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What is the purpose and usage of #ModelAttribute in Spring MVC?
#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".
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.
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<String> 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 Arrays.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"&countries=Lesoto&sponsor.organization="SilkRoad"&authorizedFunds=&authorizedHours=&
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...
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
<form:form commandName="Book" action="" methon="post">
<form:input type="text" path="title"></form:input>
</form:form>
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
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:
<form:form action="processForm" modelAttribute="student">
First Name : <form:input path="firstName" />
<br/><br/>
Last Name : <form:input path="lastName" />
<br/><br/>
<input type="submit" value="submit"/>
</form:form>
<form:input path="firstName" /> <form:input path="lastName" /> These are the fields/properties in the Student class. When the form is called/initialized their getters are invoked. On form submit their setters are invoked, and their values are transferred in the bean that was indicated with modelAttribute="student" in the form tag.
We have StudentController that includes the following methods:
#RequestMapping("/showForm")
// `Model` is used to pass data between controllers and views
public String showForm(Model theModel) {
// attribute name, value
theModel.addAttribute("student", new Student());
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:
<b>Student Information</b>
${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 like the #RequestParameter where we only get a parameter and assign the value to some method argument.
The difference is that #ModelAttribute holds all form data rather than a single parameter. It creates a bean for you which holds the data submitted in the form.
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 and forwarded to the form:
theModel.addAttribute("student", new Student());
Step 2:
Form attribute modelAttribute="student" defines that on form submission the model will update the student and will hold all parameters of the form.
Step 3:
On form submission the processForm() method is invoked with parameter #ModelAttribute("student") Student theStudent: the values being hold in the form with modelAttribute="student" were fetched and assigned to the fields in the Student object.
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
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!
#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/
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
<form:form action="test-example" method="POST" commandName="testModelAttribute"> </form:form>
(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<String, Object> map,...) {
}
Now when you will submit the form the form fields values will be available to you.
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) {
...
}
#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.
#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.
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
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";
}
First of all, Models are used in MVC Spring (MVC = Model, View, Controller). This said, models are used together with "views".
What are these views? Views can be though as "html pages that are generated by our backend framework (Spring in our case) with some variable data in some parts of the html page".
So we have the model, which is an entity containing the data do be "injected" into the view.
There are several "view" libraries that you can work with Spring: among which, JSP, Thymeleaf, Mustache and others.
For example, let us assume we are using Thymeleaf (they are all similar. What's more, Spring does not even know, except for JSP, with which view libraries he is working with. All the models are served through the Servlet of Spring. This means that the Spring code will be the same for all these view libraries. The only thing you need to change is the syntax of such html pages, which are located in resources/static/templates)
resources/static/templates //All our view web pages are here
A Controller takes care of the routes. Let's say for example we have our site hosted on localhost:8080. We want a route (URL) showing us the students. Let us say that this is available at localhost:8080/students. The controller that will do this is StudentController:
#Controller //Not #RestController
public class StudentController {
#GetMapping(/students)
public String getStudentView() {
return "student";
}
}
What this code does, is saying that, if we are going to
localhost:8080/students
then the method getStudentView() is called. But notice it should return a String. However, when working with a view library, and the controller is annotated with #Controller (and not #RestController), what spring does is looking for an html view page with the name of the String that the method returns, in our case it will look for the view at
/resources/static/templates/student.html
This is good enough for static pages without data. However, if we need a dynamic page with some data, Spring offers another big advantage: the method above getStudentView(), will also pass, under the hood, a model to our view "student.html". Our model will contain data that we can access in the file "student.html" using the specific syntax from our view library. E.g., with thymeleaf:
<div th:text="${attribute1}"> </div>
This will access the attribute "attribute1" of our model.
We can pass different data through our model. This is done by assigning it various attributes. There are different ways of assigning attributes, with #ModelAttribute:
#Controller //Not #RestController
public class StudentController {
#ModelAttribute(name = "attribute1")
public int assignAttribute1() {
return 123454321
} // Think it as "model.attribute1 = 123454321"
#GetMapping(/students)
public String getStudentView() {
return "student";
}
}
The code above will assign to the model (created under the hood), an attribute with name "attribute1" (think it as of a key), with value 12354321. Something like "model.attribute1 = 123454321".
Finally, the model is passed to the view when we go to the url
localhost:8080/students
Notice: all the methods annotated with #ModelAttribute are invoked before a view is returned. The model, once all the attributes are created, is passed to our view. simply put, after the method getStudentView() is called, all the method with #ModelAttribute are called.
This being said, the html code written above, will be viewed from the browser as:
<div> 123454321 </div> // th:text is a command of
//THymeleaf, and says to substitute the text
// between the tags with the attribute "attribute1"
// of our model passed to this view.
This is the basic usage of #ModelAttribute.
There is also another important use case:
The model might be needed in the opposite direction: i.e., from the view to the controller. In the case described above, the model is passed from the controller to the view. However, let us say that the user, from our html page, sends back some data. We can catch it with out model attributes, #ModelAttribute. This has already been described by others
I am going to implement following function:
As front-end tool, HtmlBox is used to edit text data and then transmit these data to back-end controller to save in database.
My question is:
How could the back-end controller receive the data edit and submit by HtmlBox?
According to the common coding way of Spring MVC I used, One class called Data is created with a private member variable named "articleContent", the same as the textarea id in the web page the HtmlBox embeded in. In my opinion, when the object of Data class is passed into the Controller as a parameter and the form, contain a HtmlBox with text data in it, is submitted, the data should be got by the controller easily as a String variable? But the fact isn't like this, the data object's articleContent is null?
How should controller get the data?
I have obtained the answwer of this question by myself!
In the front end Jsp view, the textarea enhanced by HtmlBox must use the "name" property or both "name" and "id" properties instead of "id" property alone! For instance:
<textarea id="theHtmlBoxName" name="theHtmlBoxName"></textarea>
In the back end controller, code as one of following:
public String saveTheHtmlBoxContent(#RequestParam String theHtmlBoxName){...};
or
public String saveTheHtmlBoxContent(HttpServletRequest request){
String data = request.getParameter("theHtmlBoxName");...};
Kenneth
Let say you have a web service that is built by applying RESTful principles. You have a URL that you can POST a new Article too. /articles is the path.
Each article consists of a title, date, description, etc... (primitive values) as well as more complex relations like the categories the article should be placed in, location, etc.
When you do a GET for the resource after it is created on e.g. /articles/123456 you get a response in JSON with all the primitive data like the title etc and simple fields that contain URLs for the location, categories etc. I do not embed those data in the article.
But what is the correct thing to do when you are creating a new article? Should I embed the categories, location etc (things that is more like relations to other compound objects) or should you first POST a new article with simple data then progressivly add data to it?
Such as /articles/123456/categories POST?
Or does this depend on the business requirements that states what data is required to create an article? What would you do?
Having mutable sub-resources (or collections) could be useful, or not. Think about the API consumer, how will he want to use your API? Also think about the lifecycle of the associated resources; can those entities live on their own? Considering these things should give you a fairly good idea of whether you need to be able to mutate those sub-resources/collections in their own right. Anyways, I'd say starting small, and POSTing those resources along with the article itself (and GETting them along with the article) in a sensible starting point. You can always add API methods to mutate/query sub-resources/collections later.
I think using using multiple POSTs to update/create each entities is simpler. If you're trying to do everything in a 'big bang' you might have to write several code to split the form object into multiple entities before adding them.
For example, assuming you're creating a new post and assign it two tags. Your HTML form is like this
<form ..>
<input name="postId" ../>
<input name="subject" ../>
<input name="content" ../>
<input name="tagIds" value="1001"../>
<input name="tagIds" value="1002"../>
</form>
(I believe) You cannot map this into Post and Tag entities in one go. So you have to create a form backing object that has all those properties:
public class NewPostForm {
private long postid;
private String subject;
private String content;
private List<String> tagIds;
..
}
Then before saving these into your persistence layer you have to slice them into a Post and Tags objects first.
#RequestMapping(..)
public String createNewPost(#ModelAttribute(..) NewPostForm form,..) {
Post newPost = .. // create post from form
List<Tag> tags = .. // create tag collections from form
// save through persistence layer
}
Whereas if you use separate POST per entity class you don't need this NewPostForm class.
I am trying to bind a spring form with a set in the command object.
In my command class AInstance I defined set as
private Set<BParameter> bParameters = new HashSet<BParameter>();
In jsp I bind it as
<form:input path="bParameters " />
<form:input path="bParameters " />
As its a Java Set so there may be many fields. When I submit this form I tried to get Set as:
Set<BParameter> bParameters = aInstance.getBParameters();
I got Set with a 0 size.
I also tried to bind as
<form:input path="bParameters[${itemsRow.index}].bParmvalues[0].parmValue" />
but got exception
Invalid property 'bParameters[0]' of bean class
What is the problem with my binding?
Use an List in the controller.
In the view you can use this straight html (not sure if this works with spring tags).
<input name="bParameters[{idx}].bParmvalues[0].parmValue" />
Its going to be an array, which Spring will translate into a List; it will also instantiate the List implementation - you don't need to do that in your Command object. Try using
private List<String> bParameters;
public void setBParameters(List<String> bParameters) {
this.bParameters= bParameters;
}
public List<String> getBParameters() {
return bParameters;
}
in your Command object. Those values are probably coming in as Strings.
I don't have problems binding as
private Set<Types> typeses = new HashSet<Types>(0);
<form:textarea path="typeses" style="width:200px;height:150px"/>
I use Spring 3.5. The only problem with this is, that it leaves []-marks on the field for some reason
I cannot find a way to bind Set into form parameters. However other solutions suggesting to "use list instead" is not good enough for me because if the field is part of JPA one-to-many relationship with eager fetching, using List will cause duplicates
Hence the best solution I find so far is by posting the form as JSON -- using Ajax -- instead. Here's how in JQuery:
var person = // .. construct form object here
$.ajax(url, {
method : 'post',
contentType : 'application/json',
data : JSON.stringify(person)
});
Note that you need to mark the contentType as application/json
On the controller side, you can bind this to a Java object by using #RequestBody annotation:
#RequestMapping(..)
public void save(#RequestBody Person person) {
..
}
More info here: https://gerrydevstory.com/2013/08/14/posting-json-to-spring-mvc-controller/
What is the purpose and usage of #ModelAttribute in Spring MVC?
#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".
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.
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<String> 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 Arrays.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"&countries=Lesoto&sponsor.organization="SilkRoad"&authorizedFunds=&authorizedHours=&
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...
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
<form:form commandName="Book" action="" methon="post">
<form:input type="text" path="title"></form:input>
</form:form>
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
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:
<form:form action="processForm" modelAttribute="student">
First Name : <form:input path="firstName" />
<br/><br/>
Last Name : <form:input path="lastName" />
<br/><br/>
<input type="submit" value="submit"/>
</form:form>
<form:input path="firstName" /> <form:input path="lastName" /> These are the fields/properties in the Student class. When the form is called/initialized their getters are invoked. On form submit their setters are invoked, and their values are transferred in the bean that was indicated with modelAttribute="student" in the form tag.
We have StudentController that includes the following methods:
#RequestMapping("/showForm")
// `Model` is used to pass data between controllers and views
public String showForm(Model theModel) {
// attribute name, value
theModel.addAttribute("student", new Student());
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:
<b>Student Information</b>
${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 like the #RequestParameter where we only get a parameter and assign the value to some method argument.
The difference is that #ModelAttribute holds all form data rather than a single parameter. It creates a bean for you which holds the data submitted in the form.
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 and forwarded to the form:
theModel.addAttribute("student", new Student());
Step 2:
Form attribute modelAttribute="student" defines that on form submission the model will update the student and will hold all parameters of the form.
Step 3:
On form submission the processForm() method is invoked with parameter #ModelAttribute("student") Student theStudent: the values being hold in the form with modelAttribute="student" were fetched and assigned to the fields in the Student object.
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
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!
#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/
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
<form:form action="test-example" method="POST" commandName="testModelAttribute"> </form:form>
(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<String, Object> map,...) {
}
Now when you will submit the form the form fields values will be available to you.
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) {
...
}
#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.
#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.
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
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";
}
First of all, Models are used in MVC Spring (MVC = Model, View, Controller). This said, models are used together with "views".
What are these views? Views can be though as "html pages that are generated by our backend framework (Spring in our case) with some variable data in some parts of the html page".
So we have the model, which is an entity containing the data do be "injected" into the view.
There are several "view" libraries that you can work with Spring: among which, JSP, Thymeleaf, Mustache and others.
For example, let us assume we are using Thymeleaf (they are all similar. What's more, Spring does not even know, except for JSP, with which view libraries he is working with. All the models are served through the Servlet of Spring. This means that the Spring code will be the same for all these view libraries. The only thing you need to change is the syntax of such html pages, which are located in resources/static/templates)
resources/static/templates //All our view web pages are here
A Controller takes care of the routes. Let's say for example we have our site hosted on localhost:8080. We want a route (URL) showing us the students. Let us say that this is available at localhost:8080/students. The controller that will do this is StudentController:
#Controller //Not #RestController
public class StudentController {
#GetMapping(/students)
public String getStudentView() {
return "student";
}
}
What this code does, is saying that, if we are going to
localhost:8080/students
then the method getStudentView() is called. But notice it should return a String. However, when working with a view library, and the controller is annotated with #Controller (and not #RestController), what spring does is looking for an html view page with the name of the String that the method returns, in our case it will look for the view at
/resources/static/templates/student.html
This is good enough for static pages without data. However, if we need a dynamic page with some data, Spring offers another big advantage: the method above getStudentView(), will also pass, under the hood, a model to our view "student.html". Our model will contain data that we can access in the file "student.html" using the specific syntax from our view library. E.g., with thymeleaf:
<div th:text="${attribute1}"> </div>
This will access the attribute "attribute1" of our model.
We can pass different data through our model. This is done by assigning it various attributes. There are different ways of assigning attributes, with #ModelAttribute:
#Controller //Not #RestController
public class StudentController {
#ModelAttribute(name = "attribute1")
public int assignAttribute1() {
return 123454321
} // Think it as "model.attribute1 = 123454321"
#GetMapping(/students)
public String getStudentView() {
return "student";
}
}
The code above will assign to the model (created under the hood), an attribute with name "attribute1" (think it as of a key), with value 12354321. Something like "model.attribute1 = 123454321".
Finally, the model is passed to the view when we go to the url
localhost:8080/students
Notice: all the methods annotated with #ModelAttribute are invoked before a view is returned. The model, once all the attributes are created, is passed to our view. simply put, after the method getStudentView() is called, all the method with #ModelAttribute are called.
This being said, the html code written above, will be viewed from the browser as:
<div> 123454321 </div> // th:text is a command of
//THymeleaf, and says to substitute the text
// between the tags with the attribute "attribute1"
// of our model passed to this view.
This is the basic usage of #ModelAttribute.
There is also another important use case:
The model might be needed in the opposite direction: i.e., from the view to the controller. In the case described above, the model is passed from the controller to the view. However, let us say that the user, from our html page, sends back some data. We can catch it with out model attributes, #ModelAttribute. This has already been described by others