創建型模式：建造者模式

什么是建造者模式

建造者模式（Builder Pattern）是一種創建型設計模式，它將一個復雜對象的構建過程與其表示分離，使得同樣的構建過程可以創建不同的表示。簡單來說，建造者模式允許您一步一步創建復雜對象，而不是一次性傳入所有參數。

建造者模式特別適合那些需要多個步驟構建、有多個可選參數或者構建過程中需要遵循特定順序的對象創建場景。

為什么需要建造者模式

假設我們有一個具有十多個屬性的Computer類，其中一些屬性是必需的，而其他屬性是可選的。使用傳統的構造函數或JavaBean模式創建對象會帶來以下問題：

伸縮構造函數問題：需要編寫多個構造函數來處理不同的參數組合
可讀性差：當參數較多時，構造函數調用難以理解
狀態不一致：使用setter方法時，對象可能處于不完整狀態
線程安全問題：構建過程中對象狀態可能發生變化

建造者模式優雅地解決了這些問題。

建造者模式的核心實現

// 產品類
public class House {// 必要參數private final String foundation;private final String structure;private final String roof;// 可選參數private final String interior;private final String exterior;private final boolean hasGarage;private final boolean hasSwimmingPool;private final boolean hasGarden;// 私有構造函數，只能通過Builder訪問private House(Builder builder) {this.foundation = builder.foundation;this.structure = builder.structure;this.roof = builder.roof;this.interior = builder.interior;this.exterior = builder.exterior;this.hasGarage = builder.hasGarage;this.hasSwimmingPool = builder.hasSwimmingPool;this.hasGarden = builder.hasGarden;}// Getterspublic String getFoundation() { return foundation; }public String getStructure() { return structure; }public String getRoof() { return roof; }public String getInterior() { return interior; }public String getExterior() { return exterior; }public boolean hasGarage() { return hasGarage; }public boolean hasSwimmingPool() { return hasSwimmingPool; }public boolean hasGarden() { return hasGarden; }@Overridepublic String toString() {return "House with " + foundation + " foundation, " + structure + " structure, " + roof + " roof, " + interior + " interior, " +exterior + " exterior. " +(hasGarage ? "Has garage. " : "") +(hasSwimmingPool ? "Has swimming pool. " : "") +(hasGarden ? "Has garden." : "");}// 靜態內部Builder類public static class Builder {// 必要參數private final String foundation;private final String structure;private final String roof;// 可選參數 - 設置默認值private String interior = "普通裝修";private String exterior = "標準外墻";private boolean hasGarage = false;private boolean hasSwimmingPool = false;private boolean hasGarden = false;// 必要參數通過構造器強制傳入public Builder(String foundation, String structure, String roof) {this.foundation = foundation;this.structure = structure;this.roof = roof;}// 可選參數通過具有鏈式調用的setter方法設置public Builder interior(String interior) {this.interior = interior;return this;}public Builder exterior(String exterior) {this.exterior = exterior;return this;}public Builder garage(boolean hasGarage) {this.hasGarage = hasGarage;return this;}public Builder swimmingPool(boolean hasSwimmingPool) {this.hasSwimmingPool = hasSwimmingPool;return this;}public Builder garden(boolean hasGarden) {this.hasGarden = hasGarden;return this;}// build方法創建最終對象public House build() {// 可以在這里添加構建驗證邏輯return new House(this);}}
}

建造者模式的關鍵點

產品類：復雜對象，通常是不可變的
Builder類：負責定義產品創建步驟的接口
鏈式調用：Builder方法返回Builder自身，支持流式API
最終構建方法：完成產品創建的方法（如build()）
參數分離：將必選參數和可選參數明確區分

使用建造者模式

public class BuilderPatternDemo {public static void main(String[] args) {// 使用Builder創建對象House simpleHouse = new House.Builder("混凝土基礎", "磚墻結構", "平頂屋頂").build();System.out.println("簡易房屋：");System.out.println(simpleHouse);// 使用Builder創建更復雜的對象，設置可選參數House luxuryHouse = new House.Builder("深層鋼筋混凝土基礎", "鋼筋混凝土框架", "坡頂屋頂").interior("豪華裝修").exterior("大理石外墻").garage(true).swimmingPool(true).garden(true).build();System.out.println("\n豪華房屋：");System.out.println(luxuryHouse);}
}

運行結果

簡易房屋：
House with 混凝土基礎 foundation, 磚墻結構 structure, 平頂屋頂 roof, 普通裝修 interior, 標準外墻 exterior. 豪華房屋：
House with 深層鋼筋混凝土基礎 foundation, 鋼筋混凝土框架 structure, 坡頂屋頂 roof, 豪華裝修 interior, 大理石外墻 exterior. Has garage. Has swimming pool. Has garden.

建造者模式的高級版本：帶Director的建造者模式

在某些情況下，我們可能需要一個額外的Director類來封裝構建過程，特別是當構建過程復雜或需要被重用時。

// 抽象Builder接口
public interface HouseBuilder {void buildFoundation();void buildStructure();void buildRoof();void buildInterior();void buildExterior();void buildGarage();void buildSwimmingPool();void buildGarden();House getResult();
}// 具體Builder實現
public class StandardHouseBuilder implements HouseBuilder {private String foundation;private String structure;private String roof;private String interior;private String exterior;private boolean hasGarage;private boolean hasSwimmingPool;private boolean hasGarden;@Overridepublic void buildFoundation() {this.foundation = "標準混凝土基礎";}@Overridepublic void buildStructure() {this.structure = "磚混結構";}@Overridepublic void buildRoof() {this.roof = "普通瓦片屋頂";}@Overridepublic void buildInterior() {this.interior = "基礎裝修";}@Overridepublic void buildExterior() {this.exterior = "普通外墻";}@Overridepublic void buildGarage() {this.hasGarage = false;}@Overridepublic void buildSwimmingPool() {this.hasSwimmingPool = false;}@Overridepublic void buildGarden() {this.hasGarden = false;}@Overridepublic House getResult() {return new House(foundation, structure, roof, interior, exterior, hasGarage, hasSwimmingPool, hasGarden);}
}// 另一個具體Builder實現
public class LuxuryHouseBuilder implements HouseBuilder {private String foundation;private String structure;private String roof;private String interior;private String exterior;private boolean hasGarage;private boolean hasSwimmingPool;private boolean hasGarden;@Overridepublic void buildFoundation() {this.foundation = "深層鋼筋混凝土基礎";}// 其他方法實現...@Overridepublic House getResult() {return new House(foundation, structure, roof, interior, exterior, hasGarage, hasSwimmingPool, hasGarden);}
}// Director類
public class HouseDirector {private HouseBuilder builder;public HouseDirector(HouseBuilder builder) {this.builder = builder;}// 更換Builderpublic void changeBuilder(HouseBuilder builder) {this.builder = builder;}// 構建最小可居住房屋public void constructMinimalHouse() {builder.buildFoundation();builder.buildStructure();builder.buildRoof();builder.buildInterior();builder.buildExterior();}// 構建完整功能房屋public void constructFullFeaturedHouse() {builder.buildFoundation();builder.buildStructure();builder.buildRoof();builder.buildInterior();builder.buildExterior();builder.buildGarage();builder.buildSwimmingPool();builder.buildGarden();}
}

實際應用示例：流暢的API構建器

下面通過一個SQL查詢構建器的例子來展示建造者模式在實際API設計中的應用：

// SQL查詢構建器
public class SQLQueryBuilder {private String table;private List<String> columns = new ArrayList<>();private List<String> conditions = new ArrayList<>();private List<String> orderBy = new ArrayList<>();private Integer limit;private Integer offset;public SQLQueryBuilder from(String table) {this.table = table;return this;}public SQLQueryBuilder select(String... columns) {if (columns.length == 0) {this.columns.add("*");} else {this.columns.addAll(Arrays.asList(columns));}return this;}public SQLQueryBuilder where(String condition) {this.conditions.add(condition);return this;}public SQLQueryBuilder orderBy(String column, boolean ascending) {this.orderBy.add(column + (ascending ? " ASC" : " DESC"));return this;}public SQLQueryBuilder limit(int limit) {this.limit = limit;return this;}public SQLQueryBuilder offset(int offset) {this.offset = offset;return this;}public String build() {// 驗證必要參數if (table == null) {throw new IllegalStateException("表名未指定");}StringBuilder query = new StringBuilder();query.append("SELECT ");// 添加列if (columns.isEmpty()) {query.append("*");} else {query.append(String.join(", ", columns));}// 添加表query.append(" FROM ").append(table);// 添加條件if (!conditions.isEmpty()) {query.append(" WHERE ");query.append(String.join(" AND ", conditions));}// 添加排序if (!orderBy.isEmpty()) {query.append(" ORDER BY ");query.append(String.join(", ", orderBy));}// 添加限制if (limit != null) {query.append(" LIMIT ").append(limit);}// 添加偏移if (offset != null) {query.append(" OFFSET ").append(offset);}return query.toString();}
}

SQL構建器的使用示例

public class SQLBuilderDemo {public static void main(String[] args) {// 創建簡單查詢String simpleQuery = new SQLQueryBuilder().from("users").select("id", "name", "email").build();System.out.println("簡單查詢：");System.out.println(simpleQuery);// 創建復雜查詢String complexQuery = new SQLQueryBuilder().select("u.id", "u.name", "COUNT(o.id) as order_count").from("users u LEFT JOIN orders o ON u.id = o.user_id").where("u.status = 'active'").where("o.create_time > '2023-01-01'").orderBy("order_count", false).limit(10).offset(20).build();System.out.println("\n復雜查詢：");System.out.println(complexQuery);}
}

建造者模式的常見應用場景

文檔生成器：構建HTML、PDF、Word文檔
復雜對象構建：具有多個配置選項的對象
不可變對象創建：需要一次性設置所有屬性的對象
流暢的API設計：提供鏈式調用的接口
配置構建：應用程序配置、網絡請求配置等
測試數據構建：創建測試用例數據
復雜GUI創建：組裝UI組件和布局

真實世界中的建造者模式應用

Java中的StringBuilder/StringBuffer：字符串構建
Lombok的@Builder注解：自動生成Builder代碼
Spring框架的UriComponentsBuilder：構建URI
Apache Camel的RouteBuilder：構建路由規則
Retrofit的RequestBuilder：構建HTTP請求
OkHttp的Request.Builder：構建HTTP請求

建造者模式的優點

參數控制：分離必選參數和可選參數
構建過程封裝：隱藏復雜構建過程的細節
可讀性：創建對象的代碼更具可讀性
靈活性：同一構建過程可創建不同表示
不變性：可以創建不可變對象
參數驗證：可在構建時檢查參數有效性

建造者模式的缺點

代碼量增加：需要創建額外的Builder類
復雜度：對于簡單對象，可能過于復雜
性能開銷：相比直接構造，略有性能損失
維護成本：當產品類變化時，需要同步修改Builder

建造者模式與工廠模式的區別

雖然建造者模式和工廠模式都是創建型模式，但它們有明顯區別：

工廠模式關注對象創建的種類(what)
建造者模式關注復雜對象的構建過程(how)

工廠用于創建整個對象，而建造者關注對象各個部分的創建過程和裝配順序。

建造者模式變體：自引用泛型流暢接口

這種高級變體使用泛型保持鏈式調用時的類型安全：

public class Email {// 郵件屬性private final String from;private final List<String> to;private final String subject;private final String body;private final List<String> attachments;private Email(Builder builder) {this.from = builder.from;this.to = new ArrayList<>(builder.to);this.subject = builder.subject;this.body = builder.body;this.attachments = new ArrayList<>(builder.attachments);}// 泛型Builder基類public static class Builder<T extends Builder<T>> {private String from;private List<String> to = new ArrayList<>();private String subject = "";private String body = "";private List<String> attachments = new ArrayList<>();// 自引用泛型類型轉換@SuppressWarnings("unchecked")protected final T self() {return (T) this;}public T from(String from) {this.from = from;return self();}public T to(String address) {this.to.add(address);return self();}public T subject(String subject) {this.subject = subject;return self();}public T body(String body) {this.body = body;return self();}public T attachment(String attachment) {this.attachments.add(attachment);return self();}public Email build() {validateRequiredFields();return new Email(this);}private void validateRequiredFields() {if (from == null || from.isEmpty()) {throw new IllegalStateException("發件人不能為空");}if (to.isEmpty()) {throw new IllegalStateException("收件人不能為空");}}}// 具體Builderpublic static class EmailBuilder extends Builder<EmailBuilder> {// 不需要額外代碼，因為基類已實現了所有功能}// 便捷的靜態工廠方法public static EmailBuilder builder() {return new EmailBuilder();}@Overridepublic String toString() {return "Email{" +"from='" + from + '\'' +", to=" + to +", subject='" + subject + '\'' +", body='" + body + '\'' +", attachments=" + attachments +'}';}
}