[FIXED] When retrieving existing Mongo DB Collection gives null pointer exception

Issue

I have a created MongoDB with data, But when retrieving all documents in a specific collection my program gives a null pointer exception.

Here is my code, UserRepository.java

package com.weatherdata.api.dbconnector;

import com.weatherdata.api.users.User;
import org.springframework.data.mongodb.repository.MongoRepository;
import org.springframework.stereotype.Repository;

import java.util.List;

@Repository
public interface UserRepository extends MongoRepository<User,String> {
    
}

UserSeeder.java

package com.weatherdata.api.users;

import com.weatherdata.api.dbconnector.UserRepository;
import org.springframework.beans.factory.annotation.Autowired;

import java.util.List;

public class UserSeeder {
    @Autowired
    UserRepository userRepository;

    public List<User> getAllUsers(){
        List <User> allUsers = this.userRepository.findAll();
        return allUsers;
    }
}

User.java

package com.weatherdata.api.users;


import org.springframework.data.annotation.Id;
import org.springframework.data.mongodb.core.mapping.DBRef;
import org.springframework.data.mongodb.core.mapping.Document;

import java.util.HashSet;
import java.util.Set;
@Document(collection = "users")
public class User {
    @Id
    private String id;
    private String username;
    private String email;
    private String mobile;
    private String method;
    private String password;

    @DBRef
    private Set<Role> roles = new HashSet<>();

    public User() {
    }

    public User(String username, String email,String mobile,String method, String password) {
        this.username = username;
        this.email = email;
        this.mobile = mobile;
        this.method = method;
        this.password = password;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getMobile() {
        return mobile;
    }

    public void setMobile(String mobile) {
        this.mobile = mobile;
    }

    public String getMethod() {
        return method;
    }

    public void setMethod(String method) {
        this.method = method;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public Set<Role> getRoles() {
        return roles;
    }

    public void setRoles(Set<Role> roles) {
        this.roles = roles;
    }
}

Error is,

java.lang.NullPointerException: Cannot invoke "com.weatherdata.api.dbconnector.UserRepository.findAll()" because "this.userRepository" is null at com.weatherdata.api.users.UserSeeder.getAllUsers(UserSeeder.java:13) ~[classes/:na] at com.weatherdata.api.filter.Alert.isExceeded(Alert.java:20) ~[classes/:na] at com.weatherdata.api.controller.SensorController.insert(SensorController.java:56) ~[classes/:na] at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na] at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:64) ~[na:na] at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na] at java.base/java.lang.reflect.Method.invoke(Method.java:564) ~[na:na] at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:197) ~[spring-web-5.3.2.jar:5.3.2] at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:141) ~[spring-web-5.3.2.jar:5.3.2] at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:106) ~[spring-webmvc-5.3.2.jar:5.3.2] at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:894) ~[spring-webmvc-5.3.2.jar:5.3.2] at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808) ~[spring-webmvc-5.3.2.jar:5.3.2] at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.3.2.jar:5.3.2] at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1061) ~[spring-webmvc-5.3.2.jar:5.3.2] at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:961) ~[spring-webmvc-5.3.2.jar:5.3.2] at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) ~[spring-webmvc-5.3.2.jar:5.3.2] at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:909) ~[spring-webmvc-5.3.2.jar:5.3.2]

But the thig is same function in rest api is working finely.

This is how I use UserSedder in Alert.java

package com.weatherdata.api.filter;

import com.weatherdata.api.users.User;
import com.weatherdata.api.users.UserSeeder;

import java.util.List;

public abstract class Alert implements AlertType {
    private double thresholdValue;
    private List<User> userList;

    @Override
    public double getThreshold() {
        return this.thresholdValue;
    }

    @Override
    public boolean isExceeded(double dataValue) {
        if(thresholdValue < dataValue){
            userList = new UserSeeder().getAllUsers();
            System.out.println(userList.stream().count());
            return true;
        }
        else{
            return false;
        }
    }

    public double getThresholdValue() {
        return thresholdValue;
    }

    public void setThresholdValue(double thresholdValue) {
        this.thresholdValue = thresholdValue;
    }
}

Solution

You must use SterioType Annotation @Service over a Classes which has business logic and @Component over a Classes which is generic to all layers of application or Utility classes cause when you annotate @Springbootapplication over the main class it has underlying @ComponentScan(basePackages=" package name where main class lies ") it automatically creates the instance of classes which are under the basepackage.

Answered By - Shivam Patil