设计模式之创建型模式中的建造者模式（Builder Pattern）

基本介绍

在java开发过程中，我们经常会创建大量的对象，这些对象有简单的也有复杂的，但是不管是简单对象还是复杂对象，构建这些对象的过程是相对稳定的，只不过它们的内部属性（成员属性）不同，这也就意味着构建的具体内部细节不一样。那么，基于这种情况，我们可以将复杂的对象的构建过程抽象出来，通过抽象过程的不同实现方法来实现不同对象的内部细节构建过程，从某种意义上来说，这也是产品构建过程复杂度的解耦，这就是建造者模式（Builder Pattern）。
建造者模式（Builder Pattern）又叫生成器模式，它把对象的创建步骤抽象成生成器，将一个产品的内部表象与产品的生产过程分割开来，一步一步构建一个复杂的对象，用户只用指定复杂对象的类型和内容，而无需知道内部的具体构建细节就可以构建它们。

建造者模式的四个角色

• 产品角色（Product）：我们所要构建的产品对象；
• 抽象建造者（Builder）：创建产品（Product）对象的接口/抽象类。它定义了创建一个产品（Product）对象所需要的各个部件的操作（抽象方法），同时包含一个获取产品（Product）对象（获取成品）的方法。
• 具体建造者（ConcreteBuilder）：抽象建造者的具体实现，实现抽象建造者（Builder）的接口，构建和装配产品（Product）对象的各个部件，对于不同的部件或者构建步骤进行不同的详细实现，来完成不同的产品。
• 指导者（Director）：主要用来使用Buider接口，构建一个使用Buider接口的对象，以一个相对稳定且统一的过程生产产品（Product）对象。

日常举例

建造小汽车的例子：小汽车主要部件：发动机（Engine）、车身框架（Frame）、轮胎（Wheel）等，不管是什么品牌的汽车，都有这些部件，只不过内部的构造和质量等不一样。代码实现，如下：

• 产品角色（Product）

public class Car {
    private String engine;
    private String frame;
    private String wheel;

    public String getEngine() {
        return engine;
    }

    public void setEngine(String engine) {
        this.engine = engine;
    }

    public String getFrame() {
        return frame;
    }

    public void setFrame(String frame) {
        this.frame = frame;
    }

    public String getWheel() {
        return wheel;
    }

    public void setWheel(String wheel) {
        this.wheel = wheel;
    }
    @Override
    public String toString() {
        return "Car{" +
                "engine='" + engine + '\'' +
                ", frame='" + frame + '\'' +
                ", wheel='" + wheel + '\'' +
                '}';
    }
}

• 抽象建造者（Builder）

public interface CarBuilder {
    /**
     * 建造发动机
     * @return
     */
    String buildEngine();

    /**
     * 建造车身框架
     * @return
     */
    String buildFrame();

    /**
     * 建造轮胎
     * @return
     */
    String buildWheel();

    /**
     * 获取小汽车成品
     * @return
     */
    Car getCar();
}

• 具体建造者（ConcreteBuilder）

public class BaoMaCarBuilder implements CarBuilder{
    Car car = new Car();
    public void buildEngine() {
        this.car.setEngine("建造宝马发动机！");
    }

    public void buildFrame() {
        this.car.setFrame("建造宝马车身框架！");
    }
    public void buildWheel() {
       this.car.setWheel("建造宝马轮胎！");
    }

    public Car getCar() {
        return this.car;
    }
}

• 指导者（Director）

public class CarDirector {
    private CarBuilder carBuilder;

    public CarDirector(CarBuilder carBuilder) {
        this.carBuilder = carBuilder;
    }

    /**
     * 构建汽车的流程交给指导者
     * @return
     */
    public Car builderCar(){
        carBuilder.buildEngine();
        carBuilder.buildFrame();
        carBuilder.buildWheel();
        return carBuilder.getCar();
    }
}

• 客户端使用

public class Client {
    public static void main(String[] args) {
        BaoMaCarBuilder baoMaCarBuilder = new BaoMaCarBuilder();
        CarDirector carDirector = new CarDirector(baoMaCarBuilder);
        Car car = carDirector.builderCar();
        System.out.println(car);
    }
}

运行结果：

建造者模式在JDK中的应用

先看一张StringBuilder类图

源码:java.lang.Appendable

public interface Appendable {

    /**
     * Appends the specified character sequence to this <tt>Appendable</tt>.
     *
     * <p> Depending on which class implements the character sequence
     * <tt>csq</tt>, the entire sequence may not be appended.  For
     * instance, if <tt>csq</tt> is a {@link java.nio.CharBuffer} then
     * the subsequence to append is defined by the buffer's position and limit.
     *
     * @param  csq
     *         The character sequence to append.  If <tt>csq</tt> is
     *         <tt>null</tt>, then the four characters <tt>"null"</tt> are
     *         appended to this Appendable.
     *
     * @return  A reference to this <tt>Appendable</tt>
     *
     * @throws  IOException
     *          If an I/O error occurs
     */
    Appendable append(CharSequence csq) throws IOException;

    /**
     * Appends a subsequence of the specified character sequence to this
     * <tt>Appendable</tt>.
     *
     * <p> An invocation of this method of the form <tt>out.append(csq, start,
     * end)</tt> when <tt>csq</tt> is not <tt>null</tt>, behaves in
     * exactly the same way as the invocation
     *
     * <pre>
     *     out.append(csq.subSequence(start, end)) </pre>
     *
     * @param  csq
     *         The character sequence from which a subsequence will be
     *         appended.  If <tt>csq</tt> is <tt>null</tt>, then characters
     *         will be appended as if <tt>csq</tt> contained the four
     *         characters <tt>"null"</tt>.
     *
     * @param  start
     *         The index of the first character in the subsequence
     *
     * @param  end
     *         The index of the character following the last character in the
     *         subsequence
     *
     * @return  A reference to this <tt>Appendable</tt>
     *
     * @throws  IndexOutOfBoundsException
     *          If <tt>start</tt> or <tt>end</tt> are negative, <tt>start</tt>
     *          is greater than <tt>end</tt>, or <tt>end</tt> is greater than
     *          <tt>csq.length()</tt>
     *
     * @throws  IOException
     *          If an I/O error occurs
     */
    Appendable append(CharSequence csq, int start, int end) throws IOException;

    /**
     * Appends the specified character to this <tt>Appendable</tt>.
     *
     * @param  c
     *         The character to append
     *
     * @return  A reference to this <tt>Appendable</tt>
     *
     * @throws  IOException
     *          If an I/O error occurs
     */
    Appendable append(char c) throws IOException;
}

源码:java.lang.AbstractStringBuilder（片段）

abstract class AbstractStringBuilder implements Appendable, CharSequence {
...
  // Documentation in subclasses because of synchro difference
    public AbstractStringBuilder append(StringBuffer sb) {
        if (sb == null)
            return appendNull();
        int len = sb.length();
        ensureCapacityInternal(count + len);
        sb.getChars(0, len, value, count);
        count += len;
        return this;
    }

    /**
     * @since 1.8
     */
    AbstractStringBuilder append(AbstractStringBuilder asb) {
        if (asb == null)
            return appendNull();
        int len = asb.length();
        ensureCapacityInternal(count + len);
        asb.getChars(0, len, value, count);
        count += len;
        return this;
    }

    // Documentation in subclasses because of synchro difference
    @Override
    public AbstractStringBuilder append(CharSequence s) {
        if (s == null)
            return appendNull();
        if (s instanceof String)
            return this.append((String)s);
        if (s instanceof AbstractStringBuilder)
            return this.append((AbstractStringBuilder)s);

        return this.append(s, 0, s.length());
    }
...
}

源码:java.lang.StringBuilder（片段）

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{
...
   @Override
     public StringBuilder append(Object obj) {
         return append(String.valueOf(obj));
     }

     @Override
     public StringBuilder append(String str) {
         super.append(str);
         return this;
     }

     /**
      * Appends the specified {@code StringBuffer} to this sequence.
      * <p>
      * The characters of the {@code StringBuffer} argument are appended,
      * in order, to this sequence, increasing the
      * length of this sequence by the length of the argument.
      * If {@code sb} is {@code null}, then the four characters
      * {@code "null"} are appended to this sequence.
      * <p>
      * Let <i>n</i> be the length of this character sequence just prior to
      * execution of the {@code append} method. Then the character at index
      * <i>k</i> in the new character sequence is equal to the character at
      * index <i>k</i> in the old character sequence, if <i>k</i> is less than
      * <i>n</i>; otherwise, it is equal to the character at index <i>k-n</i>
      * in the argument {@code sb}.
      *
      * @param   sb   the {@code StringBuffer} to append.
      * @return  a reference to this object.
      */
     public StringBuilder append(StringBuffer sb) {
         super.append(sb);
         return this;
     }

     @Override
     public StringBuilder append(CharSequence s) {
         super.append(s);
         return this;
     }
...
}

分析： 接口Appendable定义了多个append抽象方法，即为抽象建造者；抽象类AbstractStringBuilder实现了接口Appendable的方法，即为具体建造者，但是不能实例化；StringBuilder继承了抽象类AbstractStringBuilder，具体的方法已经由AbstractStringBuilder实现，StringBuilder对部分方法进行了覆盖，由此可以看出，StringBuilder既充当了具体建造者，也是指导者的角色。

附：本次演示的项目地址
https://github.com/syshlang/java-design-patterns