JavaWeb开发之Servlet中的线程有关问题及使用ServletConfig对象操作Servlet中的参数
Servlet的一些细节(7)—线程安全
当多个客户端并发访问同一个Servlet时,web服务器会为每一个客户端的访问请求创建一个线程,并在这个线程上调用Servlet的service方法,因此service方法内如果访问了同一个资源的话,就有可能引发线程安全问题。
如果某个Servlet实现了SingleThreadModel接口,那么Servlet引擎将以单线程模式来调用其service方法。
SingleThreadModel接口中没有定义任何方法,只要在Servlet类的定义中增加实现SingleThreadModel接口的声明即可。
对于实现了SingleThreadModel接口的Servlet,Servlet引擎仍然支持对该Servlet的多线程并发访问,其采用的方式是产生多个Servlet实例对象,并发的每个线程分别调用一个独立的Servlet实例对象。
实现SingleThreadModel接口并不能真正解决Servlet的线程安全问题,因为Servlet引擎会创建多个Servlet实例对象,而真正意义上解决多线程安全问题是指一个Servlet实例对象被多个线程同时调用的问题。事实上,在Servlet API 2.4中,已经将SingleThreadModel标记为Deprecated(过时的)。
实例:
出现线程问题:
package net.csdn; importjava.io.IOException; importjava.io.PrintWriter; importjavax.servlet.ServletException; importjavax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; importjavax.servlet.http.HttpServletResponse; public class NewServlet2 extendsHttpServlet { privateint count = 0; publicvoid doGet(HttpServletRequest request, HttpServletResponse response) throwsServletException, IOException { try{ count++; Thread.sleep(1000*5); PrintWriterout = response.getWriter(); out.println(count); }catch (InterruptedException e) { //TODO Auto-generated catch block e.printStackTrace(); } } publicvoid doPost(HttpServletRequest request, HttpServletResponse response) throwsServletException, IOException { doGet(request,response); } @Override publicvoid init() throws ServletException { System.out.println("验证请求时调用init()"); } }
此种方法便会出现线程问题,因为如果再多个客户端向服务器发送多个请求时,这多个客户端会处理同一个临界资源count,这时就容易产生一些线程问题。
解决线程问题:
packagenet.csdn; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; importjavax.servlet.http.HttpServletResponse; public class NewServlet2 extendsHttpServlet { privateint count = 0; publicvoid doGet(HttpServletRequest request, HttpServletResponse response) throwsServletException, IOException { //同步代码块 synchronized(this) { try{ count++; Thread.sleep(1000*5); //必须锁在一起 PrintWriterout = response.getWriter(); out.println(count); }catch (InterruptedException e) { //TODO Auto-generated catch block e.printStackTrace(); } } } publicvoid doPost(HttpServletRequest request, HttpServletResponse response) throwsServletException, IOException { doGet(request,response); } @Override publicvoid init() throws ServletException { System.out.println("刚一加载时init()"); } }
上述做法中将容易引发线程问题的代码放在了同步代码块内,解决了这个问题。
ServletConfig对象
在Servlet的配置文件中,可以使用一个或多个<init-param>标签为servlet配置一些初始化参数。
获取WEB应用的初始化参数。
<context-param> <param-name> data</param-name> <param-value> xxxx</param-value> </context-param>
如:
<servlet> <servlet-name>NewServlet</servlet-name> <servlet-class>net.csdn.NewServlet</servlet-class> <init-param> <param-name>date</param-name> <param-value>lijizh1013</param-value> </init-param> <init-param> <param-name>name</param-name> <param-value>lijizh</param-value> </init-param> </servlet>
当servlet配置了初始化参数后,web容器在创建servlet实例对象时,会自动将这些初始化参数封装到ServletConfig对象中,并在调用servlet的init方法时,将ServletConfig对象传递给servlet。进而,程序员通过ServletConfig对象就可以得到当前servlet的初始化参数信息。
阅读ServletConfig API,并举例说明该对象的作用:
– 获得字符集编码
– 获得数据库连接信息
– 获得配置文件
获取配置文件信息:
1.通过成员变量在初始化时获取ServletConfig对象。
packagenet.csdn; 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; publicclass NewServlet extends HttpServlet { //1.使用成员变量 privateServletConfig config; public void doGet(HttpServletRequestrequest, HttpServletResponse response) throws ServletException,IOException { PrintWriter out =response.getWriter(); //通过获取到的成员变量的config对象逐个获取参数名称及对应的参数值 String date =config.getInitParameter("date"); String name =config.getInitParameter("name"); String date =config.getInitParameter("date"); String name =config.getInitParameter("name"); out.println(names); out.println(values); } public void doPost(HttpServletRequestrequest, HttpServletResponse response) throws ServletException,IOException { doGet(request, response); } //通过inint方法为config成员变量赋值 @Override public void init(ServletConfig config)throws ServletException { super.init(config); this.config = config; } }
2.通过Servlet的this对象获取
packagenet.csdn; 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; publicclass NewServlet extends HttpServlet { //1.使用成员变量 public void doGet(HttpServletRequestrequest, HttpServletResponse response) throws ServletException,IOException { PrintWriter out =response.getWriter(); //通过获取到的this的config对象逐个获取参数名称及对应的参数值 String date =this.getServletConfig().getInitParameter("date"); String name =this.getServletConfig().getInitParameter("name"); String date =this.getServletConfig().getInitParameter("date"); String name =this.getServletConfig().getInitParameter("name"); out.println(names); out.println(values); } public void doPost(HttpServletRequestrequest, HttpServletResponse response) throws ServletException,IOException { doGet(request, response); } }
3.获取到config对象后获取参数值的其他方式:
例:使用this对象获取到config对象后的取值过程
package net.csdn; 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; publicclass NewServlet extends HttpServlet { public void doGet(HttpServletRequestrequest, HttpServletResponse response) throws ServletException,IOException { PrintWriter out =response.getWriter(); //通过获取到的this的config对象逐个获取参数名称及对应的参数值 String date = this.getServletConfig().getInitParameter("date"); String name =this.getServletConfig().getInitParameter("name"); String date =this.getServletConfig().getInitParameter("date"); String name =this.getServletConfig().getInitParameter("name"); out.println(names); out.println(values); //通过获取Enumeration枚举对象然后再逐个遍历参数及参数值 Enumeration e =this.getServletConfig().getInitParameterNames(); while(e.hasMoreElements()){ String names = (String)e.nextElement(); String values =(String)this.getServletConfig().getInitParameter(names); out.println(names); out.println(values); } } public void doPost(HttpServletRequestrequest, HttpServletResponse response) throws ServletException,IOException { doGet(request, response); } }