java - 使用 Spring 注解读取文件属性

Question

我正在尝试学习如何使用 spring 读取属性文件。在互联网搜索后，我发现我可以使用 @value 和 @PropertySource 注释来实现这一点。我创建了一个具有以下结构和类代码的项目:

项目结构:

AppConfigMongoDB.java 实现:

package com.mongodb.properties;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.PropertySource;


@PropertySource("classpath:config/config.properties")
public class AppConfigMongoDB {

@Value("#{mongodb.url}")
private String mongodbUrl;


@Value("#{mongodb.db}")
private String defaultDb;

public String getMongoDb()
{
    return defaultDb;
}

public String getMongoDbUrl()
{
    return mongodbUrl;
}
}

SpringConfiguration.java 实现:

 package com.mongodb.properties;

 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;

 @Configuration
 public class SpringConfiguration {
 @Bean
 public AppConfigMongoDB getAppConfigMongoDB(){
        return new AppConfigMongoDB();
  }
 }

Main.java

package com.mongodb.properties;

import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class Main {

public static void main(String[] args) {
    ApplicationContext applicationContext = new AnnotationConfigApplicationContext(SpringConfiguration.class);
    AppConfigMongoDB mongo = applicationContext.getBean(AppConfigMongoDB.class);
    System.out.println("db= "+mongo.getMongoDb());
    System.out.println("URL= "+mongo.getMongoDbUrl());
  }
}

我正在读取的属性文件名为 config.properties，它包含以下几行:

mongodb.url=1.2.3.4
mongodb.db=dataBase

我测试了这个小项目，我得到了一个堆栈跟踪，其中包含以下异常:

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'getAppConfigMongoDB': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: private java.lang.String com.mongodb.properties.AppConfigMongoDB.mongodbUrl; nested exception is org.springframework.beans.factory.BeanExpressionException: Expression parsing failed; nested exception is org.springframework.expression.spel.SpelEvaluationException: EL1008E:(pos 0): Property or field 'mongodb' cannot be found on object of type 'org.springframework.beans.factory.config.BeanExpressionContext' - maybe not public?
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:334)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1202)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:537)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:476)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:303)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:299)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:755)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:757)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:480)
at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:84)
at com.mongodb.properties.Main.main(Main.java:9)

 Caused by: org.springframework.expression.spel.SpelEvaluationException: EL1008E:(pos 0): Property or field 'mongodb' cannot be found on object of type 'org.springframework.beans.factory.config.BeanExpressionContext' - maybe not public?
at org.springframework.expression.spel.ast.PropertyOrFieldReference.readProperty(PropertyOrFieldReference.java:226)
at org.springframework.expression.spel.ast.PropertyOrFieldReference.getValueInternal(PropertyOrFieldReference.java:93)
at org.springframework.expression.spel.ast.PropertyOrFieldReference.getValueInternal(PropertyOrFieldReference.java:81)
at org.springframework.expression.spel.ast.CompoundExpression.getValueRef(CompoundExpression.java:51)
at org.springframework.expression.spel.ast.CompoundExpression.getValueInternal(CompoundExpression.java:87)
at org.springframework.expression.spel.ast.SpelNodeImpl.getValue(SpelNodeImpl.java:120)
at org.springframework.expression.spel.standard.SpelExpression.getValue(SpelExpression.java:242)
at org.springframework.context.expression.StandardBeanExpressionResolver.evaluate(StandardBeanExpressionResolver.java:161)
... 18 more

是不是Spring调用bean的问题？或者可能是属性文件路径或其他问题？

Answer 1

我可以在代码中看到几个问题。

1) 值的占位符应采用 ${mogodb.url} 格式，而不是 #{mongodb.url}。 “#”有不同的含义(见 Spring Expressions)。

2) 您将需要一个 PropertySourcesPlaceholderConfigurer bean 来进行值的注入(inject)

3) 迟早会有许多 Bean 漂浮在周围，我会使用 @ComponentScan 让上下文知道这些，而不必一一提及

4) 如果你使用 ComponentScan 来获取 bean，你将不得不提供一次 AppConfigMongoDB bean

在完成所有这些之后，我最终得到了这些类(class):

Main.java

import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class Main {

public static void main(String[] args) {
    ApplicationContext applicationContext = new AnnotationConfigApplicationContext(SpringConfiguration.class);
    AppConfigMongoDB mongo = applicationContext.getBean(AppConfigMongoDB.class);
    System.out.println("db= "+mongo.getMongoDb());
    System.out.println("URL= "+mongo.getMongoDbUrl());
  }
}

SpringConfiguration.java

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.support.PropertySourcesPlaceholderConfigurer;

@Configuration
@ComponentScan
public class SpringConfiguration {

  @Bean
  public static PropertySourcesPlaceholderConfigurer propertyConfigInDev() {
    return new PropertySourcesPlaceholderConfigurer();
  }
}

AppConfigMongoDB.java

import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.PropertySource;

@Configuration
@PropertySource("classpath:config/config.properties")
public class AppConfigMongoDB {

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

  @Value("${mongodb.db}")
  private String defaultDb;

  public String getMongoDb() {
    return defaultDb;
  }

  public String getMongoDbUrl() {
    return mongodbUrl;
  }
}