Servlet学习

Posted 2020-11-06 bsfl-study

servlet线程安全：单实例，多线程。

解决方法：将全局变量改为局部变量。

 

不安全：

package servlet;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ServletDemo5
 */
@WebServlet("/ServletDemo5")
public class ServletDemo5 extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public ServletDemo5() {
        super();
        // TODO Auto-generated constructor stub
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
    int num = 1;
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		num ++;
		
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
		System.out.println(num);
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}
	
	//运行结果，不停刷新页面出现：12345678889910101111

}

安全写法：

package servlet;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ServletDemo5
 */
@WebServlet("/ServletDemo5")
public class ServletDemo5 extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public ServletDemo5() {
        super();
        // TODO Auto-generated constructor stub
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
    
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		int num = 1;
		num ++;
		
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
		System.out.println(num);
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}
	
	//运行结果，不停刷新页面出现：22222222222

}

　　因为每个用户访问时候都开一个线程，每个线程都会执行一次dopost/doget方法，而num是局部变量，每次访问都是新的num。

 

servlet获取配置信息(web.xml)：

1.servletConfig.getInitParamter   自己创建sevletConfig对象，再getInitParamter方法获取
2.this.getInitParamter/super.getInitParamter 直接使用父类的getInitParamter方法获取
3.this.getServletConfig().getInitParamter 继承父类sevletConfig对象，再getInitParamter方法获取

第一种方法：

package servlet;

import java.io.IOException;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ServletDemo6
 */
@WebServlet("/ServletDemo6")
public class ServletDemo6 extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public ServletDemo6() {
       this.config = config;
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		response.getWriter().append("Served at: ").append(request.getContextPath());
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}
	
	private ServletConfig config;
	
	@Override
	public void init(ServletConfig config) throws ServletException {
		String encoding = config.getInitParameter("encoding");
		System.out.println(encoding);
	}

}

　　第二种和第三种方法：

package servlet;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class ServletDemo7
 */
@WebServlet("/ServletDemo7")
public class ServletDemo7 extends HttpServlet {
    private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public ServletDemo7() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //第二种方式
        //        String encoding = this.getInitParameter("encoding");//或者用super，继承父类方法
//        System.out.println(encoding);
        
        //第三种方式
        String value = this.getServletConfig().getInitParameter("encoding");
        System.out.println(value);
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

 web.xml

  <servlet>
    <servlet-name>ServletDemo6</servlet-name>
    <servlet-class>servlet.ServletDemo6</servlet-class>
    <init-param>
      <param-name>encoding</param-name>
      <param-value>UTF-8</param-value>
    </init-param>
  </servlet>
  <servlet-mapping>
    <servlet-name>ServletDemo6</servlet-name>
    <url-pattern>/demo6</url-pattern>
  </servlet-mapping>
    <servlet>
    <servlet-name>ServletDemo7</servlet-name>
    <servlet-class>servlet.ServletDemo7</servlet-class>
    <init-param>
      <param-name>encoding</param-name>
      <param-value>UTF-8</param-value>
    </init-param>
  </servlet>
  <servlet-mapping>
    <servlet-name>ServletDemo7</servlet-name>
    <url-pattern>/demo7</url-pattern>
  </servlet-mapping>