Get random elements from lists

Sometimes we may need to get a number of random elements from lists. To do this, we can use a function to get the indices of these elements first.

The function getIndices takes two parameters: num is the number of elements to get, max is the maximum value of index.

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Fixing Spring Boot error 'Unable to start ServletWebServerApplicationContext due to missing ServletWebServerFactory bean'

I was building a new Spring WebFlux application with Spring Boot. After downloading the project template from, I added some third-party dependencies and tried to start the application. Then I met the error org.springframework.context.ApplicationContextException: Unable to start ServletWebServerApplicationContext due to missing ServletWebServerFactory bean. Actually this is a common issue when I searched the solution. All the solutions are related to missing dependencies of Spring Boot starter spring-boot-starter-web.

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Use Private NPM Registry and Packages

When building large-scale NodeJS and front-end projects, it’s necessary to use private NPM registry to share private packages across different projects and team members. This post shows the essential parts of how to build the infrastructure and set up the workflow to use them.

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Apache Cassandra in Docker with JMX enabled

It’s a bit tricky to enable JMX for Cassandra when running in Docker. I create a Cassandra Docker image alexcheng/cassandra of version 3.11.2 with JMX authentication enabled, see the Dockerfile. The username and password to access JMX are both cassandra. The environment variable JMX_HOST has the default value of localhost, and it should work most of the time. If localhost doesn’t work , you can use the host’s IP address. After starting the container, you can access JMX using VisualVM or JConsole with address service:jmx:rmi://localhost/jndi/rmi://localhost:7199/jmxrmi. Change localhost to the same value as JMX_HOST if localhost is not used. Since SSL is not enabled, you need to use insecure connections.

On Mac, the local IP address can be obtained using ipconfig getifaddr en0, so we can use following command to start the container when using IP address is required.

docker run --rm -p 7199:7199 -e "JMX_HOST=`ipconfig getifaddr en0`" alexcheng/cassandra

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