Guide to JavaServer Pages (JSP) – Java服务器页面(JSP)指南

最后修改: 2016年 12月 1日


Table of Contents


1. Overview


JavaServer Pages (JSP) allows dynamic content injection into static contents using Java and Java Servlets. We can make requests to a Java Servlet, perform relevant logic, and render a specific view server-side to be consumed client-side. This article will provide a thorough overview of JavaServer Pages using Java 8 and Jave 7 EE.

JavaServer Pages(JSP)允许使用Java和Java Servlet将动态内容注入静态内容。我们可以向Java Servlet发出请求,执行相关的逻辑,并在服务器端渲染一个特定的视图,以便在客户端消费。本文将对使用Java 8和Jave 7 EE的JavaServer Pages进行全面介绍。

We’ll start by exploring a few key concepts relevant to JSP: namely, the difference between dynamic and static contents, the JSP lifecycle, and JSP syntax as well as directives and the implicit objects created at compilation!


2. JavaServer Pages

2.JavaServer Pages

JavaServer Pages (JSP) enabled Java-specific data to be passed into or placed within a .jsp view and consumed client-side.

JavaServer Pages (JSP)使Java特定的数据能够被传入或放置在.jsp视图中,并在客户端被消耗。

JSP files are essentially .html files with some extra syntax, and a couple of minor initial differences:


  1. the .html suffix is replaced with .jsp (it’s considered a .jsp filetype) and
  2. the following tag is added to the top of the .html markup elements:
<%@ page contentType="text/html;charset=UTF-8" language="java" %>

Let’s go over some of the key concepts in JSP.


2.1. JSP Syntax


There are two ways to add Java code to a .jsp. First, we can use basic Java Scriptlet syntax which involves placing Java code blocks within two Scriptlet tags:

有两种方法可以在.jsp中添加Java代码。首先,我们可以使用基本的Java Scriptlet语法,即把Java代码块放在两个Scriptlet标签中。

<% Java code here %>

The second method is specific to XML:


    Java code here

Importantly, one can use conditional logic clientside with JSP by using if, then, and else clauses and then wrapping the relevant blocks of markup with those brackets.


<% if (doodad) {%>
<% } else { %>
<% } %>

For example, if doodad is true, we would display the first, div element otherwise we would display the second, p element!


2.2. Static and Dynamic Contents


Static web contents are fixed assets that are consumed independently of RESTful, SOAP, HTTP, HTTPS requests, or other user-submitted information.


Static content, however, is fixed and is not modified by user inputs. Dynamic web contents are those assets that respond to, are modified by, or change in light of user actions or information!


JSP technology allows for the clean separation of responsibilities between dynamic and static contents.


The server (servlet) manages the dynamic contents and the client (the actual .jsp page) is the static context into which dynamic contents are injected.


Let’s take a look at the implicit objects that are created by JSP and which allow you to access JSP-relevant data server-side!


2.3. Implicit Objects


Implicit objects are generated by the JSP engine automatically during compilation.


Implicit objects include the HttpRequest and HttpResponse objects and expose various serverside functionalities for use in your servlet and for interacting with your .jsp! Here’s the list of implicit objects that are created:


request belongs to the class javax.servlet.http.HttpServletRequest. The request object exposes all user inputs data and makes it available serverside.


response belongs to the class javax.servlet.http.HttpServletResponse and determines what is passed back clientside after a request is made.


Let’s take a closer look at the request and response implicit objects since they’re the most important and heavily-used ones.


The example below demonstrates a very simple, incomplete, servlet method to handle GET requests. I’ve omitted most of the details so that we can focus on how to use the request and response objects:


protected void doGet(HttpServletRequest request, 
  HttpServletResponse response) throws ServletException, IOException {
    String message = request.getParameter("message");
    . . .

First, we see that the request and response objects are passed in as parameters into the method making them available within its scope.


We can access request parameters using the .getParameter() function. Above, we snag the message parameter and initialize a string variable so we can use it in our server-side logic. We can also access the response object which determines what and how the data passed into the view will be.


Above we set the content type on it. We don’t need to return the response object to have it’s payload display on the JSP page at render!


out belongs to the class javax.servlet.jsp.JspWriter and is used to write content to the client.


There are at least two ways to print to your JSP page and its worth discussing both here. out is created automatically and allows you to write to memory and then to the response object:



That’s it!


The second approach can be more performant since it allows you to write directly to the response object! Here, we use PrintWriter:


PrintWriter out = response.getWriter();
out.println("Hello World");

2.4. Other Implicit Objects


Here are some other Implicit objects that are also good to know!


session belongs to the class javax.servlet.http.HttpSession maintains user data for the duration of the session.


application belongs to the class javax.servlet.ServletContext stores application-wide parameters set at initialization or that need to be accessed application-wide.


exception belongs to the class javax.servlet.jsp.JspException is used to display error messages on JSP pages which have the tag <%@ page isErrorPage=”true” %>.

exception属于javax.servlet.jsp.JspException类,用于在有<%@ page isErrorPage=”true” %>标签的JSP页面上显示错误信息。

page belongs to the class java.lang.Object allows one to access or reference current servlet information.


pageContext belongs to the class javax.servlet.jsp.PageContext defaults to page scope but can be used for accessing request, application, and session attributes.


config belongs to the class javax.servlet.ServletConfig is the servlet configuration object allowing one to get the servlet context, name, and configuration parameters.


Now that we’ve covered the implicit objects provided by JSP, let’s turn to directives which allow .jsp pages to (indirectly) access some of these objects.


2.5. Directives


JSP supplies out of the box directives that can be used to specify core functionalities for our JSP files. There are two parts to JSP directives: (1) the directive itself and (2) the attribute of that directive which is assigned a value.


The three kinds of directives that can be referenced using directive tags are <%@ page … %> which defines dependencies and attributes of the JSP including content type and language, <%@ include … %> which specifies an import or file to be used, and <%@ taglib …%> which specifies a tag library defining custom actions to be used by a page.

可以使用指令标签引用的三种指令是:<%@ page … %>,它定义了JSP的依赖性和属性,包括内容类型语言<%@ include …%>,它指定了要使用的导入或文件,以及<%@ taglib …%>,它指定了一个标签库,定义了要被页面使用的自定义动作。

So, as an example, a page directive would be specified using JSP tags in the following way: <%@ page attribute=”value” %>

因此,作为一个例子,一个页面指令将用JSP标签以如下方式指定。<%@ page attribute=”value” %>

And, we can do that using XML as follows: < attribute=”value” />

而且,我们可以使用XML来实现,如下所示。< attribute=”value” />

2.6. Page Directive Attributes


There are a lot of attributes that can be declared within a page directive:


autoFlush <%@ page autoFlush=”false” %>

自动冲洗<%@ page autoFlush=”false” %>/em>

autoFlush controls the buffer output, clearing it out when the buffer size is reached. The default value is true.


buffer <%@ page buffer=”19kb” %>

buffer <%@ page buffer=”19kb” %>

buffer sets the size of the buffer used by our JSP page. The default value is 8kb.


errorPage <%@ page errorPage=”errHandler.jsp” %>

errorPage <%@ page errorPage=”errHandler.jsp” %>

errorPage specifies a JSP page as an error page.


extends <%@ page extends=”org.apache.jasper.runtime.HttpJspBase” %>

extends <%@ page extends=”org.apache.jasper.runtime.HttpJspBase” %>

extends specifies the super class of the corresponding servlet code.


info <%@ page info=”This is my JSP!” %>

info <%@ page info=”这是我的JSP!”%>

info is used to set a text-based description for the JSP.


isELIgnored <%@ page isELIgnored=”true” %>

isELIgnored <%@ page isELIgnored=”true” %>/em>

isELIgnored states whether or not the page will ignore Expression Language (EL) in JSP. EL enables the presentation layer to communicate with Java managed beans and does so using ${…} syntax and while we won’t get into the nitty-gritties of EL here, there are several examples found below that are sufficient to build our example JSP app! The default value for isELIgnored is false.

isELIgnored说明页面是否会忽略JSP中的Expression Language(EL)。EL使表现层能够与Java管理Bean进行通信,并使用${…}语法,虽然我们不会在这里讨论EL的细枝末节,但下面有几个例子足以建立我们的JSP应用实例isELIgnored的默认值是false

isErrorPage <%@ page isErrorPage=”true” %>

isErrorPage <%@ page isErrorPage=”true” %>/em>

isErrorPage says whether or not a page is an error page. We must specify an error page if we create an error handler for our page within the application.


isThreadSafe <%@ page isThreadSafe=”false” %>

isThreadSafe <%@ page isThreadSafe=”false” %>/em>

isThreadSafe has a default value of true. isThreadSafe determines whether or not the JSP can use Servlet multi-threading. In general, you would never want
to turn off that functionality.


language <%@ page language=”java” %>

language <%@ page language=”java” %>

language determines what scripting language to use in the JSP. The default value is Java.


session <%@ page session=”value”%>

session <%@ page session=”value”%>

session determines whether or not to maintain the HTTP session. It defaults to true and accepts values of true or false.


trimDirectiveWhitespaces <%@ page trimDirectiveWhitespaces =”true” %>

trimDirectiveWhitespaces <%@ page trimDirectiveWhitespaces =”true” %>

trimDirectiveWhitespaces stripes out white-spaces in the JSP page condensing the code into a more compact block at compile-time. Setting this value to true may help to reduce the size of the JSP code. The default value is false.


3. Three Examples


Now that we’ve reviewed the concepts central to JSP, let’s apply those concepts to some basic examples that will help you to get your first JSP-serving servlet up and running!


There are three main ways to inject Java into a .jsp and we’ll explore each of those ways below using native functionalities in Java 8 and Jakarta EE.

在.jsp中注入Java有三种主要方式,下面我们将利用Java 8和Jakarta EE中的原生功能来探讨每一种方式。

First, we’ll render our markup server-side to be displayed client-side. Second, we’ll look at how to add Java code directly into our .jsp file independent of javax.servlet.http‘s request and response objects.


Third, we’ll demonstrate how to both forward an HttpServletRequest to a specific .jsp and bind server-side processed Java to it.


Let’s set up our project in Eclipse using the File/New/Project/Web/Dynamic web project/ type to be hosted in Tomcat! You should see after creating the project:

让我们在Eclipse中使用File/New/Project/Web/Dynamic web project/类型来设置我们的项目,以便在Tomcat中托管!创建项目后,你应该看到。

  |- WebContent
    |- META-INF
      |- MANIFEST.MF
    |- WEB-INF
      |- lib
      |- src

We’re going to add a few files to the application structure so that we end up with:


  |- WebContent
    |- META-INF
      |- MANIFEST.MF
    |- WEB-INF
        |- ExampleTree.jsp
        |- ExampleTwo.jsp
        *- index.jsp
      |- src
        |- com
          |- baeldung

Let’s set up index.jsp which will be displayed when we access the URL context in Tomcat 8:

让我们来设置index.jsp,当我们在Tomcat 8中访问URL上下文时将会显示。

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
        <title>JSP Examples</title>
        <h1>Simple JSP Examples</h1>
        <p>Invoke HTML rendered by Servlet: <a href="ExampleOne" target="_blank">here</a></p>
        <p>Java in static page: <a href="ExampleTwo.jsp" target="_blank">here</a></p>
        <p>Java injected by Servlet: <a href="ExampleThree?message=hello!" target="_blank">here</a></p>

There are three a, each linking to one of the examples we’ll go through below in sections 4.1 through 4.4.


We also need to make sure that we’ve got our web.xml set up:



A main note here is – how to correctly map each of our servlets to a particular servlet-mapping.Doing so associates each servlet with a specific endpoint where it can consumed! Now, we’ll go through each of the other files below!


3.1. HTML Rendered in Servlet


In this example, we’ll actually skip building a .jsp file!


Instead, we’ll create a string representation of our markup and then write it to the GET response with PrintWriter after ExampleOne Servlet receives a GET request:

相反,我们将为我们的标记创建一个字符串表示,然后在ExampleOne Servlet收到GET请求后用PrintWriter将其写入GET响应。

public class ExampleOne extends HttpServlet {
  protected void doGet(HttpServletRequest request, 
    HttpServletResponse response) throws ServletException, IOException {
      PrintWriter out = response.getWriter();
	"<!DOCTYPE html><html>" +
	"<head>" +
	"<meta charset=\"UTF-8\" />" +
	"<title>HTML Rendered by Servlet</title>" +
	"</head>" +
	"<body>" +
	"<h1>HTML Rendered by Servlet</h1></br>" +
	"<p>This page was rendered by the ExampleOne Servlet!</p>" +
	"</body>" +

What we’re doing here is injecting our markup through our servlet request handling directly. Instead of a JSP tag, we generate our HTML, along with any and all Java-specific data to be inserted, purely server-side without a static JSP!


Earlier, we reviewed the out object which is a feature of JspWriter.


Above, I used the PrintWriter object instead which writes directly to the response object.


JspWriter actually buffers the string to be written into memory which is then written to the response objects after the in-memory buffer is flushed.


PrintWriter is already attached to the response object. I’ve preferred to write directly to the response object in the examples above and below for those reasons.


3.2. Java in a JSP Static Content


Here we create a JSP file named ExampleTwo.jsp with a JSP tag. As seen above, this allows Java to be added directly into our markup. Here, we randomly print an element of a String[]:


<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<!DOCTYPE html>
        <title>Java in Static Page Example</title>
        <h1>Java in Static Page Example</h1>
              String[] arr = {"What's up?", "Hello", "It's a nice day today!"}; 
	      String greetings = arr[(int)(Math.random() * arr.length)];	
        <p><%= greetings %></p>

Above, you’ll see that variable declaration within JSP tags objects: type variableName and an initialization just like regular Java.

上面,你会看到JSP标签内的变量声明对象。type variableNameinitialization就像普通Java一样。

I’ve included the above example to demonstrate how to add Java to a static page without making recourse to a specific servlet. Here, Java is simply added to a page and the JSP lifecycle takes care of the rest.


3.3. JSP With Forwarding


Now, for our final and most involved example! Here, we’re going to use the @WebServlet annotation on ExampleThree which eliminates the need for servlet mappings in server.xml.


  name = "ExampleThree",
  description = "JSP Servlet With Annotations",
  urlPatterns = {"/ExampleThree"}
public class ExampleThree extends HttpServlet {
  protected void doGet(HttpServletRequest request, HttpServletResponse response) 
    throws ServletException, IOException {
      String message = request.getParameter("message");
      request.setAttribute("text", message);
      request.getRequestDispatcher("/ExampleThree.jsp").forward(request, response);

ExampleThree takes a URL parameter passed in as message, binds that parameter to the request object, and then redirects that request object to ExampleThree.jsp.


Thus, we’ve not only accomplished a truly dynamic web experience but we’ve also done so within an application containing multiple .jsp files.


getRequestDispatcher().forward() is a simple way to ensure that the correct .jsp page is rendered.


All the data bound to the request object sent its (the .jsp file’s) way will then be displayed! Here’s how we handle that last part:


<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<!DOCTYPE html>
        <title>Java Binding Example</title>
        <h1>Bound Value</h1>
	    <p>You said: ${text}</p>

Note the JSP tag added to the top of ExampleThree.jsp. You’ll notice that I switched the JSP tags here. I’m using Expression Language (which I mentioned before) to render our set parameter (which is bound as ${text})!


3.4. Try It Out!


Now, we’ll export our application into a .war to be launched and hosted in Tomcat 8! Find your server.xml and we’ll update our Context to:

现在,我们将把我们的应用程序导出为一个.war,以便在Tomcat 8中启动和托管找到你的server.xml,我们将更新我们的Context到。

<Context path="/spring-mvc-xml" docBase="${catalina.home}/webapps/spring-mvc-xml">

Which will allow us to access our servlets and JSP’s on localhost:8080/spring-mvc-xml/jsp/index.jsp! Pick up a working copy over at: GitHub. Congrats!


4. Conclusion


We’ve covered quite a bit of ground! We’ve learned about what JavaServer Pages are, what they were introduced to accomplish, their lifecycle, how to create them, and finally a few different ways to implement them!


This concludes the introduction to JSP! Be well and code on!