设计模式之抽象工厂模式

2019-05-31

在抽象工厂模式中，不仅有抽象工厂，还有抽象零件和抽象产品。抽象工厂的作用是将抽象零件组装为抽象产品。

类图：

这里大约分为3部分。左上角Item类是抽象的零件，便于统一处理Tray和Link，使其具有可替换性。ListLink和ListTray是具体的零件。ListTray是具有多个Link的集合。

右上角的Page是抽象的产品，代表一个页面。页面上有许多Item(可以是Link或者TrayLink)。ListPage是具体的产品，实现了MakeHTML()方法。

左下角Factory是抽象工厂，ListFactory是具体工厂，用于创建一个列表风格的页面。

结果：

代码：

package factory;

public abstract class Factory {
    public static Factory getFactory(String classname) {
        Factory factory = null;
        try {
            factory = (Factory)Class.forName(classname).newInstance();
        } catch (ClassNotFoundException e) {
            System.err.println("没有找到 " + classname + "类。");
        } catch (Exception e) {
            e.printStackTrace();
        }
        return factory;
    }
    public abstract Link createLink(String caption, String url);
    public abstract Tray createTray(String caption);
    public abstract Page createPage(String title, String author);
}

package factory;

public abstract class Item {
    protected String caption;
    public Item(String caption) {
        this.caption = caption;
    }
    public abstract String makeHTML();
}

package factory;

public abstract class Link extends Item {
    protected String url;
    public Link(String caption, String url) {
        super(caption);
        this.url = url;
    }
}

package factory;
import java.util.ArrayList;

public abstract class Tray extends Item {
    protected ArrayList tray = new ArrayList();
    public Tray(String caption) {
        super(caption);
    }
    public void add(Item item) {
        tray.add(item);
    }
}

package factory;
import java.io.*;
import java.util.ArrayList;

public abstract class Page {
    protected String title;
    protected String author;
    protected ArrayList content = new ArrayList();
    public Page(String title, String author) {
        this.title = title;
        this.author = author;
    }
    public void add(Item item) {
        content.add(item);
    }
    public void output() {
        try {
            String filename = title + ".html";
            Writer writer = new FileWriter(filename);
            writer.write(this.makeHTML());
            writer.close();
            System.out.println(filename + " 编写完成。");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    public abstract String makeHTML();
}

package listfactory;
import factory.*;

public class ListFactory extends Factory {
    public Link createLink(String caption, String url) {
        return new ListLink(caption, url);
    }
    public Tray createTray(String caption) {
        return new ListTray(caption);
    }
    public Page createPage(String title, String author) {
        return new ListPage(title, author);
    }
}

package listfactory;
import factory.*;

public class ListLink extends Link {
    public ListLink(String caption, String url) {
        super(caption, url);
    }
    public String makeHTML() {
        return "  <li><a href=\"" + url + "\">" + caption + "</a></li>\n";
    }
}

package listfactory;
import factory.*;
import java.util.Iterator;

public class ListTray extends Tray {
    public ListTray(String caption) {
        super(caption);
    }
    public String makeHTML() {
        StringBuffer buffer = new StringBuffer();
        buffer.append("<li>\n");
        buffer.append(caption + "\n");
        buffer.append("<ul>\n");
        Iterator it = tray.iterator();
        while (it.hasNext()) {
            Item item = (Item)it.next();
            buffer.append(item.makeHTML());
        }
        buffer.append("</ul>\n");
        buffer.append("</li>\n");
        return buffer.toString();
    }
}

package listfactory;
import factory.*;
import java.util.Iterator;

public class ListPage extends Page {
    public ListPage(String title, String author) {
        super(title, author);
    }
    public String makeHTML() {
        StringBuffer buffer = new StringBuffer();
        buffer.append("<html><head><title>" + title + "</title></head>\n");
        buffer.append("<body>\n");
        buffer.append("<h1>" + title + "</h1>\n");
        buffer.append("<ul>\n");
        Iterator it = content.iterator();
        while (it.hasNext()) {
            Item item = (Item)it.next();
            buffer.append(item.makeHTML());
        }
        buffer.append("</ul>\n");
        buffer.append("<hr><address>" + author + "</address>");
        buffer.append("</body></html>\n");
        return buffer.toString();
    }
}

import factory.*;

public class Main {
    public static void main(String[] args) {
        if (args.length != 1) {
            System.out.println("Usage: java Main class.name.of.ConcreteFactory");
            System.out.println("Example 1: java Main listfactory.ListFactory");
            System.out.println("Example 2: java Main tablefactory.TableFactory");
            System.exit(0);
        }
        Factory factory = Factory.getFactory(args[0]);

        Link people = factory.createLink("人民日报", "http://www.people.com.cn/");
        Link gmw = factory.createLink("光明日报", "http://www.gmw.cn/");

        Link us_yahoo = factory.createLink("Yahoo!", "http://www.yahoo.com/");
        Link jp_yahoo = factory.createLink("Yahoo!Japan", "http://www.yahoo.co.jp/");
        Link excite = factory.createLink("Excite", "http://www.excite.com/");
        Link google = factory.createLink("Google", "http://www.google.com/");

        Tray traynews = factory.createTray("日报");
        traynews.add(people);
        traynews.add(gmw);

        Tray trayyahoo = factory.createTray("Yahoo!");
        trayyahoo.add(us_yahoo);
        trayyahoo.add(jp_yahoo);

        Tray traysearch = factory.createTray("检索引擎");
        traysearch.add(trayyahoo);
        traysearch.add(excite);
        traysearch.add(google);

        Page page = factory.createPage("LinkPage", "杨文轩");
        page.add(traynews);
        page.add(traysearch);
        page.output();
    }
}

增加新的工厂

如果只用到ListFactory，那就使用抽象工厂模式没有意义了。假如现在需要一个表格格式的页面，那么就可以使用抽象工厂模式整体的替换工厂、产品、零件。

类图：

具体的代码就不上了，体会这种思想即可。

结果：

与工厂模式的区别：

抽象工厂模式多了一个零件的层级，而工厂模式产出产品。假如有一个汽车工厂，生产多个品牌的车，这时就需要抽象工厂模式。因为汽车由发动机、底盘、变速箱等部件组成。每生产一个具体品牌的车就整体替换工厂、产品、零件。又假如有一家供应商向汽车厂供应零部件比如挡风玻璃，那么使用工厂模式即可，产出适合不同品牌的挡风玻璃。

缺点

难以增加具体的零件。以本题例子，假如现在需要在网页中增加一个图片零件，那么就需要添加新的代表图片的类，更改所有的具体工厂才行。