A filter is an object used to intercept the HTTP requests and responses of your application. By using filter, we can perform two operations at two instances −
• Before sending the request to the controller
• Before sending a response to the client.
The following code shows the sample code for a Servlet Filter implementation class with @Component annotation.
@Component
public class SimpleFilter implements Filter {
@Override
public void destroy() {}

@Override
public void doFilter
(ServletRequest request, ServletResponse response, FilterChain filterchain)
throws IOException, ServletException {}

@Override
public void init(FilterConfig filterconfig) throws ServletException {}
}
The following example shows the code for reading the remote host and remote address from the ServletRequest object before sending the request to the controller.
In doFilter() method, we have added the System.out.println statements to print the remote host and remote address.

package com.tecklearn.demo;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

import org.springframework.stereotype.Component;

@Component
public class SimpleFilter implements Filter {
@Override
public void destroy() {}

@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterchain) 
throws IOException, ServletException {

System.out.println("Remote Host:"+request.getRemoteHost());
System.out.println("Remote Address:"+request.getRemoteAddr());
filterchain.doFilter(request, response);
}

@Override
public void init(FilterConfig filterconfig) throws ServletException {}
}
In the Spring Boot main application class file, we have added the simple REST endpoint that returns the “Hello World” string.
package com.tecklearn.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
@RestController
public class DemoApplication {
public static void main(String[] args) {
SpringApplication.run(DemoApplication.class, args);
}
@RequestMapping(value = "/")
public String hello() {
return "Hello World";
}
}
The code for Maven build – pom.xml is given below −
<?xml version = "1.0" encoding = "UTF-8"?>
<project xmlns = "http://maven.apache.org/POM/4.0.0" xmlns:xsi = "
http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation = "http://maven.apache.org/POM/4.0.0 
http://maven.apache.org/xsd/maven-4.0.0.xsd">

<modelVersion>4.0.0</modelVersion>
<groupId>com.tecklearn</groupId>
<artifactId>demo</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>
<name>demo</name>
<description>Demo project for Spring Boot</description>

<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>1.5.8.RELEASE</version>
<relativePath/> 
</parent>

<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
<java.version>1.8</java.version>
</properties>

<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>

<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-test</artifactId>
<scope>test</scope>
</dependency>
</dependencies>

<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
</plugins>
</build>

</project>
The code for Gradle Build – build.gradle is given below −
buildscript {
ext {
springBootVersion = '1.5.8.RELEASE'
}
repositories {
mavenCentral()
}
dependencies {
classpath("org.springframework.boot:spring-boot-gradle-plugin:${springBootVersion}")
}
}

apply plugin: 'java'
apply plugin: 'eclipse'
apply plugin: 'org.springframework.boot'

group = 'com.tecklearn'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = 1.8

repositories {
mavenCentral()
}
dependencies {
compile('org.springframework.boot:spring-boot-starter-web')
testCompile('org.springframework.boot:spring-boot-starter-test')
}

You can create an executable JAR file, and run the Spring Boot application by using the Maven or Gradle commands shown below −
For Maven, use the command as shown below −
mvn clean install
After BUILD SUCCESS, you can find the JAR file under the target directory.
For Gradle, use the command as shown below −
gradle clean build
After BUILD SUCCESSFUL, you can find the JAR file under the build/libs directory.
Now, run the JAR file by using the following command
java –jar <JARFILE>
You can see the application has started on the Tomcat port 8080.
Now hit the URL http://localhost:8080/ and see the output Hello World. It should look as shown below −

Then, you can see the Remote host and Remote address on the console log as shown below −

So, this brings us to the end of blog. This Tecklearn ‘Creating Servlet Filter using Spring Boot’ blog helps you with commonly asked questions if you are looking out for a job in Java Programming. If you wish to learn Spring Boot and build a career Java Programming domain, then check out our interactive, Java and JEE Training, that comes with 24*7 support to guide you throughout your learning period. Please find the link for course details:

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• Introduction to J2EE
• Client Server architecture
• URL, Port Number, Request, Response
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• Servlet fundamentals
• Setting up a web project in Eclipse
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• GET and POST request in web application with demo
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• Servlets Continued
• Session tracking and filter
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