Java Web Development--filter Filter

1.1 Definitions

A filter is a server-side component that intercepts request and response information from the client and filters the information.

1.2 Working principle

1. When the project starts, load the filter from the Web container;

2, the filter exists between the user request and the Web resources;

3, the user request and the Web response between the sending and receiving through the filter filter according to filter rules.

1.3 Life cycle of filters

Instantiation (Web. XML load) → Initialize (init method) → filter (Dofilter method) → Destroy (Destroy method)

1 . Initialize: The init () method is called when the container loads the filter for the first time. The class contains a reference to the Filter Config object in this method. Our filters do not actually need to do this because the initialization information is not used, and this is only for demonstration purposes.

2, Filter: Most of the time the filter is consumed here. The Dofilter method is called by the container, passing in a reference to the servlet request, Servlet Response, and Filter Chain objects that are pointed to in the requests/response chain respectively. The filter then has the opportunity to process the request, pass the processing task to the next resource in the chain (by calling the Dofilter method on the Filter Chain object reference), and then handle the response when processing control returns the filter.

3 . Destroy: The container immediately calls the Destroy () method before garbage collection, so that any necessary cleanup code can be executed.

Filter chain: The server is assembled into a chain in the order defined by the filter in Web. xml

1.4 Basic configuration of the filter in Web. xml

Configuration of the filter in Web. xml:

<Filter><Filter-name> Filter Name</Filter-name><Filter-class> Filter Full class name</Filter-class><Init-param><Description> Description information, can be omitted</Description><Param-name> Parameter name</Param-name><Param-value> Parameter values</Param-value></Init-param></Filter><Filter-mapping> <filter-name< Span style= "COLOR: #0000ff" >> Filter name </filter-name< Span style= "COLOR: #0000ff" >> <url-pattern>url</url-pattern> <dispatcher>[request| include| forward| Error]</dispatcher ></filter-mapping                 

1.5 Classification of filters

Classification Table of filters

Type	Role
REQUEST	When a user accesses a page directly, the Web container invokes the filter
FORWARD	When the target resource is accessed through the RequestDispatcher forward method, the filter is called
INCLUDE	When the target resource is called through the include method of RequestDispatcher, the filter is called
ERROR	When a target resource is called through a declarative exception handling mechanism, the filter is called
ASYNC (Servlet3.0 new addition)	Supports asynchronous processing

Second, the basic use of the filter

2.1 Design of Filter class

Implement the filter interface, overriding the Init, DoFilter, Destroy methods

PublicClass MyfilterImplementsfilter{Publicvoid init (Filterconfig filterconfig)Throwsservletexception{}/** * make the browser do not cache the filter of the page */public Span style= "COLOR: #0000ff" >void doFilter (servlerrequest request,servletresponse response,filterchain Filterchian) Span style= "COLOR: #0000ff" >throws ioexception,servletexception{( HttpServletResponse) response). SetHeader ("Cache-control", "No-cache" // Pass the processing task to the next resource in the filter chain } Span style= "COLOR: #0000ff" >public void Destroy () {}} 

2.2 Registration of filters in Web. xml

1, REQUEST, FORWARD, include and other types of filter registration

<!--Filter configuration-<Filter><Filter-name>myfilter</Filter-name><Filter-class>util.filter.myfilter</Filter-class></Filter><Filter-mapping> <filter-name< Span style= "COLOR: #0000ff" >>myfilter</ Filter-name> < Url-pattern>/main.jsp</ url-pattern> << Span style= "COLOR: #800000" >dispatcher>[request| forward| Include]</dispatcher ></filter-mapping                 

2. Error Type Filter Registration

<!--Configure error page, jump to/error.jsp page when 404 error occurs-<Error-page><Error-code>404</Error-code><Location>/error.jsp</Location><Error-page><!--Filter configuration-<Filter><Filter-name>errorfilter</Filter-name><Filter-class>util.filter.errorfilter</Filter-class></Filter><Filter-mapping> <filter-name< Span style= "COLOR: #0000ff" >>errorfilter</ Filter-name> < Url-pattern>/error.jsp</ url-pattern> << Span style= "COLOR: #800000" >dispatcher>error</< Span style= "COLOR: #800000" >dispatcher></filter-mapping>       

Third, the use of Servlet3.0 filter

Add @WebFilter annotations without registering in Web. XML

@WebFilter (servletnames={"Simpleservlet"},filtername= "Simplefilter")implements filter{} 

@WebFilter common property sheet

Property name	Type	Describe
FilterName	String	Specify the Name property of the filter, equivalent to <filter-name>
Value	String[]	Equivalent to the Urlpatterns attribute, but not both, and if used at the same time, use the Value property first
Urlpatterns	String[]	Specifies the URL matching pattern for a set of filters, equivalent to the <utl-pattern> tag
Servletnames	String[]	Specifies which servlets the filter will be applied to. Value is the name attribute value in @webservlet, or the <servlet-name> value in Web. xml
Dispatchertypes	Dispatchertype	Specifies the forwarding mode of the filter. Specific values include async, ERROR, FORWARD, include, REQUEST
InitParams	Webinitparam[]	Specifies a set of filter initialization parameters, equivalent to the <init-param> tag
asyncsupported	Boolean	Declares whether the filter supports asynchronous operation mode, equivalent to <async-supported> label
Description	String	The filter description information, equivalent to the <description> label
DisplayName	String	The display name of the filter, equivalent to the <display-name> label

Java Web Development--filter Filter