监听器相关知识汇总

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#Java #Listener #监听器 #spring #javaee

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本文详细介绍了Servlet监听器的概念、分类及使用方式，并通过具体示例展示了如何实现ServletContextListener和ServletContextAttributeListener，以及如何处理对象的钝化和活化。

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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