Template method pattern

Introduction to template methods and patterns

• Template method pattern (Template Method Pattern), Also called template mode (Template Pattern), In an abstract class, the template that defines the method to execute it is exposed . Its subclass can override method implementation as needed , But the call will take place in the way defined in the abstract class .
• In short , The template method pattern defines the skeleton of an algorithm in operation , Instead, defer some steps to subclasses , So that the subclass can not change the structure of an algorithm , You can redefine some specific steps of the algorithm
• Template method pattern belongs to behavior pattern .

Case a ： Template method pattern

The procedure for preparing soybean milk is as follows :

• Process and material selection for making soybean milk -- Add ingredients – soak – Break it in the soymilk machine
• By adding different ingredients , You can make soymilk with different flavors
• Material selection 、 Soaking and smashing in a soymilk machine are the same for every flavor of soymilk
  

// abstract class , It means soybean milk 
public abstract class SoyaMilk {
    
	// Template method , make ,  Template method can be made into final ,  Don't let subclasses cover .
	final void make() {
    
		select(); 
		addCondiments();
		soak();
		beat();
	}
	// Selected materials 
	void select() {
     System.out.println(" First step ： Choose good fresh soybeans  "); }
	// Add different ingredients ,  Abstract method ,  Subclass specific implementation 
	abstract void addCondiments();
	// soak 
	void soak() {
     System.out.println(" The third step ,  Soybeans and ingredients begin to soak ,  need 3 Hours  "); }
	void beat() {
     System.out.println(" Step four ： Put the soybeans and ingredients into the soymilk machine to break them  "); }
}

public class RedBeanSoyaMilk extends SoyaMilk {
    
	@Override
	void addCondiments() {
    
		System.out.println("  Add the best red beans  ");
	}
}

public class PeanutSoyaMilk extends SoyaMilk {
    
	@Override
	void addCondiments() {
    
		System.out.println("  Add good peanuts  ");
	}
}

public class Client {
    
	public static void main(String[] args) {
    
		System.out.println("---- Make red bean milk ----");
		SoyaMilk redBeanSoyaMilk = new RedBeanSoyaMilk();
		redBeanSoyaMilk.make();
		
		System.out.println("---- Making peanuts and soymilk ----");
		SoyaMilk peanutSoyaMilk = new PeanutSoyaMilk();
		peanutSoyaMilk.make();
	}
}

Case 2 ： Improved hook method of template method pattern

• In the parent class of the template method pattern , We can define a method , It does nothing by default , Subclasses can override it as appropriate , This method is called “ hook ”.
• such as , We also want to make pure soymilk , No ingredients added , Use the hook method to transform the previous template method
  

// abstract class , It means soybean milk 
public abstract class SoyaMilk {
    
	// Template method , make ,  Template method can be made into final ,  Don't let subclasses cover .
	final void make() {
    
		select(); 
		if(customerWantCondiments()) {
    
			addCondiments();
		}
		soak();
		beat();
	}
	// Selected materials 
	void select() {
     System.out.println(" First step ： Choose good fresh soybeans  "); }
	// Add different ingredients ,  Abstract method ,  Subclass specific implementation 
	abstract void addCondiments();
	// soak 
	void soak() {
     System.out.println(" The third step ,  Soybeans and ingredients begin to soak ,  need 3 Hours  "); }
	void beat() {
     System.out.println(" Step four ： Put the soybeans and ingredients into the soymilk machine to break them  "); }
	// Hook method , Decide if ingredients need to be added 
	boolean customerWantCondiments() {
     return true; }
}

public class PureSoyaMilk extends SoyaMilk{
    
	@Override
	void addCondiments() {
    
		// Empty implementation 
	}
	@Override
	boolean customerWantCondiments() {
    
		return false;
	}
}

public class Client {
    
	public static void main(String[] args) {
    
		System.out.println("---- Make pure soybean milk ----");
		SoyaMilk pureSoyaMilk = new PureSoyaMilk();
		pureSoyaMilk.make();
	}
}

The template method pattern is in Spring Source code analysis of framework application

Spring IOC The template method pattern used in container initialization

public interface ConfigurableApplicationContext extends ApplicationContext, Lifecycle, Closeable {
    
    void refresh();
}

postProcessBeanFactory It's the hook method
onRrfresh Also a hook method .
They are in the template method refresh By using

public abstract class AbstractApplicationContext extends DefaultResourceLoader implements ConfigurableApplicationContext {
    

    public void refresh() throws BeansException, IllegalStateException {
    
        synchronized (this.startupShutdownMonitor) {
    
            StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");
            this.prepareRefresh();
            ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
            this.prepareBeanFactory(beanFactory);

            try {
    
                this.postProcessBeanFactory(beanFactory);
                StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
                this.invokeBeanFactoryPostProcessors(beanFactory);
                this.registerBeanPostProcessors(beanFactory);
                beanPostProcess.end();
                this.initMessageSource();
                this.initApplicationEventMulticaster();
                this.onRefresh();
                this.registerListeners();
                this.finishBeanFactoryInitialization(beanFactory);
                this.finishRefresh();
            } catch (BeansException var10) {
    
                if (this.logger.isWarnEnabled()) {
    
                    this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var10);
                }

                this.destroyBeans();
                this.cancelRefresh(var10);
                throw var10;
            } finally {
    
                this.resetCommonCaches();
                contextRefresh.end();
            }
        }
    }
}

summary

• The basic idea is : Algorithms only exist in one place , That is, in the parent class , Easy to modify . When you need to modify the algorithm , Just modify the template method of the parent class or some steps that have been implemented , Subclasses inherit these changes
• Achieve maximum code reuse . The template method of the parent class and some implemented steps will be inherited by the subclass and used directly .
• It unifies the algorithm , It also provides a lot of flexibility . The template method of the parent class ensures that the structure of the algorithm remains unchanged , At the same time, the implementation of some steps is provided by subclasses .
• The shortcomings of this model : Each different implementation requires a Subclass implementation , Causes the number of classes to increase , Make the system bigger
• General template methods add final keyword , Prevent subclasses from overriding template methods .
• Template method pattern usage scenarios : When a process is to be completed , The process involves a series of steps , This series of steps is basically the same , But its individual steps may be different when implemented , The template method pattern is usually considered to handle