Design Patterns

责任链模式与if-else 这个模式对于繁琐的if-else判断非常好用，也比较适合写框架代码。 1、首先if-else会让代码变得丑陋，模块与流程耦合，对于复杂的判断直接影响主业务流程，不利于扩展。 2、引入链（chain）则是让模块与流程解耦，如果A不能做，就让B做，依次类推。 举个例子，对于一个公司采购的审批，比如金额在1000以内让经理审批，1000-5000之前让主管审批。。。可以用if-else写，当然也可以用责任链

public class UglyPurchasePowerHandler {

    private PurchaseRequest request;

    public UglyPurchasePowerHandler(PurchaseRequest request) {
        this.request = request;
    }

    public void doHandler(PurchaseRequest request) {
        double totalAmount = request.getAmount();
        if (totalAmount <= 1000) {
            System.out.println("your purchase request will send to manager!");
        } else if (totalAmount > 1000 && totalAmount <= 5000) {
            System.out.println("your purchase request will send to director!");
        } else {
            System.out.println("your purchase request can't be approved!");
        }
    }

    public PurchaseRequest getRequest() {
        return request;
    }

    public void setRequest(PurchaseRequest request) {
        this.request = request;
    }

}

代码里面使用分支判断，这样做的弊端就是doHandler里面处理了所有的逻辑判断，将来扩展的时候要在这里面加入，其实这里面每一个分支判断就是一个不同的流程，对应不能的角色，完全可以分开，而主流程就是调用handler并且传递。

 @Override
    public void doHandler() {
        if (request.getAmount() <= 1000) {
            System.out.println("your purchase request will send to manager!");
        } else if (null != successorHandler) {
            successorHandler.doHandler();
        }

    }

上面是一个manager角色的handler。代码调用区别见下面

public class PurchaseHandlerTest {
    public static void main(String[] args) {
        PurchaseRequest request = new PurchaseRequest(2000, "test");
        UglyPurchasePowerHandler handler = new UglyPurchasePowerHandler(request);
        handler.doHandler(request);
       //////////////////////////////////////////////////
        DefaultPurchaseHandler defaultHandler = new DefaultPurchaseHandler(null, request);
        DirectorPurchaseHandler directorHandler = new DirectorPurchaseHandler(defaultHandler,
            request);
        ManagerPurchaseHandler managerHandler = new ManagerPurchaseHandler(directorHandler, request);

        managerHandler.doHandler();
    }
}

解耦，关键是解耦，利于扩展。比如将来需要增加角色审批，加一个handler，不需要修改主业务流程。 优点：对于分支比较多的流程建议采用这种方式，加了分支后也仅仅是增加一节“链”，调用流程不会修改，这样也方便单元测试 缺点：代码量增加，增加一个handler需要增加一个类。 当handler变多时最好抽象出模版方法，减少重复代码。

1.1 What is a Design Pattern?

Each pattern describes a problem which occur over and over again in our environment, and then describes the core of the solution to that problem.

1.5 Organizing the Catalog

Creational patterns concern the process of object creation.

Structural patterns deal with the composition of classes or objects.

Behavirial patterns characterize the ways in which classes or objects interact and distribure responsibility.

First, it shows the role that patterns can play in architexting complex system.

Second, it provides a very pragmatic reference to a set of will-engineered patterns.