public class BeanServiceImpl implements BeanService {
}
@Configuration
public class ConfigurationBean {
@Bean
public BeanService beanService() {
BeanServiceImpl beanService = new BeanServiceImpl();
System.out.println("beanService: " + beanService);
return beanService;
}
@Bean
//@Bean 如果不知道name，默认bean的name为方法名，下面等同于@Bean("beanService1")
public BeanService beanService1() {
BeanServiceImpl beanService1 = new BeanServiceImpl();
System.out.println("beanService1: " + beanService1);
return beanService1;
}
}
@Component
public class SetterBean {
private BeanService bs;
public SetterBean(BeanService beanService) {
bs = beanService;
System.out.println("有参构造：" + bs);
}
}
绝大部分是通过反射构造器去创建，详情源码见
AbstractAutowireCapableBeanFactory.doCreateBean中的createBeanInstance方法
如果没有有参构造器，默认使用无参的构造方法反射创建实例，详情见
createBeanInstance – instantiateBean方法
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
... ...
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);
}
如果有唯一有参构造器，并且参数能在Spring容器里找到，则通过该有参构造器反射创建
详情见createBeanInstance – autowireConstructor方法
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
... ...
// Candidate constructors for autowiring?
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
... ...
}
如果参数不能在Spring容器中找到，就会报
NoUniqueBeanDefinitionException异常
有多个有参构造器，Spring在创建Bean实例时，就不知道用哪个构造器去创建，就会报如下异常:
BeanInstantiationException: Failed to instantiate [XXX]: No default constructor found;
@Component
public class SetterBean {
private BeanService bs;
public SetterBean(BeanService beanService) {
bs = beanService;
System.out.println("有参构造：" + bs);
}
//@Autowired
public SetterBean(BeanService beanService, BeanService beanService1) {
bs = beanService;
System.out.println("有参构造：" + bs + "--------" + beanService1);
}
}
可以通过
像3.2 中，把@Autowired注释打开，就会使用该有参构造创建实例，启动就不会报异常，@Autowired来指定有参构造来创建实例
这个方法主要是挑选构造器，然后创建实例的
