【Java设计模式】五、建造者模式

1、建造者模式

• 某个对象的构建复杂
• 将复杂的对象的创建 和 属性赋值所分离，使得同样的构建过程可以创建不同的表示
• 建造的过程和细节调用者不需要知道，只需要通过构建者去进行操作

如，主机这个负责对象的构建，分离承诺内存条、主板这些部件，再组装构建。内存条替换一个别的，出来就是一个不同的对象。

建造者模式相关角色：

• 产品类：复杂对象，主机
• 抽象建造者类Builder：规定要实现复杂对象的哪些部件的创建
• 具体建造者类ConcreteBuilder：实现Builder，完成各个部件的具体创建
• 指挥者类Director：保证对象的各个部分按照某种顺序创建

2、案例：共享单车的创建

摩拜单车和ofo单车。Bike是产品，包含车架，车座等组件；Builder是抽象建造者，MobikeBuilder和OfoBuilder是具体的建造者，Director是指挥者。

具体的代码如下：

//自行车类
@Data
public class Bike {private String frame;  //车架private String seat;   //座椅}

抽象构建者Builder定义哪些部分要构建：

// 抽象 builder 类
public abstract class Builder {protected Bike mBike = new Bike();public abstract void buildFrame();   //构建车架public abstract void buildSeat();    //构建座椅public abstract Bike createBike();   //构建自行车}

写具体的构建者：

//摩拜单车Builder类
public class MobikeBuilder extends Builder {@Overridepublic void buildFrame() {mBike.setFrame("铝合金车架");}@Overridepublic void buildSeat() {mBike.setSeat("真皮车座");}@Overridepublic Bike createBike() {return mBike;}
}

//ofo单车Builder类
public class OfoBuilder extends Builder {@Overridepublic void buildFrame() {mBike.setFrame("碳纤维车架");}@Overridepublic void buildSeat() {mBike.setSeat("橡胶车座");}@Overridepublic Bike createBike() {return mBike;}
}

指挥者类：

//指挥者类
public class Director {private Builder mBuilder;   //声明Builder类型的变量public Director(Builder builder) {mBuilder = builder;}//组装自行车的方法public Bike construct() {mBuilder.buildFrame();mBuilder.buildSeat();return mBuilder.createBike();}
}

客户端测试：

public class Client {public static void main(String[] args) {showBike(new OfoBuilder());showBike(new MobikeBuilder());}private static void showBike(Builder builder) {Director director = new Director(builder);   //创建指挥者Bike bike = director.construct();   //让指挥者指挥组装System.out.println(bike.getFrame());System.out.println(bike.getSeat());}
}

再有新的单车公司加入，只需实现一个新的建造者类即可，符合开闭原则。但如果创建的产品组成部分差异很大，则不适合用建造者模式，比如要再造个电脑对象。

3、其他用途

@Setter
@Getter
public class Phone {private String cpu;private String screen;private String memory;private String mainboard;public Phone(String cpu, String screen, String memory, String mainboard) {this.cpu = cpu;this.screen = screen;this.memory = memory;this.mainboard = mainboard;}
}

对于属性多的对象，创建其对象往往可读性很差，如：

//构建Phone对象
Phone phone = new Phone("intel","三星屏幕","金士顿","华硕");

重构一下（@Builder注解的思路）：

public class Phone {private String cpu;private String screen;private String memory;private String mainboard;//私有的构造方法private Phone(Builder builder) {cpu = builder.cpu;screen = builder.screen;memory = builder.memory;mainboard = builder.mainboard;}public static final class Builder {private String cpu;private String screen;private String memory;private String mainboard;public Builder() {}public Builder cpu(String val) {cpu = val;return this;}public Builder screen(String val) {screen = val;return this;}public Builder memory(String val) {memory = val;return this;}public Builder mainboard(String val) {mainboard = val;return this;}//!!!!!public Phone build() {return new Phone(this);}}@Overridepublic String toString() {return "Phone{" +"cpu='" + cpu + '\'' +", screen='" + screen + '\'' +", memory='" + memory + '\'' +", mainboard='" + mainboard + '\'' +'}';}
}

此时就可链式编程创建这个对象（new Phone.Builder()创建静态内部类对象）：

public class Client {public static void main(String[] args) {Phone phone = new Phone.Builder().cpu("intel").mainboard("华硕主板").memory("金士顿内存").screen("三星屏幕").build();System.out.println(phone);}
}