Spring中的Properties

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摘自Environment注释：

Properties play an important role in almost all applications, and may originate from a variety of sources: properties files, JVM system properties, system environment variables, JNDI, servlet context parameters, ad-hoc Properties objects, Maps, and so on. The role of the environment object with relation to properties is to provide the user with a convenient service interface for configuring property sources and resolving properties from them.

配置

XML方式

这种应该是最常见的方式了，在spring配置文件中：

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<context:property-placeholder location="classpath:bean.properties" />

注解方式

如果不是用xml方式，也可以使用注解方式，使用@PropertySource：

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@Configuration
@PropertySource("classpath:bean.properties")
public class Application {

}

其他方式

PropertyPlaceholderConfigurer

XML方式：

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<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
    <property name="locations" value="classpath:bean.properties"/>
</bean>

注解方式：

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@Bean
public static PropertyPlaceholderConfigurer propertyPlaceholderConfigurer() {
    PropertyPlaceholderConfigurer ppc = new PropertyPlaceholderConfigurer();
    ppc.setLocation(new ClassPathResource("bean.properties"));
    return ppc;
}

PropertySourcesPlaceholderConfigurer

XML方式：

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<bean class="org.springframework.context.support.PropertySourcesPlaceholderConfigurer">
    <property name="locations" value="classpath:bean.properties"/>
</bean>

注解方式：

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@Bean
public static PropertySourcesPlaceholderConfigurer propertySourcesPlaceholderConfigurer() {
    PropertySourcesPlaceholderConfigurer pspc = new PropertySourcesPlaceholderConfigurer();
    pspc.setLocation(new ClassPathResource("bean.properties"));
    return pspc;
}

使用

XML

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<bean id="dataSource">
    <property name="url" value="${jdbc.url}"/>
</bean>

@Value

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@Value("${jdbc.url}")
private String url;

Environment

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@Autowired
private Environment env;
...
dataSource.setUrl(env.getProperty("jdbc.url"));

实现

PropertySourcesPlaceholderConfigurer

在看实现xml中占位符替换之前，先看下PropertySourcesPlaceholderConfigurer继承关系：

可以看到PropertySourcesPlaceholderConfigurer实现了BeanFactoryPostProcessor，而其中的postProcessBeanFactory()方法可以对BeanDefinition做属性更改：

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// PropertySourcesPlaceholderConfigurer.java
@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
    if (this.propertySources == null) {
        // 新建一个`propertySources`
        this.propertySources = new MutablePropertySources();
        if (this.environment != null) {
            // 将`environment`作为一个`propertySource`加入`propertySources`
            this.propertySources.addLast(
                new PropertySource<Environment>(ENVIRONMENT_PROPERTIES_PROPERTY_SOURCE_NAME, this.environment) {
                    @Override
                    public String getProperty(String key) {
                        return this.source.getProperty(key);
                    }
                }
            );
        }
        try {
            // `mergeProperties()`从配置文件中读取(以例子来说，就是加载`bean.properties`)
            // 之前说过`propertySources`存在顺序，此处根据参数是否覆盖，决定插入的位置
            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);
        }
    }

    // 处理占位符字符串，`PropertySourcesPropertyResolver`之前已经介绍过，是属性处理类(包括占位符替换)
    processProperties(beanFactory, new PropertySourcesPropertyResolver(this.propertySources));
    this.appliedPropertySources = this.propertySources;
}

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

    // 可以在`bean`配置中自定义占位符格式，作用同于`PropertySourcesPropertyResolver`相关属性
    propertyResolver.setPlaceholderPrefix(this.placeholderPrefix);
    propertyResolver.setPlaceholderSuffix(this.placeholderSuffix);
    propertyResolver.setValueSeparator(this.valueSeparator);

    // 此处定义字符串处理类
    // `propertyResolver`相关见`Spring中的PropertyResolver`
    StringValueResolver valueResolver = new StringValueResolver() {
        @Override
        public String resolveStringValue(String strVal) {
            String resolved = ignoreUnresolvablePlaceholders ?
                    propertyResolver.resolvePlaceholders(strVal) :
                    propertyResolver.resolveRequiredPlaceholders(strVal);
            return (resolved.equals(nullValue) ? null : resolved);
        }
    };

    // 父类`PlaceholderConfigurerSupport`中实现
    doProcessProperties(beanFactoryToProcess, valueResolver);
}

可以看到PropertySourcesPlaceholderConfigurer在处理配置文件时，并没有将配置文件加入到environment，而是自己内部进行维护propertySource，因此无法通过environment中获取到配置文件内容

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// PlaceholderConfigurerSupport.java
protected void doProcessProperties(ConfigurableListableBeanFactory beanFactoryToProcess, StringValueResolver valueResolver) {
    
    BeanDefinitionVisitor visitor = new BeanDefinitionVisitor(valueResolver);
    
    // 获取所有Bean名称
    String[] beanNames = beanFactoryToProcess.getBeanDefinitionNames();
    // 循环所有Bean，做属性占位符替换
    for (String curName : beanNames) {
        // 此处检查防止陷入死循环
        if (!(curName.equals(this.beanName) && beanFactoryToProcess.equals(this.beanFactory))) {
            BeanDefinition bd = beanFactoryToProcess.getBeanDefinition(curName);
            try {
                // 处理BeanDefinition属性中的占位符
                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.
    beanFactoryToProcess.resolveAliases(valueResolver);

    // New in Spring 3.0: resolve placeholders in embedded values such as annotation attributes.
    beanFactoryToProcess.addEmbeddedValueResolver(valueResolver);
}

我们知道，Spring在注册Bean时会先将Bean转化为BeanDefinition，而这个BeanDefinition是未做任何属性上的处理(以例子来说，PropertiesService.name的值就是${properties.name})，而BeanDefinitionVisitor做的事就是将BeanDefinition的相关属性值做占位符替换处理

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public class BeanDefinitionVisitor {
    
    // 省略部分代码...

    public void visitBeanDefinition(BeanDefinition beanDefinition) {
        visitParentName(beanDefinition);
        visitBeanClassName(beanDefinition);
        visitFactoryBeanName(beanDefinition);
        visitFactoryMethodName(beanDefinition);
        visitScope(beanDefinition);
        visitPropertyValues(beanDefinition.getPropertyValues());
        // 构造方法占位符处理
        ConstructorArgumentValues cas = beanDefinition.getConstructorArgumentValues();
        visitIndexedArgumentValues(cas.getIndexedArgumentValues());
        visitGenericArgumentValues(cas.getGenericArgumentValues());
    }

    // 例子中使用的是构造方法属性名称注入
    protected void visitGenericArgumentValues(List<ConstructorArgumentValues.ValueHolder> gas) {
        for (ConstructorArgumentValues.ValueHolder valueHolder : gas) {
            // valueHolder.getValue()就是例子中的`${properties.name}`
            Object newVal = resolveValue(valueHolder.getValue());
            if (!ObjectUtils.nullSafeEquals(newVal, valueHolder.getValue())) {
                valueHolder.setValue(newVal);
            }
        }
    }

    @SuppressWarnings("rawtypes")
    protected Object resolveValue(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.equals(ref.getBeanName())) {
                return new RuntimeBeanReference(newBeanName);
            }
        }
        else if (value instanceof RuntimeBeanNameReference) {
            RuntimeBeanNameReference ref = (RuntimeBeanNameReference) value;
            String newBeanName = resolveStringValue(ref.getBeanName());
            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) {
                // 处理占位符并且重新设值
                String visitedString = resolveStringValue(stringValue);
                typedStringValue.setValue(visitedString);
            }
        }
        else if (value instanceof String) {
            return resolveStringValue((String) value);
        }
        return value;
    }

    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");
        }
        // valueResolver即前面定义的`StringValueResolver`
        String resolvedValue = this.valueResolver.resolveStringValue(strVal);
        return (strVal.equals(resolvedValue) ? strVal : resolvedValue);
    }

}

可以看到visitBeanDefinition()对很多属性都做了占位符替换，每个属性替换方法实现都类似。以例子来说，通过构造方法属性注入，所以看visitGenericArgumentValues()，该方法实现也是比较简单，循环获取每个属性对应的属性值(${properties.name})，然后调用PropertyResolver实现占位符替换，最后对属性重新设值

context:property-placeholder

context:property-placeholder看下解析注册部分，因为占位符替换逻辑其实就是通过注册PropertySourcesPlaceholderConfigurer实现，部分代码如下：

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// AbstractBeanDefinitionParser.java
// AbstractBeanDefinitionParser可以理解为一个`xml`解析类，从`xml`中解析`BeanDefinition `
@Override
public final BeanDefinition parse(Element element, ParserContext parserContext) {
    // 解析`context:property-placeholder`应该使用的`BeanDefinition`
    // 方法在子类`AbstractSingleBeanDefinitionParser`中实现
    AbstractBeanDefinition definition = parseInternal(element, parserContext);
    if (definition != null && !parserContext.isNested()) {
        try {
            // 省略部分代码...

            BeanDefinitionHolder holder = new BeanDefinitionHolder(definition, id, aliases);
            // 注册`BeanDefinition`，其实就是注册了`PropertySourcesPlaceholderConfigurer`
            registerBeanDefinition(holder, parserContext.getRegistry());
            
            // 省略部分代码...
        }
        catch (BeanDefinitionStoreException ex) {
            parserContext.getReaderContext().error(ex.getMessage(), element);
            return null;
        }
    }
    return definition;
}

parseInternal()在子类AbstractSingleBeanDefinitionParser中实现：

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// AbstractSingleBeanDefinitionParser.java
protected final AbstractBeanDefinition parseInternal(Element element, ParserContext parserContext) {
    BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition();
    String parentName = getParentName(element);
    if (parentName != null) {
        builder.getRawBeanDefinition().setParentName(parentName);
    }
    // 
    Class<?> beanClass = getBeanClass(element);
    if (beanClass != null) {
        builder.getRawBeanDefinition().setBeanClass(beanClass);
    }
    
    // 省略部分方法...
    return builder.getBeanDefinition();
}

看下getBeanClass()方法，决定了AbstractBeanDefinition类型：

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// PropertyPlaceholderBeanDefinitionParser.java
@Override
protected Class<?> getBeanClass(Element element) {
    
    // Spring3.1之后默认使用`PropertySourcesPlaceholderConfigurer`，Spring3.1之前默认使用`PropertyPlaceholderConfigurer`
    // 可以通过设置`system-properties-mode`属性强制修改
    if (SYSTEM_PROPERTIES_MODE_DEFAULT.equals(element.getAttribute(SYSTEM_PROPERTIES_MODE_ATTRIBUTE))) {
        return PropertySourcesPlaceholderConfigurer.class;
    }

    return PropertyPlaceholderConfigurer.class;
}

在BeanFactory中注册了PropertySourcesPlaceholderConfigurer之后，占位符替换的逻辑就和上面一样了

@PropertySources

ConfigurationClassParser在注解启动时(使用@Configuration)是一个非常重要的类，功能就是解析使用@Configuration的配置类，包括@PropertySources，@ComponentScan等类注解，以及@Bean等方法注解，此处看下@PropertySources解析：

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// ConfigurationClassParser.java
// 解析@PropertySource，@Import，@ImportResource，@ComponentScan，@Bean
protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass) throws IOException {
    
    // 省略部分代码...

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

    // 省略部分代码...

    return null;
}

private void processPropertySource(AnnotationAttributes propertySource) throws IOException {
    // 获取资源文件名称
    String name = propertySource.getString("name");
    // 获取资源文件路径
    String[] locations = propertySource.getStringArray("value");
    // 是否忽略不存在的资源
    boolean ignoreResourceNotFound = propertySource.getBoolean("ignoreResourceNotFound");
    Assert.isTrue(locations.length > 0, "At least one @PropertySource(value) location is required");
    for (String location : locations) {
        try {
            // 路径支持占位符替换
            String resolvedLocation = this.environment.resolveRequiredPlaceholders(location);
            Resource resource = this.resourceLoader.getResource(resolvedLocation);
            ResourcePropertySource rps = (StringUtils.hasText(name) ?
                    new ResourcePropertySource(name, resource) : new ResourcePropertySource(resource));
            // 将资源文件添加到`environment`中
            addPropertySource(rps);
        }
        catch (IllegalArgumentException ex) {
            // from resolveRequiredPlaceholders
            if (!ignoreResourceNotFound) {
                throw ex;
            }
        }
        catch (FileNotFoundException ex) {
            // from ResourcePropertySource constructor
            if (!ignoreResourceNotFound) {
                throw ex;
            }
        }
    }
}

因此，在这种配置下，资源文件中的内容可以通过注入Environment获取：

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@Autowired
private Environment env;
...
dataSource.setUrl(env.getProperty("jdbc.url"));

参考

Properties with Spring