What is ModelAttribute in Spring MVC

Question

What is the purpose and usage of  ModelAttribute in Spring MVC

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ModelAttribute can be used as the method arguments   parameter or before the method declaration   The primary objective of this annotation to bind the request parameters or form fields to an model object  Ref  http   www javabeat net modelattribute-spring-mvc

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ModelAttribute refers to a property of the Model object  the M in MVC    so let s say we have a form with a form backing object that is called  Person  Then you can have Spring MVC supply this object to a Controller method by using the  ModelAttribute annotation   public String processForm  ModelAttribute  person   Person person       person getStuff        On the other hand the annotation is used to define objects which should be part of a Model  So if you want to have a Person object referenced in the Model you can use the following method    ModelAttribute  person   public Person getPerson        return new Person        This annotated method will allow access to the Person object in your View  since it gets automatically added to the Models by Spring    See  Using  ModelAttribute

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ModelAttribute will create a attribute  with the name specified by you   ModelAttribute  Testing   Test test  as Testing  in the given example  Test being the bean test being the reference to the bean and Testing will be available in model so that you can further use it on jsp pages for retrieval of values that you stored in you ModelAttribute

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For my style  I always use  ModelAttribute to catch object from spring form jsp  for example  I design form on jsp page  that form exist with commandName   lt form form commandName  Book  action    methon  post  gt         lt form input type  text  path  title  gt  lt  form input gt   lt  form form gt    and I catch the object on controller with follow code  public String controllerPost  ModelAttribute  Book   Book book    and every field name of book must be match with path in sub-element of form

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Take any web application whether it is Gmail or Facebook or Instagram or any other web application  it s all about exchanging data or information between the end user and the application or the UI and the back end application  Even in Spring MVC world there are two ways to exchange data    from the Controller to the UI  and  from the UI to the Controller    What we are interested here is how the data is communicated from the UI to Controller  This can also be done in 2 ways    Using an HTML Form Using Query Parameters    Using an HTML Form  Consider the below scenario      When we submit the form data from the web browser  we can access that data in our Controller class as an object  When we submit an HTML form  the Spring Container does four things  It will    first read all the data that is submitted that comes in the request using the request getParameter method  once it reads them  it will convert them into the appropriate Java type using integer parseInt  double parseDouble and all the other parse methods that are available based on the data type of the data   once parsed  it will create a object of the model class that we created  For example  in this scenario  it is the user information that is being submitted and we create a class called User  which the Container will create an object of and it will set all the values that come in automatically into that object    it will then handover that object by setting the values to the Controller    To get this whole thing to work  we ll have to follow certain steps     We first need to define a model class  like User  in which the number of fields should exactly match the number of fields in the HTML form  Also  the names that we use in the HTML form should match the names that we have in the Java class  These two are very important  Names should match  the number of fields in the form should match the number of fields in the class that we create  Once we do that  the Container will automatically read the data that comes in  creates an object of this model  sets the values and it hands it over to the Controller  To read those values inside the Controller  we use the  ModelAttribute annotation on the method parameters  When we create methods in the Controller  we are going to use the  ModelAttribute and add a parameter to it which will automatically have this object given by the Container   Here is an example code for registering an user    RequestMapping value    registerUser   method   RequestMethod POST  public String registerUser  ModelAttribute  user   User user  ModelMap model        model addAttribute  user   user       return  regResult       Hope this diagrammatic explanation helped

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ModelAttribute simply binds the value from jsp fields to Pojo calss to perform our logic in controller class  If you are familiar with struts  then this is like populating the formbean object upon submission

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At the Method Level  1 When the annotation is used at the method level it indicates the purpose of that     method is to add one or more model attributes   ModelAttribute public void addAttributes Model model    model addAttribute  india    india        At the Method Argument  1  When used as a method argument  it indicates the argument should be retrieved from the model  When not present and  should be first instantiated and then added to the model and once present in the model  the arguments fields should be populated from all  request parameters that have matching names So  it binds the form data with a bean     RequestMapping value     addEmployee   method   RequestMethod POST    public String submit  ModelAttribute  employee   Employee employee      return  employeeView

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This is used for data binding purposes in Spring MVC  Let you have a jsp having a form element in it e g     on JSP    lt form form  action  test-example  method  POST     commandName  testModelAttribute  gt   lt  form form gt    Spring Form method  Simple form element can also be used      On Controller Side    RequestMapping value     test-example   method   RequestMethod POST  public ModelAndView testExample  ModelAttribute  testModelAttribute   TestModel testModel  Map lt String  Object gt  map             Now when you will submit the form the form fields values will be available to you

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So I will try to explain it in simpler way  Let s have   public class Person       private String name       public String getName             return name             public void setName final String name            this name   name            As described in the Spring MVC documentation - the  ModelAttribute annotation can be used on methods or on method arguments  And of course we can have both use at the same time in one controller   1 Method annotation   ModelAttribute    cities      public List lt String gt  checkOptions     return new Arrays asList new      Sofia       Pleven   Ruse        and so on     Purpose of such method is to add attribute in the model  So in our case cities key will have the list new Arras asList new      Sofia       Pleven   Ruse      as value in the Model  you can think of Model as map key value      ModelAttribute methods in a controller are invoked before  RequestMapping methods  within the same controller    Here we want to add to the Model common information which will be used in the form to display to the user  For example it can be used to fill a HTML select     2 Method argument  public String findPerson  ModelAttriute value  person   Person person            Some logic with person     return  person jsp       An  ModelAttribute on a method argument indicates the argument should be retrieved from the model  So in this case we expect that we have in the Model  person object as key and we want to get its value and put it to the method argument Person person  If such does not exists or  sometimes you misspell the  value  persson    then Spring will not find it in the Model and will create empty Person object using its defaults  Then will take the request parameters and try to data bind them in the Person object using their names   name  Dmitrij  amp countries Lesoto amp sponsor organization  SilkRoad  amp authorizedFunds  amp authorizedHours  amp    So we have name and it will be bind to Person name using setName String name   So in       Some logic with person   we have access to this filled name with value  Dimitrij    Of course Spring can bind more complex objects like Lists  Maps  List of Sets of Maps and so on but behind the scene it makes the data binding magic    We can have at the same time model annotated method and request method handler with  ModelAttribute in the arguments  Then we have to union the rules   Of course we have tons of different situations -  ModelAttribute methods can also be defined in an  ControllerAdvice and so on

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I know this is an old thread  but I thought I throw my hat in the ring and see if I can muddy the water a little bit more     I found my initial struggle to understand  ModelAttribute was a result of Spring s decision to combine several annotations into one  It became clearer once I split it into several smaller annotations   For parameter annotations  think of  ModelAttribute as the equivalent of  Autowired    Qualifier i e  it tries to retrieve a bean with the given name from the Spring managed model  If the named bean is not found  instead of throwing an error or returning null  it implicitly takes on the role of  Bean i e  Create a new instance using the default constructor and add the bean to the model   For method annotations  think of  ModelAttribute as the equivalent of  Bean    Before  i e  it puts the bean constructed by user s code in the model and it s always called before a request handling method   Figuratively  I see  ModelAttribute as the following  please don t take it literally       Bean  person    Before public Person createPerson      return new Person        RequestMapping      public xxx handlePersonRequest    Autowired  Qualifier  person      Bean  person    Person person  xxx             As you can see  Spring made the right decision to make  ModelAttribute an all-encompassing annotation  no one wants to see an annotation smorgasbord

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I know I am late to the party  but I ll quote like they say   better be late than never   So let us get going  Everybody has their own ways to explain things  let me try to sum it up and simple it up for you in a few steps with an example  Suppose you have a simple form  form jsp   lt form form action  processForm  modelAttribute  student  gt  First Name    lt form input path  firstName    gt    lt br gt  lt br gt  Last Name    lt form input path  lastName    gt   lt br gt  lt br gt   lt input type  submit  value  submit   gt   lt  form form gt    path  firstName  path  lastName  These are the fields properties in the StudentClass when the form is called their getters are called but once submitted their setters are called and their values are set in the bean that was indicated in the modelAttribute  student  in the form tag   We have StudentController that includes the following methods    RequestMapping   showForm   public String showForm Model theModel     Model is used to pass data between    controllers and views     theModel addAttribute  student   new Student       attribute name  value return  form       RequestMapping   processForm   public String processForm  ModelAttribute  student   Student theStudent       System out println  theStudent     theStudent getLastName     return  form-details         ModelAttribute  student   Student theStudent   Spring automatically populates the object data with form data all behind the    scenes    now finally we have a form-details jsp   lt b gt Student Information lt  b gt    student firstName    student lastName    So back to the question What is  ModelAttribute in Spring MVC  A sample definition from the source for you  http   www baeldung com spring-mvc-and-the-modelattribute-annotation The  ModelAttribute is an annotation that binds a method parameter or method return value to a named model attribute and then exposes it to a web view   What actually happens is it gets all the values of your form those were submitted by it and then holds them for you to bind or assign them to the object   It works same like the  RequestParameter where we only get a parameter and assign the value to some field   Only difference is  ModelAttribute holds all form data rather than a single parameter  It creates a bean for you that holds form submitted data to be used by the developer later on   To recap the whole thing  Step 1    A request is sent and our method showForm runs and a model  a temporary bean is set with the name student is forwarded to the form  theModel addAttribute  student   new Student      Step 2   modelAttribute  student   on form submission model changes the student and now it holds all parameters of the form  Step 3    ModelAttribute  student   Student theStudent We fetch the values being hold by  ModelAttribute and assign the whole bean object to Student   Step 4   And then we use it as we bid  just like showing it on the page etc like I did  I hope it helps you to understand the concept  Thanks

User · Answer

Annotation that binds a method parameter or method return value to a named model attribute  exposed to a web view    public String add  ModelAttribute  specified   Model model

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The ModelAttribute annotation is used as part of a Spring MVC Web application and can be used in two scenarios   First of all  it can be used to inject data into a pre-JSP load model  This is especially useful in ensuring that a JSP is required to display all the data itself  An injection is obtained by connecting one method to the model   Second  it can be used to read data from an existing model and assign it to the parameters of the coach s method   refrence https   dzone com articles using-spring-mvc E2 80 99s

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