PropertySourcesPlaceholderConfigurer工作原理

使用场景

配置类,配置一个PropertySourcesPlaceholderConfigurer Bean

@Configuration
public class CommonConfig {

    @Bean
    public PropertySourcesPlaceholderConfigurer propertySourcesPlaceholderConfigurer(){
        return new PropertySourcesPlaceholderConfigurer();
    }
}

需要进行占位符填充的类

@Component
@PropertySource("classpath:application.properties")
public class User {

    @Value("${suser.name}")
    private String name;

    @Value("${suser.age}")
    private Integer age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

启动类

@ComponentScan
public class PropertySourcesPlaceholderDemo2 {
    public static void main(String[] args) {
        AnnotationConfigApplicationContext context=new AnnotationConfigApplicationContext();
        context.register(PropertySourcesPlaceholderDemo2.class);
        context.refresh();
        User user = context.getBean(User.class);
        System.out.println(user);
    }
}

配置文件

suser.name=lili
suser.age=12

运行结果

image.png

PropertySourcesPlaceholderConfigurer 加载时序图

image.png

PropertySourcesPlaceholderConfigurer#postProcessBeanFactory()

@Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        if (this.propertySources == null) {
            this.propertySources = new MutablePropertySources();
              //配置环境属性源
            if (this.environment != null) {
                this.propertySources.addLast(
                    new PropertySource<Environment>(ENVIRONMENT_PROPERTIES_PROPERTY_SOURCE_NAME, this.environment) {
                        @Override
                        @Nullable
                        public String getProperty(String key) {
                            return this.source.getProperty(key);
                        }
                    }
                );
            }
            try {
              //配置本地属性源
                PropertySource<?> localPropertySource =
                        new PropertiesPropertySource(LOCAL_PROPERTIES_PROPERTY_SOURCE_NAME, mergeProperties());
                if (this.localOverride) {
                    this.propertySources.addFirst(localPropertySource);
                }
                else {
                    this.propertySources.addLast(localPropertySource);
                }
            }
            catch (IOException ex) {
                throw new BeanInitializationException("Could not load properties", ex);
            }
        }

        processProperties(beanFactory, new PropertySourcesPropertyResolver(this.propertySources));
        this.appliedPropertySources = this.propertySources;
    }

通过上面的时序图,以及PropertySourcesPlaceholderConfigurer#postProcessBeanFactory(),我们其实不难发现,属性来源分为两种.

  1. 以StandardEnvironment为属性源的environmentProperties
  2. 通过loadProperties(Properties props)加载本地资源文件作为属性源的localProperties.
    属性源加载完毕后,将占位符替换为属性源中的属性

属性源装配

1. StandardEnvironment propertySources装配

打断点我们会发现 propertySouces中按顺序分别是systemProperties,systemEnvironment,以及我们通过@PropertySource("classpath:application.properties")指定的本地属性源,在这里可以看出配置的优先级:

  1. Java System Properties
  2. OS Environment Variables
  3. application.properties
image.png
  • 其中systemProperties,systemEnvironment是在AbstractEnvironment#AbstractEnvironment()中加载:
    public AbstractEnvironment() {
        customizePropertySources(this.propertySources);
    }

StandardEnvironment 对customizePropertySources进行了重写

    @Override
    protected void customizePropertySources(MutablePropertySources propertySources) {
        propertySources.addLast(
                new PropertiesPropertySource(SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME, getSystemProperties()));
        propertySources.addLast(
                new SystemEnvironmentPropertySource(SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, getSystemEnvironment()));
    }

  • 接下来重点说一说通过@PropertySource("classpath:application.properties")指定的本地属性源是如何加载到propertySouces中的.

    老规矩先看一下@PropertySource工作原理时序图

    image.png

通过时序图,不难发现主要的解析过程都是在ConfigurationClassParser类中实现的,看一下代码,这里我只展示了与PropertySources注解相关的代码

  1. 获取所有@PropertySources,进行遍历
@Nullable
    protected final SourceClass doProcessConfigurationClass(
            ConfigurationClass configClass, SourceClass sourceClass, Predicate<String> filter)
            throws IOException {

            ...

        // 获取 @PropertySource 注解,进行遍历,解析
        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");
            }
        }
        ...
    }
  1. 解析propertySource,通过value属性,获取资源地址location,构建Resource ,factory.createPropertySource(name, new EncodedResource(resource, encoding))构建ResourcePropertySource,通过addPropertySource添加到StandardEnvironment propertySources中
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;
                }
            }
        }
    }
  1. propertySource添加到environment中
private void addPropertySource(PropertySource<?> propertySource) {
        String name = propertySource.getName();
        MutablePropertySources propertySources = ((ConfigurableEnvironment) this.environment).getPropertySources();

        if (this.propertySourceNames.contains(name)) {
            // We've already added a version, we need to extend it
            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);
    }

到这里StandardEnvironment 的propertySources装配完成.

2. localProperties装配

1.PropertiesLoaderSupport#mergeProperties

protected Properties mergeProperties() throws IOException {
        Properties result = new Properties();

        if (this.localOverride) {
            // Load properties from file upfront, to let local properties override.
            loadProperties(result);
        }

        if (this.localProperties != null) {
            for (Properties localProp : this.localProperties) {
                CollectionUtils.mergePropertiesIntoMap(localProp, result);
            }
        }

        if (!this.localOverride) {
            // Load properties from file afterwards, to let those properties override.
            loadProperties(result);
        }

        return result;
    }

2.PropertiesLoaderSupport#loadProperties

将this.locations指定的本地资源解析成键值对存入props中,子类可以重写loadProperties(),实现自己的加载策略,比如通过获取'spring.profiles.active'变量,选择性的加载资源

protected void loadProperties(Properties props) throws IOException {
        if (this.locations != null) {
            for (Resource location : this.locations) {
                if (logger.isTraceEnabled()) {
                    logger.trace("Loading properties file from " + location);
                }
                try {
                  //这里会将location 解析成键值对存入props中
                    PropertiesLoaderUtils.fillProperties(
                            props, new EncodedResource(location, this.fileEncoding), this.propertiesPersister);
                }
                catch (FileNotFoundException | UnknownHostException ex) {
                    if (this.ignoreResourceNotFound) {
                        if (logger.isDebugEnabled()) {
                            logger.debug("Properties resource not found: " + ex.getMessage());
                        }
                    }
                    else {
                        throw ex;
                    }
                }
            }
        }
    }

loadProperties装配完成,到这里属性源都加载完毕,接下来就是占位符的填充

占位符解析

回过头来再看一下PropertySourcesPlaceholderConfigurer#postProcessBeanFactory(),前面一大半都是属性源的装配,接下来
processProperties(beanFactory, new PropertySourcesPropertyResolver(this.propertySources))是对beanFactory里的beanDefinition中包含的占位符解析

@Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        if (this.propertySources == null) {
            this.propertySources = new MutablePropertySources();
              //配置环境属性源
            if (this.environment != null) {
                this.propertySources.addLast(
                    new PropertySource<Environment>(ENVIRONMENT_PROPERTIES_PROPERTY_SOURCE_NAME, this.environment) {
                        @Override
                        @Nullable
                        public String getProperty(String key) {
                            return this.source.getProperty(key);
                        }
                    }
                );
            }
            try {
              //配置本地属性源
                PropertySource<?> localPropertySource =
                        new PropertiesPropertySource(LOCAL_PROPERTIES_PROPERTY_SOURCE_NAME, mergeProperties());
                if (this.localOverride) {
                    this.propertySources.addFirst(localPropertySource);
                }
                else {
                    this.propertySources.addLast(localPropertySource);
                }
            }
            catch (IOException ex) {
                throw new BeanInitializationException("Could not load properties", ex);
            }
        }
      //占位符的解析操作都在这里,配置PropertySourcesPropertyResolver
        processProperties(beanFactory, new PropertySourcesPropertyResolver(this.propertySources));
        this.appliedPropertySources = this.propertySources;
    }

1. PropertySourcesPlaceholderConfigurer#processProperties

    protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess,
            final ConfigurablePropertyResolver propertyResolver) throws BeansException {

        propertyResolver.setPlaceholderPrefix(this.placeholderPrefix);
        propertyResolver.setPlaceholderSuffix(this.placeholderSuffix);
        propertyResolver.setValueSeparator(this.valueSeparator);
     //构建字符串值解析器,ignoreUnresolvablePlaceholders是否忽略无法解析的占位符,其实这里还是委托PropertySourcesPropertyResolver去实现占位符的解析
        StringValueResolver valueResolver = strVal -> {
            String resolved = (this.ignoreUnresolvablePlaceholders ?
                    propertyResolver.resolvePlaceholders(strVal) :
                    propertyResolver.resolveRequiredPlaceholders(strVal));
            if (this.trimValues) {
                resolved = resolved.trim();
            }
            return (resolved.equals(this.nullValue) ? null : resolved);
        };

        doProcessProperties(beanFactoryToProcess, valueResolver);
    }

2. PlaceholderConfigurerSupport#doProcessProperties

获取ConfigurableListableBeanFactory 中注册的所有除自身之外的BeanDefinition ,进行占位符的解析


    protected void doProcessProperties(ConfigurableListableBeanFactory beanFactoryToProcess,
            StringValueResolver valueResolver) {

        BeanDefinitionVisitor visitor = new BeanDefinitionVisitor(valueResolver);

        String[] beanNames = beanFactoryToProcess.getBeanDefinitionNames();
        for (String curName : beanNames) {
            //排除自身&&必须是同一个beanFactory
            if (!(curName.equals(this.beanName) && beanFactoryToProcess.equals(this.beanFactory))) {
                BeanDefinition bd = beanFactoryToProcess.getBeanDefinition(curName);
                try {
                                      //核心逻辑都在这里,这里对BeanDefinition 进行占位符的解析
                    visitor.visitBeanDefinition(bd);
                }
                catch (Exception ex) {
                    throw new BeanDefinitionStoreException(bd.getResourceDescription(), curName, ex.getMessage(), ex);
                }
            }
        }

        // 解析别名目标名称和别名中的占位符
        beanFactoryToProcess.resolveAliases(valueResolver);

        // 在嵌入值（例如注释属性）中解析占位符。
        beanFactoryToProcess.addEmbeddedValueResolver(valueResolver);
    }

3. BeanDefinitionVisitor#visitBeanDefinition

对BeanDefinition的parentName,beanClassName,factoryBeanName,factoryMethodName,scope,propertyValues,constructorArgumentValues中包含的占位符进行解析,这里我们主要看一下propertyValues的解析,其他的都是差不多的.

public void visitBeanDefinition(BeanDefinition beanDefinition) {
        visitParentName(beanDefinition);
        visitBeanClassName(beanDefinition);
        visitFactoryBeanName(beanDefinition);
        visitFactoryMethodName(beanDefinition);
        visitScope(beanDefinition);
        if (beanDefinition.hasPropertyValues()) {
            visitPropertyValues(beanDefinition.getPropertyValues());
        }
        if (beanDefinition.hasConstructorArgumentValues()) {
            ConstructorArgumentValues cas = beanDefinition.getConstructorArgumentValues();
            visitIndexedArgumentValues(cas.getIndexedArgumentValues());
            visitGenericArgumentValues(cas.getGenericArgumentValues());
        }
    }

4. BeanDefinitionVisitor#visitPropertyValues

对BeanDefinition的propertyValues进行遍历,解析可能存在的占位符,并替换属性值

protected void visitPropertyValues(MutablePropertyValues pvs) {
        PropertyValue[] pvArray = pvs.getPropertyValues();
        for (PropertyValue pv : pvArray) {
            Object newVal = resolveValue(pv.getValue());
            if (!ObjectUtils.nullSafeEquals(newVal, pv.getValue())) {
                pvs.add(pv.getName(), newVal);
            }
        }
    }

5 .BeanDefinitionVisitor#resolveValue

解析给定的value中可能存在的占位符,这我们主要看一下 resolveStringValue((String) value)对字符串值的解析,其他的基本上也是一样的套路.

@Nullable
    protected Object resolveValue(@Nullable Object value) {
        if (value instanceof BeanDefinition) {
            visitBeanDefinition((BeanDefinition) value);
        }
        else if (value instanceof BeanDefinitionHolder) {
            visitBeanDefinition(((BeanDefinitionHolder) value).getBeanDefinition());
        }
        else if (value instanceof RuntimeBeanReference) {
            RuntimeBeanReference ref = (RuntimeBeanReference) value;
            String newBeanName = resolveStringValue(ref.getBeanName());
            if (newBeanName == null) {
                return null;
            }
            if (!newBeanName.equals(ref.getBeanName())) {
                return new RuntimeBeanReference(newBeanName);
            }
        }
        else if (value instanceof RuntimeBeanNameReference) {
            RuntimeBeanNameReference ref = (RuntimeBeanNameReference) value;
            String newBeanName = resolveStringValue(ref.getBeanName());
            if (newBeanName == null) {
                return null;
            }
            if (!newBeanName.equals(ref.getBeanName())) {
                return new RuntimeBeanNameReference(newBeanName);
            }
        }
        else if (value instanceof Object[]) {
            visitArray((Object[]) value);
        }
        else if (value instanceof List) {
            visitList((List) value);
        }
        else if (value instanceof Set) {
            visitSet((Set) value);
        }
        else if (value instanceof Map) {
            visitMap((Map) value);
        }
        else if (value instanceof TypedStringValue) {
            TypedStringValue typedStringValue = (TypedStringValue) value;
            String stringValue = typedStringValue.getValue();
            if (stringValue != null) {
                String visitedString = resolveStringValue(stringValue);
                typedStringValue.setValue(visitedString);
            }
        }
        else if (value instanceof String) {
            return resolveStringValue((String) value);
        }
        return value;
    }

6. BeanDefinitionVisitor#resolveStringValue

委托valueResolver进行字符串值的解析,其实就是之前构建好的StringValueResolver ,默认ignoreUnresolvablePlaceholders =false,
所以this.valueResolver.resolveStringValue(strVal)会调用propertyResolver.resolveRequiredPlaceholders(strVal))

    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        ...
        processProperties(beanFactory, new PropertySourcesPropertyResolver(this.propertySources));
        ...
    }

protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess,
            final ConfigurablePropertyResolver propertyResolver) throws BeansException {

    ...
        StringValueResolver valueResolver = strVal -> {
            String resolved = (this.ignoreUnresolvablePlaceholders ?
                    propertyResolver.resolvePlaceholders(strVal) :
                    propertyResolver.resolveRequiredPlaceholders(strVal));
            if (this.trimValues) {
                resolved = resolved.trim();
            }
            return (resolved.equals(this.nullValue) ? null : resolved);
        };
      ...
    }


@Nullable
    protected String resolveStringValue(String strVal) {
        if (this.valueResolver == null) {
            throw new IllegalStateException("No StringValueResolver specified - pass a resolver " +
                    "object into the constructor or override the 'resolveStringValue' method");
        }
        String resolvedValue = this.valueResolver.resolveStringValue(strVal);
        // Return original String if not modified.
        return (strVal.equals(resolvedValue) ? strVal : resolvedValue);
    }

7. AbstractPropertyResolver#resolveRequiredPlaceholders

    @Override
    public String resolveRequiredPlaceholders(String text) throws IllegalArgumentException {
        if (this.strictHelper == null) {
           //  创建一个新的PropertyPlaceholderHelper使用所提供的前缀和后缀。
            this.strictHelper = createPlaceholderHelper(false);
        }
        return doResolvePlaceholders(text, this.strictHelper);
    }

    private boolean ignoreUnresolvableNestedPlaceholders = false;

    private String placeholderPrefix = "${";

    private String placeholderSuffix = "}";

    @Nullable
    private String valueSeparator = ":";
    //创建PropertyPlaceholderHelper,对占位符解析
    private PropertyPlaceholderHelper createPlaceholderHelper(boolean ignoreUnresolvablePlaceholders) {
        return new PropertyPlaceholderHelper(this.placeholderPrefix, this.placeholderSuffix,
                this.valueSeparator, ignoreUnresolvablePlaceholders);
    }

8.AbstractPropertyResolver#doResolvePlaceholders

    private String doResolvePlaceholders(String text, PropertyPlaceholderHelper helper) {
        return helper.replacePlaceholders(text, this::getPropertyAsRawString);
    }

9.PropertyPlaceholderHelper#replacePlaceholders

public String replacePlaceholders(String value, PlaceholderResolver placeholderResolver) {
        Assert.notNull(value, "'value' must not be null");
        return parseStringValue(value, placeholderResolver, null);
    }

10.PropertyPlaceholderHelper#parseStringValue

到这里就是占位符的解析逻辑了,简单来说就是通过循环和递归找到占位符中的变量,最终替换成属性源中对应的值,举个例子 :

${a.${b}.${c}} ,"b:d","c:e" ,"a.d.e:success"
1. a.${b}.${c}
2.递归: ${b}->d
3.递归解析${b}对应的值中存在的占位符,显然"d"中不含占位符,直接返回,替换${b}
4.得到 a.d.${c}
5.通过循环找到下一个${c}
6.递归:${c}->e
7.递归解析${c}对应的值中存在的占位符,显然"e"中不含占位符,直接返回,替换${c}
8.得到a.b.e
9.最后一步自然就是找到a.b.e对应的属性值"success"


protected String parseStringValue(
            String value, PlaceholderResolver placeholderResolver, @Nullable Set<String> visitedPlaceholders) {

        int startIndex = value.indexOf(this.placeholderPrefix);
        if (startIndex == -1) {
            return value;
        }

        StringBuilder result = new StringBuilder(value);
        while (startIndex != -1) {
            int endIndex = findPlaceholderEndIndex(result, startIndex);
            if (endIndex != -1) {
                String placeholder = result.substring(startIndex + this.placeholderPrefix.length(), endIndex);
                String originalPlaceholder = placeholder;
                if (visitedPlaceholders == null) {
                    visitedPlaceholders = new HashSet<>(4);
                }
//解决占位符的循环引用,例如${name}-->${name}
                if (!visitedPlaceholders.add(originalPlaceholder)) {
                    throw new IllegalArgumentException(
                            "Circular placeholder reference '" + originalPlaceholder + "' in property definitions");
                }
                // 递归调用，解析占位符键中包含的占位符
                placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders);
                // 现在获取完全解析的键的值.
                String propVal = placeholderResolver.resolvePlaceholder(placeholder);
                if (propVal == null && this.valueSeparator != null) {
                    int separatorIndex = placeholder.indexOf(this.valueSeparator);
                    if (separatorIndex != -1) {
                        String actualPlaceholder = placeholder.substring(0, separatorIndex);
                        String defaultValue = placeholder.substring(separatorIndex + this.valueSeparator.length());
                        propVal = placeholderResolver.resolvePlaceholder(actualPlaceholder);
                        if (propVal == null) {
                            propVal = defaultValue;
                        }
                    }
                }
                if (propVal != null) {
                    // 递归调用，解析先前解析的占位符值中包含的占位符。
                    propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
                    result.replace(startIndex, endIndex + this.placeholderSuffix.length(), propVal);
                    if (logger.isTraceEnabled()) {
                        logger.trace("Resolved placeholder '" + placeholder + "'");
                    }
                    startIndex = result.indexOf(this.placeholderPrefix, startIndex + propVal.length());
                }
                else if (this.ignoreUnresolvablePlaceholders) {
                    // 如果忽略不能解析的值,跳过不能解析的占位符找到下一个占位符
                    startIndex = result.indexOf(this.placeholderPrefix, endIndex + this.placeholderSuffix.length());
                }
                else {
                    throw new IllegalArgumentException("Could not resolve placeholder '" +
                            placeholder + "'" + " in value \"" + value + "\"");
                }
                visitedPlaceholders.remove(originalPlaceholder);
            }
            else {  
                startIndex = -1;
            }
        }
        return result.toString();
    }

11 .PropertySourcesPropertyResolver#getPropertyAsRawString

解析占位符对应的值:String propVal = placeholderResolver.resolvePlaceholder(placeholder);

回顾一下:

    private String doResolvePlaceholders(String text, PropertyPlaceholderHelper helper) {
        return helper.replacePlaceholders(text, this::getPropertyAsRawString);
    }

public String replacePlaceholders(String value, PlaceholderResolver placeholderResolver) {
        Assert.notNull(value, "'value' must not be null");
        return parseStringValue(value, placeholderResolver, null);
    }

可以看出parseStringValue(
String value, PlaceholderResolver placeholderResolver, @Nullable Set<String> visitedPlaceholders),使用的是this::getPropertyAsRawString,

@Override
    @Nullable
    protected String getPropertyAsRawString(String key) {
        return getProperty(key, String.class, false);
    }

12. PropertySourcesPropertyResolver#getProperty

getProperty()的逻辑就很简单了,根据我们解析好的占位符的key在属性源中找到对应的属性值.

@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;
    }

总结

上面就是对PropertySourcesPlaceholderConfigurer工作原理的源码解析,概括来说分为两步:

  • 属性源装配
    • environmentProperties
    • localProperties
  • 占位符解析
    • 解析占位符中的key
    • 将key替换成对应的属性值

以上就是我对PropertySourcesPlaceholderConfigurer的理解,如果理解有误,也希望大家能帮我指出问题,大家共同成长.

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