11. spring-容器: @Value, @PropertySource详解

在使用Java config配置Bean以前，即xml配置时代，bean的属性值可以通过 <property name=“xxx” value=""/>的形式。 value既可以使用固定值，也可以使用占位符的形式${xxx}。占位符中的配置的值由一个特殊的bean的解析（PropertyPlaceholderConfigurer）。

而到了java config时代，给bean属性配置值，可以使用的方式是@Value，本章就来重点讲下@Value如何使用。

@Value定义

先看源码：

package org.springframework.beans.factory.annotation;import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;/*** Annotation at the field or method/constructor parameter level* that indicates a default value expression for the affected argument.** <p>Typically used for expression-driven dependency injection. Also supported* for dynamic resolution of handler method parameters, e.g. in Spring MVC.** <p>A common use case is to assign default field values using* <code>#{systemProperties.myProp}</code> style expressions.** <p>Note that actual processing of the {@code @Value} annotation is performed* by a {@link org.springframework.beans.factory.config.BeanPostProcessor* BeanPostProcessor} which in turn means that you <em>cannot</em> use* {@code @Value} within* {@link org.springframework.beans.factory.config.BeanPostProcessor* BeanPostProcessor} or* {@link org.springframework.beans.factory.config.BeanFactoryPostProcessor BeanFactoryPostProcessor}* types. Please consult the javadoc for the {@link AutowiredAnnotationBeanPostProcessor}* class (which, by default, checks for the presence of this annotation).** @author Juergen Hoeller* @since 3.0* @see AutowiredAnnotationBeanPostProcessor* @see Autowired* @see org.springframework.beans.factory.config.BeanExpressionResolver* @see org.springframework.beans.factory.support.AutowireCandidateResolver#getSuggestedValue*/
@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Value {/*** The actual value expression &mdash; for example, <code>#{systemProperties.myProp}</code>.*/String value();}

从源码注释提取一下关键信息：

@Value可注解在Field, 方法，方法参数，或注解。通常用于表达式驱动的依赖注入，也支持动态方法参数的解析。
一个通常的用法是SpEL, 如：#{systemProperties.myProp}。（SpEL下一章单独讲解）。
注意：事实上@Value注解是通过BeanPostProcessor实现的，所以BeanPostProcessor本身不能注入@Value的Field。
确切的说@Value是通过AutowiredAnnotationBeanPostProcessor实现，即它既处理@Autowired注解，也处理@Value。

@Value注解的用法

不通过配置文件将外部的值动态注入到Bean中

注入普通字符串

@Value("panda")
private String name;

注入操作系统属性

@Value("#{systemProperties['os.name']}")
private String os;

注入表达式结果

@Value("#{T(java.lang.Math).random() * 100.0}")
private double randomNumber;

注入其他Bean的属性

@Value("#{animal.name}")
private String animalName;

注入文件资源

@Value("classpath:application.properties")
private Resource classpathResource;

注入URL资源

@Value("https://baidu.com")
private Resource urlResource;

示例：

自定义Bean class, 演示了各种通过@Value的注入场景

package win.elegentjs.spring.ioc.value;import lombok.Data;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.core.io.Resource;/*** 自定义Bean class, 演示了各种通过@Value的注入场景* 注：不使用外部配置属性*/
@Data
public class CustomerValue {@Value("panda")private String name;@Value("#{systemProperties['os.name']}")private String os;@Value("#{T(java.lang.Math).random() * 100.0}")private double randomNumber;@Value("#{animal.name}")private String animalName;@Value("classpath:application.properties")private Resource classpathResource;@Value("https://baidu.com")private Resource urlResource;
}

java config 配置类，定义了两个bean,animal会被customerValue依赖

package win.elegentjs.spring.ioc.value;import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;/*** java config 配置类，定义了两个bean,animal会被customerValue依赖*/
@Configuration
public class CustomerValueConfig {@Beanpublic CustomerValue customerValue() {return new CustomerValue();}@Beanpublic Animal animal() {Animal animal = new Animal();animal.setName("dog");return animal;}
}

测试类，演示测试结果

package win.elegentjs.spring.ioc.value;import lombok.extern.slf4j.Slf4j;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;@Slf4j
public class CustomerValueSample {public static void main(String[] args) {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(CustomerValueConfig.class);CustomerValue customerValue = context.getBean(CustomerValue.class);log.info("==> customerValue: {}", customerValue);}
}// result:
2021-05-31 19:36:06.919 [main] INFO  win.elegentjs.spring.ioc.value.CustomerValueSample-==> customerValue: CustomerValue(name=panda, os=Mac OS X, randomNumber=54.4727006010814, animalName=dog, classpathResource=class path resource [application.properties], urlResource=URL [https://baidu.com])

通过配置文件注入属性

先通过@PropertySource指定属性配置文件源，@PropertySource源码如下：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Repeatable(PropertySources.class)
public @interface PropertySource {/*** Indicate the name of this property source. If omitted, a name will* be generated based on the description of the underlying resource.* @see org.springframework.core.env.PropertySource#getName()* @see org.springframework.core.io.Resource#getDescription()*/String name() default "";/*** Indicate the resource location(s) of the properties file to be loaded.* <p>Both traditional and XML-based properties file formats are supported* &mdash; for example, {@code "classpath:/com/myco/app.properties"}* or {@code "file:/path/to/file.xml"}.* <p>Resource location wildcards (e.g. **/*.properties) are not permitted;* each location must evaluate to exactly one {@code .properties} resource.* <p>${...} placeholders will be resolved against any/all property sources already* registered with the {@code Environment}. See {@linkplain PropertySource above}* for examples.* <p>Each location will be added to the enclosing {@code Environment} as its own* property source, and in the order declared.*/String[] value();/*** Indicate if failure to find the a {@link #value() property resource} should be* ignored.* <p>{@code true} is appropriate if the properties file is completely optional.* Default is {@code false}.* @since 4.0*/boolean ignoreResourceNotFound() default false;/*** A specific character encoding for the given resources, e.g. "UTF-8".* @since 4.3*/String encoding() default "";/*** Specify a custom {@link PropertySourceFactory}, if any.* <p>By default, a default factory for standard resource files will be used.* @since 4.3* @see org.springframework.core.io.support.DefaultPropertySourceFactory* @see org.springframework.core.io.support.ResourcePropertySource*/Class<? extends PropertySourceFactory> factory() default PropertySourceFactory.class;}

原始的源码注释很长，这里没有贴出来，感兴趣的自己去看一下，总结一下有用信息如下：

@PropertySource是一个实用注解用于添加一个PropertySource至Spring的Environment对象：即通过@PropertySource将properties配置文件中的值存储到spring的Environment对象中。
典型用法1：使用Environment对象取属性值

 @Configuration@PropertySource("classpath:/com/myco/app.properties")public class AppConfig {@Autowiredprivate Environment env;@Beanpublic TestBean testBean() {TestBean testBean = new TestBean();testBean.setName(env.getProperty("testbean.name"));return testBean;}}

典型用法2：使用@Value("${…}")解析

@Configuration
@PropertySource("classpath:/com/myco/app.properties")
public class AppConfig {@Value("${testbean.name}")private String beanName;}

在xml中能解析${…}占位符是因为配置了<context:property-placeholder>, 而@Configuration是因为配置了PropertySourcesPlaceholderConfigurer，一般情况下不需要自己配置，除非默认的规则不满足，需要自己定制。
可配置缺省值。如 “${testbean.name:zhangsan}”。如果未设置缺省值，取不到的情况下会抛出IllegalArgumentException异常。
同一个属性项，后注册的会覆盖前面的，优先级更高。

以下是一个简单示例：

在classpath下新增一个配置文件：application.properties

weather=sunning
wind=5

propertySource java config配置示例

package win.elegentjs.spring.ioc.propertysource;import lombok.ToString;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.PropertySource;@Configuration
@PropertySource({"classpath:application.properties"})
@ToString
public class MyPropertySourceConfig {@Value("${weather}")private String weather;@Value("${wind}")private Integer wind;
}

测试，查看结果

package win.elegentjs.spring.ioc.propertysource;import lombok.extern.slf4j.Slf4j;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;@Slf4j
public class MyPropertySourceSample {public static void main(String[] args) {ApplicationContext context = new AnnotationConfigApplicationContext(MyPropertySourceConfig.class);MyPropertySourceConfig config = context.getBean(MyPropertySourceConfig.class);log.info("==> config: {}", config);}
}// result:
2021-05-31 19:38:35.508 [main] INFO  w.e.s.ioc.propertysource.MyPropertySourceSample-==> config: MyPropertySourceConfig(weather=sunning, wind=5)

#{…} vs ${…}

${}用于从属性配置文件中获取属性值。
#{}里面可以写SpEL表达式，如#{T(java.lang.Math.random}, #{‘Hello world’.bytes.length}。类似于OGNL。

两者可以混用，如：#{${…}.split(’,’)}
但需要注意：必须#{}在外面，${}在里面, 因为${}会优先解析

小结

本节学习了如何使用@Value加载属性值，属性值可以来源于外部资源文件，也可以来自内部的属性，spEL。
学习了如何使用@PropertySource加载外部配置文件。至于背后的原理部分如Environment， PropertySourcesPlaceholderConfigurer等后面会开专门的章节分析。