Explain why constructor inject is better than other options

Question

In a Pro Spring 3 Book  Chapter 4 - Introduction IOC and DI in Spring - Page 59  In  Setter Injection vs  Constructor Injection  section  a paragraph says     Spring included  provide a mechanism for ensuring that all dependencies are defined when   you use Setter Injection  but by using Constructor Injection  you assert the requirement for the dependency in a container-agnostic manner    Could you explain with examples

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by using Constructor Injection  you assert the requirement for the dependency in a container-agnostic manner   This mean that you can enforce requirements for all injected fields without using any container specific solution   Setter injection example With setter injection special spring annotation  Required is required    Required Marks a method  typically a JavaBean setter method  as being  required   that is  the setter method must be configured to be dependency-injected with a value   Usage import org springframework beans factory annotation Required   import javax inject Inject  import javax inject Named    Named public class Foo        private Bar bar        Inject      Required     public void setBar Bar bar            this bar   bar           Constructor injection example All required fields are defined in constructor  pure Java solution  Usage import javax inject Inject  import javax inject Named    Named public class Foo        private Bar bar        Inject     public Foo Bar bar            this bar   bar            Unit testing This is especially useful in Unit Testing  Such kind of tests should be very simple and doesn t understand annotation like  Required  they generally not need a Spring for running simple unit test  When constructor is used  setup of this class for testing is much easier  there is no need to analyze how class under test is implemented

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A class that takes a required dependency as a constructor argument can only be instantiated if that argument is provided  you should have a guard clause to make sure the argument is not null   A constructor therefore enforces the dependency requirement whether or not you re using Spring  making it container-agnostic    If you use setter injection  the setter may or may not be called  so the instance may never be provided with its dependency  The only way to force the setter to be called is using  Required or  Autowired   which is specific to Spring and is therefore not container-agnostic   So to keep your code independent of Spring  use constructor arguments for injection   Update  Spring 4 3 will perform implicit injection in single-constructor scenarios  making your code more independent of Spring by potentially not requiring an  Autowired annotation at all

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Constructor injection is used when the class cannot function without the dependent class   Property injection is used when the class can function without the dependent class   As a concrete example  consider a ServiceRepository which depends on IService to do its work  Since ServiceRepository cannot function usefully without IService  it makes sense to have it injected via the constructor   The same ServiceRepository class may use a Logger to do tracing  The ILogger can be injected via Property injection    Other common examples of Property injection are ICache  another aspect in AOP terminology  or IBaseProperty  a property in the base class

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To make it simple  let us say that we can use constructor based dependency injection for mandatory dependencies and setter based injection for optional dependencies  It is a rule of thumb    Let s say for example   If you want to instantiate a class you always do it with its constructor  So if you are using constructor based injection  the only way to instantiate the class is through that constructor  If you pass the dependency through constructor it becomes evident that it is a mandatory dependency   On the other hand  if you have a setter method in a POJO class  you may or may not set value for your class variable using that setter method  It is completely based on your need  i e  it is optional  So if you pass the dependency through setter method of a class it implicitly means that it is an optional dependency  Hope this is clear

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By using Constructor Injection  you assert the requirement for the dependency in a container-agnostic manner   We need the assurance from the IoC container that  before using any bean  the injection of necessary beans must be done    In setter injection strategy  we trust the IoC container that it will first create the bean first but will do the injection right before using the bean using the setter methods  And the injection is done according to your configuration  If you somehow misses to specify any beans to inject in the configuration  the injection will not be done for those beans and your dependent bean will not function accordingly when it will be in use   But in constructor injection strategy  container imposes  or must impose  to provide the dependencies properly while constructing the bean  This was addressed as   container-agnostic manner   as we are required to provide dependencies while creating the bean  thus making the visibility of dependency  independent of any IoC container   Edit   Q1  And how to prevent container from creating bean by constructor with null values instead of missing beans   You have no option to really miss any  lt constructor-arg gt   in case of Spring   because you are imposed by IoC container to provide all the constructor arguments needed to match a provided constructor for creating the bean  If you provide null in your  lt constructor-arg gt  intentionally  Then there is nothing IoC container can do or need to do with it

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This example may help   Controller class    RestController  RequestMapping   abc dev    Scope value   WebApplicationContext SCOPE REQUEST  public class MyController     Setter Injection  Resource name  configBlack   public void setColor Color c        System out println  Injecting setter        this blackColor   c     public Color getColor         return this blackColor     public MyController         super       Color nred  Color nblack     Constructor injection  Autowired public MyController  Qualifier  constBlack  Color b   Qualifier  constRed  Color r        this nred   r      this nblack   b     private Color blackColor     Field injection  Autowired private Color black     Field injection  Resource name  configRed   private Color red    RequestMapping value     customers   produces      application text     method   RequestMethod GET   ResponseStatus value   HttpStatus CREATED  public String createCustomer         System out println  Field injection red      red getName         System out println  Field injection      black getName         System out println  Setter injection black      blackColor getName          System out println  Constructor inject nred      nred getName         System out println  Constructor inject nblack      nblack getName           MyController mc   new MyController        mc setColor new Red  No injection red         System out println  No injection       mc getColor   getName          return  Hello         Interface Color   public interface Color       public String getName        Class Red    Component public class Red implements Color  private String name    Override public String getName         return name     public void setName String name        this name   name     public Red String name        System out println  Red color     name       this name   name     public Red         System out println  Red color default constructor           Class Black    Component public class Black implements Color  private String name    Override public String getName         return name     public void setName String name        this name   name     public Black String name        System out println  Black color     name       this name   name     public Black         System out println  Black color default constructor           Config class for creating Beans    Configuration public class Config     Bean name    configRed   public Red getRedInstance         Red red   new Red        red setName  Config red        return red      Bean name    configBlack   public Black getBlackInstance         Black black   new Black        black setName  config Black        return black      Bean name    constRed   public Red getConstRedInstance         Red red   new Red        red setName  Config const red        return red      Bean name    constBlack   public Black getConstBlackInstance         Black black   new Black        black setName  config const Black        return black        BootApplication  main class     SpringBootApplication  ComponentScan basePackages     com    public class BootApplication    public static void main String   args        SpringApplication run BootApplication class  args            Run Application and hit URL    GET 127 0 0 1 8080 abc dev customers    Output  Injecting setter Field injection red  Config red Field injection  null Setter injection black  config Black Constructor inject nred  Config const red Constructor inject nblack  config const Black Red color  No injection red Injecting setter No injection   No injection red

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With examples   Here s a simple one   public class TwoInjectionStyles       private Foo foo          Constructor injection     public TwoInjectionStyles Foo f            this foo   f                Setting injection     public void setFoo Foo f    this foo   f        Personally  I prefer constructor injection when I can   In both cases  the bean factory instantiates the TwoInjectionStyles and Foo instances  and gives the former its Foo dependency

[java] Explain why constructor inject is better than other options

Setter injection example

Constructor injection example

Unit testing

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