October 09, 2020

## Matrix Rotation in Java

In this article, we will explore a few different ways to rotate a matrix clock by 90 degrees.

## Rotating Matrix to 90 Clockwise

Following is our Matrix.

[    1        2        3    ]

[    4        5        6    ]

[    7        8        9    ]

We will explore multiple ways to rotate this matrix clockwise 90

### With Matrix Transpose

Matrix transpose is a flipped version of the matrix along its diagonal. In short, it is the same as interchanging the rows with columns. For the above matrix the transposed matrix will look like.

[    1        4        7    ]

[    2        5        8    ]

[    3        6        9    ]

Also, the transposed matrix is equivalent to 90 Left rotation of the original array.
RoateMatrixWithTranspose
Here the Rotation of the matrix is done in two steps
1) We transpose the matrix 2) And we interchange the columns
``````public void rotateMatrixRight90Transpose(int[][] mat) {

int m = mat.length;
int n = mat[0].length;

for (int i = 0; i < m; i++) {
for (int j = i; j < n; j++) {
int x = mat[i][j];
mat[i][j] = mat[j][i];
mat[j][i] = x;
System.out.println("IC" + i + ":" + j);
}
}

// swap cols
for (int i = 0; i < m; i++) {
for (int j = 0; j < n / 2; j++) {
// swap mat[i][j] with mat[N-i-1][j]
int temp = mat[i][j];
mat[i][j] = mat[i][n - j - 1];
mat[i][n - j - 1] = temp;
}
}
}``````
Output

Matrix Rotation with Transpose

<= Original Matrix  =>

[    1        2        3    ]

[    4        5        6    ]

[    7        8        9    ]

<= After Transpose =>

[    1        4        7    ]

[    2        5        8    ]

[    3        6        9    ]

<= After Rotation =>

[    7        4        1    ]

[    8        5        2    ]

[    9        6        3    ]

## Matrix Rotation in Single pass

MatrixRotation.java
The following code will rotate the matrix in a single pass.
``````public void rotate(int[][] matrix) {
int n = matrix.length;
for (int i = 0; i < (n + 1) / 2; i++) {
for (int j = 0; j < n / 2; j++) {
int temp = matrix[n - 1 - j][i];
matrix[n - 1 - j][i] = matrix[n - 1 - i][n - j - 1];
matrix[n - 1 - i][n - j - 1] = matrix[j][n - 1 - i];
matrix[j][n - 1 - i] = matrix[i][j];
matrix[i][j] = temp;
}
}
}``````
Output

Matrix Rotation with single pass

<= Original Matrix  =>

[    1        2        3    ]

[    4        5        6    ]

[    7        8        9    ]

<= After Rotation =>

[    7        4        1    ]

[    8        5        2    ]

[    9        6        3    ]

October 07, 2020

## Eclipse Google Java Style Guide

This article shows how to use the Google Java style with Eclipse IDE. Every organization has its own style guide, in the open-source community different projects have a slightly different style guide. In this article, we will see how to use the official google style guide in Eclipse IDE.

### Get the Style Guide

Save the style guide which is a XML file to your local drive.

### Open Eclipse Preference

Opening Eclipse settings varies per OS. For example, on macOS open Eclipse -> Preferences.

### Configure save actions to automatically format code while saving files.

In the Preferences menu type “save actions”, and select Java -> Editor -> Save Actions.

## Junit 5 Example : Display Name

September 30, 2020

## JUnit 5 Tutorial : Display name

In JUnit 5, we can use @DisplayName to declare custom display names for test classes and test methods.

### Display Name Example

``````package com.bootng.display;

import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;

public class DisplayCustomNameTest {

@Test
public void test_if_it_will_rain() {
}

@Test
public void test_if_it_will_be_cloudy() {
}

@Test
public void test_if_it_will_be_sunny() {
}

}

``````
Output

### Display CustomName Example

``````package com.bootng.display;
@DisplayName("Vacation Weather Test")
public class DisplayCustNameTest {
@DisplayName("ðŸŒ§")
@Test
public void test_if_it_will_rain() {
}

@DisplayName("ðŸŒ¨")
@Test
public void test_if_it_will_be_cloudy() {
}

@DisplayName("ðŸŒž")
@Test
public void test_if_it_will_be_sunny() {
}
}
``````
Output

### Custom Name Generator Example

We can also use a Custom Name generator to generate the test names. As of Version 5.4 it comes with the following generators out of the box.

DisplayNameGenerator.IndicativeSentences
DisplayNameGenerator.ReplaceUnderscores
DisplayNameGenerator.Simple
DisplayNameGenerator.Standard

Each of the above generators has different implementations of how to generate test name.

In the Following example we are using ReplaceUnderscores.  As a result the test names will be generated by replacing the "_" from respective names.
``````package com.bootng.display;

import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;

@DisplayName("Weather Test DisplayExample ")
@DisplayNameGeneration(DisplayNameGenerator.ReplaceUnderscores.class)
public class DisplayGeneratorExampleTest {

@Test
public void test_if_it_will_rain() {
}

@Test
public void test_if_it_will_be_cloudy() {
}

@Test
public void test_if_it_will_be_sunny() {
}
}
``````

Output

## JUnit5 Important Annotations

September 30, 2020

## JUnit5 Important Annotations

Junit 5 comes with some new annotations that were not available in previous releases. Understanding these Annotations will help in writing efficient tests. This article covers some of the most commonly used annotations. In the next section, we will example of those annotations.
The following are some of the important annotations in JUnit 5.

### @Test

@Test : Denotes that a method is a test method. Without this annotation, JUnit will ignore the method

### @ParameterizedTest

Denotes a method to be a parameterized test method. Parameterized tests make it possible to run a test multiple times with different arguments.

### @RepeatedTest

@RepeatedTest : Test methods marked with @RepeatedTest can be executed repeatedly

### @TestMethodOrder

@TestMethodOrder: Helps maintain order in which test methods are executed.

### @DisplayName

@DisplayName : Helps in using a custom name to be printed either in the console or test report for the test method.

### @DisplayNameGeneration

@DisplayNameGeneration : Declares a custom display name generator for the test class

### @BeforeEach

@BeforeEach: Analogous to JUnit 4 @Before. Denotes the annotated method should be executed before each test. The method should not return any value, should not be private.

### @AfterEach

@AfterEach : Analogous to JUnit 4 @After. Denotes the annotated method should be executed after each test. The method should not return any value, should not be private.

### @BeforeAll

@BeforeAll: Denotes that the annotated method should be executed before all @Test, @RepeatedTest, @ParameterizedTest, and @TestFactory methods in the current class; analogous to JUnit 4’s @BeforeClass. Such methods are inherited (unless they are hidden or overridden) and must be static (unless the "per-class" test instance lifecycle is used).

### @AfterAll

@AfterAll: Denotes that the annotated method should be executed after all @Test, @RepeatedTest, @ParameterizedTest, and @TestFactory methods in the current class; analogous to JUnit 4’s @AfterClass. A method marked with @AfterAll is inherited (unless they are hidden or overridden) and must be static.

### @Nested

@Nested:Denotes that the annotated class is a non-static nested test class. @BeforeAll and @AfterAll methods cannot be used directly in a @Nested test class unless the "per-class" test instance lifecycle is used. Such annotations are not inherited.

### @Tag

@Tag: Used to declare tags for filtering tests, either at the class or method level; analogous to test groups in TestNG or Categories in JUnit 4. Such annotations are inherited at the class level but not at the method level.

### @Disabled

@Disabled : Used to disable a test class or test method; analogous to JUnit 4’s @Ignore. Such annotations are not inherited.

### @Timeout

@Timeout: Used to fail a test, test factory, test template, or lifecycle method if its execution exceeds a given duration. Such annotations are inherited.

## JUnit5 Test Suites

September 30, 2020

## JUnit 5 Test Suites

The test suite is a container that has a set of tests that can be executed together. For instance, say we have multiple tests and want to execute all of them or subset of them under certain conditions. Like we want to run all the tests in the integration stage, a subset of tests in the developer environment before pushing code. All these can be done using Test Suite. Using JUnit 5 test suites, we can run tests spread into multiple test classes and different packages. JUnit 5 provides two annotations: @SelectPackages and @SelectClasses to create test suites. Additionally, you can use other annotations for filtering test packages, classes, or even test methods.

## Filtering Tests in Suites

Tests in JUnit5 can be executed with @RunWith. The same annotation is also used for Junit 4. The only difference Is in Junit 5 we pass JUnitPlatform as a parameter. In this article we will go though some sample use cases of creating TestSuite in JUnit 5 and how to filter test classes, test methods in the test suite etc.

### @SelectPackages

With this annotation, we can tell the Test Suite to scan all the mention packages and its sub-packages and execute all the Test Classes. We can also specify a signal package or multiple packages. Following example demonstrates this.
Select a single package
With @SelectPackages we can select test classes only from a specified package or list of packages. In the following example, all the Test classes from package com.bootng will be executed.
``````import org.junit.platform.runner.JUnitPlatform;
import org.junit.platform.suite.api.SelectPackages;
import org.junit.runner.RunWith;

@RunWith(JUnitPlatform.class)
@SelectPackages({ "com.bootng" })
public class TestSuiteWithPackageSingle {

}``````
Select multiple packages
Similarly, if we want to support multiple packages then we can specify multiple package names. The following example will execute tests from the two packages.
``````@RunWith(JUnitPlatform.class)
@SelectPackages({ "com.bootng.assertions", "com.bootng.display" })
public class TestSuiteWithPackageMultiple {

}``````

### @SelectClasses

Select Two Classes
In this example rather than selecting packages, we can specify individual test classes that we want to include in the Test Suite. In the following example essentially we will have only two Test classes in the Test Suite.
``````@RunWith(JUnitPlatform.class)
@SelectClasses({ DisplayExampleTest.class, DisplayGeneratorExampleTest.class })
public class TestSuiteWithSelectClass {

}
``````

### @IncludePackages and @ExcludePackages

@IncludePackages and @ExcludePackages work only at the specified package level and it won't scan sub-packages. This annotation is used with @SelectPackages annotation to fine-tune the filtering. Son in the following example Test Classes from only com.bootng.assertions will be included.
IncludePackages Example
Here
``````@RunWith(JUnitPlatform.class)
@SelectPackages({ "com.bootng" })
@IncludePackages({ "com.bootng.assertions" })
public class TestSuite_IncludePackage {

}
``````
ExcludePackages Example
Includes all the test form package com.bootng, but excludes tests from sub package com.bootng.assertions.
``````@RunWith(JUnitPlatform.class)
@SelectPackages({ "com.bootng" })
@IncludePackages({ "com.bootng.assertions" })
public class TestSuite_IncludePackage {

}
``````

### @IncludeClassNamePatterns and @ExcludeClassNamePatterns

Rather than filtering the test class from packages, we can also filter them based on the name of the Test Classes. To specify test class names patterns to exclude or include, you can use @IncludeClassNamePatterns and @ExcludeClassNamePatterns annotations.
IncludeClassNamePatterns Example
In this example, we can including only the test whose name ends with "Test" from the package (and sub packages) of com.bootng
``````@RunWith(JUnitPlatform.class)
@SelectPackages({ "com.bootng" })
@IncludeClassNamePatterns("^.*Test*\$")
public class TestSuite_NamePattern {

}
``````

### @IncludeTags and @ExcludeTags

IncludeTags Example
With IncludeTags and Exclude Tags we can filter Classes or Methods that are Tagged with the specified tag. For Instance, the following example, we are scanning package com.bootng.assertions and then only interested in including Test Classes or Test Methods tagged with Weather.
``````@RunWith(JUnitPlatform.class)
@SelectPackages({ "com.bootng.assertions" })
@IncludeTags({ "Weather" })
public class TestSuite_TagInclude {

}``````
ExcludeTags Example
Similar to IncludeTags tag to filter Test Methods or Test Classes but for exclusion. The following example will exclude all the Tests tagged with "Weather"
``````@RunWith(JUnitPlatform.class)
@SelectPackages({ "com.bootng.assertions" })
@ExcludeTags({ "Weather" })
public class TestSuite_TagExclude {

}
``````

## Summary

JUnit 5 provides powerful ways to filter Test cases in Test Suites. We have several options starting from including/excluding a specific packages to include/exclude specific Classes to Include/Exclude specific methods.

## Test Classes and Methods Requirement

September 21, 2020

## Test Classes and Methods Requirement

JUnit 5 is the latest version of Junit. It has a brand new architecture and more capabilities compared to the previous generation of Junit. In this article, we will quickly go through some of the main features and concepts of JUnit 5. In this article we go over the high-level structural requirement for writing Unit Tests. Generally, we write test method to represent a test case or a certain scenario which we want to validate. Such test methods would reside inside a Class. With Junit5 we don't need to mark or annotate the main class with any special JUnit specific annotations. Junit5 will be able to

### Test Class

Any top-level class, static member class, or @Nested class that contains at least one test method. We don't need to annotate the Class, JUnit5 will be able to find test methods defined inside it.
Class Requirement
The Class should contain only one Constructor and the Class should not be an abstract Class.

### Test Method

Any not abstract instance method that is annotated with @Test, @RepeatedTest, @ParameterizedTest, @TestFactory, or @TestTemplate.
Test Method
The test method should not return any value and should not be abstract.

### Lifecycle Method

Any method that is directly annotated or meta-annotated with @BeforeAll, @AfterAll, @BeforeEach, or @AfterEach Lifecycle methods can be inherited from parent class. Lifecycle methods should not be private and should not return any value.
Lifecycle Method
Should not be private and should not return value. @BeforeAll and @AfterAll should be used on a static method

### Example

Following Example shows a Test Class with one test method annotated with @Test and four Lifecylcle emthods
SimpleHelloTest3
``````package com.bootng.start;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;

import org.junit.jupiter.api.AfterAll;
import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.BeforeEach;

public class SimpleHelloTest3 {

@BeforeAll
public static void method1() {
System.out.println("method 1");
}

@BeforeEach
public void method2() {
System.out.println("method 2");
}

@AfterAll
public static void method3() {
System.out.println("method 3");
}

@AfterEach
public void method4() {
System.out.println("method 4");
}

@Test
public void test_hello() {
String msg = "hello";
assertEquals(msg, "hello", " message is equal Hello");
System.out.println("");
}

}
``````
Running the above test would output the following. method1 and method2 would be executed before the actual test and then method4 and method3 would be executed.
Console Output
method 1 method 2 test_hello method 4 method 3

## JUnit5 Writing first Test

September 21, 2020

## JUnit5 Writing first Test

JUnit 5 is the latest version of Junit. It has a brand new architecture and more capabilities compared to the previous generation of Junit. In this article, we will quickly go through some of the main features and concepts of JUnit 5. In this Article we will start with our first JUnit 5 Test.
SimpleHelloTest
First Junit5 Test. This test has only one test method which asserts that the message string is equal to "hello"
``````package com.bootng.start;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;

public class SimpleHelloTest {

@Test
public void test_hello() {
String msg = "hello";
assertEquals(msg, "hello", " message is equal Hello");

}

}
``````
When running this test, the test will surely pass since the assert condition will be successfull.

August 21, 2020

## JUnit 5 Introduction

JUnit 5 is the latest version of JUnit. It has a brand new architecture and more capabilities compared to the previous generation of JUnit. In this article, we will quickly go through some of the main features and concepts of JUnit 5 through hands on examples.

### JUnit 5 Project

JUnit 5 is composed of mainly three sub-projects JUnit Platform, JUnit Jupiter, and JUnit Vintage. junit-jupiter-api (JUnit Platform): JUnit Jupiter API for writing tests and extensions.
junit-jupiter-engine(JUnit Jupiter) : JUnit Jupiter test engine implementation, only required atruntime.
junit-vintage-engine:(JUnit Vintage): To run tests wittern in older version of JUnit on JUnit 5 Platform.

Java 8
JUnit 5 requires a minimum Java 8 version

### JUnit 5 Artifacts

If you’re using Maven you can add the corresponding subprojects in your pom file (juinit-jupiter-api, junit-jupiter-engine, junit-vintage-engine). Starting with Juinit 5.4 we can also simply add aggregate artifact named junit-jupiter in our pom.xml to include all the JUnit 5 related dependencies to our project.
JUnit 5 Maven Dependencies
``````<dependency>
<groupId>org.junit.jupiter</groupId>
<artifactId>junit-jupiter</artifactId>
<version>5.7.0-M1</version>
<scope>test</scope>
</dependency>``````

### Main Features of JUnit 5

Following are the main features in JUnit 5
Supports Lambda expressions.
Nested Unit Tests
Parameterized Unit Tests
Conditional Test inclusion or exclusion
Extension Model

### Main Annotations

Tests written in JUnit5 follow some standard approach and it is done by utilizing the capabilities provided by the JUnit 5 framework. The following are some of the annotations provided by JUnit 5. This is just to provide some quick pointers to the rich feature sets of JUnit 5.
Common annotations
@Test – denotes a test method, it does not take any arguments.
@DisplayName – specifies a custom name for the test class or method.
@DisplayNameGeneration - specify a generator to generate a custom name for method or class.
@Nested - it marks a class a nested test
@Disabled – prevents a test class or method from executing;
@BeforeEach,
@AfterEach – runs the annotated method before or after each test method in the same class.
@BeforeAll,
@AfterAll – runs the annotated method before any or after all of the test methods in the class.

July 28, 2020

## Assert in Java Code

An assertion is a java statement. An assertion statement is used in the code to ensures the correctness of any assumptions on which the program depends.  When the java runtime executes the assert statement it will throw assertion exception. Assertion helps in maintaining the invariants in a program under check. We will explore how to use the assertion statement with an example.

### Syntax for assertion

The following are the two different syntaxes for using assertion.
First way
If the expression is false the assert statement will throw java.lang.AssertionError
``assert expression;``
Second way
With two expressions where expression1 must be a boolean expression. and expression2 must return a value (must not return void). if exprerssion1 is false, the assert statement will throw java.lang.AssertError with expression2 as the message
``assert expression1 : expression2;``
By default assertion is disabled we can enable it by setting the jvm parameter ea or enableassertions.

### Enabling assertion

By default, assertions are disabled in Java. If assertions are disabled, assertion statements do not have any effects. In order to enable them, we use the following command
``````java -ea JavaAssertionTest
(or)
java -enableassertions JavaAssertionTest``````

### Example

In the following example, we have a Movie object containing movie name and minAge. From the main class we create a movie object and call the play method only if the minimum age requirement is met.
Movie.java
``````class Movie {
private String title;
private int minAge;

public Movie(String title, int age) {
this.title = title;
this.minAge = age;
}

public void play() {
System.out.print("Playing movie:  " + title);
}

public String getTitle() {
return title;
}

public int getAgeRestriction() {
return minAge;
}
}``````
JavaAssertionTest
``````public class JavaAssertionTest {
public static void main(String[] args) {
Movie movieA = new Movie("movie 007 James bond", 11);
Movie movieB = new Movie("movie Transformers", 18);

//We want to play only movies suitable for age 11 or under
assert (movieA.getAgeRestriction() <= 18);
movieA.play();

assert (movieB.getAgeRestriction() <= 11): "Underage for movie" + movieB.getTitle();
movieB.play();

System.out.println("played with all movies");
}
}``````
Running the above example will result in the following output.
Console Output
Playing movie movie 007 James bond Exception in thread "main" java.lang.AssertionError: Underage for moviemovie Transformers at JavaAssertionTest.main(JavaAssertionTest.java:12)
Note in the above case the moment the assertion error is thrown, the program will exit.

### Handling AssertionErrror

If an assertion error occurs the program will throw exception and exit. If we want to handle the assertion error, we can put the assert statement within a try-catch block. And in the catch block, we can have our logic if assert statement. The following example shows the same.
With Try Catch block
``````public static void main(String[] args) {
Movie movieA = new Movie("movie 007 James bond", 11);
Movie movieB = new Movie("movie Transformers", 18);

System.out.println("Start playing movies");

//We want to play only movies suitable for age 11 or under
assert (movieA.getAgeRestriction() <= 18);
movieA.play();

try {
assert (movieB.getAgeRestriction() <= 11): "Underage for movie with title:: " + movieB.getTitle();
movieB.play();
}catch (AssertionError e) {
System.out.println("Oops this movie cannot be played because " + e.getMessage());
}

System.out.println("played with all movies");
}``````
Console output
Start playing movies Playing movie: movie 007 James bond Oops this movie cannot be played because Underage for movie with title:: movie Transformers played with all movies
Summary
• assertions are a great way to confirm program invariants hold true.
• we have two different syntaxes for writing assertion in java.
• by default assertions are turned off, we can turn on using JVM flag.
• if an assert statement fails, the program will throw AssertionError and exit.
• we can catch AssertionError, and still get hold control of the program.

June 18, 2020

## Consuming RESTful API in Spring Boot

In our earlier articles, we learned how to create rest full API's . In this article, we will learn how to invoke or call Rest full API's in spring boot. Essentially we are going to write a simple client to consume a few public RESTful API's. RESTTemplate is for just that, Spring's RESTTemplate is used to write client applications to consume RESTful API
In this article, we will consume two different public API. For that, we will write a standalone Spring Boot App, to consumes the REST API's as follows
API 1: Get all GitHub API's
``GET https://api.github.com``
API 2: Get all Github repositories for user bootng
``GET https://api.github.com/users/bootng/repos``

### Technologies Used

• Java 11
• Apache Maven 3.5.0
• Spring Boot 2.2.6
• Eclipse IDE
Main Application Class
``````package com.javaexp;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
@SpringBootApplication
public class ResttemplateApplication {
private static final Logger log = LoggerFactory.getLogger(ResttemplateApplication.class);
public static void main(String args[]) {
SpringApplication.run(ResttemplateApplication.class, args);
log.info("completed executing ResttemplateApplication.run()");
}
}
``````
Our first REST client is as bellow, which calls a REST endpoint and then displays results in the console.
``````package com.bootng;

import java.util.Iterator;
import java.util.Map.Entry;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.core.annotation.Order;
import org.springframework.http.ResponseEntity;
import org.springframework.stereotype.Component;
import org.springframework.web.client.RestTemplate;
import com.fasterxml.jackson.databind.JsonNode;

/**
* This Class List all the endpoints from URL https://api.github.com
*
*/
@Component
@Order(1)
public class LoadAllGithubEndpoints implements CommandLineRunner {
private static final Logger log = LoggerFactory.getLogger(ResttemplateApplication.class);
@Override
public void run(String... args) throws Exception {
RestTemplate restTemplate = new RestTemplateBuilder().build();
ResponseEntity<JsonNode> apis =
restTemplate.getForEntity("https://api.github.com", JsonNode.class);
StringBuilder result = new StringBuilder("\n List of Public API's");
apis.getBody().fields().next().getValue();
Iterator<Entry<String, JsonNode>> it = apis.getBody().fields();
while (it.hasNext()) {
result.append("\n").append(it.next().getValue().asText());
}
log.info(result.toString());
}
}
``````
Our second REST client is as bellow, which calls a REST endpoint and then displays all the repositories for user bootng in the console.
``````package com.bootng;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.core.annotation.Order;
import org.springframework.http.ResponseEntity;
import org.springframework.stereotype.Component;
import org.springframework.web.client.RestTemplate;
import com.fasterxml.jackson.databind.JsonNode;

@Component
@Order(2)
public class LoadGithubRepo implements CommandLineRunner {
private static final Logger log = LoggerFactory.getLogger(ResttemplateApplication.class);
@Override
public void run(String... args) throws Exception {
RestTemplate restTemplate = new RestTemplateBuilder().build();
ResponseEntity<JsonNode> repos = restTemplate
.getForEntity("https://api.github.com/users/bootng/repos", JsonNode.class);
int counter = 1;
StringBuilder result = new StringBuilder("\n List of Repositories");
if (repos.getBody().isArray()) {
for(JsonNode jsonNode : repos.getBody()) {
result.append("\n Repo ").append(counter++).append("::");
result.append(jsonNode.get("name").asText());
}
}
log.info(result.toString());
}
}
``````
Notice that LoadGithubRepo is marked with annotation Order(2) and LoadAllGithubEndpoints is marked with annotation Order(1). That means Spring will execute LoadAllGithubEndpoints first and then LoadGithubRepo.
Building and Running the application
``````mvn clean install
mvn spring-boot:run``````
Console Output
```20-June-18 16:25:17:627  INFO main c.b.ResttemplateApplication:27 - about to call LoadAllEndpoint.run()
20-June-18 16:25:18:570  INFO main c.b.ResttemplateApplication:40 -
List of Public API's
https://api.github.com/user
https://github.com/settings/connections/applications{/client_id}
https://api.github.com/authorizations
https://api.github.com/search/code?q={query}{&page,per_page,sort,order}
https://api.github.com/search/commits?q={query}{&page,per_page,sort,order}
https://api.github.com/user/emails
https://api.github.com/emojis
https://api.github.com/events
https://api.github.com/feeds
https://api.github.com/user/followers
https://api.github.com/user/following{/target}
https://api.github.com/gists{/gist_id}
https://api.github.com/hub
https://api.github.com/search/issues?q={query}{&page,per_page,sort,order}
https://api.github.com/issues
https://api.github.com/user/keys
https://api.github.com/search/labels?q={query}&repository_id={repository_id}{&page,per_page}
https://api.github.com/orgs/{org}
https://api.github.com/orgs/{org}/repos{?type,page,per_page,sort}
https://api.github.com/orgs/{org}/teams
https://api.github.com/gists/public
https://api.github.com/rate_limit
https://api.github.com/repos/{owner}/{repo}
https://api.github.com/search/repositories?q={query}{&page,per_page,sort,order}
https://api.github.com/user/repos{?type,page,per_page,sort}
https://api.github.com/user/starred{/owner}{/repo}
https://api.github.com/gists/starred
https://api.github.com/users/{user}
https://api.github.com/user/orgs
https://api.github.com/users/{user}/repos{?type,page,per_page,sort}
https://api.github.com/search/users?q={query}{&page,per_page,sort,order}
20-June-18 16:25:19:069  INFO main c.b.ResttemplateApplication:36 -
List of Repositories
Repo 1::angular-word-merger
Repo 2::hansini-static-deploy
Repo 3::JUnit5-examples
Repo 4::okhttp
Repo 5::spring-boot-web-start
Repo 6::springboot_docker
20-June-18 16:25:19:071  INFO main c.b.ResttemplateApplication:57 - Started ResttemplateApplication in 3.382 seconds (JVM running for 6.625)
20-June-18 16:25:19:071  INFO main c.b.ResttemplateApplication:17 - completed executing ResttemplateApplication.run()
```
Get source code and build
• git clone https://github.com/siddharthagit/spring-boot-resttemplate
• cd spring-boot-resttemplate
• mvn clean install
• mvn spring-boot:run
Summary
• In this article, we saw how to write a simple REST client using RESTTEmplate in Spring Boot.
• In our next article, we will see more use cases of using RESTTemplate like for POST/PUT, etc.

### References

June 15, 2020

ControllerAdvice is an annotation provided by Spring. It is used to manage Exception handling in a Spring application. It allows us to handle any exception from any Classes in the Application and not just a single Class or one method. This article explains the advantages of using ControllerAdvice and some important points to be considered while using it.
In this article, we will walk through a few examples to demonstrate the functionality of ControllerAdvice annotation. To start will we will use the Spring Boot Rest application. This application has many endpoints specifically to drive a blog backed which can serve APIs for CRUD operations on Blog, Tags, Categories, etc.
Let's examine the following endpoints from the application which allows us to retrieve a specific Blog or Comments by its id.
Example REST API
``````GET /blogapi/blogs/:Blog_ID
GET /blogapi/categories/:CAT_ID``````
All these endpoints return the respective Objects if they exist with the specified ID or throws NotFoundException. If we think about any other Resource that has a GET endpoint will have a similar use case, that is throwing NotFoundException if Object is not found with the specified id.
So basically for any Resource Controller will need to deal with common exceptions that can occur while invoking the API. ControllerAdvice helps in handling all these exceptions from all the controllers globally in a separate class.
We will see the implementation of a Blog controller without Controlleradvice and with Controlleradvice
Let's see the Controller for the GET /blogs/id endpoint without Controlleradvice.

### Example 1

Controller for Blog
This controller method implements the GET blog by id. The controller uses BlogService to get the BlogStory object with the specified id. If the service can't find any blog with the specified id, it throws NotFoundException. In the controller, we catch that exception and return ResponseEntity with HTTP Status Not Found. The service can also throw AppExceptions which is also needed to be handled at the controller.
``````@RequestMapping(value = {"/v1/blogs/{id}"}, method = RequestMethod.GET,
produces = MediaType.APPLICATION_JSON_VALUE)
public ResponseEntity getBlogStory(@PathVariable(value = "") String id) {
ResponseEntity apiResponse;
BlogStory blogStory;
try {
blogStory = blogService.getBlogStory(id);
apiResponse = new ResponseEntity(blogStory, HttpStatus.OK);
} catch (NotFoundException e) {
//Handle NotFoundException
apiResponse = new ResponseEntity(HttpStatus.NOT_FOUND);
e.printStackTrace();
}
catch (AppException e) {
//Handle AppException
apiResponse = new ResponseEntity(HttpStatus.INTERNAL_SERVER_ERROR);
log.error(e.getMessage());
}
return apiResponse;
}``````
Nothing is wrong with this controller but, we are doing the same kind of stuff probably in all the controller methods, that are handling NotFoundException or AppException. For instance think about the implementations of other endpoints GET /categories/id and GET /comments/id.
That's where ControllerAdvice comes into the picture. We will create a Global Exception handing class that can handle the specific type of Exceptions for example in our case it is NotFoundException. We mark the class with @ControllerAdvice so that Spring can apply the exception handling implemented in this class globally to all the controllers in this application.

### Example 2

GlobalExceptionHandler
``````package com.bootng.handler.exception;

import java.util.Collections;
import java.util.List;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.HttpMediaTypeNotSupportedException;
import org.springframework.web.HttpRequestMethodNotSupportedException;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.context.request.WebRequest;
import com.bootng.beans.AppException;

public class GlobalExceptionHandler {

private static final Logger log = LoggerFactory.getLogger(GlobalExceptionHandler.class);
/** Customize the response for NotFoundException. */
@ExceptionHandler({NotFoundException.class})
protected ResponseEntity<Object> handleNotFoundException(Exception ex, WebRequest request) {
log.info("global exception handler " + ex.getMessage());
List<String> errors = Collections.singletonList(ex.getMessage());
ResponseEntity<Object> ret = ResponseEntity.status(HttpStatus.NOT_FOUND).body(errors);
return ret;
}

/** Customize the response for AppException. */
@ExceptionHandler({AppException.class})
protected ResponseEntity<Object> handleException(Exception ex, WebRequest request) {
log.info("global exception handler " + ex.getMessage());
String errors = "We cannot serve the request now, please try later";
ResponseEntity<Object> ret = ResponseEntity.status(HttpStatus.FORBIDDEN).body(errors);
return ret;
}
}
``````
In this ControllerAdvice implementations, we are handing two exceptions, NotFoundException and AppException. If you see the source code in git, this ControllerAdvice has actually more methods to handle more Exceptions, but here we are just showing handling just these two Exceptions.
Now let us write the same controller taking advantage of the Global Exception Handler class we defined above. Now we don't have to catch the exception in our controller, we can simply work with the core business logic and delegate the exception handling to the Global Exception Handler Class.
Get Blog By ID v2
``````@RequestMapping(value = {"/v2/blogs/{id}"}, method = RequestMethod.GET,
produces = MediaType.APPLICATION_JSON_VALUE)
public ResponseEntity<BlogStory> getBlogStoryV2(@PathVariable(value = "") String id)
throws AppException, NotFoundException {
BlogStory blogStory = blogService.getBlogStory(id);
return new ResponseEntity<BlogStory>(blogStory, HttpStatus.OK);
}``````
For the end-user both the controller works exactly the same. But the implementation is different with regards to handling exceptions. If we have a ControllerAdvice and any of the controllers throws exception handled by ControllerAdvice then, Spring will invoke the respective method from the ControllerAdvice to handle the exception

Summary
• The @ControllerAdvice annotation was first introduced in Spring 3.2
• We can have a single or multiple controller advice class per application. Having a single controller advice class is recommended per application.
• ControllerAdvice should contain methods to handle specific errors in the application.
• Using ControllerAdvice results in more clean and maintainable codes.
• We can handle both custom Exceptions as well as Spring Exceptions like HttpMediaTypeNotSupportedException, HttpRequestMethodNotSupportedException etc.
Git Source Code
• git clone https://github.com/siddharthagit/spring-boot-references
• cd spring-rest
• mvn clean install
• mvn spring-boot:run