Detailed explanation of the use examples of @Value annotation in Spring Boot

1. Preface

In Spring Boot projects, you usually need toorRead configuration information from the configuration file.@ValueAnnotations provide an easy way to inject configuration values ​​into Spring components.

2. Introduction to @Value Annotation

@ValueIs annotation provided by Spring to read values ​​from configuration files or other sources (such as system environment variables, SpEL expressions) and inject them into fields or method parameters of the bean. The basic syntax is as follows:

@Value("${}")
private String propertyValue;

3. Common usage of @Value annotation

1. Read or configure the value

(1) Configuration file example

=Spring Boot Demo
=1.0.0

app:
  name: Spring Boot Demo
  version: 1.0.0

(2) Java code example

@Component
public class AppConfig {
    @Value("${}")
    private String appName;
    @Value("${}")
    private String appVersion;
    public void printConfig() {
        ("Application Name: " + appName);
        ("Application Version: " + appVersion);
    }
}

(3) Test output

Application Name: Spring Boot Demo

Application Version: 1.0.0

2. Use @Value to set the default value

If no attribute is defined in the configuration file,@ValueAnnotations can provide a default value. The default syntax is:${defaultValue}：

@Component
public class DefaultValueExample {
    @Value("${:Unknown Author}")
    private String author;
    public void printAuthor() {
        ("Application Author: " + author);
    }
}

iforNot configured, then output:

Application Author: Unknown Author

3. Read system environment variables and Java runtime parameters

@ValueIt can also be used to read system environment variables or-DJVM running parameters passed in the method:

@Component
public class EnvConfig {
    @Value("${JAVA_HOME}")
    private String javaHome;
    @Value("${}")
    private String userName;
    public void printSystemProperties() {
        ("JAVA_HOME: " + javaHome);
        ("User Name: " + userName);
    }
}

If the running environment variable is setJAVA_HOME=/usr/lib/jvm/java-11-openjdk, then output:

JAVA_HOME: /usr/lib/jvm/java-11-openjdk
User Name: admin

4. Combined with Spring Expression Language (SpEL)

@ValueAnnotations support the Spring Expression Language (SpEL) and can be used to calculate values ​​dynamically.

(1) Basic SpEL expression

@Component
public class SpELExample {
    @Value("#{2 * 5}")
    private int result;
    public void printResult() {
        ("Result: " + result);
    }
}

Output:

Result: 10

(2) Quote the properties of the Bean

@Component
public class AnotherBean {
    private String message = "Hello from AnotherBean";
    public String getMessage() {
        return message;
    }
}
@Component
public class BeanReferenceExample {
    @Autowired
    private AnotherBean anotherBean;
    @Value("#{}")
    private String messageFromAnotherBean;
    public void printMessage() {
        (messageFromAnotherBean);
    }
}

Output:

Hello from AnotherBean

5. Read array, collection, and Map type data

(1) Read the array

=192.168.1.1,192.168.1.2,192.168.1.3

@Component
public class ArrayConfig {
    @Value("${}")
    private String[] servers;
    public void printServers() {
        ((servers));
    }
}

Output:

[192.168.1.1, 192.168.1.2, 192.168.1.3]

(2) ReadList

@Component
public class ListConfig {
    @Value("#{'${}'.split(',')}")
    private List<String> serverList;
    public void printServerList() {
        (serverList);
    }
}

Output:

[192.168.1.1, 192.168.1.2, 192.168.1.3]

(3) ReadMap

=username:admin,password:123456,url:jdbc:mysql://localhost:3306/test

@Component
public class MapConfig {
    @Value("#{${}}")
    private Map<String, String> dbConfig;
    public void printDbConfig() {
        (dbConfig);
    }
}

Output:

{username=admin, password=123456, url=jdbc:mysql://localhost:3306/test}

4. Comparison between @Value and @ConfigurationProperties

Example:@ConfigurationPropertiesusage

=Spring Boot Demo
=1.0.0

@Configuration
@ConfigurationProperties(prefix = "app")
public class AppProperties {
    private String name;
    private String version;
    // getter & setter
}

5. Summary

• The @Value annotation is used to inject values ​​from the configuration file into Spring components.
• @Value can be used to read values ​​in or , and supports default values.
• @Value can also be used to read environment variables, JVM running parameters, and SpEL expressions.
• @Value can parse arrays, Lists, Map and other data structures.
• @ConfigurationProperties is recommended for complex configurations, while @Value is suitable for simple value injection.

This is the end of this article about the detailed explanation of the use examples of @Value annotation in Spring Boot. For more related contents of using @Value annotation, please search for my previous articles or continue browsing the related articles below. I hope everyone will support me in the future!