【Spring注解系列12】@Value与@PropertySource注解

秒速五厘米 2022-02-24 11:56 311阅读 0赞

# 1.@Value与@PropertySource注解 #

**@Value注解：**主要用于赋值，该值可以是取值配置文件中的，也可以直接赋值，也可以使用SpEl表达式进行计算的结果，抑或直接从环境变量中获取。 该注解不能处理日期类赋值

1、基本数值
    2、可以写SpEL； #{}
    3、可以写${}；取出配置文件【properties】中的值（在运行环境变量里面的值）

**原理是**底层使用了后置处理器AutowiredAnnotationBeanPostProcessor。

* <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).

**@PropertySource注解：**主要用于加载配置文件，value值为classpath

等价于 <context:property-placeholder location="classpath:student.properties"/>

# 2.实例 #

**@PropertySource放置的位置问题：**

1.若使用@Component方式注入对象，则@PropertySource防止在普通类Student或配置类中都可以

2.若使用@Bean方式注入Student对象，则@PropertySource必须放在配置类上加载

**方式一：使用@Component方式注入Student对象**

/**
     *
     * 	使用@Value赋值；
     * 	1、基本数值
     * 	2、可以写SpEL； #{}
     * 	3、可以写${}；取出配置文件【properties】中的值（在运行环境变量里面的值）
     */
    @PropertySource({"classpath:/student.properties"})//指定配置文件classPath
    @Component //此时使用的是@Component方式注入，则@PropertySource放在哪都可以
    public class Student {
    
        @Value("张三")//直接赋值
        private String name;
        @Value("${stu.name}")//从配置文件中取值
        private String alias;
        @Value("${os.name}")//从容器中读取环境变量
        private String system;
        @Value("#{33-12}")//使用SpEL 计算
        private Integer age;
    //    @Value("${stu.birthDate}")//不能处理日期
        private Date birthDate;
        @Value("${stu.hasNickName}") //布尔值
        private Boolean hasNickName;
    
        @Override
        public String toString() {
            return "Student{" +
                    "name='" + name + '\'' +
                    ", alias='" + alias + '\'' +
                    ", system='" + system + '\'' +
                    ", age=" + age +
                    ", birthDate=" + birthDate +
                    ", hasNickName=" + hasNickName +
                    '}';
        }
    
        public Boolean getHasNickName() {
            return hasNickName;
        }
    
        public void setHasNickName(Boolean hasNickName) {
            this.hasNickName = hasNickName;
        }
    
        public String getAlias() {
            return alias;
        }
    
        public void setAlias(String alias) {
            this.alias = alias;
        }
    
        public String getSystem() {
            return system;
        }
    
        public void setSystem(String system) {
            this.system = system;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public Integer getAge() {
            return age;
        }
    
        public void setAge(Integer age) {
            this.age = age;
        }
    
        public Date getBirthDate() {
            return birthDate;
        }
    
        public void setBirthDate(Date birthDate) {
            this.birthDate = birthDate;
        }
    }
    
    
    @Configuration
    @ComponentScan(basePackages = "com.java.model")
    //@PropertySource({"classpath:/student.properties"}) //放在这里也可以
    public class PropertyValueConfig {
    
        
    }
    
    
    
    public class PropertyValueTest {
    
        public static void main(String[] args) {
            AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(PropertyValueConfig.class);
            Student s = applicationContext.getBean(Student.class);
            System.out.println(s);
        }
    }
    
    
    测试结果：
    Student{name='张三', alias='张三', system='Windows 7', age=21, birthDate=null, hasNickName=true}

**方式二：使用@Bean方式注入Student对象**

/**
     *
     * 	使用@Value赋值；
     * 	1、基本数值
     * 	2、可以写SpEL； #{}
     * 	3、可以写${}；取出配置文件【properties】中的值（在运行环境变量里面的值）
     */
    public class Student {
    
        @Value("张三")//直接赋值
        private String name;
        @Value("${stu.name}")//从配置文件中取值
        private String alias;
        @Value("${os.name}")//从容器中读取环境变量
        private String system;
        @Value("#{33-12}")//使用SpEL 计算
        private Integer age;
    //    @Value("${stu.birthDate}")//不能处理日期
        private Date birthDate;
        @Value("${stu.hasNickName}") //布尔值
        private Boolean hasNickName;
    
        @Override
        public String toString() {
            return "Student{" +
                    "name='" + name + '\'' +
                    ", alias='" + alias + '\'' +
                    ", system='" + system + '\'' +
                    ", age=" + age +
                    ", birthDate=" + birthDate +
                    ", hasNickName=" + hasNickName +
                    '}';
        }
    
        public Boolean getHasNickName() {
            return hasNickName;
        }
    
        public void setHasNickName(Boolean hasNickName) {
            this.hasNickName = hasNickName;
        }
    
        public String getAlias() {
            return alias;
        }
    
        public void setAlias(String alias) {
            this.alias = alias;
        }
    
        public String getSystem() {
            return system;
        }
    
        public void setSystem(String system) {
            this.system = system;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public Integer getAge() {
            return age;
        }
    
        public void setAge(Integer age) {
            this.age = age;
        }
    
        public Date getBirthDate() {
            return birthDate;
        }
    
        public void setBirthDate(Date birthDate) {
            this.birthDate = birthDate;
        }
    }
    
    
    @Configuration
    @PropertySource({"classpath:/student.properties"}) //此时只能放在这里加载
    public class PropertyValueConfig {
    
         @Bean
        public Student student(){
            return new Student();
        }
    }
    
    
    
    public class PropertyValueTest {
    
        public static void main(String[] args) {
            AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(PropertyValueConfig.class);
            Student s = applicationContext.getBean(Student.class);
            System.out.println(s);
        }
    }
    
    
    测试结果：
    Student{name='张三', alias='张三', system='Windows 7', age=21, birthDate=null, hasNickName=true}

# 3.EmbeddedValueResolverAware方式实现和@Value一样的效果 #

EmbeddedValueResolverAware是一个String型的value解析器，想要通过这个获取值的任何对象，都可以实现这个接口。

/**
     * Interface to be implemented by any object that wishes to be notified of a
     * {@code StringValueResolver} for the resolution of embedded definition values.
     *
     * <p>This is an alternative to a full ConfigurableBeanFactory dependency via the
     * {@code ApplicationContextAware}/{@code BeanFactoryAware} interfaces.
     *
     * @author Juergen Hoeller
     * @author Chris Beams
     * @since 3.0.3
     * @see org.springframework.beans.factory.config.ConfigurableBeanFactory#resolveEmbeddedValue(String)
     * @see org.springframework.beans.factory.config.ConfigurableBeanFactory#getBeanExpressionResolver()
     * @see org.springframework.beans.factory.config.EmbeddedValueResolver
     */
    public interface EmbeddedValueResolverAware extends Aware {
    
    	/**
    	 * Set the StringValueResolver to use for resolving embedded definition values.
    	 */
    	void setEmbeddedValueResolver(StringValueResolver resolver);
    
    }

**使用方法如下：**

@Configuration
    //@ComponentScan(basePackages = "com.java.model")
    @PropertySource({"classpath:/student.properties"})//指定配置文件classPath
    public class PropertyValueConfig implements EmbeddedValueResolverAware{
    
        @Bean
        public Student student(){
            System.out.println("EmbeddedValueResolverAware----->setEmbeddedValueResolver--->stu = "+stuName);
            return new Student();
        }
    
        private String stuName;
    
        @Override
        public void setEmbeddedValueResolver(StringValueResolver resolver) {
            this.stuName = resolver.resolveStringValue("${stu.name}");
        }
    }