Spring DependencyObjectProvider

	private class DependencyObjectProvider implements BeanObjectProvider<Object> {

    /**
     * 依赖描述
     */
    private final DependencyDescriptor descriptor;

    /**
     * 是否是 Optional 类型
     */
    private final boolean optional;

    @Nullable
    private final String beanName;
}

		@Override
		public Object getObject() throws BeansException {
			// 获取bean实例
			// 是 optional 的情况下创建
			if (this.optional) {
				return createOptionalDependency(this.descriptor, this.beanName);
			}
			else {
				// 解析依赖进行创建对象
				Object result = doResolveDependency(this.descriptor, this.beanName, null, null);
				if (result == null) {
					throw new NoSuchBeanDefinitionException(this.descriptor.getResolvableType());
				}
				return result;
			}
		}

    @Override
    public Object getObject(final Object... args) throws BeansException {
        // 获取bean实例
        // 是 optional 的情况下创建
        if (this.optional) {
            return createOptionalDependency(this.descriptor, this.beanName, args);
        }
        else {
            // 创建 依赖描述对象， 重写方法 resolveCandidate , 从容器(BeanFactory)中获取
            DependencyDescriptor descriptorToUse = new DependencyDescriptor(this.descriptor) {
                @Override
                public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory) {
                    return beanFactory.getBean(beanName, args);
                }
            };

            // 解析依赖进行创建对象
            Object result = doResolveDependency(descriptorToUse, this.beanName, null, null);
            if (result == null) {
                throw new NoSuchBeanDefinitionException(this.descriptor.getResolvableType());
            }
            return result;
        }
    }

@Override
		@Nullable
		public Object getIfAvailable() throws BeansException {
			if (this.optional) {
				return createOptionalDependency(this.descriptor, this.beanName);
			}
			else {
				DependencyDescriptor descriptorToUse = new DependencyDescriptor(this.descriptor) {
					@Override
					public boolean isRequired() {
						return false;
					}
				};
				return doResolveDependency(descriptorToUse, this.beanName, null, null);
			}
		}

		@Override
		@Nullable
		public Object getIfUnique() throws BeansException {
			DependencyDescriptor descriptorToUse = new DependencyDescriptor(this.descriptor) {
				@Override
				public boolean isRequired() {
					return false;
				}

				@Override
				@Nullable
				public Object resolveNotUnique(ResolvableType type, Map<String, Object> matchingBeans) {
					return null;
				}
			};
			if (this.optional) {
				return createOptionalDependency(descriptorToUse, this.beanName);
			}
			else {
				return doResolveDependency(descriptorToUse, this.beanName, null, null);
			}
		}

		@Nullable
		protected Object getValue() throws BeansException {
			if (this.optional) {
				return createOptionalDependency(this.descriptor, this.beanName);
			}
			else {
				return doResolveDependency(this.descriptor, this.beanName, null, null);
			}
		}