Do the back-end web engineer, can not escape the web.xml, we all know that this configuration file is the daily work, then to our knowledge must know some sort.
We started the web project, the first thing is to load the web.xml file, the file only after all the configuration information Yes, web project one truly up and running.
Then we open the door with configuration information web.xml it.
First, we need to be sure one is, regardless of the configuration sequence web.xml which is kind of how the information as a whole, our execution order is not executed according to your configuration file written in the order, have their own web.xml execution order
Initialization process:
- When starting Web items, containers (such as Tomcat) will read two nodes <listener> and <contex-param> web.xml configuration file.
- Then the container creates a ServletContext (context), that can be used throughout the WEB project within the context of this application.
- Then the container will be read into the <context-param> into key-value pairs and to ServletContext.
- Create a container class instance <listener> </ listener>, that is create a listener (Note: listener classes can be defined in a custom class needs to inherit but must ServletContextListener).
- Listening class have a contextInitialized (ServletContextEvent event) initialization method, in this method may be a value obtained by the context-param set event.getServletContext (). GetInitParameter ( "contextConfigLocation"). There must also be in this class a contextDestroyed (ServletContextEvent event) destruction methods. For releasing resources before closing the application, for example, close the database connection.
- After obtaining the value of context-param, you can do some operate. Note that this time your WEB project has not been fully completed to start. This action will be earlier than all of the Servlet.
Is performed under the same sequence is performed in accordance with types of the mapping information corresponding to
In accordance with the order of execution xml, let's look at each in turn execute the order of the label is what is meant
Worth noting that, web project at startup, container (Tomcat) creates a domain object ServletContext, we briefly explain
ServletContext
The official explanation is that the servlet context, when activated, will give each server a project to create an object that is ServletContext object, and the object is a globally unique. And the whole project within the servlet share this object.
First, let's remember, ServletContext is a domain object, do not rush, which gives you tell us about the domain objects
By the figure, I believe you can understand in this domain objects are to act as what role
So exactly what is it the domain object
Storage server domain object is created in memory for the transfer and sharing of resources between different dynamic (the servlet) Data
Any domain object has the following three methods
| SetAttribute(name,value) |
Adding data to the field inside the object, is added in the form of key-value added time |
| GetAttribute(name); |
Reading data inside the object domain according to the specified key |
| RemoveAttribute(name) |
Delete data from the inside of the object with the specified key field |
I believe that here, played a lot of children's shoes have learned the code is valid and whether the domain object thing, SetAttribute these methods, when we pass data to the domain object, is frequently used.
So then we can come to find out
context-param
Action: This element is used within the context initialization parameters declared application (the entire WEB item).
Param-name in there with param-value
param-name setting parameter name context. The name must be unique
param-value parameter setting name
After the reading is completed, we are transforming this information into key-value pairs, handed over ServletContext
According to the order, we need to know that next Listener
Listener translates to the listener, as the name suggests is used to monitor our web activity, this is well understood, servlet provided a total of eight listeners
Three domain objects are created and destroyed listeners
| Object Types |
Corresponding listener |
| ServletContext |
ServletContextListener |
| HttpSession |
HttpSessionListener |
| HttpServletRequest |
HttpServletRequestListener |
Listener object property changes three listener ( property to add, remove property, property replacement )
| Object Types |
Corresponding listener |
| ServletContext |
ServletContextAttributeListener |
| HttpServletRequest |
ServletRequestAttributeListener |
| HttpSession |
HttpSessionAttributeListener |
Monitor the change of state JavaBean HttpSession object (binding, unbinding the passivation and activation)
| Object Types |
Corresponding listener |
| HttpSession |
HttpSessionBindingListener (bind, unbind) |
| HttpSession |
HttpSessionActivationListener (passivation and activation) |
About then listen to it here, in-depth content I will write a blog in addition to our in-depth introduction
Next is about the filter , that is, filter, filter by definition is used to filter some of the information needed
filter
Data on the web filter is actually submitted over, the information processed by the server, re-delivered to the other party. You can be understood as a place of intermediate processing.
They are often used to intercept request, and response intercept filtering
通过上图，我们可以更直观地了解到过滤器的工作角色，值得我们注意的是，过滤器可能会有多个，经过过滤器一过滤完之后，还需要通过滤器二进行过滤处理，因为每个过滤器都有自己特定的过滤功能
关于多个过滤器的执行顺序，我们请求到达Servlet之间是可以经过多个过滤器的，这些过滤器的执行顺序是依赖于<filter-mapping>里面配置的顺序。
servlet
servlet全称Java Servlet，是用Java编写的服务端程序，其主要功能在于交互式地浏览和修改数据，生成动态Web内容。
在web.xml这里主要是将你所写的servlet对应的类配置到当中去，具体的servlet的详解可以看我关于servlet详解的文章