java springboot how to autowired in prometheus

Question

i have a controller for my http url with autowired event for my database (everything work fine)

@RestController public class CalculateDistance {

@Autowired MyDatabase mydb

some code

@GetMapping(value = "/url")
public Strng get() {
    return mydb.fetch("my query");
}

now i have the same autowired but its not working, i get null instead of my object

 @Component public class PrometheusMonitor {

     @Autowired MyDatabase mydb

     public PrometheusMonitor(MeterRegistry registry) {
         meterRegistry = registry;

         mydb =  null ...

i get an exception becuase mydb = null

but it works for my http controller

Do you create a PrometheusMonitor with `new`? If so, then it's a duplicate of https://stackoverflow.com/questions/19896870/why-is-my-spring-autowired-field-null — JB Nizet, Nov 27 '19 at 17:47
no since springboot does it, i followed this https://blog.autsoft.hu/defining-custom-metrics-in-a-spring-boot-application-using-micrometer/ — yenk, Nov 27 '19 at 17:49
Ah. OK. I see it now. You can't expect Spring to have autowired the field of an object if the object doesn't exist yet. And since you're trying to access `mydb` in the constructor, the object isn't constructed yet. Stop using field injection. Everything would be much clearer if you posted actual code, instead of pseudo-code. — JB Nizet, Nov 27 '19 at 17:54
Stop using field injection. Use constructor injection. Inject the mydb using the constructor, just as you're doing it to inject registry. — JB Nizet, Nov 27 '19 at 17:56
@JBNizet sorry, I am not an expert, can you give an example, im no familar with consutrct injector — yenk, Nov 27 '19 at 17:57
Remove the Autowired annotation on the field. Add an argument of type MyDatabase to your constructor, just like you're already doing it with MeterRegistry. It's just a constructor. You construct an object by passing what it needs as constructor arguments. That's constructor injection. — JB Nizet, Nov 27 '19 at 17:59

score 1 · Accepted Answer · answered Nov 28 '19 at 06:32

To put into works what @JB said,

Constructor Injection will:

Supports immutability
State Safe. The object is instantiated to a full state or is not instantiated at all.
Easy to tests and mocks object injections
Constructors are more suitable for mandatory dependencies

IntelliJ IDEA support:

So for your example, you need to pass it in the constructor like this:

 @Component public class PrometheusMonitor {

 @Autowired
 public PrometheusMonitor(MeterRegistry registry, MyDatabase mydb) {
     meterRegistry = registry;

     assertNotNull(mydb);

     // rest of code

Read more about this:

https://www.vojtechruzicka.com/field-dependency-injection-considered-harmful/

im am missing the autowire at the function Danke dir – yenk Nov 28 '19 at 20:12 — yenk, Nov 28 '19 at 20:12

score 0 · Answer 2 · answered Nov 27 '19 at 18:04

First make sure you dont doing the following:

PrometheusMonitor monitor = new PrometheusMonitor(registry);

that will not property autowire your database and would give you errors if you try to autowire the PrometheusMonitor unless you have a bean created for MeterRegistry

Here are things you can do

1) Have a no-argument constructor for PrometheusMonitor

 @Component public class PrometheusMonitor {

   @Autowired MyDatabase mydb

   public PrometheusMonitor() {}

   public void initializeMonitor(MeterRegistry registry) {
     meterRegistry = registry;
   }
}

Then in your class you can do something like:

@Service class MyService {
  @Autowire
  private PrometheusMonitor monitor;

  @PostConstruct
  privte void init() {
    MeterRegistry registry = getRegistry();
    monitor.initializeMonitor(registry);
  }
}

2) Create a MeterRegistry Bean for your argumented constructor

@Bean
public MeterRegistry registry() {
   return getRegistry();
}

Then during creation of your PrometheusMonitor it will autowire the registry for the argument when you autowire the mintor

https://blog.marcnuri.com/field-injection-is-not-recommended/ — Book Of Zeus, Nov 28 '19 at 06:23

java springboot how to autowired in prometheus

2 Answers2