從Springboot中的application.properties檔案中獲取屬性時獲取空值

目標：從我的 Java 類中的 application.properties 檔案中讀取屬性值。

當前行為：獲取空值

預期行為：

value1
value3

應用程式屬性

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);
    }
}

試過： 我嘗試從 @RestController 類中訪問相同的值，它正在作業。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();
    }
}

環境值.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>

這種行為差異背后的原因是什么？謝謝 。

uj5u.com熱心網友回復：

您正在 main 方法中創建 TryValueApplication 的第二個非托管實體，因此不會將屬性注入其中。

    @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);
    }

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