Servlet learning diary 8 - servlet life cycle and thread safety

Catalog

One 、 There are four stages in the life cycle

1.1 Instantiation

1.2 initialization

1.3 service

1.4 The destruction

1.5 Code

Two 、 characteristic

2.1 Thread safety problem

2.2 How to ensure thread safety

2.3 Realization

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One 、 There are four stages in the life cycle

1.1 Instantiation

When the user first accesses Servlet when , Called by container Servlet The constructor of creates a concrete Servlet object . You can also create instances as soon as the container starts . Use the following code to set Servlet Whether to create... When the server starts .

<load-on-starup>1</load-on-starup>

Be careful ： Only once

1.2 initialization

In the initialization phase ,init() Method will be called . This method is javax.servlet.Servlet Interface . among , Method with a ServletConfig Object of type as parameter .

Be careful ：init Method is executed only once

1.3 service

When the client has a request , The container will request ServletRequest And response ServletResponse Object turn Servlet, In the form of parameters to service Method .

This method executes multiple times

1.4 The destruction

When Servlet Containers （Tomcat） Stopping or restarting will lead to | Destroy from Servlet Object and call destroy Method .

destroy Method execution one Time

 

1.5 Code

package com.ha.servlet;

import java.io.IOException;
import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class LifeServlet implements Servlet{
    public LifeServlet(){
        System.out.println("1、 Instantiation ");
    }

    public void init(ServletConfig servletConfig) throws ServletException {
        System.out.println("2、 initialization ");
    }

    public ServletConfig getServletConfig() {
        return null;
    }

    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
        System.out.println("3、 Receiving request , In response to the results ");
    }

    public String getServletInfo() {
        return null;
    }

    public void destroy() {
        System.out.println("4、 The destruction ");
    }
    
}

  perform shutdown.bat, This tomcat The destroyed content will be printed in a flash in the window of

Two 、 characteristic

2.1 Thread safety problem

Servlet After the visit , Will execute instantiation operation , Create a Servlet object . And we Tomcat The container can access the same server concurrently by multiple threads at the same time Servlet, If you modify a member variable in a method , There will be thread safety problems .

Code interpretation , More than two people visit at the same time ：

package com.ha.servlet;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

public class SafeServlet extends HttpServlet {
    private String message = "";

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // hypothesis 1、 Receiving parameters 
        //2、 Call business logic   Get login results 
        message = " Login successful ";// Login failed ！
        PrintWriter printWriter = resp.getWriter();
        printWriter.println(message);

    }

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req,resp);
    }
}

2.2 How to ensure thread safety

1、synchronized

         Put the code with thread safety problems into the synchronization code block

2、 Realization singleThreadModel Interface

        servlet Realization singleThreadModel After the interface , Every thread creates servlet example , In this way, each client request does not have the problem of sharing resources , however servlet Too inefficient to respond to client requests , So it has been eliminated .

3、 Use local variables as much as possible

2.3 Realization

1、 If locks are used

The efficiency of the code will be very low , Only one person is allowed to access  

2、

3、