Using JSP and JAVA

Question

I’m starting to mess with JSP. But I’ve been using JAVA for some time.

I have the index.jsp file which is my page and I have my indexJava.java which is the program itself. I would like to know how to join the two. For example, when I click on a button, it sends a variable to the . java and executes certain method that is related to this button.

Answer 1

The middle man is Servlet.

First, let’s name things. No project is too small to use the MVC design standard, which is short for Model - View - Controller, and that means your project must have these three elements.

Your application should look like this:

Model: Java indexes.
^{(I took the liberty of capitalizing on the first letter of your class to follow the Java nomenclature pattern that says which classes should start with uppercase)}
View: jsp result.
^{do not confuse with your index, which may be . html}
Controller: Meucontroller.java
^{(just an example name, I suggest you put something more intuitive depending on what your application is about)}

The Controller is what we call the Servlet, and the Servlet is a Java class. In the example above I called the Servlet Meucontroller.java. He, being a Java class, can communicate normally with his Model, as he said he has known Java for some time I believe that will not have difficulties in this part.

For a Java class to be a Servlet it must extend Httpservlet (or some other class that is a subclass in the Servlet interface hierarchy, but this is not the case). I recommend putting this class in a separate package of your Model to be very clear the distinction.

Your Servlet, by extending the Httpservlet class will have the possibility to override the methods doGet() and doPost() (among others, which are usually not used), which has the parameters Httpservletrequest and Httpservletresponse. These two parameters take two objects as argument, and who calls this method is Container (Tomcat), which is also responsible for creating the objects in question, consequently, Container has a reference to them, and when updating them in their Servlet class the Container will be aware of these modifications. This is how you communicate your Model classes with your View.

You still need to let your View know where your Servlet address, a flexible way to do this is by mapping the Servlet path through Deployment Descriptor, which is an XML file.

So that your homepage communicates with the methods doGet() and doPost() of your Servlet you must make a <form> in a . html file and submit directing to your Servlet, example:

<form method=POST action=SelectCoffee.do> 

By clicking the button submit you will send a POST type request to your Servlet, so your method doPost() will be triggered if you have not implemented the doPost() in your Servlet you will receive an error while trying to submit the form.

If you make your form so:

<form action=SelectCoffee.do> 

It will be of type GET, which is the default type. To work properly you will need to have implemented the method doGet() in his Servlet.

Selectcoffeee.do is the name I chose to give to Servlet, it is redirected to the class indicated in Deployment Descriptor.

Translating all this theory into practice, here is an example for you to use as a guide:

This model I created based on chapter 3 of the book "Head First - Servlets & JSP", which is my object of study.

And the contents of the files are:

Coffeeexpert.java

package com.example.model;
import java.util.ArrayList;
import java.util.List;
import javax.servlet.http.HttpServlet;

public class CoffeeExpert extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public List<String> getBrands(String type) {
        List<String> brands = new ArrayList<>();
        if(type.toLowerCase().equals("dark")) {
            brands.add("Toco");
            brands.add("3 Corações");
            brands.add("Pilão");
        }
        else if(type.toLowerCase().equals("cappuccino")) {
            brands.add("3 Corações");
            brands.add("Nescafé");
        }
        else if(type.toLowerCase().equals("expresso")) {
            brands.add("Do bar");
            brands.add("Da loja de carros");
        }
        return brands;
    }
}

Coffeeselect.java

package com.example.web;
import java.io.IOException;
import java.util.List;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.example.model.CoffeeExpert;

public class CoffeeSelect extends HttpServlet {
    private static final long serialVersionUID = 1L;

    public void doGet(HttpServletRequest request, 
            HttpServletResponse response) throws IOException, ServletException {
        String t = request.getParameter("type");
        CoffeeExpert ce = new CoffeeExpert();
        List<String> result = ce.getBrands(t);

        //--- LET THE FUN BEGIN! ---//
        request.setAttribute("styles", result);

        RequestDispatcher view = request.getRequestDispatcher("result.jsp");
        view.forward(request, response);
    }
}

web xml. (Deployment Descriptor)

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://java.sun.com/xml/ns/javaee"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    id="WebApp_ID" version="3.0">
    <servlet>
        <servlet-name>Ch3 Coffee</servlet-name>
        <servlet-class>com.example.web.CoffeeSelect</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>Ch3 Coffee</servlet-name>
        <url-pattern>/SelectCoffee.do</url-pattern>
    </servlet-mapping>
    <welcome-file-list>
        <welcome-file>form.html</welcome-file>
    </welcome-file-list>
</web-app>

html form.

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
    <h1 align=center>Coffee Selection Page</h1>
    <form method=GET action=SelectCoffee.do>
        <p>Select coffee type:</p>
        Type:
        <select name=type size=1>
        <option>Dark
        <option>Cappuccino
        <option>Expresso
        </select>
        <center><input type=submit></center>
    </form>
</body>
</html>

result.jsp

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1" import="java.util.*"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Meu JSP</title>
</head>
<body>
    <h1 align=center>Coffee Recomendations JSP</h1>
    <p>
        <%
            @SuppressWarnings("unchecked")
            List<String> styles = (List<String>) request.getAttribute("styles");
            for(String s: styles) {
                out.print("<p>Try > " + s + "</p>");
            }
        %>
    </p>
</body>

^{PS: in the book it was Beer instead of Coffee, but I changed it because I didn’t find it very appropriate in a work environment :)}

---

Receiving a vector in Servlet

In the above example I used:

String t = request.getParameter("type");

To receive the value of select in the initial html:

<select name=type size=1>

To take an array instead of a single value you basically have to change the getParameter() for getParameterValues(). See example:

<form method=GET action=Vector.do>
    <p>Select more than one option:</p>
    <input type=checkbox name=info value="opcao 1">op1
    <input type=checkbox name=info value="segunda opcao">op2
    <input type=checkbox name=info value=terceira>op3
    <input type=checkbox name=info value="ultima opcao">op4
    <center><input type=submit></center>
</form>

Servlet:

protected void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    String[] info = req.getParameterValues("info");
    for(String s: info) {
        System.out.println(s);
    }
}

Selecting the four options the program prints in the console:

option 1
second option
third
last option

Note that I added another Servlet to Deployment Descriptor, and its URL is "Vector.do".

Answer 2

Servlets are responsible for processing accesses to a given URL. Therefore, you need to create a Servlet to process the requests that will be made to the URL that your button points to.

In the code below, for example. All accesses to the URL http://meusite.com/home will be processed by the defined.

@WebServlet("/home")
public class ControllerServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // processar requisições GET aqui.
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // processar requisições POST aqui.
    }
}

You do not necessarily need to overwrite the methods listed above. There are other ways to process requests made to the URL assigned to Servlet. To better understand what these methods are, I recommend you take a look at Httpservlet class documentation.