java设计模式--Builder模式

一.Builder模式

二.使用例子

三.Spring中的Builder模式

Builder模式，构建者、构造者模式，在《图解设计模式》中归为 生成实例 一栏，该模式用于组装具有复杂结构的实例；

　　　　当需要逐步获取对象的初始值时，可以使用Builder模式；

一.Builder模式角色

Builder：建造者，Builder角色负责定义用来生成实例的接口API，Builder角色中准备了用于生成实例的具体方法；

ConcreteBuilder：具体的建造者，负责实现Builder接口的的类，定义了生成实例时实际调用的方法，定义了获取最终生成结果的方法；

Director：监工，负责使用Builder角色的接口来生成实例，不依赖于ConcreteBuilder；

二.使用例子

Demo：比如有一段预约信息，将其解析为  预约 对象； 预约信息如下："Date, November 5, Headcount, 250, City, Shanghai, DollarsPerHead, 60,HasSite, false"；

date为预约日期，headCount为预期来人数，city为城市，DollarsPerHead为人均预算，hasSite为是否提供地址(比如预约某场演出，是否给表演者提供地点了)；

类结构图：

                                           

预约信息  Reservation.java

@Getter
@Setter
@ToString   //lombok的注解，方便代码查看以及编写
public class Reservation {
    //Date, November 5, Headcount, 250, City, Shanghai, DollarsPerHead, 60,HasSite, false
    private Date date;
    private int headCount;
    private String city;
    private double dollarsPerHead;
    private boolean hasSite;
}

ReservationBuilder.java

public interface ReservationBuilder {
    
    public ReservationBuilder futurize(Date date) ;
    
    public ReservationBuilder setCity(String city);
    
    public ReservationBuilder setDollarsPerHead(double dollarsPerHead);
    
    public ReservationBuilder setSite(boolean hasSite);
    
    public ReservationBuilder setHeadCount(int headCount);
    
    Reservation build();
}

DefaultReservationBuilder.java

public class DefaultReservationBuilder implements ReservationBuilder{
    private Date date;
    private int headCount;
    private String city;
    private double dollarsPerHead;
    private boolean hasSite;
    
    public ReservationBuilder futurize(Date date) {
        this.date=date;
        return this;
    }
    
    public ReservationBuilder setCity(String city) {
        this.city=city;
        return this;
    }
    
    public ReservationBuilder setDollarsPerHead(double dollarsPerHead) {
        this.dollarsPerHead=dollarsPerHead;
        return this;
    }
    
    public ReservationBuilder setSite(boolean hasSite) {
        this.hasSite=hasSite;
        return this;
    }
    
    public ReservationBuilder setHeadCount(int headCount) {
        this.headCount=headCount;
        return this;
    }
    
    public Reservation build() {
        Reservation reservation = new Reservation();
        reservation.setCity(this.city);
        reservation.setDate(this.date);
        reservation.setDollarsPerHead(this.dollarsPerHead);
        reservation.setHasSite(this.hasSite);
        reservation.setHeadCount(this.headCount);
        return reservation;
    }
}

ReservationDirector.java

public class ReservationDirector {

    private ReservationBuilder builder;
    public ReservationDirector(ReservationBuilder builder) {
        this.builder=builder;
    }
    
    public Reservation construct(String input) throws Exception {
        String[] strings = input.split(",\s*");
        for(int i=0;i<strings.length-1;i++) {
            String type=strings[i];
            String val=strings[i+1];
            if("date".equalsIgnoreCase(type)) {
                int year = Calendar.getInstance().get(Calendar.YEAR);
                String res=year+ " "+val.substring(0, 3)+" "+val.substring(val.length()-2);
                SimpleDateFormat sdf = new SimpleDateFormat("yyyy MMM dd", Locale.ENGLISH);
                Date date = sdf.parse(res);
                builder.futurize(date);
            }else if("headcount".equalsIgnoreCase(type)) {
                builder.setHeadCount(Integer.valueOf(val));
            }else if("city".equalsIgnoreCase(type)) {
                builder.setCity(val);
            }else if("dollarsperHead".equalsIgnoreCase(type)) {
                builder.setDollarsPerHead(Double.parseDouble(val));
            }else if("hassite".equalsIgnoreCase(type)) {
                builder.setSite(Boolean.parseBoolean(val));
            }
        }
        Reservation reservation = builder.build();
        return reservation;
    }
}

测试方法：

    public static void main(String[] args) throws Exception {
        //Date, November 5, Headcount, 250, City, Shanghai, DollarsPerHead, 60,HasSite, false
        String input="Date, November 5, Headcount, 20, City, Shanghai, DollarsPerHead, 60,HasSite, false";
        ReservationBuilder builder=new DefaultReservationBuilder();
        ReservationDirector director=new ReservationDirector(builder);
        Reservation reservation = director.construct(input);
        System.out.println(reservation);
    }

测试输出：

三.Spring中的Builder模式

　　Spring中 RequestMappingInfo中内部接口Builder 、内部静态类DefaultBuilder等，就是采用了Builer模式，RequestMappingInfo就是监工Director对象；类结构图：

              

    public interface Builder {
        /* Set the path patterns*/
        Builder paths(String... paths);

        /* Set the request method conditions*/
        Builder methods(RequestMethod... methods);

        /* Set the request param conditions*/
        Builder params(String... params);

        /*Set the header conditions*/
        Builder headers(String... headers);

        /* Set the consumes conditions*/
        Builder consumes(String... consumes);

        /* Set the produces conditions*/
        Builder produces(String... produces);

        /* Set the mapping name*/
        Builder mappingName(String name);

        /* Set a custom condition to use*/
        Builder customCondition(RequestCondition<?> condition);

        /* Provide additional configuration needed for request mapping purposes*/
        Builder options(BuilderConfiguration options);

        /** Build the RequestMappingInfo*/
        RequestMappingInfo build();
    }

创建RequestMappingInfo对象地方：作用就是解析RequestMapping注解生成对应RequestMappingInfo对象；    

Builder方法大部分返回值为Builder类型，作用是为了链式执行，看起来生成RequestMappingInfo对象代码只有一句简洁；