Filter filter

1 Concept

1. filter The filters are javaWeb One of the three components .
2. It's also an interface
3. The main function is to intercept requests , Filtering response

The main application scenarios are .

• Permission check
• Diary operation
• Business management
• ....

1.1 Filter example

This is a filter used to determine whether the user has logged in .

public class ManagerFilter implements Filter {
    

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    

    }

	// Conduct Filter Business 
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
    
        HttpServletRequest httpServletRequest = (HttpServletRequest) servletRequest;
        Object user = httpServletRequest.getSession().getAttribute("user");
        if (user == null) {
    
          	httpServletRequest.getRequestDispatcher("/pages/user/login.jsp").forward(servletRequest,servletResponse);
        } else {
    
        	//filterChain  The function is similar to request forwarding .
            filterChain.doFilter(servletRequest,servletResponse);
        }
    }
    @Override
    public void destroy() {
    

    }
}

web.xml Configuration in

<servlet>
   <servlet-name>BookServlet</servlet-name>
    <servlet-class>com.atguigu.web.BookServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>BookServlet</servlet-name>
    <url-pattern>/manager/bookServlet</url-pattern>
</servlet-mapping>

<filter>
    <filter-name>ManagerFilter</filter-name>
    <filter-class>com.atguigu.filter.ManagerFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>ManagerFilter</filter-name>
    <url-pattern>/pages/manager/*</url-pattern>
    <!-- Here, you need to ensure that the address of the filter and the request service are the same , This is the access address to be blocked by the filter -->
    <url-pattern>/manager/bookServlet</url-pattern>
</filter-mapping>

As can be seen from the above example

1. xml in Of an object url There can be more than one .
2. The filter has no specific access address , All addresses must be the addresses of other components that already exist , It's the same with Filter Corresponding to , A project will not have only one place to check whether the user logs in .

1.2 working principle

This is how the filter example above works . Of course , A real project cannot be so simple .

1.3 Life cycle

1. Construction method
2. init Initialization method
   front 1、2 Bu huizai web Execute... At startup . Namely Servlet Not yet created FIlter And we're done .
3. doFilter Filtration method
   The first 3 Step will be executed for each interception
4. destroy The destruction
   The first 4 Step , stop it web Execution of the project .

1.4 FilterConfig class

FilterConfig when Filter Configuration file for
and Servlet be similar Tomcat Each creation Filter A corresponding FilterConfig class , Contains Filter Configuration information .

FilterConfig The role of

1. get Filter The name of filter-name
2. get Filter Is the initialization parameter of init-param Initialize parameters
3. obtain ServletContext object

java Code ：

@Override 
public void init(FilterConfig filterConfig) throws ServletException {
     
	System.out.println("2.Filter  Of  init(FilterConfig filterConfig) initialization "); 
	// 1、 obtain  Filter  The name of  filter-name  The content of  
	System.out.println("filter-name  The value of is ：" + filterConfig.getFilterName()); 
	// 2、 To get in  web.xml  Configured in  init-param  Initialize parameters  
	System.out.println(" Initialize parameters  username  The value of is ：" + filterConfig.getInitParameter("username")); 
	System.out.println(" Initialize parameters  url  The value of is ：" + filterConfig.getInitParameter("url")); 
	// 3、 obtain  ServletContext  object  
	System.out.println(filterConfig.getServletContext()); 
}

A complete point Filter xml To configure

<!--filter  The tag is used to configure a  Filter  filter --> 
<filter> 
	<filter-name>AdminFilter</filter-name> 
	<filter-class>com.atguigu.filter.AdminFilter</filter-class>
	
	<init-param> 
		<param-name>username</param-name> 
		<param-value>root</param-value> 
	</init-param> 
	
	<init-param> 
		<param-name>url</param-name> 
		<param-value>jdbc:mysql://localhost3306/test</param-value> 
	</init-param> 
</filter>

1.5 FilterChain Filter chain

You can pack here FilterChain The image is called a two-way Chain .

This chain is linked and the order cannot be changed . If Filter02 If there is an error, it will return directly , No more to Servlet send data , If Servlet error , The system returns no direct error .
Post code has no format restrictions . The only thing to notice , He wrote that filterChain.doFilter() hinder ,Filter The order of execution of is determined by ,Filter stay xml The order of definitions in determines , Cannot be changed in the program .

Add post code

@Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
    
        try {
    
            filterChain.doFilter(servletRequest,servletResponse);
            // The program runs all the way from the last business to here , Ensure that the database operation is absolutely correct .
            JdbcUtils.commitAndClose();//  Commit transaction 
        } catch (Exception e) {
    
            JdbcUtils.rollbackAndClose();// Roll back the transaction 
            e.printStackTrace();
            throw new RuntimeException(e);// Throw the anomaly to Tomcat Manage and display friendly error pages 
        }
    }

1.6 Filter Configuration of connection path

At the beginning of the path / It stands for http://ip:port/ Project path

1.6.1 Precise configuration

<url-pattern>/target.jsp</url-pattern>

1.6.2 Directory configuration

<url-pattern>/pages/manager/*</url-pattern>

1.6.3 Suffix path

<url-pattern>/*.html</url-pattern>

Filter The filter only cares about whether the requested address matches , Does not care if the requested resource exists , That's about the wrong page .

1.7 ThreadLocal Use

ThreadLocal characteristic ：

1. You can associate a data for the current thread , But only one （ It's OK map And access data ）
2. Every ThreadLocal object , Only one data can be associated for the current thread , If you want to associate multiple , You can only use multiple ThreadLocal Object instances .
3. Every ThreadLocal Object instances are defined in static type
4. ThreaLocal Data stored in , When the thread is destroyed . Will be JVM The virtual machine is automatically released .

Usage method

// establish ThreadLocal  object 
public static ThreadLocal<Object> threadLocal = new ThreadLocal<Object>();
// Set up  threadLocal  Value 
threadLocal.set("value");
// get  threadLocal  Value 
threadLocal.get(); // Values can be obtained without parameters 

Use ThreadLocal The benefits of , No matter what ThreadLocal new Where is the , Each thread is unique , It can prevent high concurrency , Release data error .

1.8 Use Filter and ThreadLocal Portfolio Management jdbc Business

Principle analysis ：

Transaction rollback is set in Filter Post code of , Be sure to throw exceptions when processing transactions , Otherwise, the transaction cannot return .

Set the error interface , There is no need to set error jump for the error page alone , Just configure , If an error occurs, the system will automatically go to the error page .

<!--error-page  Label configuration , After server error , Auto jump page --> 
<error-page> 
	<!--error-code  Is the wrong type --> 
	<error-code>500</error-code> 
	<!--location  The label indicates . The page path to jump to --> 
	<location>/pages/error/error500.jsp</location> 
</error-page>