模板方法模式詳解

一、模式定義

模板方法模式(Template Method Pattern)定義一個操作中的算法骨架，將某些步驟延遲到子類實現。

二、核心結構

1. 抽象模板類

public abstract class AbstractTemplate {// 模板方法（final防止子類覆蓋）public final void templateMethod() {step1();step2();step3();if(hook()) {step4();}}// 抽象方法（必須由子類實現）protected abstract void step2();// 具體方法（已有默認實現）protected void step1() {System.out.println("執行步驟1");}protected void step3() {System.out.println("執行步驟3");}// 鉤子方法（可選覆蓋）protected boolean hook() {return true;}protected void step4() {System.out.println("執行步驟4");}
}

2. 具體實現類

public class ConcreteClassA extends AbstractTemplate {protected void step2() {System.out.println("A實現-步驟2");}protected boolean hook() {return false; // 關閉步驟4}
}public class ConcreteClassB extends AbstractTemplate {protected void step2() {System.out.println("B實現-步驟2");}protected void step4() {System.out.println("B定制-步驟4");}
}

三、完整示例：飲料制作系統

1. 抽象飲料類

public abstract class BeverageTemplate {// 模板方法（final）public final void prepareBeverage() {boilWater();brew();pourInCup();if(customerWantsCondiments()) {addCondiments();}}protected abstract void brew();protected abstract void addCondiments();protected void boilWater() {System.out.println("煮沸水");}protected void pourInCup() {System.out.println("倒入杯中");}// 鉤子方法protected boolean customerWantsCondiments() {return true;}
}

2. 具體飲料實現

// 咖啡
public class Coffee extends BeverageTemplate {protected void brew() {System.out.println("沖泡咖啡粉");}protected void addCondiments() {System.out.println("加糖和牛奶");}protected boolean customerWantsCondiments() {String answer = getUserInput();return answer.toLowerCase().startsWith("y");}private String getUserInput() {System.out.print("要加糖和牛奶嗎(y/n)? ");BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));try {return reader.readLine();} catch (IOException e) {return "no";}}
}// 茶
public class Tea extends BeverageTemplate {protected void brew() {System.out.println("浸泡茶葉");}protected void addCondiments() {System.out.println("加檸檬");}
}

3. 客戶端使用

public class BeverageTest {public static void main(String[] args) {System.out.println("制作咖啡...");BeverageTemplate coffee = new Coffee();coffee.prepareBeverage();System.out.println("\n制作茶...");BeverageTemplate tea = new Tea();tea.prepareBeverage();}
}

四、高級應用：數據庫操作模板

1. 抽象DAO模板

public abstract class JdbcTemplate {// 模板方法public final <T> T execute(String sql, RowMapper<T> rowMapper) {Connection conn = null;PreparedStatement stmt = null;ResultSet rs = null;try {conn = getConnection();stmt = conn.prepareStatement(sql);setParameters(stmt);rs = stmt.executeQuery();return rowMapper.mapRow(rs);} catch (SQLException e) {throw new RuntimeException(e);} finally {closeResources(conn, stmt, rs);}}protected abstract void setParameters(PreparedStatement stmt) throws SQLException;protected Connection getConnection() throws SQLException {return DriverManager.getConnection("jdbc:mysql://localhost:3306/test");}protected void closeResources(Connection conn, Statement stmt, ResultSet rs) {try { if (rs != null) rs.close(); } catch (SQLException e) {}try { if (stmt != null) stmt.close(); } catch (SQLException e) {}try { if (conn != null) conn.close(); } catch (SQLException e) {}}
}

2. 行映射接口

public interface RowMapper<T> {T mapRow(ResultSet rs) throws SQLException;
}

3. 具體DAO實現

public class UserDao extends JdbcTemplate {public User findById(long id) {return execute("SELECT * FROM users WHERE id = ?", rs -> {User user = new User();user.setId(rs.getLong("id"));user.setName(rs.getString("name"));return user;});}protected void setParameters(PreparedStatement stmt) throws SQLException {stmt.setLong(1, 1L); // 設置查詢參數}
}

五、模式優勢

1. 提高代碼復用性
2. 實現反向控制（好萊塢原則）
3. 便于擴展和維護
4. 符合開閉原則

六、適用場景

1. 多個類有相同算法結構
2. 需要控制子類擴展點
3. 存在公共行為需要抽取
4. 框架設計中的流程控制

七、注意事項

1. 模板方法應該聲明為final
2. 合理設計抽象方法和鉤子方法
3. 避免過度抽象導致復雜度增加
4. 與策略模式區分使用場景

八、最佳實踐

1. 使用鉤子方法提供靈活擴展點
2. 保持模板方法簡潔
3. 合理命名抽象方法
4. 考慮與工廠方法模式結合使用
5. 為常用操作提供默認實現

九、完整示例代碼結構

src/
├── main/
│ ├── java/
│ │ ├── template/
│ │ │ ├── AbstractTemplate.java
│ │ │ ├── ConcreteClassA.java
│ │ │ ├── ConcreteClassB.java
│ │ │ ├── BeverageTemplate.java
│ │ │ ├── Coffee.java
│ │ │ ├── Tea.java
│ │ │ ├── JdbcTemplate.java
│ │ │ ├── UserDao.java
│ │ │ └── BeverageTest.java