关于Spring属性处理器PropertyResolver以及应用运行环境Environment的深度分析，强大的StringValueResolver使用和解析【享学Spring】（下）

public class ClassPathScanningCandidateComponentProvider implements EnvironmentCapable, ResourceLoaderAware {
  ...
  @Override
  public final Environment getEnvironment() {
    if (this.environment == null) {
      this.environment = new StandardEnvironment();
    }
    return this.environment;
  }
  ...
}

// @since 2.5  该接口非常简单，就是个函数式接口~
@FunctionalInterface
public interface StringValueResolver {
  @Nullable
  String resolveStringValue(String strVal);
}

// @since 4.3  这个类出现得还是蛮晚的   因为之前都是用内部类的方式实现的~~~~这个实现类是最为强大的  只是SpEL
public class EmbeddedValueResolver implements StringValueResolver {
  // BeanExpressionResolver之前有非常详细的讲解，简直不要太熟悉~  它支持的是SpEL  可以说非常的强大
  // 并且它有BeanExpressionContext就能拿到BeanFactory工厂，就能使用它的`resolveEmbeddedValue`来处理占位符~~~~
  // 双重功能都有了~~~拥有了和@Value一样的能力，非常强大~~~
  private final BeanExpressionContext exprContext;
  @Nullable
  private final BeanExpressionResolver exprResolver;
  public EmbeddedValueResolver(ConfigurableBeanFactory beanFactory) {
    this.exprContext = new BeanExpressionContext(beanFactory, null);
    this.exprResolver = beanFactory.getBeanExpressionResolver();
  }
  @Override
  @Nullable
  public String resolveStringValue(String strVal) {
    // 先使用Bean工厂处理占位符resolveEmbeddedValue
    String value = this.exprContext.getBeanFactory().resolveEmbeddedValue(strVal);
    // 再使用el表达式参与计算~~~~
    if (this.exprResolver != null && value != null) {
      Object evaluated = this.exprResolver.evaluate(value, this.exprContext);
      value = (evaluated != null ? evaluated.toString() : null);
    }
    return value;
  }
}

public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {
  ...
  @Override
  @Nullable
  public String resolveEmbeddedValue(@Nullable String value) {
    if (value == null) {
      return null;
    }
    String result = value;
    // embeddedValueResolvers是个复数：因为我们可以自定义处理器添加到bean工厂来，增强它的能力
    for (StringValueResolver resolver : this.embeddedValueResolvers) {
      result = resolver.resolveStringValue(result);
      // 只要处理结果不为null，所以的处理器都会执行到~~~~
      if (result == null) {
        return null;
      }
    }
    return result;
  }
  ...
}

public abstract class AbstractApplicationContext extends DefaultResourceLoader
    implements ConfigurableApplicationContext {
  protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    ...
    // 如果从来没有注册过，Spring容器默认会给注册一个这样的内部类
    // 可以看到，它最终还是委托给了Environment去干这件事~~~~~~
    // 显然它最终就是调用PropertyResolver#resolvePlaceholders
    if (!beanFactory.hasEmbeddedValueResolver()) {
      beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
    }
    ...
  } 
}

class ApplicationContextAwareProcessor implements BeanPostProcessor {
  // 这里new的一个EmbeddedValueResolver，它持有对beanFactory的引用~~~
  // 所以调用者直接使用的是EmbeddedValueResolver：它支持解析占位符（依赖于Enviroment上面有说到）并且支持SpEL的解析  非常强的
  public ApplicationContextAwareProcessor(ConfigurableApplicationContext applicationContext) {
    this.applicationContext = applicationContext;
    this.embeddedValueResolver = new EmbeddedValueResolver(applicationContext.getBeanFactory());
  }
}

<bean id="propertyConfigurer" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
   <property name="location">
      <!-- classpath:conf/jdbc.properties  -->
       <value>conf/jdbc.properties</value>
   </property>
    <property name="fileEncoding">
        <value>UTF-8</value>
    </property>
</bean>

public class Main {
    public static void main(String[] args) {
        StandardEnvironment environment = new StandardEnvironment();
        MutablePropertySources mutablePropertySources = environment.getPropertySources();
        MapPropertySource mapPropertySource = new MapPropertySource("diy", new HashMap<String, Object>() {{
            put("app.name", "fsx");
            put("app.key", "${user.home1}"); // 注意这里是user.home1 特意让系统属性里不存在的
            put("app.full", "${app.key} + ${app.name}");
        }});
        mutablePropertySources.addFirst(mapPropertySource);
        // 正常使用
        String s = environment.resolvePlaceholders("${app.full}");
        System.out.println(s);
        s = environment.getProperty("app.full");
        System.out.println(s);
    }
}

${user.home1} + fsx
Exception in thread "main" java.lang.IllegalArgumentException: Could not resolve placeholder 'user.home1' in value "${user.home1}"
  at org.springframework.util.PropertyPlaceholderHelper.parseStringValue(PropertyPlaceholderHelper.java:172)
  at org.springframework.util.PropertyPlaceholderHelper.parseStringValue(PropertyPlaceholderHelper.java:160)

beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));

server.port=8080