Java IoC 容器原理手寫實現：工廠模式 + 配置文件 + 反射

手寫迷你 IoC 容器

1. 統一接口 ApplicationContext

public interface ApplicationContext {
    Object getBean(String name);
    Object getBean(String name, String className);
}

2. 配置文件讀取器 PropertiesReader

public class PropertiesReader {
    private final String fileName;
    public PropertiesReader(String fileName) { this.fileName = fileName; }

    public Map<String, String> load() {
        Properties props = new Properties();
        try (InputStream is = getClass().getClassLoader().getResourceAsStream(fileName)) {
            props.load(is);
        } catch (IOException e) { throw new RuntimeException(e); }
        Map<String, String> map = new HashMap<>();
        for (String key : props.stringPropertyNames()) {
            map.put(key, props.getProperty(key));
        }
        return map;
    }
}

3. 基於類路徑的實現 ClassPathXmlApplicationContext

public class ClassPathXmlApplicationContext implements ApplicationContext {
    private final Map<String, String> beanMap;
    public ClassPathXmlApplicationContext(String configFile) {
        this.beanMap = new PropertiesReader(configFile).load();
    }

    @Override
    public Object getBean(String name) {
        String className = beanMap.get(name);
        if (className == null) throw new IllegalArgumentException("No bean: " + name);
        try {
            System.err.println("創建 Bean: " + name + " -> " + className);
            return Class.forName(className).getDeclaredConstructor().newInstance();
        } catch (ReflectiveOperationException e) {
            throw new RuntimeException("Bean 實例化失敗: " + name, e);
        }
    }
    @Override public Object getBean(String name, String className) { return null; }
}

4. 配置文件 applicationContext.properties

student=com.example.Student
service=com.example.UserService
dao=com.example.UserDao

5. 測試

public class IoCTest {
    public static void main(String[] args) {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext.properties");
        Student s = (Student) ctx.getBean("student");
        s.setSid("15301");
        s.setSname("張三");
        System.out.println(s.getSid() + " " + s.getSname());
    }
}

關鍵點總結

• 配置文件映射 beanName → 全限定類名
• 反射 Class.forName().newInstance() 實例化 (需無參構造)
• 容器管理單例/原型作用域 (此處簡化為每次新建)
• Spring 進階：BeanDefinition 元數據、BeanPostProcessor 擴展點、循環依賴檢測、AOP 代理創建

擴展方向

• 支持構造器注入 / Setter 注入 / @Autowired 字段注入
• XML / 注解 / Java Config 多種配置源
• 生命周期回調 (InitializingBean, DisposableBean)
• 三級緩存解決循環依賴