springboot自定义属性文件通过value注解引入，和@autowired不同的是，它是由ConfigurationClassPostProcessor这个BeanDefinitionRegistryPostProcessor来处理，属性文件的读取和注入是在BeanDefinition级别，对象实例化之前。

我们建一个简单的类的梳理一下。

@Component
@Data
@PropertySource(name="myprod",value = "classpath:config/product.properties")
public class Product {
    @Value("${prod.name}")
    private String name;
    @Value("${product.title}")
    private String title;
}

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调用堆栈 从refresh开始，主要走了这几个方法：
invokeBeanFactoryPostProcessors
invokeBeanDefinitionRegistryPostProcessors
ConfigurationClassPostProcessor->processConfigBeanDefinitions
ConfigurationClassParser->doProcessConfigurationClass
ConfigurationClassParser->processPropertySource
ConfigurationClassParser->addPropertySource

public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
        List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
        String[] candidateNames = registry.getBeanDefinitionNames();

        for (String beanName : candidateNames) {
            BeanDefinition beanDef = registry.getBeanDefinition(beanName);
            if (ConfigurationClassUtils.isFullConfigurationClass(beanDef) ||
                    ConfigurationClassUtils.isLiteConfigurationClass(beanDef)) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Bean definition has already been processed as a configuration class: " + beanDef);
                }
            }"代码会走这里因为com.timmy.app没有Configuration注解，只是一个轻量级的配置类"
            else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {
                configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
            }
        }

        // Return immediately if no @Configuration classes were found
        if (configCandidates.isEmpty()) {
            return;
        }


        "这个就是用来处理各种配置类的parser"
        ConfigurationClassParser parser = new ConfigurationClassParser(
                this.metadataReaderFactory, this.problemReporter, this.environment,
                this.resourceLoader, this.componentScanBeanNameGenerator, registry);

        Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
        Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
        do {  "开始解析主类com.timmy.app的bean定义了"
            parser.parse(candidates);
            parser.validate();

            Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
            configClasses.removeAll(alreadyParsed);

            // Read the model and create bean definitions based on its content
            if (this.reader == null) {
                this.reader = new ConfigurationClassBeanDefinitionReader(
                        registry, this.sourceExtractor, this.resourceLoader, this.environment,
                        this.importBeanNameGenerator, parser.getImportRegistry());
            }
           "准备解析@Configuration标签的配置类，就是之前忽略的"
            this.reader.loadBeanDefinitions(configClasses);
            alreadyParsed.addAll(configClasses);

           省略代码...

ConfigurationClassParser主要方法：
doProcessConfigurationClass->processPropertySource->addPropertySource

逻辑主要集中在doProcessConfigurationClass方法
doProcessConfigurationClass负责解析@PropertySource，@Import annotations，@ComponentScan等注解 。
1 调用processPropertySource处理自身的propertySource
2 扫描类上的ComponentScan，对扫出的类继续调用parse
3 处理@Import annotations等其他标签

    protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
            throws IOException {

        if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
            // Recursively process any member (nested) classes first
            processMemberClasses(configClass, sourceClass);
        }

        "处理sourceClass上的@PropertySource注解"，
        for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
                sourceClass.getMetadata(), PropertySources.class,
                org.springframework.context.annotation.PropertySource.class)) {
            if (this.environment instanceof ConfigurableEnvironment) {
                processPropertySource(propertySource);
            }
            else {
                logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
                        "]. Reason: Environment must implement ConfigurableEnvironment");
            }
        }

        "扫描sourceClass上@ComponentScan下所有类,继续parse "      
        Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
                sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
        if (!componentScans.isEmpty() &&
                !this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
            for (AnnotationAttributes componentScan : componentScans) {
                // The config class is annotated with @ComponentScan -> perform the scan immediately
                Set<BeanDefinitionHolder> scannedBeanDefinitions =
                        this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
                // Check the set of scanned definitions for any further config classes and parse recursively if needed
                for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
                    BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
                    if (bdCand == null) {
                        bdCand = holder.getBeanDefinition();
                    }
                    if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
                    "开始解析每个扫出的类"                    
                        parse(bdCand.getBeanClassName(), holder.getBeanName());
                    }
                }
            }
        }

        // Process any @Import annotations
        processImports(configClass, sourceClass, getImports(sourceClass), true);

        // Process any @ImportResource annotations
        AnnotationAttributes importResource =
                AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
        if (importResource != null) {
            String[] resources = importResource.getStringArray("locations");
            Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
            for (String resource : resources) {
                String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
                configClass.addImportedResource(resolvedResource, readerClass);
            }
        }

        // Process individual @Bean methods
        Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
        for (MethodMetadata methodMetadata : beanMethods) {
            configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
        }

        // Process default methods on interfaces
        processInterfaces(configClass, sourceClass);

        // Process superclass, if any
        if (sourceClass.getMetadata().hasSuperClass()) {
            String superclass = sourceClass.getMetadata().getSuperClassName();
            if (superclass != null && !superclass.startsWith("java") &&
                    !this.knownSuperclasses.containsKey(superclass)) {
                this.knownSuperclasses.put(superclass, configClass);
                // Superclass found, return its annotation metadata and recurse
                return sourceClass.getSuperClass();
            }
        }

        // No superclass -> processing is complete
        return null;
    }

processPropertySource结构很简单:
1根据注解里的location属性载入配置文件
2调用addPropertySource处理每个属性文件

private void processPropertySource(AnnotationAttributes propertySource) throws IOException {
        String name = propertySource.getString("name");
        if (!StringUtils.hasLength(name)) {
            name = null;
        }
        String encoding = propertySource.getString("encoding");
        if (!StringUtils.hasLength(encoding)) {
            encoding = null;
        }
        String[] locations = propertySource.getStringArray("value");
        Assert.isTrue(locations.length > 0, "At least one @PropertySource(value) location is required");
        boolean ignoreResourceNotFound = propertySource.getBoolean("ignoreResourceNotFound");

        Class<? extends PropertySourceFactory> factoryClass = propertySource.getClass("factory");
        PropertySourceFactory factory = (factoryClass == PropertySourceFactory.class ?
                DEFAULT_PROPERTY_SOURCE_FACTORY : BeanUtils.instantiateClass(factoryClass));

        for (String location : locations) {
            try {
                String resolvedLocation = this.environment.resolveRequiredPlaceholders(location);
                Resource resource = this.resourceLoader.getResource(resolvedLocation);
                addPropertySource(factory.createPropertySource(name, new EncodedResource(resource, encoding)));
            }
            catch (IllegalArgumentException | FileNotFoundException | UnknownHostException ex) {
                // Placeholders not resolvable or resource not found when trying to open it
                if (ignoreResourceNotFound) {
                    if (logger.isInfoEnabled()) {
                        logger.info("Properties location [" + location + "] not resolvable: " + ex.getMessage());
                    }
                }
                else {
                    throw ex;
                }
            }
        }
    }

addPropertySource这个类才是真正处理@value属性：
1把用户定义的properties文件加到Eniverment中去
2如果有相同name的属性文件，需要合并

private void addPropertySource(PropertySource<?> propertySource) {
        String name = propertySource.getName();
        MutablePropertySources propertySources = ((ConfigurableEnvironment) this.environment).getPropertySources();
        if (this.propertySourceNames.contains(name)) {
            // 可能会有多个地方注解同一个属性文件，
            PropertySource<?> existing = propertySources.get(name);
            if (existing != null) {
                PropertySource<?> newSource = (propertySource instanceof ResourcePropertySource ?
                        ((ResourcePropertySource) propertySource).withResourceName() : propertySource);
                if (existing instanceof CompositePropertySource) {
                    ((CompositePropertySource) existing).addFirstPropertySource(newSource);
                }
                else {
                    if (existing instanceof ResourcePropertySource) {
                        existing = ((ResourcePropertySource) existing).withResourceName();
                    }
                    CompositePropertySource composite = new CompositePropertySource(name);
                    composite.addPropertySource(newSource);
                    composite.addPropertySource(existing);
                    propertySources.replace(name, composite);
                }
                return;
            }
        }

        if (this.propertySourceNames.isEmpty()) {
            propertySources.addLast(propertySource);
        }
        else {
            String firstProcessed = this.propertySourceNames.get(this.propertySourceNames.size() - 1);
            propertySources.addBefore(firstProcessed, propertySource);
        }
        this.propertySourceNames.add(name);
    }
    @Override
    public PropertySource<?> createPropertySource(@Nullable String name, EncodedResource resource) throws IOException {
        return (name != null ? new ResourcePropertySource(name, resource) : new ResourcePropertySource(resource));
    }

CompositePropertySource 的场景其实是你有两个不同的文件，但是 @PropertySource中设置同样的name属性，这样CompositePropertySource 会做一个合并，按加入的时间顺序取。

增加一个product2，PropertySource name都设置为myprod

@Component
@Data
@PropertySource(name="myprod",value = "classpath:config/product.properties")
public class Product {
    @Value("${prod.name}")
    private String name;
    @Value("${product.title}")
    private String title;
}


@Component
@Data
@PropertySource(name="myprod",value = "classpath:config/product2.properties")
public class Product2 {
    @Value("${prod.name}")
    private String name;
    @Value("${product.title}")
    private String title;
}

debug到addPropertySource，newSource和existing已经不一样了。
environment的propertySources里也有两份文件了。

image.png

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