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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Java Servlet Client Request

Client request from java servlet can contain a lot of data and most of the data can be retrieved using different methods. Here are those methods. Most methods are self explanatory so their explanation is not provided.

1. getLocale: Returns the preferred locale that the client will accept the locale in.
2. getAttribute(String name): Returns the value of the attribute.
3. getInputStream(): Returns body of the request in binary data.
4. getAuthType: Returns BASIC, SSL or NULL
5. getCharacterEncoding: Returns encoding type like ASCII, 8-bit, UTF-8, UTF-16
6. getContentType: Returns MIME type(Type of data like application/json/xml, etc). Specifying it will help the application to decode it.
7. getContextPath: Returns URI's portion that returns context of the request.
8. getHeader(String name): Returns values of the specified header.
9. getMethod: Returns Get, post, put, delete
10. getRequestURI()
11. getRequestedSessionId()
12. getServletPath()
13. getParameterValues(String name)
14. isSecure()
15. getContentLength(): Returns the length of the request body or -1 if the length is not know.
16. getIntHeader(String name): Returns value of the header as an int.
17. getServerPort(): Returns the port number on which this request was received.
18. getParameter(String name): Returns the parameter value or null if the param doesn't exist.
19. getPathInfo(): Returns any extra path info associated with the URL.
20. getProtocol(): Returns protocol like HTTP, HTTPS, etc
21. getQueryString():
22. getCookies(): Returns all the cookies from the client's request as an array.
23. getAttributeNames(): Returns an enum of all the attributes from the request.
24. getHeaderNames: Returns an enum of all the headers from the request.
25. getParameterNames: Returns an enum of all the parameters from the request.
26. getSession: Returns the current session or creates a new one.
27. getSession(boolean create): Returns the current session if one exists and will create a new one only if(boolean = true).
28. getRemoteAddr()
29. getRemoteHost()
30. getRemoteUser()

Example
This is how any method is needed to be applied to the request to retrieve a specific value.
request.getHeaderNames();

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