){}
    public static synchronized Singleton getInstance(){
        //If it has not been instantiated, instantiate one and return
        if(instance == null){
            instance = new Singleton();
        }
        return instance;
    }
}

The keyword synchronized is used to ensure the thread safety of the getInstance method, but this method has great disadvantages. Because all threads need to queue after arriving at the method, it has a great loss on performance. This treatment solves the thread safety problem and does not solve the efficiency problem.

Lazy mode places the instantiation time when it needs to be used (hungry people have instances when the class is loaded), that is, "delayed loading". Compared with hungry people, it can avoid instantiating instances that may not be used during loading, but the problem is also obvious. We need to spend energy to solve the problem of thread safety.

1. Hungry man model

public class Singleton {
    //When the class is loaded, instance already points to an instance
    private static Singleton instance = new Singleton();
    private Singleton(){}
    public static Singleton getInstance(){
        return instance;
    }
}

Compared with the lazy mode, the hungry mode already has an instance when the class is loaded. The disadvantage of the hungry man: it may produce many useless examples.

1. Double check lock

public class Singleton {
    private static Singleton instance = null;
    private Singleton(){}
    public static Singleton getInstance(){
        if(instance == null){
            synchronized (Singleton.class){
                if(instance == null){
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

Double check lock is also a kind of delayed loading, and it solves the problem of low efficiency when ensuring thread safety. In contrast to the lazy mode, if you lock the entire getInstance method, you have to obtain the lock, release the lock and wait every time you call this method... While the double check lock only locks part of the code. After entering the method, if the check is empty, it will enter the synchronization code block, which is obviously much more efficient.

1. Static inner class

public class Singleton {
    private static class SingletonHolder{
        private static Singleton instance = new Singleton();
    }
    private Singleton(){}
    public static Singleton getInstance(){
        return SingletonHolder.instance;
    }
}

Static internal classes can not only enjoy the convenience of class loading to ensure thread safety, but also delay loading.

The characteristics of Java static internal classes are: internal static classes will not be loaded when loading, but will be loaded when using. When used, class loading is thread safe, which perfectly achieves our expected effect.

3, Factory mode

1. What is the factory model?

Define a factory interface for creating product objects, and postpone the actual creation of product objects to specific sub factory classes. This meets the requirement of "separation of creation and use" in the creation mode. In the actual development, we can try to use factory pattern instead of complex objects.

2. Simple factory mode

We call the created object "product" and the object that creates the product "factory". If there are not many products to be created, only one factory class can be completed. This mode is called "simple factory mode".
The method of creating an instance in the simple factory pattern is usually static.
Therefore, Simple Factory Pattern is also called Static Factory Method Pattern.

1. Implementation method

public class Client {
    //Abstract product
    public interface Product {
        void show();
    }
    //Specific product: ProductA
    static class ConcreteProduct1 implements Product {
        public void show() {
            System.out.println("Specific product 1 display...");
        }
    }
    //Specific product: ProductB
    static class ConcreteProduct2 implements Product {
        public void show() {
            System.out.println("Specific product 2 display...");
        }
    }
    final class Const {
        static final int PRODUCT_A = 0;
        static final int PRODUCT_B = 1;
        static final int PRODUCT_C = 2;
    }
    static class SimpleFactory {
        public static Product makeProduct(int kind) {
            switch (kind) {
                case Const.PRODUCT_A:
                    return new ConcreteProduct1();
                case Const.PRODUCT_B:
                    return new ConcreteProduct2();
            }
            return null;
        }
    }
}

1. Advantages and disadvantages

advantage:

• The factory class contains the necessary logical judgment to decide when to create an instance of which product. The client can avoid the responsibility of directly creating product objects and easily create corresponding products. The responsibilities of the factory and products are clearly distinguished.
• The client does not need to know the class name of the specific product created, but only needs to know the parameters.
• You can also import a configuration file to replace and add new specific product classes without modifying the client code.

Disadvantages:

• The factory class of simple factory mode is single, which is responsible for the creation of all products. The responsibility is too heavy. Once it is abnormal, the whole system will be affected. Moreover, the factory class code will be very bloated and violate the principle of high aggregation.
• Using the simple factory mode will increase the number of classes in the system (introduce new factory classes), and increase the complexity and understanding difficulty of the system
• It is difficult to expand the system. Once new products are added, the factory logic has to be modified. When there are many product types, the logic may be too complex
• The simple factory mode uses the static factory method, which makes the factory role unable to form an inheritance based hierarchical structure.

However, every time a product is added in the simple factory mode, a specific product class and a corresponding specific factory class will be added, which increases the complexity of the system and violates the "opening and closing principle".

3. Factory method mode

"Factory method pattern" is a further abstraction of the simple factory pattern. Its advantage is that the system can introduce new products without modifying the original code, that is, meet the opening and closing principle.

1. Implementation method

public class AbstractFactoryTest {
    public static void main(String[] args) {
        try {
            Product a;
            AbstractFactory af;
            af = (AbstractFactory) ReadXML.getObject();
            //Abstract factory contents are put into external configuration files, such as xml/properties, and loaded through I/O streams to create abstract factories
            a = af.newProduct();
            a.show();
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }
    }
}
//Abstract product: provides the interface of the product
interface Product {
    public void show();
}
//Concrete product 1: implement abstract methods in abstract products
class ConcreteProduct1 implements Product {
    public void show() {
        System.out.println("Specific product 1 display...");
    }
}
//Concrete product 2: implement abstract methods in abstract products
class ConcreteProduct2 implements Product {
    public void show() {
        System.out.println("Specific product 2 display...");
    }
}
//Abstract factory: provides the generation method of factory products
interface AbstractFactory {
    public Product newProduct();
}
//Specific factory 1: the generation method of factory products is realized
class ConcreteFactory1 implements AbstractFactory {
    public Product newProduct() {
        System.out.println("Specific factory 1 generation-->Specific products 1...");
        return new ConcreteProduct1();
    }
}
//Specific factory 2: the generation method of factory products is realized
class ConcreteFactory2 implements AbstractFactory {
    public Product newProduct() {
        System.out.println("Specific plant 2 generation-->Specific products 2...");
        return new ConcreteProduct2();
    }
}

public class ReadXML {
    //This method is used to extract the specific class name from the XML configuration file and return an instance object
    public static Object getObject() {
        try {
            //Create document object
            DocumentBuilderFactory dFactory = DocumentBuilderFactory.newInstance();
            DocumentBuilder builder = dFactory.newDocumentBuilder();
            Document doc;
            doc = builder.parse(new File("src/FactoryMethod/config1.xml"));
            //Gets the text node containing the class name
            NodeList nl = doc.getElementsByTagName("className");
            Node classNode = nl.item(0).getFirstChild();
            String cName = "FactoryMethod." + classNode.getNodeValue();
            //System.out.println("new class name:" + cName);
            //Generate an instance object from the class name and return it
            Class<?> c = Class.forName(cName);
            Object obj = c.newInstance();
            return obj;
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }

}

1. Advantages and disadvantages

advantage:

• Users only need to know the name of the specific factory to get the desired product, without knowing the specific creation process of the product.
• Flexibility is enhanced. For the creation of new products, you only need to write one more corresponding factory class.
• Typical decoupling framework. The high-level module only needs to know the abstract class of the product, does not need to care about other implementation classes, and meets the Demeter rule, dependency inversion principle and Richter substitution principle.

Disadvantages:

• The number of classes is easy to be too many, which increases the complexity
• It increases the abstraction and understanding difficulty of the system
• Abstract products can only produce one product, which can be solved by using abstract factory mode.

4. Abstract factory mode

Abstract factory pattern definition: it provides an interface for access classes to create a group of related or interdependent objects, and the access class can obtain the pattern structure of products of different levels of the same family without specifying the specific class of the product. Here are two concepts: same family and same level.

1. Implementation method

public class FarmTest {
    public static void main(String[] args) {
        try {
            Farm f;
            Animal a;
            Plant p;
            //Read the corresponding configuration information for the production plant
            f = (Farm) ReadXML.getObject();
            a = f.newAnimal();
            p = f.newPlant();
            a.show();
            p.show();
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }
    }
}
//Abstract products: Animals
interface Animal {
    public void show();
}
//Specific products: Horses
class Horse implements Animal {
    public Horse() {
        System.out.println("Generation of specific horses");
    }
    public void show() {
        System.out.println("Perform corresponding operations for horses");
    }
}
//Specific products: Cattle
class Cattle implements Animal {

    public Cattle() {
        //Generation of specific cattle
        System.out.println("Generation of specific cattle");
    }
    public void show() {
        System.out.println("Perform corresponding operations for horses");
    }
}
//Abstract products: Plants
interface Plant {
    public void show();
}
//Specific products: Fruits
class Fruitage implements Plant {

    public Fruitage() {
        System.out.println("Specific fruit generation");
    }
    public void show() {
        System.out.println("Perform corresponding operations for fruits");
    }
}
//Specific products: Vegetables
class Vegetables implements Plant {
    public Vegetables() {
        System.out.println("Specific vegetable generation");
    }
    public void show() {
        System.out.println("Perform the corresponding operations of vegetables");
    }
}
//Abstract factory: Farm class
interface Farm {
    public Animal newAnimal();
    public Plant newPlant();
}
//Specific factory: Farm 1
class SGfarm implements Farm {
    public Animal newAnimal() {
        System.out.println("A new cow is born!");
        return new Cattle();
    }
    public Plant newPlant() {
        System.out.println("Vegetables grow!");
        return new Vegetables();
    }
}
//Specific factory: Farm 2
class SRfarm implements Farm {
    public Animal newAnimal() {
        System.out.println("The new horse was born!");
        return new Horse();
    }
    public Plant newPlant() {
        System.out.println("Fruit grows!");
        return new Fruitage();
    }
}

public class ReadXML2 {
    public static Object getObject() {
        try {
            DocumentBuilderFactory dFactory = DocumentBuilderFactory.newInstance();
            DocumentBuilder builder = dFactory.newDocumentBuilder();
            Document doc;
            doc = builder.parse(new File("src/AbstractFactory/config.xml"));
            NodeList nl = doc.getElementsByTagName("className");
            Node classNode = nl.item(0).getFirstChild();
            String cName = "AbstractFactory." + classNode.getNodeValue();
            System.out.println("New class name:" + cName);
            Class<?> c = Class.forName(cName);
            Object obj = c.newInstance();
            return obj;
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }
}

1. Advantages and disadvantages

advantage:

• Abstract factory pattern is optimized based on factory method pattern. All factory method patterns have advantages.

• The multi-level products associated in the product family can be managed together within the class, instead of introducing multiple new classes for management.

• When a product family is required, the abstract factory can ensure that the client always uses only the product group of the same product.

• Abstract factory enhances the scalability of the program. When adding a new product family, there is no need to modify the original code to meet the opening and closing principle.

Disadvantages:

• When a new product needs to be added to the product family, all factory classes need to be modified. It increases the abstraction and understanding difficulty of the system.

summary

The first is to have a certain understanding of software design pattern. It is a series of routines to solve specific problems, a summary of code design experience, and has a certain universality. Its purpose is to improve the reusability, readability and reliability of the code. Software design pattern can be divided into single instance pattern and factory pattern, and factory pattern can also be divided into simple factory pattern, factory method pattern and abstract factory pattern. For different application scenarios, we can choose different design patterns.

reference resources:
https://segmentfault.com/a/1190000040289636.
https://segmentfault.com/a/1190000040297407.
https://segmentfault.com/a/1190000040319351.