Spring Boot外部属性未加载

Question

I'm starting with Spring Boot, so am first going through the Spring Boot reference Guide

I'm currently having a look at the Externalized Configuration

My goal is to have a simple Component to read the value out of application.properties , which, if I'm not mistaken, should be loaded automatically, without any further configuration.

The example in the reference is as follows:

import org.springframework.stereotype.*
import org.springframework.beans.factory.annotation.*

@Component
public class MyBean {

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

    // ...

}

with the following explanation:

On your application classpath (eg inside your jar) you can have an application.properties that provides a sensible default property value for name. When running in a new environment, an application.properties can be provided outside of your jar that overrides the name; and for one-off testing, you can launch with a specific command line switch (eg java -jar app.jar --name="Spring").

Seems simple enough. So, I have created following Component:

@Component
public class Admin {

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

    public String getName(){
        return "ReadName_" + name;
    }
}

And, in my recources folder, the following application.properties file (which, I verified, is copied into my .jar)

name=myName

So, if I follow the logic in the reference (which is also mentioned in this post: Read application.properties , and run my application.

The code compiles and runs without error, but each time I print the value of getName() from an instance of Admin on my screen, it just gives a "ReadName_null" instead of "ReadName_myName", the value in the properties file.

As the reference states, in 24.2:

By default SpringApplication will convert any command line option arguments (starting with '--', eg --server.port=9000) to a property and add it to the Spring Environment. As mentioned above, command line properties always take precedence over other property sources.

So, just to check, I run my .jar file with both the application.properties with the key present, and add --name=passedName as a command line argument.

Even though this value should be automatically added to the environment, and overrule anything (or in my case: nothing) that is currently there, and I expect the getName() to return "ReadName_passedName", it still returns "ReadName_null".

EDIT: I print the command line arguments I pass while booting, so I know that the argument is indeed read by the system as "--name=passedName"

Is there anything obvious I'm missing here?

Answer 1

If you do new Admin () , you are creating a new instance of Admin and not a spring bean. So you don't get any advantages which spring provides(for example Dependency Injection) And hence the value is null.

Instead you should Autowire it in your class. This way spring will inject the instance of Admin that it has created (with injected values)

@Autowired
Admin admin;  

And before you ask, yes by default all beans are singleton(unless specified otherwise). So no matter whereever you Autowire Admin, you will get the same instance.

This is a broad topic, you should read about it.