如何在我的 Eclipse 项目中导入 javax.servlet/jakarta.servlet API? [英] How do I import the javax.servlet / jakarta.servlet API in my Eclipse project?

问题描述

我想在 Eclipse 中使用 Servlets 进行开发，但它说无法解析包 javax.servlet/jakarta.servlet.如何将 javax.servlet/jakarta.servlet 包添加到我的 Eclipse 项目中?

解决方案

确保您拥有正确的 Eclipse 和 Server 版本

确保您至少使用

B.或者，通过 Eclipse 首选项

• 打开窗口>首选项 >服务器 >运行时环境.

• 您可以在此处添加、编辑和删除服务器.

将服务器与项目关联

一个.在新项目中

• 打开左侧的 Project Navigator/Explorer.

• 在那里右击并选择 New >项目，然后在菜单 Web >动态网络项目.

• 在向导中，将目标运行时设置为集成服务器.

B.或者，在现有项目中

• 右键单击项目并选择属性.

• 在Targeted Runtimes部分，选择集成服务器.

无论哪种方式，Eclipse 都会在构建路径中自动获取 servletcontainer 的库.这样您就可以导入和使用 Servlet API.

永远不要携带松散的特定于服务器的 JAR 文件

在任何情况下，您都不需要在项目的 Build Path 属性中摆弄.最重要的是，您应该永远手动复制/下载/移动/包含各个 servletcontainer 特定的库，例如 servlet-api.jar、jsp-api.jar、el-api.jar、j2ee.jar、javaee.jar 等，只会导致未来的可移植性、兼容性、类路径和可维护性问题，因为当您的 web 应用程序部署到与最初获取这些库的位置不同的制造商/版本的 servlet 容器时，它将无法工作.

如果您使用 Maven，您需要绝对确保目标运行时已经提供的 servletcontainer 特定库被标记为 provided.您可以在以下答案中找到适用于 Tomcat 10+、Tomcat 9-、JEE 9+ 和 JEE 8- 的正确 pom.xml 依赖项声明示例:Tomcat 9 将 servlet 转换为 javax.servlet.Servlet 而不是 jakarta.servlet.http.HttpServlet

这里有一些典型的异常，当你乱扔 /WEB-INF/lib 甚至 /JRE/lib, /JRE/lib/ext 等带有 servletcontainer 特定库的粗心尝试修复编译错误:

• org.apache.jsp.index_jsp._jspInit 处的 java.lang.NullPointerException
• java.lang.NoClassDefFoundError: javax/el/ELResolver
• java.lang.NoSuchFieldError: IS_DIR
• java.lang.NoSuchMethodError: javax.servlet.jsp.PageContext.getELContext()Ljavax/el/ELContext;
• java.lang.AbstractMethodError: javax.servlet.jsp.JspFactory.getJspApplicationContext(Ljavax/servlet/ServletContext;)Ljavax/servlet/jsp/JspApplicationContext;
• org.apache.jasper.JasperException:getJspApplicationContext(ServletContext) 方法未定义类型JspFactory
• java.lang.VerifyError: (class: org/apache/jasper/runtime/JspApplicationContextImpl, method: createELResolver signature: ()Ljavax/el/ELResolver;) 函数参数不兼容
• jar 未加载.请参阅 Servlet 规范 2.3，第 9.7.2 节.违规类:javax/servlet/Servlet.class

I want to develop with Servlets in Eclipse, but it says that the package javax.servlet / jakarta.servlet cannot be resolved. How can I add javax.servlet / jakarta.servlet package to my Eclipse project?

解决方案

Ensure you've the right Eclipse and Server version

Ensure that you're using at least Eclipse IDE for Enterprise Java developers (with the Enterprise). It contains development tools to create dynamic web projects and easily integrate servletcontainers (those tools are part of Web Tools Platform, WTP). In case you already had Eclipse IDE for Java (without Enterprise), and manually installed some related plugins, then chances are that it wasn't done properly. You'd best trash it and grab the real Eclipse IDE for Enterprise Java one.

You also need to ensure that you already have a servletcontainer installed on your machine which implements at least the same Servlet API version as the servletcontainer in the production environment, for example Apache Tomcat, Oracle GlassFish, JBoss AS/WildFly, etc. Usually, just downloading the ZIP file and extracting it is sufficient. In case of Tomcat, do not download the EXE format, that's only for Windows based production environments. See also a.o. Several ports (8005, 8080, 8009) required by Tomcat Server at localhost are already in use.

A servletcontainer is a concrete implementation of the Servlet API. Note that the Java EE SDK download at Oracle.com basically contains GlassFish. So if you happen to already have downloaded Java EE SDK, then you basically already have GlassFish. Also note that for example GlassFish and JBoss AS/WildFly are more than just a servletcontainer, they also supports JSF, EJB, JPA and all other Java EE fanciness. See also a.o. What exactly is Java EE?

Ensure that you're using the right Servlet package

The javax.* package has been renamed to jakarta.* package since Servlet API version 5.0 which is part of Jakarta EE 9 (Tomcat 10, TomEE 9, WildFly 22 Preview, GlassFish 6, Payara 6, Liberty 22, etc). So if you're targeting these server versions or newer, then you need to replace

import javax.servlet.*;
import javax.servlet.http.*;

by

import jakarta.servlet.*;
import jakarta.servlet.http.*;

in order to get it to compile, else you might risk to face this build error

The superclass "javax.servlet.http.HttpServlet" was not found on the Java Build Path

Integrate Server in Eclipse and associate it with Project

Once having installed both Eclipse for Enterprise Java and a servletcontainer on your machine, do the following steps in Eclipse:

1. Integrate servletcontainer in Eclipse

   a. Via Servers view

   • Open the Servers view in the bottom box.

   • Rightclick there and choose New > Server.

   • Pick the appropriate servletcontainer make and version and walk through the wizard.

   b. Or, via Eclipse preferences

   • Open Window > Preferences > Server > Runtime Environments.

   • You can Add, Edit and Remove servers here.

2. Associate server with project

   a. In new project

   • Open the Project Navigator/Explorer on the left hand side.

   • Rightclick there and choose New > Project and then in menu Web > Dynamic Web Project.

   • In the wizard, set the Target Runtime to the integrated server.

   b. Or, in existing project

   • Rightclick project and choose Properties.

   • In Targeted Runtimes section, select the integrated server.

   Either way, Eclipse will then automatically take the servletcontainer's libraries in the build path. This way you'll be able to import and use the Servlet API.

Never carry around loose server-specific JAR files

You should in any case not have the need to fiddle around in the Build Path property of the project. You should above all never manually copy/download/move/include the individual servletcontainer-specific libraries like servlet-api.jar, jsp-api.jar, el-api.jar, j2ee.jar, javaee.jar, etc. It would only lead to future portability, compatibility, classpath and maintainability troubles, because your webapp would not work when it's deployed to a servletcontainer of a different make/version than where those libraries are originally obtained from.

In case you're using Maven, you need to make absolutely sure that servletcontainer-specific libraries which are already provided by the target runtime are marked as <scope>provided</scope>. You can find examples of proper pom.xml dependency declarations for Tomcat 10+, Tomcat 9-, JEE 9+ and JEE 8- in this answer: Tomcat 9 casting servlets to javax.servlet.Servlet instead of jakarta.servlet.http.HttpServlet

Here are some typical exceptions which you can get when you litter the /WEB-INF/lib or even /JRE/lib, /JRE/lib/ext, etc with servletcontainer-specific libraries in a careless attempt to fix the compilation errors:

• java.lang.NullPointerException at org.apache.jsp.index_jsp._jspInit
• java.lang.NoClassDefFoundError: javax/el/ELResolver
• java.lang.NoSuchFieldError: IS_DIR
• java.lang.NoSuchMethodError: javax.servlet.jsp.PageContext.getELContext()Ljavax/el/ELContext;
• java.lang.AbstractMethodError: javax.servlet.jsp.JspFactory.getJspApplicationContext(Ljavax/servlet/ServletContext;)Ljavax/servlet/jsp/JspApplicationContext;
• org.apache.jasper.JasperException: The method getJspApplicationContext(ServletContext) is undefined for the type JspFactory
• java.lang.VerifyError: (class: org/apache/jasper/runtime/JspApplicationContextImpl, method: createELResolver signature: ()Ljavax/el/ELResolver;) Incompatible argument to function
• jar not loaded. See Servlet Spec 2.3, section 9.7.2. Offending class: javax/servlet/Servlet.class

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