Autowired在Bean类的Spring中不起作用

Question

I am trying to create a bean and instantiate the bean properties overriding default constructor and using an object of Environment [org.springframework.core.env.Environment] to fetch and assign properies from property file.

Below is my property file [mi.properties]

mi.name=GB
mi.grade=13

Below is my Simple bean Class

public class School implements EnvironmentAware {

private String schoolName;
private int schoolGrade;
private int totalStudents;

public School(Environment env) {
    this.schoolName = env.getProperty("mi.name", "MT");
    this.schoolGrade = env.getProperty("mi.grade", Integer.class, 10);
    this.totalStudents = env.getProperty("mi.total", Integer.class, 1000);
}

public School() {
    this.schoolName = this.env.getProperty("mi.name", "MT");
    this.schoolGrade = this.env.getProperty("mi.grade", Integer.class, 10);
    this.totalStudents = this.env.getProperty("mi.total", Integer.class, 1000);
}
//Getter Setters
}

Below is my Java config class

@Configuration
@PropertySource("classpath:/cross/mi.properties")
public class JavaConfig {
@Autowired
private Environment env;

@Bean
public School getSchool()
{
    School obj = new School(env);
    return obj;
}
}

This would create School bean properly . However when I try to autowire Environment in School bean it is not creating bean.

Below is what I have tried

public class School implements EnvironmentAware {

private String schoolName;
private int schoolGrade;
private int totalStudents;

@Autowired
private Environment env;

public School(Environment env) {
    this.schoolName = env.getProperty("mi.name", "MT");
    this.schoolGrade = env.getProperty("mi.grade", Integer.class, 10);
    this.totalStudents = env.getProperty("mi.total", Integer.class, 1000);
}

public School() {
    this.schoolName = this.env.getProperty("mi.name", "MT");
    this.schoolGrade = this.env.getProperty("mi.grade", Integer.class, 10);
    this.totalStudents = this.env.getProperty("mi.total", Integer.class, 1000);
}
//Getter Setters
}

And Java config changed as below

@Configuration
@PropertySource("classpath:/cross/mi.properties")
@ComponentScan
public class JavaConfig {
@Bean
public School getSchool()
{
    School obj = new School();
    return obj;
}
}

This is not creating School bean in Spring context and when I debugged, Environment instance inside overridden default constructor is hence method is failing.

I am bit confused on this .

1. As per my understanding the Spring life cycle context would autowire all @autowired properties before constructor call . Is it correct ?

2. If above statement in wrong when all autowired properties are resolved in spring life cycle?

3. As per JVM architecture , it allocates memory and assign default value to properties of a class when class is loaded for first time . Does this means by default when School class is loaded its property is defaulted to null ?

4. How autowired Environment in JavaConfig class is working ? How Spring autowires this values and in what stage of its life cycle ?

As suggested in some forums I have tried implementing EnvironmentAware interface in class and added @Component annotation to class.Still No result.

Answer 1

Spring injects @Autowired properties using reflection after constructor is executed and the instance is created. That's why you always get null in the constructor - the properties of the class are not set by Spring yet.

You can achieve what you want in a several ways. One would be to use the

afterPropertiesSet() method which is executed as the name says after the properties are set :) There you will have env

Another solution which would be the preferred one by me is to add the autowired beans in the constructor. Some people call it constructor injection.

It would be marking the constructor using Environment as autowired like that (or having just one constructor):

@Autowired
public School(Environment env) {
    this.schoolName = env.getProperty("mi.name", "MT");
    this.schoolGrade = env.getProperty("mi.grade", Integer.class, 10);
    this.totalStudents = env.getProperty("mi.total", Integer.class, 1000);
}

This way you can also set your Environment variable as final (which would be a good idea). This will work with the @component annotation as other have suggested in order to remove the manual creation of bean using new School()

Check this link for more

Answer 2

Injection happens after construction, unless you explicitly use constructor injection:

@Autowired
public School(Environment env) {
    this.schoolName = env.getProperty("mi.name", "MT");
    this.schoolGrade = env.getProperty("mi.grade", Integer.class, 10);
    this.totalStudents = env.getProperty("mi.total", Integer.class, 1000);
}

Another option is to use a @PostConstruct method to do your initialising instead:

@Autowired
private Environment env;

public School() {
}

@PostConstruct
public void initialise() {
    this.schoolName = this.env.getProperty("mi.name", "MT");
    this.schoolGrade = this.env.getProperty("mi.grade", Integer.class, 10);
    this.totalStudents = this.env.getProperty("mi.total", Integer.class, 1000);
}