监听器相关知识汇总

监听器：监听对象的状态变化的组件。

监听器的相关要素：

1.事件源：被监听的对象，如request，session，servletContext

2.监听器：监听时间源对象，事件源对象的状态的变化都会触发监听器

3.注册监听器：将监听器与事件源进行绑定

4.响应行为：监听器监听到事件源的状态变化时，所触发的相关行为。

监听器的划分：

1.按照被监听的对象：

a.ServletRequest

b.HttpSession

c.ServletContext

2.按照监听的状态分：

a.监听域对象的创建与销毁

b.监听域对象的属性变化

监听器demo：

1.创建自定义监听器，实现ServletContextListener接口

public class MyServletContextListener implements ServletContextListener {

/**
* Default constructor.
*/
public MyServletContextListener() {

}

/**
* @see ServletContextListener#contextDestroyed(ServletContextEvent)
*/
public void contextDestroyed(ServletContextEvent arg0)  {
System.out.println("contextDestroyed...");
}

/**
* @see ServletContextListener#contextInitialized(ServletContextEvent)
*/
public void contextInitialized(ServletContextEvent arg0)  {
System.out.println("contextInitialized...");
}

}

2.在web.xml中配置监听器

<listener>
<listener-class>com.milan.listener.MyServletContextListener</listener-class>
</listener>

ServletContextListener的主要作用：

a.对象的初始化，如加载数据库驱动，连接池等

b.配置文件的初始化，如spring的相关配置文件

c.定时器，如Timer/TimerTask

属性监听器demo：

创建自定义的属性监听器，实现ServletContextAttributeListener

public class MyServletContextAttributeListener implements ServletContextAttributeListener {

/**
* Default constructor.
*/
public MyServletContextAttributeListener() {

}

/**
* @see ServletContextAttributeListener#attributeAdded(ServletContextAttributeEvent)
*/
public void attributeAdded(ServletContextAttributeEvent arg0)  {
System.out.println("attributeAdded...");
System.out.println("attributeAdded..."+arg0.getName());
System.out.println("attributeAdded..."+arg0.getValue());
}

/**
* @see ServletContextAttributeListener#attributeRemoved(ServletContextAttributeEvent)
*/
public void attributeRemoved(ServletContextAttributeEvent arg0)  {
System.out.println("attributeRemoved。。。");
}

/**
* @see ServletContextAttributeListener#attributeReplaced(ServletContextAttributeEvent)
*/
public void attributeReplaced(ServletContextAttributeEvent arg0)  {
System.out.println("attributeReplaced...");
}

}

web.xml中配置属性监听器：

<listener>
<listener-class>com.milan.listener.MyServletContextAttributeListener</listener-class>
</listener>

编写操作属性的Servlet：

public class TestServletContextAttributeListener extends HttpServlet {
private static final long serialVersionUID = 1L;

/**
* @see HttpServlet#HttpServlet()
*/
public TestServletContextAttributeListener() {
super();
// TODO Auto-generated constructor stub
}

/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
request.getServletContext().setAttribute("name", "kaka");
request.getServletContext().setAttribute("name", "milan");
request.getServletContext().removeAttribute("name");
}

/**
* @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);
}

}

控制台输出如下：

attributeAdded...

attributeAdded...name

attributeAdded...kaka

attributeReplaced...name

attributeRemoved...name

绑定解绑监听器：此监听器无需再web.xml中进行配置

Person类上绑定HttpSessionBindingListener

public class Person implements HttpSessionBindingListener{

private String username;
private int age;

public Person(String username, int age) {
super();
this.username = username;
this.age = age;
}

@Override
public String toString() {
return "Person [username=" + username + ", age=" + age + "]";
}

public String getUsername() {
return username;
}

public void setUsername(String username) {
this.username = username;
}

public int getAge() {
return age;
}

public void setAge(int age) {
this.age = age;
}

@Override
public void valueBound(HttpSessionBindingEvent arg0) {
System.out.println("person:valueBound");
}

@Override
public void valueUnbound(HttpSessionBindingEvent arg0) {
System.out.println("person:valueUnbound");
}

}
测试监听器的绑定与解绑的类TestPersonBinding
public class TestPersonBinding extends HttpServlet {
private static final long serialVersionUID = 1L;

/**
* @see HttpServlet#HttpServlet()
*/
public TestPersonBinding() {
super();
// TODO Auto-generated constructor stub
}

/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
HttpSession session = request.getSession();
Person p = new Person("milan", 1988);
session.setAttribute("person", p);
session.removeAttribute("person");
}

/**
* @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);
}

}运行后，测试结果如下：控制台打印了
person:valueBound

person:valueUnbound

钝化活化监听器：

创建Student类，实现Serializable,HttpSessionActivationListener

public class Student implements Serializable,HttpSessionActivationListener{

private String username;
private int age;

public Student(String username, int age) {
super();
this.username = username;
this.age = age;
}

@Override
public String toString() {
return "Student [username=" + username + ", age=" + age + "]";
}

public String getUsername() {
return username;
}

public void setUsername(String username) {
this.username = username;
}

public int getAge() {
return age;
}

public void setAge(int age) {
this.age = age;
}

@Override
public void sessionDidActivate(HttpSessionEvent arg0) {
System.out.println("Student：活化");
}

@Override
public void sessionWillPassivate(HttpSessionEvent arg0) {
System.out.println("Student：钝化");
}

}
创建测试类TestStudentActivation
public class TestStudentActivation extends HttpServlet {
private static final long serialVersionUID = 1L;

/**
* @see HttpServlet#HttpServlet()
*/
public TestStudentActivation() {
super();
// TODO Auto-generated constructor stub
}

/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
HttpSession session = request.getSession();
Student s = new Student("milan", 1988);
session.setAttribute("student", s);
System.out.println("student存入session");
}

/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}

}
运行Tomcat：控制台输出如下
contextInitialized...

地址栏输入：http://localhost:8081/Tomcat/TestStudentActivation，控制台输出如下

student存入session

关闭tomcat，控制台输出如下

Student：钝化

contextDestroyed...

启动tomcat：控制台输出如下

contextInitialized...

Student：活化

此时可以访问session中的Student的相关属性