What is ServletContext

ServletContext is the largest interface in Servlet, which presents the Servlet view of the web application, and the ServletContext instance is obtained through the "getServletContext()" method.

#This article will introduce the role of ServletContext, its methods and its uses.

Recommended course: Java Tutorial.

Each application will have a ServletContext object associated with it. When the container is distributed on multiple virtual machines, the web application will have a ServletContext object on each virtual machine where it is distributed. ServletContext instance. By default, the ServletContext is not distributed and only exists on one virtual machine.

ServletContext is officially called servlet context. The server will create an object for each project, which is the ServletContext object. This object is globally unique and shared by all servlets within the project. So it is called global application shared object.

ServletContext: Represents the current web application (very important)

When the WEB container starts, it will create a corresponding ServletContext object for each WEB application. It represents the current web application.

The ServletConfig object maintains a reference to the ServletContext object. When writing a servlet, developers can obtain the ServletContext object through the ServletConfig.getServletContext method. Since all Servlets in a WEB application share the same ServletContext object, Communication between Servlet objects can be achieved through the ServletContext object.

The ServletContext object is usually also called the context domain object.

Application of ServletContext:

Commonly used methods in context are:

void setAttribute(String,Object);

Object getAttribute(String );

void removeAttribute(String);

1. As a domain object, data can be shared throughout the entire web application.

Domain objects: objects used to share data within a visible range

Scope: share data within the entire web application scope

Life cycle: when the server starts After the ServletContext object is created after the web application is loaded, the domain is generated. When the web application is removed from the container or the server is shut down, the domain is destroyed along with the destruction of the web application

.

2. Obtain the initialization parameters of the WEB application

In the first paragraph, we add configuration information to a single servlet through the tag. This configuration information is in It is not accessible from other Servlets. But if we use the tag (parallel with the Servlet tag) to configure properties for the entire Web application, then all Servlets can access the parameters inside. For example: you can put the database configuration information here.

Do not confuse some concepts involved here:

Request parameter parameter --- Parameter information in the request sent by the browser Initialization parameter initparameter --- In web.xml for Servlet or The basic parameter domain attributes included in the initialization of the ServletContext configuration --- key-value pairs accessed in the four major scopes

Code example:

In web.xml, it is the entire web The application adds initialization parameters: username, password. The code location is as follows:

package com.vae.servlet;
 
 
import java.io.IOException;
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 ServletTest03 extends HttpServlet {
 
 
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        ServletContext context = this.getServletContext(); // 得到上下文对象
 
 
        // 获取单个的Context里面的初始化参数
        String value1 = context.getInitParameter("username");
        String value2 = context.getInitParameter("password");
        System.out.println(value1 + ";" + value2);
        System.out.println();
 
 
        // 一次性获取Context里所有的初始化参数
        Enumeration enumeration = context.getInitParameterNames();
        while (enumeration.hasMoreElements()) {
            String name = (String) enumeration.nextElement();
            String value = context.getInitParameter(name);
            System.out.println(name + ";" + value);
 
 
        }
 
 
    }
 
 
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }
 
 
}

As you can see from the above code, we can obtain the initialization parameters through the context.getInitParameter() method.

The operation effect is as follows:

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