Spring版本

5.2.5.RELEASE

参考

《芋道源码》

在平时的项目中，我们经常会使用配置文件来根据不同的环境动态加载配置项。那么，spring是如何使用配置项替换掉bean中的占位符的呢？这一切的神奇之处都在PropertySourcesPlaceholderConfigurer（5.2之前是使用PropertyPlaceholderConfigurer，5.2废弃该类，官方建议使用PropertySourcesPlaceholderConfigurer），它负责加载并替换bean中的占位符。首先从一个小demo来认识这个类。

1. DEMO

1.1 Student

public class Student {

    private String id;

    private String name;

    private String desc;

   // 省略 getter、setter
}

1.2 CustomPropertyConfig

public class CustomPropertyConfig extends PropertySourcesPlaceholderConfigurer {

    private Resource [] locations;

    private PropertiesPersister propertiesPersister = new DefaultPropertiesPersister();

    @Override
    public void setLocations(Resource... locations) {
        this.locations = locations;
    }

    @Override
    public void setLocalOverride(boolean localOverride) {
        this.localOverride = localOverride;
    }

    @Override
    protected void loadProperties(Properties props) throws IOException {
        if (this.locations != null) {
            for (Resource location : this.locations) {
                InputStream inputStream = null;
                try {
                    String fileName = location.getFilename();
                    String env = "application-" + System.getProperty("spring.profiles.active", "dev") + ".properties";
                    if (fileName.contains(env)) {
                        logger.info("loading properties file from " + location);
                        inputStream = location.getInputStream();
                        this.propertiesPersister.load(props,inputStream);
                    }
                } catch (Exception e) {
                    logger.info("error",e);
                } finally {
                    if (inputStream != null) {
                        inputStream.close();
                    }
                }
            }
        }
    }
}

1.3 环境配置文件

application-dev.properties

student.name=student-dev

application-test.properties

student.name=student-test
student.value=student-test-value

1.4 spring.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="propertyConfig" class="com.kungyu.custom.element.CustomPropertyConfig">
        <property name="locations">
            <list>
                <value>classpath:application-dev.properties</value>
                <value>classpath:application-test.properties</value>
            </list>
        </property>
    </bean>
    <bean id="student" class="com.kungyu.custom.element.Student">
        <property name="name" value="${student.name}"/>
    </bean>
</bean>

1.5 测试

  public class Test {
    public static void main(String[] args) throws ClassNotFoundException {
        ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
        Student student = (Student) context.getBean("student");
        System.out.println(student.getName());
}

启动参数配置如下：

启动参数配置

用于指定加载的配置文件，输出如下：

输出结果

可以看到，student这个bean中name属性原本使用的是${student.name}这个占位符，如今已经被替换成配置文件application-dev.properties中的student-name的属性值student-dev

当加载测试环境配置的时候，修改VM options即可

2. 源码解读

通过debug，我们可以发现，入口在于PropertySourcesPlaceholderConfigurer#postProcessBeanFactory

找到该入口方法的debug方法可以查看《Spring源码解析（十六）-BeanFactoryPostProcessor》

2.1 PropertySourcesPlaceholderConfigurer#postProcessBeanFactory

    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
        // propertySources：属性资源，内部包含一个propertySourceList，用于存放各个环境下的资源
        if (this.propertySources == null) {
            this.propertySources = new MutablePropertySources();
            // environment系统环境的一些配置
            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 {
                // 通过mergeProperties加载用户配置的资源文件
                PropertySource<?> localPropertySource =
                        new PropertiesPropertySource(LOCAL_PROPERTIES_PROPERTY_SOURCE_NAME, mergeProperties());
                // localOverride为true代表优先加载本地资源，也就是优先级高（所以使用addFirst），false代表最后加载本地资源，也就是本地资源优先级地（所以使用addLast）
                // 这种优先级的高低在PropertySourcesPropertyResolver#getProperty中体现
                if (this.localOverride) {
                    this.propertySources.addFirst(localPropertySource);
                }
                else {
                    this.propertySources.addLast(localPropertySource);
                }
            }
            catch (IOException ex) {
                throw new BeanInitializationException("Could not load properties", ex);
            }
        }

        // 进行资源解析，同时替换${}占位符
        processProperties(beanFactory, new PropertySourcesPropertyResolver(this.propertySources));
        this.appliedPropertySources = this.propertySources;
    }

首先判断类属性propertySources是否为空，为空则构造一个对象，接着主要分俩步：

• 加载系统环境配置，加入到propertySources中
• 加载用户的配置，也就是DEMO中的配置文件，同样加入到propertySources中

这里需要留意多份环境配置文件之间存在优先级之分，通过addFirst加入的配置文件优先级最高：

    /**
     * Add the given property source object with highest precedence.
     */
    public void addFirst(PropertySource<?> propertySource) {
        removeIfPresent(propertySource);
        this.propertySourceList.add(0, propertySource);
    }

通过addLast加入的优先级最低：

    /**
     * Add the given property source object with lowest precedence.
     */
    public void addLast(PropertySource<?> propertySource) {
        removeIfPresent(propertySource);
        this.propertySourceList.add(propertySource);
    }

这个优先级的区分，将体现在2.12节的getProperty方法中

处理完配置文件之后，调用processProperties方法正式开始占位符相关处理

2.2 PropertySourcesPlaceholderConfigurer#processProperties

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

        // 设置解析器的占位符前缀，占位符后缀，和默认值分割符，对应值如下
        // placeholderPrefix   ${
        // placeholderSuffix   }
        // valueSeparator  :
        propertyResolver.setPlaceholderPrefix(this.placeholderPrefix);
        propertyResolver.setPlaceholderSuffix(this.placeholderSuffix);
        propertyResolver.setValueSeparator(this.valueSeparator);

        // 构造一个函数式接口的解析器
        StringValueResolver valueResolver = strVal -> {
            // ignoreUnresolvablePlaceholders 是否无视不可解析的占位符，如果设置为false，那么碰到不可解析的占位符的时候，会抛出异常
            String resolved = (this.ignoreUnresolvablePlaceholders ?
                    propertyResolver.resolvePlaceholders(strVal) :
                    propertyResolver.resolveRequiredPlaceholders(strVal));
            if (this.trimValues) {
                resolved = resolved.trim();
            }
            return (resolved.equals(this.nullValue) ? null : resolved);
        };

        // 真正解析占位符
        doProcessProperties(beanFactoryToProcess, valueResolver);
    }

逻辑分为三步：

• 设置占位符前缀、后缀和默认值分隔符

propertyResolver.setPlaceholderPrefix(this.placeholderPrefix);
propertyResolver.setPlaceholderSuffix(this.placeholderSuffix);
propertyResolver.setValueSeparator(this.valueSeparator);

• 构造占位符解析器的函数式接口

StringValueResolver valueResolver = strVal - > {
    /* ignoreUnresolvablePlaceholders 是否无视不可解析的占位符，如果设置为false，那么碰到不可解析的占位符的时候，会抛出异常 */
    String resolved = (this.ignoreUnresolvablePlaceholders ?
               propertyResolver.resolvePlaceholders( strVal ) :
               propertyResolver.resolveRequiredPlaceholders( strVal ) );
    if ( this.trimValues )
    {
        resolved = resolved.trim();
    }
    return(resolved.equals( this.nullValue ) ? null : resolved);
};

该函数式接口在2.7节中将会被使用，通过resolvePlaceholders和resolveRequiredPlaceholders对占位符strVal进行解析

• 真正开始解析占位符

doProcessProperties(beanFactoryToProcess, valueResolver);

2.3 PropertySourcesPlaceholderConfigurer#doProcessProperties

protected void doProcessProperties(ConfigurableListableBeanFactory beanFactoryToProcess, StringValueResolver valueResolver) {
    // 传入valueResolver构造一个vistor，之后将使用valueResolver来解析beanDefinition中的占位符属性
    BeanDefinitionVisitor visitor = new BeanDefinitionVisitor(valueResolver);
    // 获取全部beanName
    String[] beanNames = beanFactoryToProcess.getBeanDefinitionNames();
    for (String curName: beanNames) {
        // Check that we're not parsing our own bean definition,
        // to avoid failing on unresolvable placeholders in properties file locations.
        // curName.equals(this.beanName)：占位符解析本质上也是注册了一个bean，所以对于占位符这个bean，需要跳过
        if (! (curName.equals(this.beanName) && beanFactoryToProcess.equals(this.beanFactory))) {
            // 获取BeanDefinition
            BeanDefinition bd = beanFactoryToProcess.getBeanDefinition(curName);
            try {
                // 对parentName、class、property等等进行解析，并且替换beanDefinition中原来的占位符
                visitor.visitBeanDefinition(bd);
            } catch(Exception ex) {
                throw new BeanDefinitionStoreException(bd.getResourceDescription(), curName, ex.getMessage(), ex);
            }
        }
    }
    // New in Spring 2.5: resolve placeholders in alias target names and aliases as well.
    // 对别名也应用解析起进行处理
    beanFactoryToProcess.resolveAliases(valueResolver);
    // New in Spring 3.0: resolve placeholders in embedded values such as annotation attributes.
    // 往嵌入式注解解析器列表增加该解析器，在创建bean的过程中，会使用该解析器去解析@Value的属性
    beanFactoryToProcess.addEmbeddedValueResolver(valueResolver);
}

• 将解析器valueResolver作为参数构造了一个BeanDefinitionVisitor对象，那么可以想象，解析的动作应该是发生在BeanDefinitionVisitor对象的成员方法中了
• 获取全部已解析的BeanDefinition，遍历，使用BeanDefinitionVisitor对象的成员方法visitBeanDefinition进行解析并且替换BeanDefinition中的占位符
• 调用resolveAliases处理别名，不是我们关心的流程，具体不展开了
• 调用addEmbeddedValueResolver，往嵌入式注解解析器列表embeddedValueResolvers增加该解析器，在《Spring源码解析（十）-填充bean属性》4.2节中解析了@Value注解，那时候会使用到该解析器列表embeddedValueResolvers

    @Override
    public void addEmbeddedValueResolver(StringValueResolver valueResolver) {
        Assert.notNull(valueResolver, "StringValueResolver must not be null");
        this.embeddedValueResolvers.add(valueResolver);
    }

2.4 BeanDefinitionVisitor#visitBeanDefinition

    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());
        }
    }

可以看到，该方法对parentName、beanClass等数据域都进行了解析，这里我们只关心属性的解析方法visitPropertyValues

2.5 BeanDefinitionVisitor#visitPropertyValues

    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);
            }
        }
    }

• 拿到属性列表pvArray
• 遍历pvArray，调用resolveValue进行占位符解析，如果解析后的新值和原值不同，通过add方法进行替换或者合并（如果可以合并的前提下）

2.6 BeanDefinitionVisitor#resolveValue

    @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;
    }

这里对占位符名称value做了各种数据类型的处理，一般传入的占位符名称都是string类型的，因此直接跳到resolveStringValue((String) value)

2.7 BeanDefinitionVisitor#resolveStringValue

    @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);
    }

这里使用了valueResolver.resolveStringValue(strVal)进行解析，而追踪下来，我们知道valueResolver其实就是2.2节中构造的解析器：

StringValueResolver valueResolver = strVal - > {
    /* ignoreUnresolvablePlaceholders 是否无视不可解析的占位符，如果设置为false，那么碰到不可解析的占位符的时候，会抛出异常 */
    String resolved = (this.ignoreUnresolvablePlaceholders ?
               propertyResolver.resolvePlaceholders( strVal ) :
               propertyResolver.resolveRequiredPlaceholders( strVal ) );
    if ( this.trimValues )
    {
        resolved = resolved.trim();
    }
    return(resolved.equals( this.nullValue ) ? null : resolved);
};

那么，明显，resolveStringValue方法也就是调用resolvePlaceholders和resolveRequiredPlaceholders中的其中一个，我们以更为严格的resolveRequiredPlaceholders为例进行解析

2.8 AbstractPropertyResolver#resolveRequiredPlaceholders

    @Override
    public String resolveRequiredPlaceholders(String text) throws IllegalArgumentException {
        if (this.strictHelper == null) {
            this.strictHelper = createPlaceholderHelper(false);
        }
        return doResolvePlaceholders(text, this.strictHelper);
    }

可以看到是交由doResolvePlaceholders去进行解析的

2.9 AbstractPropertyResolver#doResolvePlaceholders

    private String doResolvePlaceholders(String text, PropertyPlaceholderHelper helper) {
        // 注意，这里的形参PlaceholderResolver是一个函数式接口，这里传入getPropertyAsRawString方法作为PlaceholderResolver#resolvePlaceholder的实现
        return helper.replacePlaceholders(text, this::getPropertyAsRawString);
    }

replacePlaceholders方法的第二个参数是函数式接口参数PlaceholderResolver：

    @FunctionalInterface
    public interface PlaceholderResolver {

        /**
         * Resolve the supplied placeholder name to the replacement value.
         * @param placeholderName the name of the placeholder to resolve
         * @return the replacement value, or {@code null} if no replacement is to be made
         */
        @Nullable
        String resolvePlaceholder(String placeholderName);
    }

所以，getPropertyAsRawString便是该函数式接口中resolvePlaceholder的实现方法，而resolvePlaceholder将会在2.11节中被调用

2.10 PropertyPlaceholderHelper#replacePlaceholders

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

2.11 PropertyPlaceholderHelper#parseStringValue

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

        // value：传入的原始占位符
        // placeholderResolver：解析器
        // visitedPlaceholders：缓存集合
        int startIndex = value.indexOf(this.placeholderPrefix);
        if (startIndex == -1) {
            return value;
        }

        // spring允许存在多个占位符，如student.name可以配置为${student.name}${student.value}
        // 所以下面的代码需要循环处理多个占位符，并对result进行替换
        StringBuilder result = new StringBuilder(value);
        while (startIndex != -1) {
            // 查找结束符的位置
            int endIndex = findPlaceholderEndIndex(result, startIndex);
            if (endIndex != -1) {
                // 进行截取，去除占位符，得到真正的属性名称，比如${student.name} -> student.name
                String placeholder = result.substring(startIndex + this.placeholderPrefix.length(), endIndex);
                String originalPlaceholder = placeholder;
                if (visitedPlaceholders == null) {
                    visitedPlaceholders = new HashSet<>(4);
                }
                if (!visitedPlaceholders.add(originalPlaceholder)) {
                    throw new IllegalArgumentException(
                            "Circular placeholder reference '" + originalPlaceholder + "' in property definitions");
                }
                // Recursive invocation, parsing placeholders contained in the placeholder key.
                // 递归处理占位符中嵌套占位符
                placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders);
                // Now obtain the value for the fully resolved key...
                // 通过解析器获取到占位符对应的值
                String propVal = placeholderResolver.resolvePlaceholder(placeholder);
                if (propVal == null && this.valueSeparator != null) {
                    // 获取不到值，说明此时placeholder可能具备默认值
                    // 查找默认值划分符号
                    int separatorIndex = placeholder.indexOf(this.valueSeparator);
                    if (separatorIndex != -1) {
                        // 查找到划分符号，截取获取属性名称
                        String actualPlaceholder = placeholder.substring(0, separatorIndex);
                        // 截取默认值
                        String defaultValue = placeholder.substring(separatorIndex + this.valueSeparator.length());
                        // 尝试解析
                        propVal = placeholderResolver.resolvePlaceholder(actualPlaceholder);
                        if (propVal == null) {
                            // 解析不到，使用默认值
                            propVal = defaultValue;
                        }
                    }
                }
                if (propVal != null) {
                    // Recursive invocation, parsing placeholders contained in the
                    // previously resolved placeholder value.
                    // 解析属性值
                    propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
                    // 替换结果中的占位符为属性值
                    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) {
                    // Proceed with unprocessed value.
                    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();
    }

方法看着很长，但逻辑其实挺好理解的，主要就是解决占位符嵌套和多个占位符这俩种情况，具体逻辑细看一下即可明白，不展开了，需要留意的是：

String propVal = placeholderResolver.resolvePlaceholder(placeholder);

这里的resolvePlaceholder实际上调用的是2.9节中的getPropertyAsRawString方法

多个占位符的情况
占位符内部嵌套的情况

2.12 PropertySourcesPropertyResolver#getPropertyAsRawString

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

    @Nullable
    protected <T> T getProperty(String key, Class<T> targetValueType, boolean resolveNestedPlaceholders) {
        if (this.propertySources != null) {
            // 遍历，从各个propertySource中查找值，找到值之后马上返回，抛弃后面的propertySource
            for (PropertySource<?> propertySource : this.propertySources) {
                // 省略
                Object value = propertySource.getProperty(key);
                if (value != null) {
                    if (resolveNestedPlaceholders && value instanceof String) {
                        value = resolveNestedPlaceholders((String) value);
                    }
                    logKeyFound(key, propertySource, value);
                    // 类型转化
                    return convertValueIfNecessary(value, targetValueType);
                }
            }
        }
        // 省略
        return null;
    }

核心逻辑就一句：

Object value = propertySource.getProperty(key);

从propertySource取出元素值，不为空的话，直接返回，这也是2.1节中优先级的代码体现

3. 总结

配置文件的运作流程整个逻辑代码是挺好理解的，并不复杂，稍微花点心，就看懂了，逻辑总结起来不外乎几步：

• 获取配置文件，转化为propertySources
• 构造基于propertySources的解析器valueResolver
• 获取BeanDefinitions，遍历，并对占位符逐一解析并替换

只不过在其中还穿插了比如占位符嵌套、值转化等场景的处理。