建造者模式-白红宇

建造者模式

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发布时间：2019-06-24

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建造者模式

标签： Java与设计模式

建造者模式: 又称生成器模式, 能够将一个产品的内部表象与产品的生成过程切割开来, 从而能够使一个建造过程生成具有不同的内部表象的产品(将一个复杂对象的构建与它的表示分离, 使得同样的构建过程能够创建不同的表示). 这样用户仅仅需指定须要建造的类型就能够得到详细产品,而不须要了解详细的建造过程和细节.

与抽象工厂的差别:
在建造者模式中,角色分指导者(Director)与建造者(Builder): 用户联系指导者, 指导者指挥建造者, 最后得到产品. 建造者模式能够强制实行一种分步骤进行的建造过程.

实现

需求: 模仿宇宙飞船的建造过程    如果宇宙飞船有非常多零部件: 引擎、轨道舱、逃逸塔、各种小零件... 因此宇宙飞船的建造/装配非常复杂(须要非常好的生产/装配技术),而建造者模式能够将部件的建造与装配分开:

产品与部件

产品AirShip由多个零部件(Engine/EscapeTower/OrbitalModule)组成:

/** * 目标对象 - 宇宙飞船 * (代表复杂对象, 拥有复杂的建造过程) * Created by jifang on 15/12/8. */public class AirShip {
          private Engine engine;    private EscapeTower escapeTower;    private OrbitalModule orbitalModule;    public Engine getEngine() {        return engine;    }    public void setEngine(Engine engine) {        this.engine = engine;    }    public EscapeTower getEscapeTower() {        return escapeTower;    }    public void setEscapeTower(EscapeTower escapeTower) {        this.escapeTower = escapeTower;    }    public OrbitalModule getOrbitalModule() {        return orbitalModule;    }    public void setOrbitalModule(OrbitalModule orbitalModule) {        this.orbitalModule = orbitalModule;    }    @Override    public String toString() {        return "AirShip{" +                "engine=" + engine +                ", escapeTower=" + escapeTower +                ", orbitalModule=" + orbitalModule +                '}';    }}class Engine {    private String description;    public Engine(String description) {        this.description = description;    }    @Override    public String toString() {        return "Engine{" +                "description='" + description + '\'' +                '}';    }}class EscapeTower {    private String description;    public EscapeTower(String description) {        this.description = description;    }    @Override    public String toString() {        return "EscapeTower{" +                "description='" + description + '\'' +                '}';    }}class OrbitalModule {    private String description;    public OrbitalModule(String description) {        this.description = description;    }    @Override    public String toString() {        return "OrbitalModule{" +                "description='" + description + '\'' +                '}';    }}

建造者(Builder)

Builder(AirShipBuilder)是为创建一个Product对象的各个部件指定的抽象接口, ConcreteBuilder(LowerAirShipBuilder/HigherAirShipBuilder)是详细的建造者, 实现Builder接口, 构造和装配各个部件.

/** * @author jifang * @since 16/8/17 下午2:13. */public interface AirShipBuilder {
          void builtEngine();    void builtEscapeTower();    void builtOrbitalModule();    AirShip getResult();}

生产低端飞船, 须要LowerAirShipBuilder; 生产高端飞船, 就须要HigherAirShipBuilder:

class LowerAirShipBuilder implements AirShipBuilder {    private AirShip airShip = new AirShip();    @Override    public void builtEngine() {        System.out.println("\t\t构造低端引擎");        airShip.setEngine(new Engine("低端 - 引擎"));    }    @Override    public void builtEscapeTower() {        System.out.println("\t\t构造低端逃逸塔");        airShip.setEscapeTower(new EscapeTower("低端 - 逃逸塔"));    }    @Override    public void builtOrbitalModule() {        System.out.println("\t\t构造低端轨道舱");        airShip.setOrbitalModule(new OrbitalModule("低端 - 轨道舱"));    }    @Override    public AirShip getResult() {        return airShip;    }}class HigherAirShipBuilder implements AirShipBuilder {    private AirShip airShip = new AirShip();    @Override    public void builtEngine() {        System.out.println("\t\t构造高端引擎");        airShip.setEngine(new Engine("高端 - 引擎"));    }    @Override    public void builtEscapeTower() {        System.out.println("\t\t构造高端逃逸塔");        airShip.setEscapeTower(new EscapeTower("高端 - 逃逸塔"));    }    @Override    public void builtOrbitalModule() {        System.out.println("\t\t构造高端轨道舱");        airShip.setOrbitalModule(new OrbitalModule("高端 - 轨道舱"));    }    @Override    public AirShip getResult() {        return airShip;    }}

指挥者(Director)

使用Director(AirShipDirector)控制建造过程, 也用它来隔离用户与建造过程的关联:

/** * @author jifang * @since 16/8/17 下午2:15. */public class AirShipDirector {
          /**     * 确定一种稳定的构造过程     *     * @param builder     */    public static void construct(AirShipBuilder builder) {        builder.builtEngine();        builder.builtEscapeTower();        builder.builtOrbitalModule();    }}

Client

全然不需知道详细的创建/装配过程, 仅仅需指定Builder:

public class Client {    @Test    public void client() {        AirShipBuilder lowBuilder = new LowerAirShipBuilder();        // 构造低端飞船        AirShipDirector.construct(lowBuilder);        AirShip lowShip = lowBuilder.getResult();        System.out.println(lowShip);        AirShipBuilder highBuilder = new HigherAirShipBuilder();        // 同样的构造过程, 不同的Builder, 能够构造出不同的飞船        AirShipDirector.construct(highBuilder);        AirShip highShip = highBuilder.getResult();        System.out.println(highShip);    }}

实例-MyBatis中的建造者模式

MyBatis的SqlSessionFactoryBuilder是对SqlSessionFactory建造过程的简单封装,他对建造者模式做了简化处理(仅仅有Builder而无Director),以减小编程复杂度:

/** * @author Clinton Begin */public class SqlSessionFactoryBuilder {
        public SqlSessionFactory build(Reader reader) {    return build(reader, null, null);  }  public SqlSessionFactory build(Reader reader, String environment) {    return build(reader, environment, null);  }  public SqlSessionFactory build(Reader reader, Properties properties) {    return build(reader, null, properties);  }  public SqlSessionFactory build(Reader reader, String environment, Properties properties) {    try {      XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);      return build(parser.parse());    } catch (Exception e) {      throw ExceptionFactory.wrapException("Error building SqlSession.", e);    } finally {      ErrorContext.instance().reset();      try {        reader.close();      } catch (IOException e) {        // Intentionally ignore. Prefer previous error.      }    }  }  public SqlSessionFactory build(InputStream inputStream) {    return build(inputStream, null, null);  }  public SqlSessionFactory build(InputStream inputStream, String environment) {    return build(inputStream, environment, null);  }  public SqlSessionFactory build(InputStream inputStream, Properties properties) {    return build(inputStream, null, properties);  }  public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {    try {      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);      return build(parser.parse());    } catch (Exception e) {      throw ExceptionFactory.wrapException("Error building SqlSession.", e);    } finally {      ErrorContext.instance().reset();      try {        inputStream.close();      } catch (IOException e) {        // Intentionally ignore. Prefer previous error.      }    }  }  public SqlSessionFactory build(Configuration config) {    return new DefaultSqlSessionFactory(config);  }}

因为SqlSessionFactory创建的SqlSession须要支持非常多操作(如selectOne()、selectList()、update()等), 因此SqlSessionFactory的构造过程是非常复杂的(可參考SqlSessionManager、DefaultSqlSessionFactory实现),因此使用SqlSessionFactoryBuilder作为Builder简化其构造过程,而且为其设置配置文件(mybatis-configuration.xml)作为Director来指导Builder的构造过程:

详细可參考博客:

小结

因为构建和装配的解耦, 不同的构建器, 同样的装配过程, 能够产生不同的产品,实现了更好的复用.因此经常使用于创建一些复杂的对象, 这些对象内部构建间的建造顺序一般是稳定的, 但内部的构建通常面临着复杂的变化. 如:

StringBuilder的append();

JDBC的PreparedStatement;

JDOM的DomBuilder、SAXBuilder;

MyBatis的SqlSessionFactoryBuilder.

參考:

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