Yet Another Blog

Constructor based dependency injection is the recommended way to use dependency injection in Spring. In this post I'm going to demonstrate how to write unit test of a service class which has dependencies on other beans e.g. repositories. Let's assume our service class is like the following: @Service class MyService { private final UserRepository userRepository;     private final AddressRepository addressRepository;     @Autowired     public MyService(UserRepository userRepository, AddressRepository addressRepository) {         this.userRepository = userRepository;         this.addressRepository = addressRepository;     }     public boolean isUserHasAddress(String username){     // omitting the method details, will return true always     return true;     } } Now we need to write unit test for the above class. Here is a way to do it: @RunWith(S...

We often need to generate a spreadsheet file from server and allow users to download the file. Here is how to do that with Spring using the Apache POI library. Dependency: We need to add the following dependencies in our spring application. Here is the Gradle dependencies. compile group: 'org.apache.poi', name: 'poi', version: '3.15' compile group: 'org.apache.poi', name: 'poi-ooxml', version: '3.15' Once we add the dependencies we'll be able to access the POI library in our application. Generate WorkBook We need to generate a Workbook object in POI. Here is a sample method that returns a Workbook object public Workbook downloadPersonList() throws IOException {     // sample list. this can be the output of a DB query     List<Person> personList = new ArrayList<>();       personList.add(new Person("nayan", 29));     personList.add(new Person("rafiq", 35)) ;     personList.add(ne...

It's possbile to map GET parameters to an object in Spring. It's really useful when we need to pass lots of parameters as query string e.g. submitting a search form with many parameters. The following controller demonstrates this.   @RestController @RequestMapping("hello") public class MyController {     @RequestMapping(value = "data", method = RequestMethod.GET)     public MyRequest getData(MyRequest request){             return request;     }     public static class  MyRequest {         private String name;          private int age;         public String getName(){return this.name;}         public int getAge(){return this.name;}         public void setName(String name){this.name=name;}          public void setAge(int age){this.age=ag...

Why background task? There are tons of examples where we often need to run a task in background e.g. sending an email, sms or notification, writing some log in file etc. If a task is not required to maintain atomicity and can be executed after some delay, it's a good candidate to run in background.   How to run tasks in background in Java? In Java, the easiest way to run a task in background is to create a new thread and execute it. It's quite simple. Here is how to do this: new Thread(new Runnable(){ @Override public void run(){ // your background task } }).start(); It's easy! But there are some maintenance problem with the above code. The major problem is - there is no way to control how many background tasks we want to run simultaneously. For example, if 1000 users registers in your application at a time and you create 1000 background threads to send them a confirmation email, maybe you'll be in trouble. So you ne...