devkuma – JSP

JSP/Servlet | Preparing for Google App Engine Development

kc@example.com (kc kim) — Sun, 15 Oct 2017 15:34:56 +0900

This section explains server-side Java and Google App Engine. It also covers the preparation needed to use GAE and start actual development.

JSP/Servlet | Preparing for Google App Engine Development | Using Server-Side Java

kc@example.com (kc kim) — Sun, 15 Oct 2017 15:34:56 +0900

As the Web has evolved and more interesting services have appeared, more people have started thinking, “I want to build a Web application myself.” Unlike a simple Web page, if you want to create an interesting or useful service, you inevitably need server-side development.

Many languages can now be used for server-side development. There are scripting languages such as Perl, PHP, Ruby, and Python, but when development reaches a certain scale, Java still stands out. Java is also used in many other fields, so many people probably think, “Looking ahead, Java is still the right choice.”

It is fine to decide on Java server development for various reasons, but where can you actually use server-side Java? In reality, not many rental servers support Java. Perl and PHP are integrated into Web servers and used there, but Java requires a dedicated Java server to be set up and operated, so rental servers do not easily support it. Even if they do support it, the cost is higher than for other languages.

Even if you want to use Java, there are very few easy-to-use rental server environments that support it. Therefore, at an individual level, server-side Java is not easy to use.

In that situation, a savior appeared for people who wanted to develop servers with Java. That was Google App Engine (GAE).

Google App Engine is a cloud service provided by Google. It is a so-called PaaS (Platform as a Service), and you can start for free by creating applications that use features such as Google’s Bigtable and Google accounts.

This was probably the first server-side Java environment that anyone could use for free. Without detailed server-side configuration, you could develop and deploy with tools provided by Google and create a Web application as-is. It is no exaggeration to say that there had not been such an easy-to-use server-side Java environment before.

If you want to try server-side Java, GAE can be considered a very suitable environment. Since such a good environment can be used for free, let’s use it to start server-side Java.

Preparation Before Starting

What is needed to start server-side Java with GAE? The required items are summarized below.

JDK

This is the Java development environment. It is currently distributed from the Oracle site, which owns Java.

Eclipse

This is a Java development tool. There are many development tools, but if you use GAE, you should use Eclipse. It is also distributed for free from the Eclipse Foundation site, its developer.

SDK/Plugin

Prepare the GAE for Java SDK, which is like a development kit, and plugins that add GAE/Java development features to Eclipse. These can be installed after starting Eclipse.

Google and GAE Accounts

In addition to software, you need a Google account and a GAE account. Because you will use GAE, these must be available.

Basic Knowledge of Java SE

The other required item is knowledge of Java SE. From here on, server-side Java will be explained on the assumption that you already know the basics of Java. If you do not yet know Java basics, first learn the fundamentals of Java.

JSP/Servlet

kc@example.com (kc kim) — Sun, 15 Oct 2017 15:34:56 +0900

JSP/Servlet | Creating a GAE Application

kc@example.com (kc kim) — Sun, 15 Oct 2017 15:34:56 +0900

Once the development setup is ready, create a GAE application in Eclipse. Then deploy it to the GAE site and run it.

JSP/Servlet | JSP(Java Server Pages)

kc@example.com (kc kim) — Sun, 15 Oct 2017 15:34:56 +0900

The most fundamental part of server-side Java can be called “JSP”. It lets you embed Java code in HTML by using special tags. First, use JSP to run server-side Java.

JSP/Servlet | Passing Values Between Client and Server

kc@example.com (kc kim) — Sun, 15 Oct 2017 15:34:56 +0900

You can now output values from the server to the client. Then how do you send the necessary information from the client to the server? The basics are “query strings” and “forms”. This section explains how to pass data with these two methods.

JSP/Servlet | Sessions and Cookies

kc@example.com (kc kim) — Sun, 15 Oct 2017 15:34:56 +0900

On the web, “cookies” are used as a feature that can store a small amount of data in the browser. There is also a feature called a “session” that keeps various data while connected to the server. This section explains these methods.

JSP/Servlet | Servlet

kc@example.com (kc kim) — Sun, 15 Oct 2017 15:34:56 +0900

The technology used on the server side together with JSP is the “Servlet”. This section explains the basics of servlets.

JSP/Servlet | Servlets and JSP/HTML

kc@example.com (kc kim) — Sun, 15 Oct 2017 15:34:56 +0900

Servlets are usually used to exchange data between HTML or JSP pages and servlets, rather than being used alone. This section explains the features needed to exchange and share data between them, including storing values in request, session, and application scopes, as well as forward and redirect.

JSP/Servlet | web.xml DTD by Version

kc@example.com (kc kim) — Sun, 15 Oct 2017 15:34:56 +0900

Servlet 2.2

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE web-app PUBLIC
"-//Sun Microsystems, Inc.//DTD Web Application 2.2//EN"
"http://java.sun.com/j2ee/dtds/web-app_2_2.dtd">

<web-app />

Servlet 2.3

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE web-app PUBLIC
"-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
"http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app />

Servlet 2.4

Starting with this version, it changed from DTD to schema.

<?xml version="1.0" encoding="UTF-8"?>
<web-app  xmlns="http://java.sun.com/xml/ns/j2ee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee
    http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
    id="servlet-2_4" version="2.4">

</web-app>

Servlet 2.5

<?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"     
    xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
    http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
   id="servlet-2_5" version="2.5">

</web-app>

Servlet 3.0

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
    http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    version="3.0">

</web-app>

Servlet 3.1

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
 http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
  version="3.1">
  
</web-app>

JSP/Servlet | HTTPServletRequest Methods

kc@example.com (kc kim) — Sun, 15 Oct 2017 15:34:56 +0900

The request object is used to get values, cookies, headers, and client information sent with an HTTP request.

This page summarizes the methods of the Request object.

Before You Start

The return value examples assume that the following URL was received.

http://www.devkuma.com:8080/app/request.jsp?name=devkuma

Main Information

Method	Example return value	Description
request.getRequestURL()	http://www.devkuma.com:8080/app/request.jsp
request.getRequestURI()	/app/request.jsp	The remaining address and parameters after excluding the scheme, server name, and port number from the URL
request.getScheme()	http	Protocol such as http, https, or ftp
request.getServerName()	www.devkuma.com	Server name
request.getServerPort()	8080	Server port
request.getContextPath()	/app	Context path
request.getMethod()	GET	HTTP method such as GET or POST
request.isSecure()	false	Whether SSL security is enabled. Returns true/false depending on whether a secure channel such as https is used
request.getLocale()	ko_KR	Locale information
request.getProtocol()	HTTP/1.1	Protocol in use. Protocol/major version.minor version

Basic Local Information (Server Information)

Method	Example return value	Description
request.getLocalAddr()	127.0.0.1
request.getLocalName()	localhost
request.getLocalPort()	8080

Remote Information (Client Information)

Method	Example return value	Description
request.getRemoteAddr()	127.0.0.1
request.getRemoteHost()	127.0.0.1
request.getRemotePort()	60361

Session ID

Method	Example return value	Description
request.getRequestedSessionId()	39AE7AC19BC174D803C2A7BD8638E382	Session ID
request.getSession().getId()	39AE7AC19BC174D803C2A7BD8638E382	Session ID
request.isRequestedSessionIdFromCookie()	true	Whether the session ID was provided by a cookie
request.isRequestedSessionIdFromURL()	false	Whether the session ID was provided as part of the URL
request.isRequestedSessionIdValid()	true	Whether the session is still valid

Parameter Information

Method	Example return value	Description
request.getParameterNames()	java.util.Enumeration	Returns all parameter names.
request.getParameter(name)	devkuma	Returns the parameter value by parameter name.

Display All Parameter Values

Enumeration params = request.getParameterNames();
while (params.hasMoreElements()) {
    String name = (String) params.nextElement();
    String value = request.getParameter(name);
    logger.debug(name + "=" + value);
}

The displayed return value is as follows.

name=devkuma

The return value is displayed as above because the request was made as /app/request.jsp?name=devkuma.

Method	Example return value	Description
request.getCookies()	Array	Returns all cookie values.

Cookie cookies[] = request.getCookies();
for(int i = 0; i < cookies.length; i++) {
    String name = cookies[i].getName();
    String value = cookies[i].getValue();
    logger.debug(name + "=" + value);
}

The displayed return value is as follows.

JSESSIONID=39AE7AC19BC174D803C2A7BD8638E382

In this return example, only JSESSIONID is displayed because no additional cookie value was set.

Attribute

Method	Example return value	Description
request.getAttributeNames()	Array	Returns all attribute names.
request.getAttribute(name)	…	Returns the value of the attribute.

Display All Attribute Values

Enumeration<String> attrs = request.getAttributeNames();
while (attrs.hasMoreElements()) {
    String name = (String)attrs.nextElement();
    String value = (String)request.getAttribute(name);
    logger.debug(name + " : " + value);
}

Header Information

Method	Example return value	Description
request.getHeaderNames()	java.util.Enumeration	Returns all header names.
request.getHeader(“user-agent”)	Mozilla/5.0 ….	Returns a header value by header name.

Display All Header Values

Enumeration<String> headers = request.getHeaderNames();
while (headers.hasMoreElements()) {
    String name = (String) headers.nextElement();
    String value = request.getHeader(name);
    logger.debug(name + "=" + value);
}

The displayed return values are as follows.

host=www.devkuma.com:8080
connection=keep-alive
cache-control=max-age=0
upgrade-insecure-requests=1
user-agent=Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.99 Safari/537.36
accept=text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8
accept-encoding=gzip, deflate
accept-language=ko-KR,ko;q=0.9,en-US;q=0.8,en;q=0.7,ja;q=0.6,es;q=0.5
cookie=JSESSIONID=26356CFACA2FFA1B4AA25C75D291E028

Request Body

You can get data passed in the request body as follows.

DataInputStream dis = new DataInputStream(request.getInputStream());
String str = null
while ((str = dis.readLine()) != null) {
    logger.debug(new String(str.getBytes("ISO-8859-1"), "utf-8") + "/n");
    // Korean text sent as euc-kr is broken.
}

JSP/Servlet | JSTL(JSP Standard Tag Library)

kc@example.com (kc kim) — Sun, 15 Oct 2017 15:34:56 +0900

JSTL stands for Jsp Standard Tag Library. It is a set of tags that standardizes syntax used in JSP pages.

Why Is JSTL Needed?

When JSP(Java) code and HTML code are mixed together in a JSP page, the code becomes complex and difficult to maintain.

Compare the following code written only with JSP and the code written with JSTL.

Written Only with JSP

<%
    if (list.size() > 0) {
        for(int i = 0; i < list.size(); i++) {
            HashMap map = (HashMap)list.get(i);
%>
            제목 : <%=map.get("title")%>
<%
        }
    }
%>

Written with JSTL

<c:if test="${!empty list}">
    <c:forEach var="item" items="${list}">
        제목 : ${item.title}
    </c:forEach>
</c:if>

The JSTL version is clearly easier to read and understand.

JSTL Types

Core

Supports variables, flow control, and URL handling.

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

fmt

Displays locale, message formats, numbers, and date formats.

<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>

functions

Handles strings and collections.

<%@ taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions" %>

How to Apply JSTL

Add the Library

You only need to add the jstl-1.2.jar file. If you use Maven, add the following dependency to pom.xml.

<!-- https://mvnrepository.com/artifact/javax.servlet/jstl -->
<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>jstl</artifactId>
    <version>1.2</version>
</dependency>

Declare the Tag Library

To use JSTL, you need a declaration inside the JSP file indicating that JSTL will be used.

The following directive declares the JSTL core, also known as the C tag.

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

Applied to a JSP file, it looks like this.

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
<head>
    <title>JSTL</title>
</head>
<body>
Hello World!! JSTL
</body>
</html>

However, if you declare it this way, you must add the directive to every JSP file, which is inconvenient.

To include it in every JSP, set <include-prelude> in web.xml as follows.

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    version="3.0">
 
... omitted ...

    <jsp-config>
        <jsp-property-group>
            <url-pattern>*.jsp</url-pattern>
            <el-ignored>false</el-ignored>
            <page-encoding>UTF-8</page-encoding>
            <scripting-invalid>false</scripting-invalid>
            <include-prelude>/WEB-INF/jsp/common/directive.jsp</include-prelude>
        </jsp-property-group>
    </jsp-config>

... omitted ...

</web-app>

The configured JSP file (directive.jsp) is always included in all JSP (*.jsp) files. Writing the JSTL directives there makes the setup easier.

<%@ page contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%@ taglib prefix="c"   uri="http://java.sun.com/jsp/jstl/core"%>
<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>
<%@ taglib prefix="fn"  uri="http://java.sun.com/jsp/jstl/functions"%>

devkuma – JSP

JSP/Servlet | Preparing for Google App Engine Development

JSP/Servlet | Preparing for Google App Engine Development | Using Server-Side Java

Preparation Before Starting

JDK

Eclipse

SDK/Plugin

Google and GAE Accounts

Basic Knowledge of Java SE

JSP/Servlet

JSP/Servlet | Creating a GAE Application

JSP/Servlet | JSP(Java Server Pages)

JSP/Servlet | Passing Values Between Client and Server

JSP/Servlet | Sessions and Cookies

JSP/Servlet | Servlet

JSP/Servlet | Servlets and JSP/HTML

JSP/Servlet | web.xml DTD by Version

Servlet 2.2

Servlet 2.3

Servlet 2.4

Servlet 2.5

Servlet 3.0

Servlet 3.1

JSP/Servlet | HTTPServletRequest Methods

Before You Start

Main Information

Basic Local Information (Server Information)

Remote Information (Client Information)

Session ID

Parameter Information

Display All Parameter Values

Cookie Information

Display All Cookie Values

Attribute

Display All Attribute Values

Header Information

Display All Header Values

Request Body

JSP/Servlet | JSTL(JSP Standard Tag Library)

Why Is JSTL Needed?

Written Only with JSP

Written with JSTL

JSTL Types

Core

fmt

functions

How to Apply JSTL

Add the Library

Declare the Tag Library