模板设计模式的使用

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1.什么是模板设计模式
定义：有一个抽象父类定义了模板方法，子类继承父类的方法，按照各自的需求来实现父类的方法。

2.类图：

3.示例：

package com.buka.designer.template;

/** 编程是门艺术，设计源于生活！
 */
public abstract class Ticket {

    //模板方法
    public void buyTicket() {
        login();
        selectTicket();
        pay();
    }

    public final void login() {
        System.out.println("登录。。。");
    }

    //子类必须实现
    public abstract void selectTicket();

    //子类可以不实现
    public void pay() {
        System.out.println("支付宝。。。");
    }
}

package com.buka.designer.template;

/**
 * 火车票
 */
public class TrainTicket extends Ticket{

    @Override
    public void selectTicket() {
        System.out.println("选择火车票。。。");
    }

    @Override
    public void pay() {

        System.out.println("使用微信支付。。。");

    }
}

package com.buka.designer.template;

/**
 * 船票
 */
public class ShipTicket extends Ticket{

    @Override
    public void selectTicket() {

        System.out.println("选择船票。。。");

    }
}

package com.buka.designer.template;

/**
 * 测试
 */
public class TemplateTest {

    public static void main(String[] args) {

        Ticket trainTicket = new TrainTicket();
        trainTicket.buyTicket();

        Ticket shipTicket = new ShipTicket();
        shipTicket.buyTicket();

    }
}

4.用用场景： spring源码中使用模板方法设计模式，例如：AbstractXmlApplicationContext作为父类，其子类有ClassPathXmlApplicationContext，FileSystemXmlApplicationContext等等。 FileSystemXmlApplicationContext集成getResourceByPath方法：

/**
	 * Resolve resource paths as file system paths.
	 * 
Note: Even if a given path starts with a slash, it will get
	 * interpreted as relative to the current VM working directory.
	 * This is consistent with the semantics in a Servlet container.
	 * @param path path to the resource
	 * @return the Resource handle
	 * @see org.springframework.web.context.support.XmlWebApplicationContext#getResourceByPath
	 */
	@Override
	protected Resource getResourceByPath(String path) {
		if (path.startsWith("/")) {
			path = path.substring(1);
		}
		return new FileSystemResource(path);
	}

}

ClassPathXmlApplicationContext类继承了AbstractXmlApplicationContext的getConfigResources方法

@Override
	@Nullable
	protected Resource[] getConfigResources() {
		return this.configResources;
	}

XmlWebApplicationContext类继承了父类的loadBeanDefinitions，getDefaultConfigLocation两个方法。

/**
	 * Loads the bean definitions via an XmlBeanDefinitionReader.
	 * @see org.springframework.beans.factory.xml.XmlBeanDefinitionReader
	 * @see #initBeanDefinitionReader
	 * @see #loadBeanDefinitions
	 */
@Override
	protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
		// Create a new XmlBeanDefinitionReader for the given BeanFactory.
		XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

		// Configure the bean definition reader with this context's
		// resource loading environment.
		beanDefinitionReader.setEnvironment(getEnvironment());
		beanDefinitionReader.setResourceLoader(this);
		beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

		// Allow a subclass to provide custom initialization of the reader,
		// then proceed with actually loading the bean definitions.
		initBeanDefinitionReader(beanDefinitionReader);
		loadBeanDefinitions(beanDefinitionReader);
	}
	
	/**
	 * The default location for the root context is "/WEB-INF/applicationContext.xml",
	 * and "/WEB-INF/test-servlet.xml" for a context with the namespace "test-servlet"
	 * (like for a DispatcherServlet instance with the servlet-name "test").
	 */
	@Override
	protected String[] getDefaultConfigLocations() {
		if (getNamespace() != null) {
			return new String[] {DEFAULT_CONFIG_LOCATION_PREFIX + getNamespace() + DEFAULT_CONFIG_LOCATION_SUFFIX};
		}
		else {
			return new String[] {DEFAULT_CONFIG_LOCATION};
		}
	}