循环依赖就是 N 个类中循环嵌套引用, 如果在日常开发中我们用 new 对象的方式发生这种循环依赖的话程序会在运行时一直循环调用, 直至内存溢出报错.
下面说一下 Spring 是如果解决循环依赖的.
第一种: 构造器参数循环依赖
Spring 容器会将每一个正在创建的 Bean 标识符放在一个 "当前创建 Bean 池" 中, Bean 标识符在创建过程中将一直保持在这个池中.
因此如果在创建 Bean 过程中发现自己已经在 "当前创建 Bean 池" 里时将抛出 BeanCurrentlyInCreationException 异常表示循环依赖; 而对于创建完毕的 Bean 将从 "当前创建 Bean 池" 中清除掉.
首先我们先初始化三个 Bean.
- public class StudentA {
- private StudentB studentB ;
- public void setStudentB(StudentB studentB) {
- this.studentB = studentB;
- }
- public StudentA() {
- }
- public StudentA(StudentB studentB) {
- this.studentB = studentB;
- }
- }
- public class StudentB {
- private StudentC studentC ;
- public void setStudentC(StudentC studentC) {
- this.studentC = studentC;
- }
- public StudentB() {
- }
- public StudentB(StudentC studentC) {
- this.studentC = studentC;
- }
- }
- public class StudentC {
- private StudentA studentA ;
- public void setStudentA(StudentA studentA) {
- this.studentA = studentA;
- }
- public StudentC() {
- }
- public StudentC(StudentA studentA) {
- this.studentA = studentA;
- }
- }
OK, 上面是很基本的 3 个类, StudentA 有参构造是 StudentB.StudentB 的有参构造是 StudentC,StudentC 的有参构造是 StudentA , 这样就产生了一个循环依赖的情况.
我们都把这三个 Bean 交给 Spring 管理, 并用有参构造实例化.
下面是测试类:
- public class Test {
- public static void main(String[] args) {
- ApplicationContext context = new ClassPathXmlApplicationContext("com/zfx/student/applicationContext.xml");
- //System.out.println(context.getBean("a", StudentA.class));
- }
- }
执行结果报错信息为:
- Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException:
- Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
如果大家理解开头那句话的话, 这个报错应该不惊讶, Spring 容器先创建单例 StudentA,StudentA 依赖 StudentB, 然后将 A 放在 "当前创建 Bean 池" 中.
此时创建 StudentB,StudentB 依赖 StudentC , 然后将 B 放在 "当前创建 Bean 池" 中, 此时创建 StudentC,StudentC 又依赖 StudentA.
但是, 此时 Student 已经在池中, 所以会报错, 因为在池中的 Bean 都是未初始化完的, 所以会依赖错误 , 初始化完的 Bean 会从池中移除.
第二种: setter 方式单例, 默认方式
如果要说 setter 方式注入的话, 我们最好先看一张 Spring 中 Bean 实例化的图
如图中前两步骤得知: Spring 是先将 Bean 对象实例化之后再设置对象属性的, Spring 中的 bean 为什么默认单例, 这篇建议大家看下.
- public class Test {
- public static void main(String[] args) {
- ApplicationContext context = new ClassPathXmlApplicationContext("com/zfx/student/applicationContext.xml");
- System.out.println(context.getBean("a", StudentA.class));
- }
- }
- /** Cache of singleton objects: bean name --> bean instance(缓存单例实例化对象的 Map 集合) */
- private final Map singletonObjects = new ConcurrentHashMap(64);
- /** Cache of singleton factories: bean name --> ObjectFactory(单例的工厂 Bean 缓存集合) */
- private final Map singletonFactories = new HashMap(16);
- /** Cache of early singleton objects: bean name --> bean instance(早期的单身对象缓存集合) */
- private final Map earlySingletonObjects = new HashMap(16);
- /** Set of registered singletons, containing the bean names in registration order(单例的实例化对象名称集合) */
- private final Set registeredSingletons = new LinkedHashSet(64);
- /**
- * 添加单例实例
- * 解决循环引用的问题
- * Add the given singleton factory for building the specified singleton
- * if necessary.
- *
- To be called for eager registration of singletons, e.g. to be able to
- * resolve circular references.
- * @param beanName the name of the bean
- * @param singletonFactory the factory for the singleton object
- */
- protected void addSingletonFactory(String beanName, ObjectFactory singletonFactory) {
- Assert.notNull(singletonFactory, "Singleton factory must not be null");
- synchronized (this.singletonObjects) {
- if (!this.singletonObjects.containsKey(beanName)) {
- this.singletonFactories.put(beanName, singletonFactory);
- this.earlySingletonObjects.remove(beanName);
- this.registeredSingletons.add(beanName);
- }
- }
- }
- public class Test {
- public static void main(String[] args) {
- ApplicationContext context = new ClassPathXmlApplicationContext("com/zfx/student/applicationContext.xml");
- // 此时必须要获取 Spring 管理的实例, 因为现在 scope="prototype" 只有请求获取的时候才会实例化对象
- System.out.println(context.getBean("a", StudentA.class));
- }
- }
- Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException:
- Error creating bean with name 'a': Requested bean is currently in creation: Is there an unresolvable circular reference?
来源: http://developer.51cto.com/art/202002/610261.htm