簡介

spring ioc的極簡實作，可以完成自動屬性注入，使用二級快取解決回圈依賴（就是把半成品物件放個map里先保存著），基于注解，可以指定組態檔，總計不到100行代碼，可以簡單對spring依賴注入有個大致的了解，專案下載鏈接在文末

大概步驟

最基本的注解

Autowired注解

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Autowired {
}

Component注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface Component {
}

ComponentScan注解（這個注解其實也可以不要，比如默認就掃描main方法目錄）

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface ComponentScan {
    String value();
}

Application類

用來決議組態檔，創建bean

public class Application {
    Map<Class<?>,Object> singletonInstance =new HashMap<>();
    Map<Class<?>,Object> earlySingletonInstance=new HashMap<>();
    Set<Class<?>> classSet=new HashSet<>();
    public Application(Class<?> configClass) throws ClassNotFoundException, InstantiationException, IllegalAccessException {
        //獲得掃描路徑
        String path = configClass.getDeclaredAnnotation(ComponentScan.class).value();
        ClassLoader classLoader = Application.class.getClassLoader();
        //獲得掃描路徑下所有檔案
        for (File file : new File(classLoader.getResource(path.replace(".","/")).getFile()).listFiles()) {
            //這里沒有遞回處理，只掃描第一層的class檔案
            if(!file.getName().endsWith(".class"))continue;
            String fullName=(path+"."+file.getName()).replace(".class","");
            Class<?> aClass = classLoader.loadClass(fullName);
            //保存所有被Component注解的類
            if(aClass.isAnnotationPresent(Component.class))classSet.add(aClass);
        }
        //生成半成品物件放到二級快取
        for (Class<?> aClass : classSet) earlySingletonInstance.put(aClass,aClass.newInstance());
        //Autowired屬性注入
        for (Class<?> aClass : classSet){
            Object instance = earlySingletonInstance.get(aClass);
            for (Field field : aClass.getDeclaredFields()) {
                if(field.isAnnotationPresent(Autowired.class)){
                    field.setAccessible(true);
                    field.set(instance,earlySingletonInstance.get(field.getType()));
                }
            }
            //放到單例池
            singletonInstance.put(aClass, instance);
        }
    }
    //按型別獲取，生成的都是單例bean
    public <T> T getBean(Class<T> clazz){
        return clazz.cast(singletonInstance.get(clazz));
    }
}

測驗

Calculate.java，Service.java這兩個檔案位于io.spring.test目錄下

@Component
public class Calculate {
    @Autowired
    private Service service;

    public int add(int x,int y){
        return service.add(x, y);
    }
}

@Component
public class  Service {
    @Autowired
    Calculate calculate;//測驗回圈依賴
    public int add(int x, int y) {
        return x+y;
    }
}

@ComponentScan("io.spring.test")//掃描路徑
public class App {
    public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {
        Application appContext = new Application(App.class);//初始化
        Calculate cal = appContext.getBean(Calculate.class);//獲得bean物件
        System.out.println(cal.add(3,5));
    }
}

測驗程式輸出8，說明依賴注入的目的達到了

專案下載

https://github.com/fuchengshun/simple-spring-ioc

標籤：Java

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