[FIXED] Error creating bean while using Autowired

Issue

I have a simple Springboot application in which I am trying to use in memory HashMap as a DB to store some info. This is my restcontroler

    @RestController
public class Parking {

    @Autowired
    ParkingServices parkingServices;

    @Autowired
    Vehicle vehicle;

    @GetMapping("/test")
    public void park() {
        vehicle = Vehicle.builder().color("Blue").plateNumber("qwew").build();
        parkingServices.parkCar(vehicle);
    }

    @GetMapping("/")
    public void Test() {
        System.out.println("test");
    }
}

This is my simple parking service

   @Service
public class ParkingServices {

    @Autowired
    ParkingLotService parkingLot;

    public ParkingServices() {
        parkingLot.init(10);
    }

    public int parkCar(Vehicle vehicle) {
        return parkingLot.parkCar(vehicle);
    }
}

And this is my ParkingLot service

  @Service
public class ParkingLotService {
    private int size = 0;
    private HashMap<Integer, Slot> parkingSlots = new HashMap<>();
    private Stack<Integer> st = new Stack<>();

    public void init(int size) {
        this.size = size;
        parkingSlots = new HashMap<>();
        for (int i = 0; i < size; i++) {
            parkingSlots.put(i, new ParkingSlot());
            st.push(i);
        }
    }

    public int parkCar(Vehicle vehicle) {
        int firstPark = st.pop();
        Slot slot = parkingSlots.get(firstPark);
        slot.parkVehicle(vehicle);
        parkingSlots.put(firstPark, slot);
        return firstPark;
    }

    public void removeCar(int parkingNumber) {
        Slot slot = parkingSlots.get(parkingNumber);

    }
}

Now when i try to run the application I am getting this error as a stack trace

    org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'parking': Unsatisfied dependency expressed through field 'parkingServices'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'parkingServices' defined in file [D:\Projects\parking\demo\target\classes\com\parking\demo\Services\ParkingServices.class]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.parking.demo.Services.ParkingServices]: Constructor threw exception; nested exception is java.lang.NullPointerException
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:660) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:640) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:119) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessProperties(AutowiredAnnotationBeanPostProcessor.java:399) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1413) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:601) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:524) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:944) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:918) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:583) ~[spring-context-5.3.5.jar:5.3.5]
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:144) ~[spring-boot-2.4.4.jar:2.4.4]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:769) [spring-boot-2.4.4.jar:2.4.4]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:761) [spring-boot-2.4.4.jar:2.4.4]
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:426) [spring-boot-2.4.4.jar:2.4.4]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:326) [spring-boot-2.4.4.jar:2.4.4]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1313) [spring-boot-2.4.4.jar:2.4.4]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1302) [spring-boot-2.4.4.jar:2.4.4]
    at com.parking.demo.ParkingApplication.main(ParkingApplication.java:11) [classes/:na]
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'parkingServices' defined in file [D:\Projects\parking\demo\target\classes\com\parking\demo\Services\ParkingServices.class]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.parking.demo.Services.ParkingServices]: Constructor threw exception; nested exception is java.lang.NullPointerException
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1316) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1214) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:564) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:524) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:335) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:234) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:333) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:208) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1380) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1300) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.resolveFieldValue(AutowiredAnnotationBeanPostProcessor.java:657) ~[spring-beans-5.3.5.jar:5.3.5]
    ... 21 common frames omitted
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.parking.demo.Services.ParkingServices]: Constructor threw exception; nested exception is java.lang.NullPointerException
    at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:225) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:87) ~[spring-beans-5.3.5.jar:5.3.5]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1308) ~[spring-beans-5.3.5.jar:5.3.5]
    ... 32 common frames omitted
Caused by: java.lang.NullPointerException: null
    at com.parking.demo.Services.ParkingServices.<init>(ParkingServices.java:15) ~[classes/:na]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_252]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_252]
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_252]
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_252]
    at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:212) ~[spring-beans-5.3.5.jar:5.3.5]
    ... 34 common frames omitted

I know its due to Autowired is not working on ParkingLotService, its ability to work if instead of Autowired in ParkingService, I directly try to initialize Object but not the other way around. Can someone help me with it ? Why Autowired is not working and how to make it work.

Solution

Spring @Autowired field injection after construction invoked.

See:

Autowired Fields
Fields are injected right after construction of a bean, before any config methods are invoked. Such a config field does not have to be public.

Spring Autowired

So for your scenario, you can use:

1. Autowired Constructors to inject ParkingLotService and invoke the init method.

    @Autowired
    public ParkingServices(ParkingLotService parkingLot) {
        parkingLot.init(10);
        this.parkingLot = parkingLot;
    }

2. use @PostConstruct method to invoke the init method

@PostConstruct
public void myInit() {
  parkingLot.init(10);
}

Answered By - chengpohi