【再探】设计模式—抽象工厂及建造者模式

抽象工厂模式和建造者模式都属于创建型模式。两者都能创建对应的对象，而创建者模式更侧重于创建复杂对象，将对象的创建过程封装起来，让客户端不需要知道对象的内部细节。

1 抽象工厂模式

需求：

在使用工厂方法模式时，当增加新的产品类型时，需要创建对应的Factory类，这导致类的数量增加，让系统变得臃肿。
需要保证同风格下生成一致的组件。比如在写页面时，有dark及light模式，我们希望button、text等组件能在不同模式下保持一致的风格。抽象工厂模式介绍

1.1 抽象工厂模式介绍

为创建一组对象提供一组解决方案。与工厂模式相比，抽象工厂模式中的具体工厂不只是创建一种产品，而是创建一族产品。

图抽象工厂模式UML

public class Button {

    private final String color;

    public Button(String color) {
        this.color = color;
    }

    @Override
    public String toString() {
        return getClass().getSimpleName() + "{" +
                "color='" + color + '\'' +
                '}';
    }
}

public class Text {

    private final String color;

    public Text(String color) {
        this.color = color;
    }

    @Override
    public String toString() {
        return getClass().getSimpleName() + "{" +
                "color='" + color + '\'' +
                '}';
    }
}

public class DarkButton extends Button{
    public DarkButton(String color) {
        super(color);
    }
}

public class DarkText extends Text{
    public DarkText(String color) {
        super(color);
    }
}

public class LightButton extends Button{
    public LightButton(String color) {
        super(color);
    }
}

public class LightText extends Text{
    public LightText(String color) {
        super(color);
    }
}

public interface ComponentsFactory {

    Button buildButton();

    Text buildText();

}

public class DarkComponentFactory implements ComponentsFactory{

    private final String color = "dark";

    @Override
    public Button buildButton() {
        return new DarkButton(color);
    }

    @Override
    public Text buildText() {
        return new DarkText(color);
    }

}

public class LightComponentFactory implements ComponentsFactory{

    private final String color = "light";

    @Override
    public Button buildButton() {
        return new LightButton(color);
    }

    @Override
    public Text buildText() {
        return new LightText(color);
    }
}

public class HtmlWeb {
    public static void main(String[] args) {
        ComponentsFactory componentsFactory = new DarkComponentFactory();
        System.out.println("-----------dark模式-----------");
        System.out.println(componentsFactory.buildButton());
        System.out.println(componentsFactory.buildText());
        System.out.println("-----------light模式-----------");
        componentsFactory = new LightComponentFactory();
        System.out.println(componentsFactory.buildButton());
        System.out.println(componentsFactory.buildText());
    }
}

1.1.1 优缺点

优点：

隔离类的实例化过程，使得客户端并不需要知道如何创建对象。
当不同产品族多个对象被设计成一起工作时，能保证客户端始终只使用同一个产品族的对象。
减少了工厂类的数量。
增加新的产品族方便，符合开闭原则。

缺点：

不符合单一职责原则，一个类创建了多个对象。
族群增加新的种类时，不符合开闭原则，需要修改全部的工厂类。

2 建造者模式

需求：

需要创建一个复杂的对象，隔离类的实例化过程，使得客户端不需要知道对象的内部细节。
创建对象时，对执行顺序有要求。

2.1 建造者模式介绍

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，不同的构建顺序可以构建不同的表示。

图构建模式UML

Director 为指挥者，用来控制Buider 的执行顺序，在实际开发中，这个角色经常会被省略，它的功能集成到Builder上；而Builder中的builderPart方法的返回值为它本身，这样就可以链式调用了。

public class JdbcConnector {

    private JdbcConnection connection;

    public void initConnection(URI uri) {
         connection = new JdbcConnection(uri);
    }

    public JdbcPreparedStatement getPreparedStatement(String query) {
        if (connection == null) throw new RuntimeException("连接器还未初始化");
        return connection.getPreparedStatement(query);
    }

    public static class JdbcConnection {
        public JdbcConnection(URI uri){
            System.out.println(uri);
        }

        public JdbcPreparedStatement getPreparedStatement(String query) {
            return new JdbcPreparedStatement(query);
        }
    }

    public static class JdbcPreparedStatement {
        String query;

        public JdbcPreparedStatement(String query) {
            this.query = query;
        }

        public String getResult() {
            return this.query;
        }
    }

}

public class JdbcConnectorBuilder {

    private String scheme;
    private String host;
    private String port;
    private String database;
    private String username;
    private String password;

    public JdbcConnectorBuilder setScheme(String scheme) {
        this.scheme = scheme;
        return this;
    }

    public JdbcConnectorBuilder setHost(String host) {
        this.host = host;
        return this;
    }

    public JdbcConnectorBuilder setPort(String port) {
        this.port = port;
        return this;
    }

    public JdbcConnectorBuilder setDatabase(String database) {
        this.database = database;
        return this;
    }

    public JdbcConnectorBuilder setUsername(String username) {
        this.username = username;
        return this;
    }

    public JdbcConnectorBuilder setPassword(String password) {
        this.password = password;
        return this;
    }

    /**
     * 产品
     */
    private JdbcConnector jdbcConnector = null;

    public JdbcConnectorBuilder() {
        jdbcConnector = new JdbcConnector();
    }

    public JdbcConnector build() {
        StringBuilder sb = new StringBuilder("jdbc:");
        sb.append(scheme == null ? "mysql" : scheme).append("://");
        if (host == null) throw new RuntimeException("host不能为空");
        sb.append(host);
        sb.append(":").append(port == null ? "3306" : port);
        if (database == null) throw new RuntimeException("数据库不能为空");
        sb.append("/").append(database);
        try {
            URI uri = new URI(sb.toString());
            if (username == null || password == null) throw new RuntimeException("账号或密码错误");
            jdbcConnector.initConnection(uri);
        } catch (URISyntaxException e) {
            throw new RuntimeException("连接失败，连接信息有误");
        }
        return jdbcConnector;
    }

}

public class JdbcConnectionTest {
    public static void main(String[] args) {
        JdbcConnectorBuilder builder = new JdbcConnectorBuilder();
        builder.setScheme("mysql")
                .setHost("localhost")
                .setPort("3307")
                .setDatabase("my-database")
                .setUsername("root")
                .setPassword("123456");
        JdbcConnector jdbcConnector = builder.build();
        JdbcConnector.JdbcPreparedStatement statement = jdbcConnector.getPreparedStatement("select * from student");
        System.out.println(statement.getResult());
    }
}

JDK 中的StringBuilder及Apache httpclient 的 URIBuilder 应用了建造者模式。

自定义StringBuilder

public class CustomStringBuilder {

    public static void main(String[] args) {
        CustomStringBuilder customStringBuilder = new CustomStringBuilder("hello customStringBuilder");
        for (int i =0; i < 100; i++) {
            customStringBuilder.append("---建造者模式---").append(i + "");
        }
        System.out.println(customStringBuilder);
    }

    char[] value;

    int count = 0;

    public CustomStringBuilder(int capacity) {
        value = new char[capacity];
    }

    public CustomStringBuilder(String str) {
        this(str.length() + 16);
        append(str);
    }

    CustomStringBuilder append(String str) {
        if (str == null) {
            throw new RuntimeException("字符串不能为空");
        }
        ensureCapacity(count + str.length());
        str.getChars(0,str.length(),value,count);
        count += str.length();
        return this;
    }

    private void ensureCapacity(int minCapacity) {
        if (minCapacity >= value.length) {
            value = Arrays.copyOf(value,value.length << 1);
        }
    }

    @Override
    public String toString() {
        return new String(value,0,count);
    }
}

自定义URIBuilder

public class CustomURIBuilder {

    public static void main(String[] args) throws URISyntaxException {
        CustomURIBuilder customURIBuilder = new CustomURIBuilder();
        customURIBuilder.setHost("localhost").setPort("8080").setEncodedPath("/userinfo");
        customURIBuilder.appendQueryParam("username","hmf");
        customURIBuilder.appendQueryParam("status","1");
        customURIBuilder.appendQueryParam("createDate","2024/04/20");
        URI uri = customURIBuilder.build();
        System.out.println(uri);
    }

    private String scheme;

    private String host;

    private String port;

    private String encodedPath;

    private List<NameValuePair> queryParams;

    public CustomURIBuilder setScheme(String scheme) {
        this.scheme = scheme;
        return this;
    }

    public CustomURIBuilder setHost(String host) {
        this.host = host;
        return this;
    }

    public CustomURIBuilder setPort(String port) {
        this.port = port;
        return this;
    }

    public CustomURIBuilder setEncodedPath(String encodedPath) {
        this.encodedPath = encodedPath;
        return this;
    }

    public CustomURIBuilder appendQueryParam(String name,String value) {
        if (queryParams == null) queryParams = new ArrayList<>();
        queryParams.add(new NameValuePair(name,value));
        return this;
    }

    public URI build() throws URISyntaxException {
        return new URI(buildStr());
    }

    public String buildStr() {
        StringBuilder sb = new StringBuilder();
        if (this.scheme != null) sb.append(this.scheme).append("://");
        if (this.host == null) throw new RuntimeException("host不能为空");
        sb.append(this.host);
        if (this.port != null) sb.append(":").append(this.port);
        if (this.encodedPath != null) sb.append(this.encodedPath);
        if (this.queryParams != null) {
            for (int i = 0; i < this.queryParams.size(); i++) {
                sb.append(i == 0 ? "?" : "&").append(this.queryParams.get(i));
            }
        }
        return sb.toString();
    }

    private static class NameValuePair {
        String name;
        String value;

        public NameValuePair(String name, String value) {
            this.name = name;
            this.value = value;
        }

        @Override
        public String toString() {
            if (this.value == null) return name;
            return name + "=" + value;
        }
    }

}

2.1.1 优缺点