介紹

先說(shuō)一下什么是循環(huán)依賴(lài)，循壞依賴(lài)即循環(huán)引用，兩個(gè)或多個(gè)bean相互引用，最終形成一個(gè)環(huán)。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(lài)有兩種場(chǎng)景

構(gòu)造器的循環(huán)依賴(lài) 屬性的循環(huán)依賴(lài)

構(gòu)造器的循環(huán)依賴(lài)，可以在構(gòu)造函數(shù)中使用@Lazy注解延遲加載。在注入依賴(lài)時(shí)，先注入代理對(duì)象，當(dāng)首次使用時(shí)再創(chuàng)建對(duì)象完成注入

屬性的循環(huán)依賴(lài)主要是通過(guò)3個(gè)map來(lái)解決的

構(gòu)造器的循環(huán)依賴(lài)

@Componentpublic class ConstructorA { private ConstructorB constructorB; @Autowired public ConstructorA(ConstructorB constructorB) { this.constructorB = constructorB; }}

@Componentpublic 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(shuō)明Spring沒(méi)辦法初始化所有的Bean，即上面這種形式的循環(huán)依賴(lài)Spring無(wú)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

@Autowiredpublic ConstructorB(@Lazy ConstructorA constructorA) {this.constructorA = constructorA;}

因?yàn)槲覀冎饕P(guān)注屬性的循環(huán)依賴(lài)，構(gòu)造器的循環(huán)依賴(lài)就不做過(guò)多分析了

屬性的循環(huán)依賴(lài)

先演示一下什么是屬性的循環(huán)依賴(lài)

@Componentpublic class FieldA { @Autowired private FieldB fieldB;}

@Componentpublic 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(lài)在面試中還是經(jīng)常被問(wèn)到的。總體來(lái)說(shuō)也不復(fù)雜，但是涉及到Spring Bean的初始化過(guò)程，所以感覺(jué)比較復(fù)雜，我寫(xiě)個(gè)demo演示一下整個(gè)過(guò)程

Spring的Bean的初始化過(guò)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(guò)程分為2部分

bean的實(shí)例化過(guò)程，即調(diào)用構(gòu)造函數(shù)將對(duì)象創(chuàng)建出來(lái) bean的初始化過(guò)程，即填充bean的各種屬性

bean初始化過(guò)程完畢，則bean就能被正常創(chuàng)建出來(lái)了

下面開(kāi)始寫(xiě)Demo，ObjectFactory接口用來(lái)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(duì)應(yīng)的工廠，此時(shí)對(duì)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(duì)象還沒(méi)有被實(shí)例化之前就放進(jìn)來(lái)了 private final Set<String> singletonsCurrentlyInCreation = Collections.newSetFromMap(new ConcurrentHashMap<>(16)); public <T> T getBean(Class<T> beanClass) throws Exception { // 類(lèi)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(kāi)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(lài)，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(lài)，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(cè)試一波

public static void main(String[] args) throws Exception {DependencyDemo dependencyDemo = new DependencyDemo();// 假裝掃描出來(lái)的對(duì)象Class[] classes = {A.class, B.class};// 假裝項(xiàng)目初始化所有beanfor (Class aClass : classes) {dependencyDemo.getBean(aClass);}// trueSystem.out.println(dependencyDemo.getBean(B.class).getA() == dependencyDemo.getBean(A.class));// trueSystem.out.println(dependencyDemo.getBean(A.class).getB() == dependencyDemo.getBean(B.class));}

是不是很簡(jiǎn)單？我們只用了2個(gè)map就搞定了Spring的循環(huán)依賴(lài)

2個(gè)Map就能搞定循環(huán)依賴(lài)，那為什么Spring要用3個(gè)Map呢？

原因其實(shí)也很簡(jiǎn)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(duì)應(yīng)的Bean。在我們的例子中ObjectFactory的實(shí)現(xiàn)很簡(jiǎn)單哈，就是將實(shí)例化好的對(duì)象直接返回，但是在Spring中就沒(méi)有這么簡(jiǎn)單了，執(zhí)行過(guò)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(duì)象緩存起來(lái)不就行了，這樣就能提高效率了

比如A依賴(lài)B和C，B和C又依賴(lài)A，如果不做緩存那么初始化B和C都會(huì)調(diào)用A對(duì)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Beanprivate final Map<String, Object> singletonObjects =new ConcurrentHashMap<>(256);// 正在初始化的Bean對(duì)應(yīng)的工廠，此時(shí)對(duì)象已經(jīng)被實(shí)例化private final Map<String, ObjectFactory<?>> singletonFactories =new HashMap<>(16);// 緩存Bean對(duì)應(yīng)的工廠生產(chǎn)好的Beanprivate final Map<String, Object> earlySingletonObjects =new HashMap<>(16);// 存放正在初始化的Bean，對(duì)象還沒(méi)有被實(shí)例化之前就放進(jìn)來(lái)了private final Set<String> singletonsCurrentlyInCreation =Collections.newSetFromMap(new ConcurrentHashMap<>(16));public <T> T getBean(Class<T> beanClass) throws Exception {// 類(lèi)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beanObject object = beanClass.getDeclaredConstructor().newInstance();singletonFactories.put(beanName, () -> {return object;});// 開(kāi)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classClass<?> 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(lài)，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(xiě)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(lài)的文章都會(huì)提到，這次你明白工作原理是什么了把

總結(jié)一波

拿bean的時(shí)候先從singletonObjects（一級(jí)緩存）中獲取 如果獲取不到，并且對(duì)象正在創(chuàng)建中，就從earlySingletonObjects（二級(jí)緩存）中獲取 如果還是獲取不到就從singletonFactories（三級(jí)緩存）中獲取，然后將獲取到的對(duì)象放到earlySingletonObjects（二級(jí)緩存）中，并且將bean對(duì)應(yīng)的singletonFactories（三級(jí)緩存）清除 bean初始化完畢，放到singletonObjects（一級(jí)緩存）中，將bean對(duì)應(yīng)的earlySingletonObjects（二級(jí)緩存）清除

參考博客

[1]https://mp.weixin.qq.com/s/gBr3UfC1HRcw4U-ZMmtRaQ[2]https://mp.weixin.qq.com/s/5mwkgJB7GyLdKDgzijyvXw比較詳細(xì)[1]https://zhuanlan.zhihu.com/p/84267654[2]https://juejin.im/post/5c98a7b4f265da60ee12e9b2

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