Spring IOC 一些常见容易忽略的问题，包括autowire/resource 注入，类型定义，循环依赖等，会后续不断补充。

Autowire注入五种模式

1.no

默认，如果没找到在调用依赖对象时会抛NullPointerException

2.byType

根据bean class类型在容器中找到唯一一个对应的给目标对象注入

当所注入的类型在容器中只有唯一对象时，较为简单不需要关注属性名或setter方法，以下分析多实现情况

场景1 annotation @Autowired example:

DAO:

public interface UserDAO {void query();
}

两个DAO实现类(UserDAO1, UserDAO2):

@Repository
public class UserDAO1 implements UserDAO {@Overridepublic void query() {System.out.println("1----User dao query");}
}

@Repository
public class UserDAO2 implements UserDAO {@Overridepublic void query() {System.out.println("2----User dao query");}
}

Service调用类(UserService):

@Service
public class UserService {@Autowiredprivate UserDAO userDAO1;public void queryUser(){userDAO1.query();}
}

测试代码:

@Configuration
@ComponentScan("com.example.demo.ioc")
public class AutowireDemo {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(AutowireDemo.class);context.getBean(UserService.class).queryUser();}}

输出:

1----User dao query

因为在Service中注入的UserDAO有多个实现的情况下，找不到该类型唯一的对象， 再次根据属性名userDAO1判断注入UserDAO1. 当属性名为UserDAO2在容器中的id时，则会注入UserDAO2.

场景2 xml autowire byType example:

xml中配置default-autowire

beans.xml:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://www.springframework.org/schema/beanshttp://www.springframework.org/schema/beans/spring-beans.xsd"default-autowire="byType"><!--<bean id="userDAO1" class="com.example.demo.ioc.dao.UserDAO1"/>--><bean id="userDAO2" class="com.example.demo.ioc.dao.UserDAO2"/><bean id="userService" class="com.example.demo.ioc.service.UserService" /></beans>

UserService:

public class UserService {private UserDAO userDAO1;public void queryUser(){userDAO1.query();}public UserDAO getUserDAO1() {return userDAO1;}public void setUserDAO1(UserDAO userDAO) {this.userDAO1 = userDAO;}
}

测试代码:

ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("classpath:beans.xml");context.getBean(UserService.class).queryUser();

输出:

2----User dao query

以xml方式使用byType方式注入时，要保证所在环境里对应类型的bean唯一性，xml会自动检测编译报出error。此时属性名、setter方法名都不具备任何特殊处理意义，只跟类型有关。

3.byName

根据name找到容器中唯一id的bean注入给对象.

场景1 annotation方式@Resource 默认将属性名作为name example 1:

当resource不指定name时，则将属性名作为name查找bean,否则按指定name查找bean，与属性名无关

@Service
public class UserService {@Resourceprivate UserDAO userDAO2;public void queryUser(){userDAO2.query();}
}

输出:

2----User dao query

@Resource 指定type注入依赖 example 2:

​ @Resource使用Type属性时，属性名不能与存在的bean Name重合，可能会导致注入对象不一致问题，see @exampl 3

@Service
public class UserService {@Resource(type = UserDAO2.class)private UserDAO userDAO;public void queryUser(){userDAO.query();}
}

输出:

2----User dao query

@Resource 指定type同时属性名对应要注入bean的类型不一致 example 3:

属性名指定要注入的为UserDAO1,但同时type指定类型为UserDAO2

@Service
public class UserService {@Resource(type = UserDAO2.class)private UserDAO userDAO1;public void queryUser(){userDAO1.query();}

输出:

Error:

警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userService': Injection of resource dependencies failed; nested exception is org.springframework.beans.factory.BeanNotOfRequiredTypeException: Bean named 'userDAO1' is expected to be of type 'com.example.demo.ioc.dao.UserDAO2' but was actually of type 'com.example.demo.ioc.dao.UserDAO1'
Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userService': Injection of resource dependencies failed; nested exception is org.springframework.beans.factory.BeanNotOfRequiredTypeException: Bean named 'userDAO1' is expected to be of type 'com.example.demo.ioc.dao.UserDAO2' but was actually of type 'com.example.demo.ioc.dao.UserDAO1'
at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessPropertyValues(CommonAnnotationBeanPostProcessor.java:321)

场景2 xml配置中配置default-autowire example:

局部为bean单独配置:

<bean id="userService" class="com.example.demo.ioc.service.UserService" autowire="">

beans.xml:

全局配置autowire，指定default-autowire时，不用再单独为bean的property进行显示传值，会自动根据配置的mode初始化bean

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://www.springframework.org/schema/beanshttp://www.springframework.org/schema/beans/spring-beans.xsd"default-autowire="byName"><bean id="userDAO1" class="com.example.demo.ioc.dao.UserDAO1"/><bean id="userDAO2" class="com.example.demo.ioc.dao.UserDAO2"/><bean id="userService" class="com.example.demo.ioc.service.UserService"></bean></beans>

UserService:

注入的UserDAO由setter方法决定，setUserDAO2将容器中名为userDAO2的bean赋给属性userDAO1

public class UserService {private UserDAO userDAO1;public void queryUser(){userDAO1.query();}public UserDAO getUserDAO1() {return userDAO1;}public void setUserDAO2(UserDAO userDAO) {this.userDAO1 = userDAO;}
}

测试代码:

public class AutowireDemo {public static void main(String[] args) {ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("classpath:beans.xml");context.getBean(UserService.class).queryUser();}}

输出:

2----User dao query

4.constructor

与byType相似，按类型注入。需要提供含参构造方法，按照类型查找对应的bean作为参数注入。否则会在使用所依赖的对象是Null Pointer

5.default

与no相同

Singleton依赖prototype Bean，原型失效引起的问题

beans.xml

<bean id="prototypeBean" class="com.example.demo.ioc.service.PrototypeBean" scope="prototype"></bean><bean id="singletonBean" class="com.example.demo.ioc.service.SingletonBean" scope="singleton"></bean>

SingletonBean:

public class SingletonBean {private PrototypeBean prototypeBean;public void print(){System.out.println("prototypeBean hashcode:"+prototypeBean.hashCode());System.out.println("singletonBean hashcode:"+this.hashCode());}public void setPrototypeBean(PrototypeBean prototypeBean) {this.prototypeBean = prototypeBean;}
}

PrototypeBean:

public class PrototypeBean {
}

测试代码:

ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("classpath:beans.xml");
for(int i=0;i<3;i++) {context.getBean(SingletonBean.class).print();
}

输出：

prototypeBean hashcode:1512981843
singletonBean hashcode:42768293
prototypeBean hashcode:1512981843
singletonBean hashcode:42768293
prototypeBean hashcode:1512981843
singletonBean hashcode:42768293

singleton中依赖的原型bean在输出中结果表现为单例.因为在spring初始化单例bean时已经构造并初始化好了依赖对象。每次调用的prototypeBean都是初始化好完成的那个bean。

解决方案

1.ApplicationContextAware

实现该接口，可获取ApplicationContext，在每次调用prototype的时候，从容器中获取即可实现每次即用即建。

但对应的会极大程度引入spring整个环境，不利于解耦

2.lookup method

beans.xml:

<bean id="prototypeBean" class="com.example.demo.ioc.service.PrototypeBean" scope="prototype"></bean><bean id="singletonBean" class="com.example.demo.ioc.service.SingletonBean" scope="singleton" ><lookup-method name="getPrototypeBean" bean="prototypeBean"></lookup-method></bean>

SingletonBean:

将获取prototypeBean的方式转为look up method

public  abstract  class SingletonBean {public void print(){System.out.println("prototypeBean hashcode:"+getPrototypeBean().hashCode());System.out.println("singletonBean hashcode:"+this.hashCode());}public void setPrototypeBean(PrototypeBean prototypeBean) {this.prototypeBean = prototypeBean;}public abstract PrototypeBean getPrototypeBean();
}

lookup method由spring负责解析返回对应的一个bean，不需要程序员手动实现，因此可作为抽象类存在与配置

输出:

prototypeBean hashcode:668849042
singletonBean hashcode:434176574
prototypeBean hashcode:2096057945
singletonBean hashcode:434176574
prototypeBean hashcode:1689843956
singletonBean hashcode:434176574