您好,登錄后才能下訂單哦!
這篇文章主要介紹“Spring怎么解決循環(huán)依賴的”,在日常操作中,相信很多人在Spring怎么解決循環(huán)依賴的問題上存在疑惑,小編查閱了各式資料,整理出簡單好用的操作方法,希望對大家解答”Spring怎么解決循環(huán)依賴的”的疑惑有所幫助!接下來,請跟著小編一起來學(xué)習(xí)吧!
介紹
先說一下什么是循環(huán)依賴,Spring在初始化A的時(shí)候需要注入B,而初始化B的時(shí)候需要注入A,在Spring啟動(dòng)后這2個(gè)Bean都要被初始化完成
Spring的循環(huán)依賴有兩種場景
構(gòu)造器的循環(huán)依賴
屬性的循環(huán)依賴
構(gòu)造器的循環(huán)依賴,可以在構(gòu)造函數(shù)中使用@Lazy注解延遲加載。在注入依賴時(shí),先注入代理對象,當(dāng)首次使用時(shí)再創(chuàng)建對象完成注入
屬性的循環(huán)依賴主要是通過3個(gè)map來解決的
構(gòu)造器的循環(huán)依賴
@Component public class ConstructorA { private ConstructorB constructorB; @Autowired public ConstructorA(ConstructorB constructorB) { this.constructorB = constructorB; } } @Component public class ConstructorB { private ConstructorA constructorA; @Autowired public ConstructorB(ConstructorA constructorA) { this.constructorA = constructorA; } } @Configuration @ComponentScan("com.javashitang.dependency.constructor") public class ConstructorConfig { } public class ConstructorMain { public static void main(String[] args) { AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(ConstructorConfig.class); System.out.println(context.getBean(ConstructorA.class)); System.out.println(context.getBean(ConstructorB.class)); } }
運(yùn)行ConstructorMain的main方法的時(shí)候會(huì)在第一行就報(bào)異常,說明Spring沒辦法初始化所有的Bean,即上面這種形式的循環(huán)依賴Spring無法解決。
我們可以在ConstructorA或者ConstructorB構(gòu)造函數(shù)的參數(shù)上加上@Lazy注解就可以解決
@Autowired public ConstructorB(@Lazy ConstructorA constructorA) { this.constructorA = constructorA; }
因?yàn)槲覀冎饕P(guān)注屬性的循環(huán)依賴,構(gòu)造器的循環(huán)依賴就不做過多分析了
屬性的循環(huán)依賴
先演示一下什么是屬性的循環(huán)依賴
@Component public class FieldA { @Autowired private FieldB fieldB; } @Component public class FieldB { @Autowired private FieldA fieldA; } @Configuration @ComponentScan("com.javashitang.dependency.field") public class FieldConfig { } public class FieldMain { public static void main(String[] args) { AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(FieldConfig.class); // com.javashitang.dependency.field.FieldA@3aa9e816 System.out.println(context.getBean(FieldA.class)); // com.javashitang.dependency.field.FieldB@17d99928 System.out.println(context.getBean(FieldB.class)); } }
Spring容器正常啟動(dòng),能獲取到FieldA和FieldB這2個(gè)Bean
屬性的循環(huán)依賴在面試中還是經(jīng)常被問到的??傮w來說也不復(fù)雜,但是涉及到Spring Bean的初始化過程,所以感覺比較復(fù)雜,我寫個(gè)demo演示一下整個(gè)過程
Spring的Bean的初始化過程其實(shí)比較復(fù)雜,為了方便理解Demo,我就把Spring Bean的初始化過程分為2部分
bean的實(shí)例化過程,即調(diào)用構(gòu)造函數(shù)將對象創(chuàng)建出來
bean的初始化過程,即填充bean的各種屬性
bean初始化過程完畢,則bean就能被正常創(chuàng)建出來了
下面開始寫Demo,ObjectFactory接口用來生產(chǎn)Bean,和Spring中定義的接口一樣
public interface ObjectFactory<T> { T getObject(); } public class DependencyDemo { // 初始化完畢的Bean private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256); // 正在初始化的Bean對應(yīng)的工廠,此時(shí)對象已經(jīng)被實(shí)例化 private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16); // 存放正在初始化的Bean,對象還沒有被實(shí)例化之前就放進(jìn)來了 private final Set<String> singletonsCurrentlyInCreation = Collections.newSetFromMap(new ConcurrentHashMap<>(16)); public <T> T getBean(Class<T> beanClass) throws Exception { // 類名為Bean的名字 String beanName = beanClass.getSimpleName(); // 已經(jīng)初始化好了,或者正在初始化 Object initObj = getSingleton(beanName, true); if (initObj != null) { return (T) initObj; } // bean正在被初始化 singletonsCurrentlyInCreation.add(beanName); // 實(shí)例化bean Object object = beanClass.getDeclaredConstructor().newInstance(); singletonFactories.put(beanName, () -> { return object; }); // 開始初始化bean,即填充屬性 Field[] fields = object.getClass().getDeclaredFields(); for (Field field : fields) { field.setAccessible(true); // 獲取需要注入字段的class Class<?> fieldClass = field.getType(); field.set(object, getBean(fieldClass)); } // 初始化完畢 singletonObjects.put(beanName, object); singletonsCurrentlyInCreation.remove(beanName); return (T) object; } /** * allowEarlyReference參數(shù)的含義是Spring是否允許循環(huán)依賴,默認(rèn)為true * 所以當(dāng)allowEarlyReference設(shè)置為false的時(shí)候,當(dāng)項(xiàng)目存在循環(huán)依賴,會(huì)啟動(dòng)失敗 */ public Object getSingleton(String beanName, boolean allowEarlyReference) { Object singletonObject = this.singletonObjects.get(beanName); if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) { synchronized (this.singletonObjects) { if (singletonObject == null && allowEarlyReference) { ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName); if (singletonFactory != null) { singletonObject = singletonFactory.getObject(); } } } } return singletonObject; } /** * 判斷bean是否正在被初始化 */ public boolean isSingletonCurrentlyInCreation(String beanName) { return this.singletonsCurrentlyInCreation.contains(beanName); } }
測試一波
public static void main(String[] args) throws Exception { DependencyDemo dependencyDemo = new DependencyDemo(); // 假裝掃描出來的對象 Class[] classes = {A.class, B.class}; // 假裝項(xiàng)目初始化所有bean for (Class aClass : classes) { dependencyDemo.getBean(aClass); } // true System.out.println( dependencyDemo.getBean(B.class).getA() == dependencyDemo.getBean(A.class)); // true System.out.println( dependencyDemo.getBean(A.class).getB() == dependencyDemo.getBean(B.class)); }
是不是很簡單?我們只用了2個(gè)map就搞定了Spring的循環(huán)依賴
2個(gè)Map就能搞定循環(huán)依賴,那為什么Spring要用3個(gè)Map呢?
原因其實(shí)也很簡單,當(dāng)我們從singletonFactories中根據(jù)BeanName獲取相應(yīng)的ObjectFactory,然后調(diào)用getObject()這個(gè)方法返回對應(yīng)的Bean。在我們的例子中 ObjectFactory的實(shí)現(xiàn)很簡單哈,就是將實(shí)例化好的對象直接返回,但是在Spring中就沒有這么簡單了,執(zhí)行過程比較復(fù)雜,為了避免每次拿到ObjectFactory然后調(diào)用getObject(),我們直接把ObjectFactory創(chuàng)建的對象緩存起來不就行了,這樣就能提高效率了
比如A依賴B和C,B和C又依賴A,如果不做緩存那么初始化B和C都會(huì)調(diào)用A對應(yīng)的ObjectFactory的getObject()方法。如果做緩存只需要B或者C調(diào)用一次即可。
知道了思路,我們把上面的代碼改一波,加個(gè)緩存。
public class DependencyDemo { // 初始化完畢的Bean private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256); // 正在初始化的Bean對應(yīng)的工廠,此時(shí)對象已經(jīng)被實(shí)例化 private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16); // 緩存Bean對應(yīng)的工廠生產(chǎn)好的Bean private final Map<String, Object> earlySingletonObjects = new HashMap<>(16); // 存放正在初始化的Bean,對象還沒有被實(shí)例化之前就放進(jìn)來了 private final Set<String> singletonsCurrentlyInCreation = Collections.newSetFromMap(new ConcurrentHashMap<>(16)); public <T> T getBean(Class<T> beanClass) throws Exception { // 類名為Bean的名字 String beanName = beanClass.getSimpleName(); // 已經(jīng)初始化好了,或者正在初始化 Object initObj = getSingleton(beanName, true); if (initObj != null) { return (T) initObj; } // bean正在被初始化 singletonsCurrentlyInCreation.add(beanName); // 實(shí)例化bean Object object = beanClass.getDeclaredConstructor().newInstance(); singletonFactories.put(beanName, () -> { return object; }); // 開始初始化bean,即填充屬性 Field[] fields = object.getClass().getDeclaredFields(); for (Field field : fields) { field.setAccessible(true); // 獲取需要注入字段的class Class<?> fieldClass = field.getType(); field.set(object, getBean(fieldClass)); } singletonObjects.put(beanName, object); singletonsCurrentlyInCreation.remove(beanName); earlySingletonObjects.remove(beanName); return (T) object; } /** * allowEarlyReference參數(shù)的含義是Spring是否允許循環(huán)依賴,默認(rèn)為true */ public Object getSingleton(String beanName, boolean allowEarlyReference) { Object singletonObject = this.singletonObjects.get(beanName); if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) { synchronized (this.singletonObjects) { singletonObject = this.earlySingletonObjects.get(beanName); if (singletonObject == null && allowEarlyReference) { ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName); if (singletonFactory != null) { singletonObject = singletonFactory.getObject(); this.earlySingletonObjects.put(beanName, singletonObject); this.singletonFactories.remove(beanName); } } } } return singletonObject; } }
我們寫的getSingleton的實(shí)現(xiàn)和org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton(java.lang.String, boolean)的實(shí)現(xiàn)一模一樣,這個(gè)方法幾乎所有分析Spring循環(huán)依賴的文章都會(huì)提到,這次你明白工作原理是什么了把
到此,關(guān)于“Spring怎么解決循環(huán)依賴的”的學(xué)習(xí)就結(jié)束了,希望能夠解決大家的疑惑。理論與實(shí)踐的搭配能更好的幫助大家學(xué)習(xí),快去試試吧!若想繼續(xù)學(xué)習(xí)更多相關(guān)知識,請繼續(xù)關(guān)注億速云網(wǎng)站,小編會(huì)繼續(xù)努力為大家?guī)砀鄬?shí)用的文章!
免責(zé)聲明:本站發(fā)布的內(nèi)容(圖片、視頻和文字)以原創(chuàng)、轉(zhuǎn)載和分享為主,文章觀點(diǎn)不代表本網(wǎng)站立場,如果涉及侵權(quán)請聯(lián)系站長郵箱:is@yisu.com進(jìn)行舉報(bào),并提供相關(guān)證據(jù),一經(jīng)查實(shí),將立刻刪除涉嫌侵權(quán)內(nèi)容。