JavaWeb开发基础ServletConfig与ServletContext¶

ServletConfig¶

package javax.servlet;

import java.util.Enumeration;

public interface ServletConfig {
    String getServletName();

    ServletContext getServletContext();

    String getInitParameter(String var1);

    Enumeration<String> getInitParameterNames();
}

ServletConfig用于Servlet初始化时从web.xml获取配置信息。主要有以下方法：

public String getInitParameter(String name): Returns the parameter value for the specified parameter name.
public Enumeration getInitParameterNames(): Returns an enumeration of all the initialization parameter names.
public String getServletName(): Returns the name of the servlet.
public ServletContext getServletContext(): Returns an object of ServletContext.

示例，web.xml

<servlet>
    <servlet-name>ExampleServlet</servlet-name>
    <servlet-class>com.example.ExampleServlet</servlet-class>
    <init-param>
        <param-name>param1</param-name>
        <param-value>value1</param-value>
    </init-param>
    <init-param>
        <param-name>param2</param-name>
        <param-value>value2</param-value>
    </init-param>
</servlet>
<servlet-mapping>
    <servlet-name>ExampleServlet</servlet-name>
    <url-pattern>/example</url-pattern>
</servlet-mapping>

ExampleServlet.java

package com.example;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Enumeration;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ExampleServlet extends HttpServlet {
    private ServletConfig config;

    @Override
    public void init(ServletConfig config) throws ServletException {
        this.config = config;
        super.init(config);
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();

        // 获取初始化参数
        String param1 = config.getInitParameter("param1");
        String param2 = config.getInitParameter("param2");

        // 获取所有初始化参数
        Enumeration<String> paramNames = config.getInitParameterNames();
        while (paramNames.hasMoreElements()) {
            String paramName = paramNames.nextElement();
            String paramValue = config.getInitParameter(paramName);
            out.println("<p>" + paramName + ": " + paramValue + "</p>");
        }

        // 获取 ServletContext
        ServletContext context = config.getServletContext();
        out.println("<p>ServletContext: " + context.getContextPath() + "</p>");

        // 获取 Servlet 名称
        String servletName = config.getServletName();
        out.println("<p>Servlet Name: " + servletName + "</p>");
    }
}

ServletContext¶

ServletContext用于在整个Web应用程序范围内共享信息和资源。每个Web应用程序都有一个唯一的ServletContext对象，该对象在应用程序启动时创建，并在应用程序关闭时销毁。

通过ServletContext，Servlet可以获取上下文初始化参数、访问资源、记录日志、共享数据等。主要有以下方法：

public String getInitParameter(String name): Returns the parameter value for the specified parameter name.
public Enumeration getInitParameterNames(): Returns the names of the context’s initialization parameters.
public void setAttribute(String name,Object object): sets the given object in the application scope.
public Object getAttribute(String name): Returns the attribute for the specified name.
public Enumeration getInitParameterNames(): Returns the names of the context’s initialization parameters as an Enumeration of String objects.
public void removeAttribute(String name): Removes the attribute with the given name from the servlet context.

ServletContext不但能从web.xml文件get属性，还能set和remove属性。

示例，web.xml

<context-param>
    <param-name>contextParam1</param-name>
    <param-value>value1</param-value>
</context-param>
<context-param>
    <param-name>contextParam2</param-name>
    <param-value>value2</param-value>
</context-param>

ExampleServlet.java

package com.example;

import java.io.IOException;
import java.io.InputStream;
import java.util.Enumeration;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ExampleServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();

        // 获取 ServletContext
        ServletContext context = getServletContext();

        // 获取上下文初始化参数
        String contextParam1 = context.getInitParameter("contextParam1");
        String contextParam2 = context.getInitParameter("contextParam2");

        // 获取所有上下文初始化参数
        Enumeration<String> paramNames = context.getInitParameterNames();
        while (paramNames.hasMoreElements()) {
            String paramName = paramNames.nextElement();
            String paramValue = context.getInitParameter(paramName);
            out.println("<p>" + paramName + ": " + paramValue + "</p>");
        }

        // 设置和获取上下文属性
        context.setAttribute("attributeName", "attributeValue");
        String attributeValue = (String) context.getAttribute("attributeName");
        out.println("<p>Attribute: " + attributeValue + "</p>");

        // 获取资源
        InputStream is = context.getResourceAsStream("/WEB-INF/config.properties");
        if (is != null) {
            out.println("<p>Resource found</p>");
        } else {
            out.println("<p>Resource not found</p>");
        }

        // 获取实际路径
        String realPath = context.getRealPath("/index.html");
        out.println("<p>Real Path: " + realPath + "</p>");

        // 记录日志
        context.log("This is a log message.");

        // 获取上下文路径
        String contextPath = context.getContextPath();
        out.println("<p>Context Path: " + contextPath + "</p>");
    }
}

ExampleServlet继承HttpServlet，getServletContext方法来自HttpServlet，HttpServlet继承GenericServlet，GenericServlet提供了getServletContext方法，内部调用的是ServletConfig的getServletContext方法：

public ServletContext getServletContext() {
    ServletConfig sc = this.getServletConfig();
    if (sc == null) {
        throw new IllegalStateException(lStrings.getString("err.servlet_config_not_initialized"));
    } else {
        return sc.getServletContext();
    }
}

总结，ServletConfig每个Servlet都有，ServletContext整个web容器只有一个。ServletConfig用于在Servlet初始化时传递配置信息，包含初始化参数和ServletContext的引用，主要用于获取初始化参数和共享上下文。ServletContext表示整个Web应用程序的上下文，提供在不同Servlet之间共享数据和资源的机制，主要用于共享数据、访问资源、记录日志和获取应用程序信息。

参考资料：

https://www.javatpoint.com/servletconfig

https://www.javatpoint.com/servletcontext

https://www.javatpoint.com/attribute

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