Spring：如何从Model类的属性文件中获取值

Question

I am using Spring MVC for project. I have some constant values which is stored in properties file and I want to fetch from properties file. Question I am unable to fetch values in Model Classes from properties file. It is getting null .

I have set property file location in servlet-context.xml

 <context:property-placeholder location="classpath:myproperties.properties" />

Now by using @Value annotation I inject value from properties file.

@Component
class ModelTest {

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

     // Default Constructor
     public ModelTest(){
          Sysout(fname);      // getting null here
     }

     @PostConstruct
     public void initMembers(){
          Sysout(fname)      // Prints fname value properly    
     }

     public void setFname(String fname){
          this.fname=fname;
     }

     public String getFname(){
          return fname;
     }

     @Override
     public String toString() {
          Sysout(fname);
          return "ModelTest [variableFirst=" + variableFirst + "]"; 
     }
}

Here is ServiceTest class.

@Service
class ServiceTest(){

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

    public String printTest(){
         sysout(fname);           // Prints fname value
         return new ModelTest().toString() // Prints null
    }
}

Here is ControllerHome Class :

@Controller
public class ControllerHome {

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

     @Autowired
     private ServiceTest service;

     @RequestMapping("/")
     public @ResponseBody String printData(){
          sysout(fname);           // Prints fname value
          return service.printTest();  // Print null
     }
}

In model class fname is getting null while In controller and service class value is coming properly.

is anyone face such issue?

Answer 1

This is how I do it :

@Component
@PropertySource("classpath:myproperties.properties") // <-Add this.
class ModelTest {

     @Autowired
     private Environment env; 

     public void test(){
        String name = env.getProperty("name"); //Assuming you have a 'name' key in your myproperties.property
     }
}

Answer 2

When you say model class, do you mean the value passed to a controller method indicated by @ModelAttribute ?

If so, that class is created by ordinary constructor invocation through reflection. It is not a spring bean, and thus @Value does nothing.

Addressing your edit, I think there is some fundamental misunderstanding about how Spring works.

@Service
class ServiceTest(){

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

    public String printTest(){
        sysout(fname);           // Prints fname value
        // Calling new here means Spring does nothing
        // ModelTest is not a Spring bean
        // `@Component`, `@PostConstruct` and `@Value` in ModelTest mean nothing.
        return new ModelTest().toString() // Prints null
    }
}

Instead, you have to do something like this:

@Service
class ServiceTest(){

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

    @Autowired
    private ModelTest modelTest;

    public String printTest(){
        sysout(fname);           // Prints fname value
        // modelTest is now a Spring bean
        return modelTest.toString() // Should not print null
    }
}

Now, Spring will create ModelTest , and @Component , @PostConstruct and @Value will be honored by Spring.

However, @Component by itself has a default singleton scope. So, you will have the same modelTest always.

So, you have to do something like this:

@Component
@Scope(value = "request", proxyMode = ScopedProxyMode.TARGET_CLASS)
class ModelTest {
    // ...
}

Now, while the modelTest reference in ServiceTest will remain constant, the use of a proxy will divert the method calls to a new instance of ModelTest , created by Spring, per request.