一文详解Spring构造函数推断

public class MyBean {
    private final MyDependency myDependency;
    public MyBean(MyDependency myDependency) {
        this.myDependency = myDependency;
    }
    public void doSomething() {
        myDependency.doSomething();
    }
}

@Configuration
public class AppConfig {
    @Bean
    public MyDependency myDependency() {
        return new MyDependency();
    }
    @Bean
    public MyBean myBean() {
        return new MyBean(myDependency());
    }
}

public class BeanWrapperImpl extends AbstractPropertyAccessor implements BeanWrapper {
    // ...
    @Override
    public Object createInstance() throws BeansException {
        // 获取Bean的构造函数
        Constructor<?> constructorToUse = this.constructorResolver.autowireConstructor;
        if (constructorToUse == null) {
            throw new BeansException("No default constructor found");
        }
        try {
            // 使用构造函数创建Bean实例，并返回
            return constructorToUse.newInstance(getConstructorArgumentValues(constructorToUse), getBeanClassLoader());
        }
        catch (Throwable ex) {
            throw new BeanCreationException("Could not instantiate bean", ex);
        }
    }
    // ...
    private Constructor<?> autowireConstructor;
    // ...
    public BeanWrapperImpl(Class<?> clazz) {
        super();
        this.objectType = clazz;
        this.propertyEditorRegistry = new SimpleTypeConverter();
        this.constructorResolver = new ConstructorResolver(this.objectType, this);
    }
    // ...
    private class ConstructorResolver {
        // ...
        public ConstructorResolver(Class<?> clazz, BeanWrapperImpl bw) {
            // 查找可以使用的构造函数并将其缓存
            Constructor<?>[] candidates = clazz.getDeclaredConstructors();
            Constructor<?> autowireCandidate = null;
            int numAutowireCandidates = 0;
            for (Constructor<?> candidate : candidates) {
                if (candidate.isAnnotationPresent(Autowired.class)) {
                    autowireCandidate = candidate;
                    numAutowireCandidates++;
                }
            }
            if (numAutowireCandidates == 1) {
                this.autowireConstructor = autowireCandidate;
            }
            else if (numAutowireCandidates > 1) {
                throw new BeansException("Multiple autowire constructors found");
            }
        }
        // ...
    }
    // ...
}