spring boot @Value源码解析

Spring boot 的@Value只能用于bean中，在bean的实例化时，会给@Value的属性赋值：如下面的例子：

 

@SpringBootApplication
@Slf4j
public class DemoApplication {

    @Value(value = "${server.port}")
    private  String config;

    public static void main(String[] args) {

        SpringApplication.run(DemoApplication.class, args);

    }
}

 

下面的类显示了bean的实例化方法：org.springframework.context.support.AbstractApplicationContext

@Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);

                // Initialize message source for this context.
                initMessageSource();

                // Initialize event multicaster for this context.
                initApplicationEventMulticaster();

                // Initialize other special beans in specific context subclasses.
                onRefresh();

                // Check for listener beans and register them.
                registerListeners();

                // 实例化方法 Instantiate all remaining (non-lazy-init) singletons.
                finishBeanFactoryInitialization(beanFactory);

                // Last step: publish corresponding event.
                finishRefresh();
            }

            catch (BeansException ex) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
                }

                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

                // Reset 'active' flag.
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                // might not ever need metadata for singleton beans anymore...
                resetCommonCaches();
            }
        }
    }

bean的具体实例过过程，就不在这里说了，具体可以看前面的文章Spring boot IOC源码解析.

这个方法中：org.springframework.beans.factory.annotation.InjectionMetadata#inject 会遍历bean中所有的成员：

public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
        Collection<InjectedElement> checkedElements = this.checkedElements;
        Collection<InjectedElement> elementsToIterate =
                (checkedElements != null ? checkedElements : this.injectedElements);
        if (!elementsToIterate.isEmpty()) {
            for (InjectedElement element : elementsToIterate) {
                if (logger.isTraceEnabled()) {
                    logger.trace("Processing injected element of bean '" + beanName + "': " + element);
                }
                element.inject(target, beanName, pvs);
            }
        }
    }

 

最终解析的代码如下：org.springframework.core.env.PropertySourcesPropertyResolver#getProperty(java.lang.String, java.lang.Class<T>, boolean)

@Nullable
    protected <T> T getProperty(String key, Class<T> targetValueType, boolean resolveNestedPlaceholders) {
        if (this.propertySources != null) {
            for (PropertySource<?> propertySource : this.propertySources) {
                if (logger.isTraceEnabled()) {
                    logger.trace("Searching for key '" + key + "' in PropertySource '" +
                            propertySource.getName() + "'");
                }
                Object value = propertySource.getProperty(key);
                if (value != null) {
                    if (resolveNestedPlaceholders && value instanceof String) {
                        value = resolveNestedPlaceholders((String) value);
                    }
                    logKeyFound(key, propertySource, value);
                    return convertValueIfNecessary(value, targetValueType);
                }
            }
        }
        if (logger.isTraceEnabled()) {
            logger.trace("Could not find key '" + key + "' in any property source");
        }
        return null;
    }

propertySource的内容，可以看上篇博客，里面记录了spring boot是如何读取配置文件信息的。

来源：https://www.cnblogs.com/hankuikui/p/11858475.html