What is order of evaluation for #Autowiring in SpringBoot project - spring-boot

Question

I'm trying to write a simple SprintBoot REST Controller to run on Websphere Liberty, and having a problem with #Autowire. Here are the relevant (I think) pieces of code:
#CrossOrigin
#RestController
#RequestMapping(path = "userSetting")
public class RESTInterface {
#Autowired
private UserSettingDAO userSettingDAO;
#RequestMapping(path = "/getUserSetting", method = { RequestMethod.GET }, produces = MediaType.APPLICATION_JSON_UTF8_VALUE)
public String getUserSetting(
#RequestParam(value="user_id", required=true ) String user_id,
#RequestParam(value="app_id", required=true ) String app_id,
#RequestParam(value="dashboard_name", required=true ) String dashboard_name
) {
if ( userSettingDAO == null ) {
System.out.println( "userSettingDAO is null" );
...
////////////////////////////////////////////////////////////////////////////////
#Component
public class UserSettingDAO {
#Autowired
private JdbcTemplate jdbcTemplate;
#Autowired
private DataSource dataSource;
public UserSettingDAO() {
System.out.println( "dataSource is " + dataSource );
System.out.println( "jdbcTemplate is " + jdbcTemplate );
When I deploy the application to Liberty, it appears to start up OK.
When I hit the appropriate endpoint, I see the following in console.log:
UserSettingDAO.jdbcTemplate = null
dataSource is null
jdbcTemplate is null
2018-04-24 16:54:19.887 INFO 8807 --- [cutor-thread-11] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/userSetting/getUserSetting],methods=[GET],produces=[application/json;charset=UTF-8]}" onto public java.lang.String com.ui.usersetting.restinterface.RESTInterface.getUserSetting(java.lang.String,java.lang.String,java.lang.String)
2018-04-24 16:54:19.890 INFO 8807 --- [cutor-thread-11] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/userSetting/upsertUserSetting],methods=[POST],produces=[application/json;charset=UTF-8]}" onto public void com.ui.usersetting.restinterface.RESTInterface.upsertUserSetting(java.lang.String,java.lang.String,java.lang.String,java.lang.String)
2018-04-24 16:54:19.891 INFO 8807 --- [cutor-thread-11] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/userSetting/deleteUserSetting],methods=[DELETE],produces=[application/json;charset=UTF-8]}" onto public void com.ui.usersetting.restinterface.RESTInterface.deleteUserSetting(java.lang.String,java.lang.String,java.lang.String)
2018-04-24 16:54:19.893 INFO 8807 --- [cutor-thread-11] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/userSetting/hello],methods=[GET],produces=[application/json;charset=UTF-8]}" onto public java.lang.String com.ui.usersetting.restinterface.RESTInterface.hello()
2018-04-24 16:54:19.924 INFO 8807 --- [cutor-thread-11] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/error],produces=[text/html]}" onto public org.springframework.web.servlet.ModelAndView org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.errorHtml(javax.servlet.http.HttpServletRequest,javax.servlet.http.HttpServletResponse)
2018-04-24 16:54:19.925 INFO 8807 --- [cutor-thread-11] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/error]}" onto public org.springframework.http.ResponseEntity<java.util.Map<java.lang.String, java.lang.Object>> org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.error(javax.servlet.http.HttpServletRequest)
2018-04-24 16:54:20.166 INFO 8807 --- [cutor-thread-11] s.w.s.m.m.a.RequestMappingHandlerAdapter : Looking for #ControllerAdvice: org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext#6a5997f7: startup date [Tue Apr 24 16:54:15 EDT 2018]; root of context hierarchy
2018-04-24 16:54:21.386 INFO 8807 --- [cutor-thread-11] o.s.j.e.a.AnnotationMBeanExporter : Registering beans for JMX exposure on startup
2018-04-24 16:54:21.389 INFO 8807 --- [cutor-thread-11] o.s.j.e.a.AnnotationMBeanExporter : Bean with name 'dataSource' has been autodetected for JMX exposure
2018-04-24 16:54:21.396 INFO 8807 --- [cutor-thread-11] o.s.j.e.a.AnnotationMBeanExporter : Located MBean 'dataSource': registering with JMX server as MBean [com.zaxxer.hikari:name=dataSource,type=HikariDataSource]
My question is: Why are the two println statements saying that their respective variables are null? Does it have something to do with the fact that the UserSettingDAO constructor appears in the log to be executed before the lines about dataSource appear in the log?
What should I do to get those variables properly initialized?

Answer 1

At the time a Spring component's constructor is called, any #Autowired objects will still be null. This is because the spring container needs to initialize an instance of the class before it can inject values into the #Autowired fields.
I don't know how Spring implements injection, but most Java injection works something like this:
Construct new instance of the bean class, normally by invoking default ctor (or initializing a dynamically generated sub-class proxy)
Obtain objects that need to be injected into the bean
Using reflection/proxy, set the fields on the bean
To validate this, I created a method on UserSettingsDAO that utilizes the #AutoWired fields:
#Component
public class UserSettingDAO {
#Autowired
private JdbcTemplate jdbcTemplate;
#Autowired
private DataSource ds;
public UserSettingDAO() {
System.out.println("ctor/jdbcTemplate is " + jdbcTemplate );
System.out.println("ctor/datasource is: " + ds);
}
public void doStuff() {
System.out.println("doStuff/jdbcTemplate is " + jdbcTemplate );
System.out.println("doStuff/datasource is: " + ds);
}
}
If we inject this DAO into another class and use it, we will see that the fields are initialized after the UserSettingDAO is constructed:
#CrossOrigin
#RestController
public class RESTInterface {
#Autowired
private UserSettingDAO dao;
#RequestMapping(path = "/jdbc", method = { RequestMethod.GET }, produces = MediaType.APPLICATION_JSON_UTF8_VALUE)
public String jdbc() {
return dao.doStuff();
}
}
Checking logs, this will produce the following output:
ctor/jdbcTemplate is null
ctor/datasource is: null
doStuff/jdbcTemplate is org.springframework.jdbc.core.JdbcTemplate#4eb46bed
doStuff/datasource is: com.ibm.ws.rsadapter.jdbc.WSJdbcDataSource#727d23c5