Servlet Filter

- Filter help to block a malicious request if they are not allowed to access a backend resource.
- Filter also help to manipulate the responses that will be sent back to the client.

Different types of filters:

1. Authentication-Blocking: Block or allow any requests based on user identity.
2. Logging and auditing: Helps in user tracking.
3. Image conversion: Helps with scaling maps
4. Data compression: Reduces download size
5. Localization: Helps support the code in various countries based on different languages and formatting. 
6. Encryption filters
7. Tokenizing filters
8. Mime-type chaining filters
9. Caching filters
10. XSL/T Filters: Helps transforming XML requests.

- Filters are specified in web.xml and are executed in the order specified in the file.
- Filter java class implements javax.servlet.Filter and implements these 3 methods: init(), doFilter(), destroy()

import javax.servlet.*;
import java.util.logging.Filter;

public class TestFilter implements Filter {
    public void  init(FilterConfig config) throws ServletException {
        //Get and print init param                
        String testParam = config.getInitParameter("param1");
        System.out.println("Test Param: " + testParam);
    }

    public void  doFilter(ServletRequest request,
                          ServletResponse response,
                          FilterChain chain)
            throws java.io.IOException, ServletException {

        // Get the method of the request
        String method = request.getMethod();
        logger.info("Request Method is: ", method);

        // Pass request back down the filter chain
        chain.doFilter(request,response);
    }
    public void destroy( ){
        //close any resources here        }
}

Compile TestFilter.java in usual way and put your class file in <Tomcat-installation-directory>/webapps/ROOT/WEB-INF/classes

Make following config change in the web.xml

<filter>
    <filter-name>TestFilter</filter-name>
    <filter-class>TestFilter</filter-class>
    <init-param>
        <param-name>param1</param-name>
        <param-value>Test</param-value>
    </init-param>
</filter>
<filter-mapping>
<filter-name>TestFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>

The TestFilter will apply to all the servlets since we specified url-pattern as /*. For example if we want to apply TestFilter to only a class called as TestFile then the config will look like

<filter>
    <filter-name>TestFilter</filter-name>
    <filter-class>TestFilter</filter-class>
    <init-param>
        <param-name>param1</param-name>
        <param-value>Test</param-value>
    </init-param>
</filter>
<filter-mapping>
<filter-name>TestFilter</filter-name>
<url-pattern>TestFile</url-pattern>
</filter-mapping>

Multiple filters

<filter>
    <filter-name>TestFilter</filter-name>
    <filter-class>TestFilter</filter-class>
    <init-param>
        <param-name>param1</param-name>
        <param-value>Value1</param-value>
    </init-param>
</filter>

<filter-mapping>
<filter-name>TestFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>

<filter>
<filter-name>TestFilter2</filter-name>
<filter-class>TestFilter2</filter-class>
<init-param>
    <param-name>param</param-name>
    <param-value>Value</param-value>
</init-param>
</filter>

<filter-mapping>
<filter-name>TestFilter2</filter-name>
<url-pattern>TestFile</url-pattern>
</filter-mapping>

In this example, TestFilter will be applied followed by TestFilter2.

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