Autowired returns null in spring boot

Question

Following is the application any pointers where the error is

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.ComponentScan;
@ComponentScan
@SpringBootApplication
public class SpringAutoWApplication {

public static void main(String[] args) {
    SpringApplication.run(SpringAutoWApplication.class, args);
    DestClass dc = new DestClass();
    dc.showTestWired();
  }
}



import org.springframework.beans.factory.annotation.Autowired;
@Component
public class DestClass {

@Autowired
private TestWired testWired;

public void showTestWired() {
    System.out.println(testWired.getTestInt());
}

}

Class to be autowired

import org.springframework.stereotype.Component;


 @Component
public class TestWired {

 int testInt = 234;

public int getTestInt() {
    return testInt;
}

public void setTestInt(int testInt) {
    this.testInt = testInt;
}
}

Error:
Exception in thread "main" java.lang.NullPointerException at DestClass.showTestWired(DestClass.java:13)

the line where i am trying to call the print the int is returning a null pointer exception

Answer: After few hours of looking around this is the way it works, as I cannot post my answer I am doing it here

public class SpringAutoWApplication {
public static void main(String[] args) {
    ConfigurableApplicationContext context = SpringApplication.run(SpringAutoWApplication.class, args);
    DestClass dc = context.getBean(DestClass.class);
    dc.showTestWired();
 }

}

Ok here goes the answer for this ` ConfigurableApplicationContext context = SpringApplication.run(SpringAutoWApplication.class, args); DestClass dc = context.getBean(DestClass.class);` With this you can create autowire. — Joe G, May 17 '18 at 20:04

Mạnh Quyết Nguyễn · Answer 1 · 2018-05-17T16:26:18.833

1

Autowire only work on spring bean.

Your DestClass is not a bean. Hence no autowire occur, @Autowire has no effect.

Mark your DestClass with @Component will do it:

@Component
public class DestClass {

Moreover, you're initialize your own instance, not get from spring bean so your test bean is null:

   DestClass dc = new DestClass();

Instead, autowire DestClass in your SpringAutoWApplication class.

@Autowired
DestClass dc;

Finally it can be test like:

@SpringBootApplication
public class SpringAutoWApplication {

 @Bean
 String test(DestClass dc) {
    dc.showTestWired();
    return "work";
 }

  public static void main(String[] args) {
    SpringApplication.run(SpringAutoWApplication.class, args);
  }
}

edited May 17 '18 at 16:26

answered May 17 '18 at 16:05

Mạnh Quyết Nguyễn

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I tried that too still the same error – Joe G May 17 '18 at 16:07
1

See my edited asnwer – Mạnh Quyết Nguyễn May 17 '18 at 16:10
Still its null `@ComponentScan @SpringBootApplication public class SpringAutoWApplication { @Autowired static DestClass dc; public static void main(String[] args) { SpringApplication.run(SpringAutoWApplication.class, args); dc.showTestWired(); } }` – Joe G May 17 '18 at 16:13
Remove the `static` modifier on `DestClass`. Can't autowire static field. – Mạnh Quyết Nguyễn May 17 '18 at 16:14
If I remove static from a static main I cannot reference a non static variable. – Joe G May 17 '18 at 16:15
Yes, of course. If you cannot set up another class for test. Define a `@Bean` method. `@Bean String getString(DestClass dc) { dc. showTestWired(); return "work";}` – Mạnh Quyết Nguyễn May 17 '18 at 16:17
And how do I call that? – Joe G May 17 '18 at 16:21
No need to call, @Bean method will be called upon start. Try it – Mạnh Quyết Nguyễn May 17 '18 at 16:21
Still a error, Is it working for you, can you pelase try and let me know SpringAutoWApplication.main(SpringAutoWApplication.java:17) – Joe G May 17 '18 at 16:23
See my edited answer – Mạnh Quyết Nguyễn May 17 '18 at 16:26
though it prints .. does that mean I cannot use these in any other classes apart from the main spring class? Also if I need multiple classes to be autowired all has to be defined in only one class? – Joe G May 17 '18 at 16:33
You can wire your `DestClass` in all bean class (annotated with @Component, @Service...). You can freely use them. My example is just used to show how it works. – Mạnh Quyết Nguyễn May 17 '18 at 16:34
What I was trying is using `TestWired` class as a autowired inside the `DestClass`, I want the `DestClass` to be normal and call methods on it as my starting string application in the question shows. `DestClass dc = new DestClass(); dc.showTestWired();` Can you let me know how I can do that? – Joe G May 17 '18 at 16:39
You cannot do that. You have to make DestClass a bean to autowire. – Mạnh Quyết Nguyễn May 17 '18 at 16:41
Adding @Component should make it a bean? or what are the other ways to make it a bean – Joe G May 17 '18 at 16:50
I have posted the answer. Thanks a lot for your support and help! – Joe G May 17 '18 at 20:10

Autowired returns null in spring boot

1 Answers1