在 Springboot 中从 application.properties 文件获取属性时获取 null 值

Question

目标：从我的 Java class 中的 application.properties 文件中读取属性值。

当前行为：获取 null 个值

预期行为：

value1
value3

应用程序.properties

category1.subcategory2=value1
category1.subcategory3=value3
greeting.salutation=Hello

TryValueApplication.java

package com.example.tryvalue;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class TryValueApplication {
    @Value("${category1.subcategory2}")
    private String category1;

    @Value("basic value")
    private String category3;

    public static void main(String[] args) {
        SpringApplication.run(TryValueApplication.class, args);
        TryValueApplication tryValueApplication = new TryValueApplication();
        System.out.println("Running main method");
        tryValueApplication.printCategory();
    }

    public void printCategory(){
        System.out.println("Print environment values");
        System.out.println(category1);
        System.out.println(category3);
    }
}

尝试过：我尝试从@RestController class 中访问相同的值并且它正在工作。 GreetingController.java

package com.example.tryvalue.controller;

import com.example.tryvalue.EnvironmentValue;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class GreetingController {
    @Value("${greeting.salutation}")
    private String greetingSalutation;

    @Autowired
    private EnvironmentValue environmentValue;

    @GetMapping
    public String greeting(){
        return greetingSalutation + " world";
    }

    @GetMapping("/category1")
    public String getCategory1(){
        return environmentValue.getCategory1();
    }
}

环境值.java

package com.example.tryvalue;

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

@Component
public class EnvironmentValue {
    @Value("${category1.subcategory2}")
    private String category1;

    @Value("${category1.subcategory3}")
    private String category2;

    @Value("basic value")
    private String category3;

    public EnvironmentValue(){
        System.out.println("Creating instance of EnvironmentValue");
    }

    public String getCategory1(){
        return category1;
    }

    public String getCategory2(){
        return category2;
    }

    public String getCategory3(){
        return category3;
    }
}

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.7.4</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.example</groupId>
    <artifactId>tryValue</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>tryValue</name>
    <description>tryValue</description>
    <properties>
        <java.version>11</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

这种行为差异背后的原因可能是什么？ 谢谢。

Answer 1

您正在 main 方法中创建 TryValueApplication 的第二个非托管实例，因此不会将属性注入其中。

TryValueApplication 的托管实例应该注入值。 将其添加到 TryValueApplication 以打印值

 @Value("${category1.subcategory2}") private String category1; @Value("${category1.subcategory3}") private String category3; public static void main(String[] args) { SpringApplication.run(TryValueApplication.class, args); } @PostConstruct public void printCategory(){ System.out.println("Print environment values"); System.out.println(category1); System.out.println(category3); }