Category Archives: spring-boot

Markov chains in Java: Suggest what Narendra Modi will say using Markov chains

Recently, I read an article[1] on Markov chains. In the post, author showed how we can build autocomplete functionality using them. The article piqued my interest to learn more about Markov chain and I started looking for an example application that I can build using it. I decided to build a web application that will suggest me what Indian prime minister Narendra Modi[2] will say after a word/pair of words/triplet for words.

I am not a supporter of Narendra Modi style of leadership. The reason I chose him is because I could easily find text of all his speeches on the web [3].

This post is divided into three sections:

  1. What is Markov chain?
  2. Create dataset for the application
  3. Build the application that uses Markov chain

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The Ultimate Dockerfile for Spring Boot Maven and Gradle applications

For Maven users, the ultimate Dockerfile is below.

FROM openjdk:8-jdk-alpine as build
WORKDIR /workspace/app

COPY mvnw .
COPY .mvn .mvn
COPY pom.xml .
RUN ./mvnw dependency:go-offline

COPY src src
RUN ./mvnw package -DskipTests
RUN mkdir -p target/dependency && (cd target/dependency; jar -xf ../*.jar)

FROM openjdk:8-jre-alpine
VOLUME /tmp
ARG DEPENDENCY=/workspace/app/target/dependency
COPY --from=build ${DEPENDENCY}/BOOT-INF/lib /app/lib
COPY --from=build ${DEPENDENCY}/META-INF /app/META-INF
COPY --from=build ${DEPENDENCY}/BOOT-INF/classes /app
ENTRYPOINT ["java","-cp","app:app/lib/*","FULL_NAME_OF_SPRING_BOOT_MAIN_CLASS"]

Please make sure to update FULL_NAME_OF_SPRING_BOOT_MAIN_CLASS with Spring Boot application main class.

For Gradle users, the ultimate Dockerfile is below.

FROM openjdk:8-jdk-alpine as build
WORKDIR /workspace/app

COPY gradlew .
COPY gradle gradle
COPY build.gradle .
RUN ./gradlew dependencies

COPY src src
RUN ./gradlew build unpack -x test
RUN mkdir -p build/dependency && (cd build/dependency; jar -xf ../libs/*.jar)

FROM openjdk:8-jre-alpine
VOLUME /tmp
ARG DEPENDENCY=/workspace/app/build/dependency
COPY --from=build ${DEPENDENCY}/BOOT-INF/lib /app/lib
COPY --from=build ${DEPENDENCY}/META-INF /app/META-INF
COPY --from=build ${DEPENDENCY}/BOOT-INF/classes /app
ENTRYPOINT ["java","-cp","app:app/lib/*","FULL_NAME_OF_SPRING_BOOT_MAIN_CLASS"]

Please make sure to update FULL_NAME_OF_SPRING_BOOT_MAIN_CLASS with Spring Boot application main class.

You can watch this video to learn more about optimizing docker images for Spring Boot applications.

Solution: ORA-12514, TNS:listener does not currently know of service requested in connect descriptor

Today, one of the teams was facing the issue ORA-12514, TNS:listener does not currently know of service requested in connect descriptor. They were trying to connect to Oracle using Spring Boot JPA application and getting the exception at application boot up.

Team was able to successfully connect to Oracle using SQLDeveloper. But, when connecting to Oracle using Spring Boot JPA application it was failing to boot up.

Like most developers, we googled around to find the answers. The popular answer that you will get is as mentioned in this stackoverflow question. The answer suggests that you have to update tnsnames.ora file and add your service to it.

I knew it is not the right answer as we are able to connect using SQL Developer.

So, I started looking into the Spring Data JPA configuration of the application. The configuration that was giving error is shown below.

spring.datasource.driverClassName=oracle.jdbc.driver.OracleDriver
spring.datasource.url=jdbc:oracle:thin:@//myhost:1521/efsdev
spring.datasource.username=myuser
spring.datasource.password=mypassword
spring.jpa.database-platform=org.hibernate.dialect.Oracle12cDialect

In the above configuration, there is only one configuration property that could be possibly wrong — spring.datasource.url.

So, I googled around to find the correct way to specify JDBC url for Oracle.

I learned that there are two ways you can specify JDBC string URL. The two ways are:

1) jdbc:oracle:thin:@[HOST][:PORT]:SID

2) jdbc:oracle:thin:@//[HOST][:PORT]/SERVICE

As you can see above, we are using the second way to specify the URL. According to second URL syntax, efsdev is the service name.

Developers mentioned that efsdev is the SID. So, we need to use the first URL.

After changing the configuration to the one mentioned below, application was successfully able to connect with Oracle.

spring.datasource.driverClassName=oracle.jdbc.driver.OracleDriver
spring.datasource.url=jdbc:oracle:thin:@myhost:1521:efsdev
spring.datasource.username=myuser
spring.datasource.password=mypassword
spring.jpa.database-platform=org.hibernate.dialect.Oracle12cDialect

That’s it for this post. I hope this saves someone’s day.

Enabling Https for local Spring Boot development with mkcert

Today, I discovered mkcert – a tool that generates valid TLS certificate. It works for any hostname or IP, including localhost. In this post, I will show you how to generate a valid PKCS12 format certificate using mkcert. Then, we will use that certificate in a Spring boot application.

We will start by installing mkcert on our local machine. If you are using Mac then we can use brew package manager. For installation instructions specific to your OS you can refer to the documentation.

brew install mkcert

Once mkcert is installed, you can use its CLI to create and install a CA. To do that, run the following command.

mkcert -install

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Configuring Spring Cache Manager with AWS ElastiCache Redis (cluster mode disabled) and Lettuce

We have Spring Boot 2 application that uses Redis as the cache manager. We deploy our application on Amazon AWS where we use AWS ElastiCache Redis service in cluster mode disabled. Our setup includes a Redis master with two Redis slaves. The default Java client for Redis with spring-boot-starter-data-redis dependency is lettuce-core. When you are working with single Redis node with no slaves, using AWS Elastic Cache Redis is as simple as providing the spring.redis.url with the value of AWS ElastiCache Redis instance URL. This was the set up that we were using till a month back. As the load on the system increased we decided to use ElastiCache Redis in replicated setup to scale our reads. In AWS, Redis implements replication in two ways:

  1. With a single shard that contains all of the cluster’s data in each node – Redis (cluster mode disabled)
  2. With data partitioned across up to 15 shards — Redis (cluster mode enabled)

In our case, cached data is less than 1 GB so it fits in RAM of single node. This made us choose cluster mode disabled setup.

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Single sign-on in Spring Boot applications with Spring Security OAuth

This week I had to dig deeper into the world of Single sign-on. I learnt a lot of things about it from basic conceptual knowledge to how to setup your own Single sign-on server with Spring Boot. In this post, I will share my learnings with you. In case something is not clear please leave a comment and I will address it.

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Using Spring Boot @SpyBean

Today, a colleague asked me to help him write a REST API integration test. We use Spring’s MockMvc API to test the REST API. The application uses MongoDB with Spring Data MongoDB. The application uses both MongoTemplate and Mongo based repositories for working with MongoDB. To make tests work independent of MongoDB, we mock Spring MongoDB repository interfaces.

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