The Backend Engineer’s Journal

How to run main program from Maven?

mvn compile exec:java -Dexec.mainClass=com.TestMain

Spring ApplicationContextAware Example

Spring provides ApplicationContextAware interface to get the ApplicationContext anywhere in our code. In order to get this feature we need to implement ApplicationContextAware interface in our beans and override setApplicationContext method.

Here is one example to get the context from CustomerGroup bean.

Customer.java

public class Customer {
     private String name;
     public String getName() {
         return name;
     }
     public void setName(String name) {
         this.name = name;
     }
     @Override
     public String toString() {
         return "Customer [name=" + name + "]";
     }
 
 
 }

CustomerGroup.java

import org.springframework.beans.BeansException;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationContextAware;
 
 public class CustmerGroup implements ApplicationContextAware {
     
     ApplicationContext context;
     public ApplicationContext getContext() {
         return context;
     }
 
     @Override
     public void setApplicationContext(ApplicationContext context)
             throws BeansException {
         this.context = context;
     }
 
 }

Application-context.xml

<?xml version="1.0" encoding="UTF-8"?>
 <beans xmlns="http://www.springframework.org/schema/beans"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://www.springframework.org/schema/beans
  http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
     <bean id="customer" class="com.pretech.test.Customer">
         <property name="name" value="Vinod Kumaran" />
     </bean>
     <bean id="customerGroupBean" class="com.pretech.test.CustmerGroup" />
 </beans>

Test class

import org.springframework.context.ApplicationContext;
 import org.springframework.context.support.ClassPathXmlApplicationContext;
  
 public class AppContextMain {
  
     public static void main(String[] args) {
  
  
         ApplicationContext context = new ClassPathXmlApplicationContext(
                 "application-context.xml");
  
         CustmerGroup customerGroupBean = (CustmerGroup) context
                 .getBean("customerGroupBean");
  
         //Getting context from customerGroup bean and getting customer from that context
         System.out.println(customerGroupBean.getContext().getBean(Customer.class));
     }
  
 }

Output

Customer [name=Vinod Kumaran

Spring BeanNameAware Example

BeanNameAware Interface to be implemented by beans that want to be aware of their bean name in a bean factory. If a bean in spring implements BeanNameAware then that bean has to implement a method that is setBeanName. And when that bean is loaded in spring container, the name is set to this method.

Here is an example to implement this interface and printing the name when it is loaded.

Student.java (Which implements BeanNameAware)

import org.springframework.beans.factory.BeanNameAware;

public class Student implements BeanNameAware {
    @Override
    public void setBeanName(String name) {
        System.out.println("Setting bean name " + name);

    }

}

application-context.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
  http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
    <bean id="student" class="com.pretech.test.Student" />
</beans>

Test class

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class BeanNameAwareTest {

    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext(
                "application-context.xml");
    }
}

Output

INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@48a930: defining beans [student]; root of factory hierarchy
Setting bean name student

Apache Camel Mongodb component Example

Apache camel provides many components to interact with external systems, here is one simple example to connect mongodb using from apache camel route.

Create a maven project and add below dependencies

<dependencies>
<dependency>
<groupId>org.apache.camel</groupId>
<artifactId>camel-core</artifactId>
<version>2.12.1</version>
</dependency>
<dependency>
<groupId>org.apache.camel</groupId>
<artifactId>camel-spring</artifactId>
<version>2.12.1</version>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-core</artifactId>
<version>3.2.4.RELEASE</version>
</dependency>

<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-beans</artifactId>
<version>3.2.4.RELEASE</version>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context</artifactId>
<version>3.2.4.RELEASE</version>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-web</artifactId>
<version>3.2.4.RELEASE</version>
</dependency>
<dependency>
<groupId>org.apache.camel</groupId>
<artifactId>camel-jetty</artifactId>
<version>2.12.1</version>
</dependency>
<dependency>
<groupId>org.apache.camel</groupId>
<artifactId>camel-mongodb</artifactId>
<version>2.12.1</version>
</dependency>
</dependencies>

Create a Camel route

package com.vinod.test;

import org.apache.camel.builder.RouteBuilder;

public class CamelMongoRoute extends RouteBuilder {
@Override
public void configure() throws Exception {
from("jetty:http://localhost:8181/mongoSelect")
.to("mongodb:myDb?database=customerdb&collection=customer&operation=findAll");
from("jetty:http://localhost:8181/mongoInsert")
.to("mongodb:myDb?database=customerdb&collection=customer&operation=insert");
}
}

Create application context.xml

<?xml version="1.0" encoding="UTF-8"?>
 <beans xmlns="http://www.springframework.org/schema/beans"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:camel="http://camel.apache.org/schema/spring"
     xsi:schemaLocation="
            http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
           http://camel.apache.org/schema/spring http://camel.apache.org/schema/spring/camel-spring.xsd">
     <camel:camelContext id="camel-client">
         <camel:routeBuilder ref="vinodroute" />
     </camel:camelContext>
     <bean id="myDb" class="com.mongodb.Mongo">
         <constructor-arg index="0" value="localhost" />
     </bean>
     <bean id="vinodroute" class="com.vinod.test.CamelMongoRoute" />
 </beans>

Create a Main program

This main program will start the routes

package com.vinod.test;
 
 import org.springframework.context.support.ClassPathXmlApplicationContext;
 
 public class CamelMongoMain {
     @SuppressWarnings("resource")
     public static void main(String[] args) {
                       new ClassPathXmlApplicationContext("application-context.xml");
     }
 
 }
  

Run it

1. Start Mongodb
2. Run the main program
3. Use rest client to give inputs

To insert data into mongodb

Url= http://localhost:8181/mongoInsert

Request body= {name:"Vinod K K"}

Retrieve data
Url= http://localhost:8181/mongoSelect

Response from mongodb = [{ "_id" : { "$oid" : "5419cefb0364771a127e352c"} , "name" : "Vinod K K"}]

Data in mongodb
> use customerdb
switched to db customerdb
> show collections
customer
system.indexes
> db.customer.find()
{ "_id" : ObjectId("5419cefb0364771a127e352c"), "name" : "Vinod K K" }
>

Done !!!!

Download example

https://github.com/kkvinodkumaran/camel

Spring 4 AsyncRestTemplate Example

Spring's central class for asynchronous client-side HTTP access. Exposes similar methods as RestTemplate, but returns ListenableFuture wrappers as opposed to concrete results. See more about AsyncRestTemplate
1. Create a maven project with below dependencies

<dependency>
 <groupid>org.springframework</groupid>
 <artifactid>spring-core</artifactid>
 <version>4.0.6.RELEASE</version>
</dependency>
<dependency>
 <groupid>org.springframework</groupid>
 <artifactid>spring-web</artifactid>
 <version>4.0.6.RELEASE</version>
</dependency>

2. Create a main class to test AsyncRestTemplate

import java.util.concurrent.ExecutionException;
import org.springframework.http.HttpEntity;
import org.springframework.http.HttpHeaders;
import org.springframework.http.HttpMethod;
import org.springframework.http.MediaType;
import org.springframework.http.ResponseEntity;
import org.springframework.util.concurrent.ListenableFuture;
import org.springframework.web.client.AsyncRestTemplate;

public class AsyncRestTemplateExample {

 public static void main(String[] args) {

  String url = "http://pretechsol.com";
  AsyncRestTemplate asyncRestTemplate = null;
  HttpMethod method = null;
  try {

   // Create AsyncRestTemplate object
   asyncRestTemplate = new AsyncRestTemplate();

   // Define http method
   method = HttpMethod.GET;

   // Define response type
   Class responseType = String.class;

   // Define headers

   HttpHeaders headers = new HttpHeaders();
   headers.setContentType(MediaType.TEXT_PLAIN);

   HttpEntity<String> requestEntity = new HttpEntity<String>("params",
     headers);
   ListenableFuture<ResponseEntity<String>> future = asyncRestTemplate
     .exchange(url, method, requestEntity, responseType);

   ResponseEntity<String> entity = future.get();
   System.out.println(entity.getBody());
  } catch (InterruptedException e) {
   e.printStackTrace();
  } catch (ExecutionException e) {
   e.printStackTrace();
  }
 }

}

Camel Timer Example

Camel Timer component is used to generate or process message exchanges when a time fires. Here is one example to print ‘Hello world ‘ in each five seconds

Example

package com.vinod.test;
 import org.apache.camel.Exchange;
 import org.apache.camel.Processor;
 import org.apache.camel.builder.RouteBuilder;
 import org.apache.camel.main.Main;
 public class TimerTest {
     public static void main(String... args) throws Exception {
         Main main = new Main();
         main.enableHangupSupport();
         main.addRouteBuilder(new TimerRoute());
         main.run(args);
     }
 }
 
 class TimerRoute extends RouteBuilder {
     @Override
     public void configure() throws Exception {
         from("timer://foo?period=5000").process(new Processor() {
             public void process(Exchange exchange) throws Exception {
                 System.out.println("Hello world  :"
                         + System.currentTimeMillis());
             }
         });
     }
 }

Output

Hello world :1453013393616

Hello world :1453013398609

Hello world :1453013403614

Hello world :1453013408617

Hello world :1453013413621

Hello world :1453013418624

Done!! download source codes from

https://github.com/kkvinodkumaran/camel

Ref: http://camel.apache.org/timer.html

How to sort list of objects with multiple attributes?

We can sort list of objects using Collections.sort(Collection c) or Collections.sort(Collection c,Comparator) based on compareTo,compare implementations. Some scenarios we need to sort objects based on multiple attributes. Here is one simple example to sort student objects based on the department, name and address using apache common api.

Create a Maven project and add below dependency.

<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-lang3</artifactId>
<version>3.0</version>
</dependency>

Create a pojo class (Student.java)

package com.test.vinod;

public class Student {
 private String name;
 private String address;
 private String dept;

 public Student(String dept, String name, String address) {
  super();
  this.dept = dept;
  this.name = name;
  this.address = address;
 }

 //getters and setters

 @Override
 public String toString() {
  return dept + " " + name + " "
    + address;
 }

}

StudentComparator.java

package com.test.vinod;

import java.util.Comparator;

import org.apache.commons.lang3.builder.CompareToBuilder;

public class StudentComparator implements Comparator<Student> {

 public int compare(Student o1, Student o2) {
  return new CompareToBuilder().append(o1.getDept(), o2.getDept())
    .append(o1.getName(), o2.getName())
    .append(o1.getAddress(), o2.getAddress()).toComparison();
 }

}

Main class (StudentSort.java)

package com.test.vinod;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class StudentSort {

 public static void main(String[] args) {
  Student s1 = new Student("Commerce", "Vinod", "Bangalore");
  Student s2 = new Student("Electronics", "Ram", "Chennai");
  Student s3 = new Student("Electronics", "Vinod", "Bangalore");
  Student s4 = new Student("Commerce", "Nirmal", "Bombay");
  Student s5 = new Student("Electronics", "Ashok", "Delhi");
  Student s6 = new Student("Electronics", "Prem", "Agra");
  Student s7 = new Student("Commerce", "Anoop", "Bangalore");
  Student s8 = new Student("Electronics", "Santhosh", "Cochin");
  Student s9 = new Student("Commerce", "Vinod", "Agra");

  List<Student> studentList = new ArrayList<Student>();
  studentList.add(s1);
  studentList.add(s2);
  studentList.add(s3);
  studentList.add(s4);
  studentList.add(s5);
  studentList.add(s6);
  studentList.add(s7);
  studentList.add(s8);
  studentList.add(s9);

  System.out.println("Before sorting:");
  for (Student st : studentList) {
   System.out.println(st);
  }
  Collections.sort(studentList, new StudentComparator());

  System.out.println("\n\nAfter sorting:");
  for (Student st : studentList) {
   System.out.println(st);
  }

 }

}

Output

Before sorting:
Commerce Vinod Bangalore
Electronics Ram Chennai
Electronics Vinod Bangalore
Commerce Nirmal Bombay
Electronics Ashok Delhi
Electronics Prem Agra
Commerce Anoop Bangalore
Electronics Santhosh Cochin
Commerce Vinod Agra 

After sorting:
Commerce Anoop Bangalore
Commerce Nirmal Bombay
Commerce Vinod Agra
Commerce Vinod Bangalore
Electronics Ashok Delhi
Electronics Prem Agra
Electronics Ram Chennai
Electronics Santhosh Cochin
Electronics Vinod Bangalore

The Backend Engineer’s Journal

How to run main program from Maven?

Spring ApplicationContextAware Example

Customer.java

CustomerGroup.java

Application-context.xml

Spring BeanNameAware Example

Student.java (Which implements BeanNameAware)

application-context.xml

Apache Camel Mongodb component Example

Create a maven project and add below dependencies

Create a Camel route

Create application context.xml

Spring 4 AsyncRestTemplate Example

Camel Timer Example

Example

How to sort list of objects with multiple attributes?

Create a Maven project and add below dependency.

StudentComparator.java

Output

Confusion Matrix + Precision/Recall (Super Simple, With Examples)

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