Website: https://howtodoinjava.com
Github: https://github.com/lokeshgupta1981/
Twitter: @HowToDoInJAVA
30 Java interview questions for experienced developers, with direct answers and code examples covering collections, concurrency, JVM memory, streams, design, Spring transactions and small system design tasks.
“Unsupported class file major version 69” means a JVM or bytecode tool older than Java 25 tried to read a Java 25 class file. Learn to read the class file version, map it to the Java release, and fix the error by upgrading the runtime, compiling with –release, or upgrading Gradle, Spring, JaCoCo, Mockito and Lombok.
Schedule jobs in Spring Boot 4 with Quartz 2.5: inject Spring beans into jobs, use simple and cron triggers, handle misfires, chain jobs with listeners, persist and cluster jobs with the JDBC job store, and manage them from the actuator endpoint.
Connect Spring Boot 4.1 to Elasticsearch 9 with Spring Data Elasticsearch 6.1 and the new Java client: run Elasticsearch in Docker, map documents, search with repositories and NativeQuery, and test with Testcontainers.
A hands-on Kubernetes guide for Java developers: deploy a Spring Boot API and PostgreSQL to a local kind cluster and learn pods, Deployments, Services, Ingress, ConfigMaps, Secrets, volumes and StatefulSets with real kubectl output.
Upload an .xlsx class roster to a Spring Boot endpoint, read it with Apache POI, validate every row with Bean Validation and save the valid rows in JDBC batches. Includes a JSON error report, duplicate checks, a SAX reader for large files with measured heap, upload limits and MockMvc tests.
Set up Hibernate 7.4 in a Spring Boot 4.1 application: the JPA starter, datasource and JPA properties, entities, Spring Data repositories, transactions, the Session API, SQL logging, tests on H2 and MySQL, and the Hibernate 7 upgrade changes.
A shared Random is thread-safe but makes every thread fight over one seed. Learn which generator each thread should use, how to keep seeded results reproducible, and what a JMH benchmark measured on Java 25.
Spring Boot @Autowired connects the classes together without tight coupling, making application modular, testable, and easy to maintain.
Learn to fix MySQL error #1194 (“Table is marked as crashed”) with Stellar Repair for MySQL. Read this detailed guide for step-by-step repair process. Discover how to quickly restore corrupted MySQL tables with minimal data loss.
JUnit @AutoClose closes test fields after each test or class. Learn custom close methods, static vs instance timing, PER_CLASS, null and failing close().
JUnit version with Spring Boot explained. Spring Boot 4.1.1 manages JUnit 6.0.3; override it with junit-jupiter.version and test with @WebMvcTest.
JUnit 6 nullability explained with JSpecify @NullMarked and @Nullable, the annotated JUnit API, NullAway compile errors and the assertNotNull() contract.
Set up the JUnit Gradle dependency with junit-bom, junit-jupiter and the launcher in Gradle 9, filter tests, add integration tests and fix common errors.
Add the JUnit Maven dependency with junit-bom 6.1.3, junit-jupiter and Surefire 3.6.0, run one test, add Failsafe and fix version errors in pom.xml.
JUnit 6 tutorial with a Maven project, a first test with assertions, mvn test runs, a failure report explained and what changed from JUnit 5.
Spring AI CallAdvisor and StreamAdvisor provide hooks to wrap around LLM invocations, both for synchronous and streaming scenarios.
Angular UI testing in Java with Selenide and JUnit 5. Wait for spinners without explicit waits, handle timeouts and failures, and add Allure reports.
The innodb_force_recovery option starts MySQL in a recovery mode that lets the InnoDB storage engine run on a corrupted database, so we can dump the data before we rebuild the damaged tables. When innodb_force_recovery is not working, the cause is either a problem in the option file or corruption that …
The jpackage is a command-line tool introduced in Java 14 and helps create native installable packages (.exe, .msi, .dmg, .pkg, .deb, .rpm) for Windows, macOS, and Linux. Jpackage bundles Java applications along with a Java runtime (JRE) image, ensuring that end users do not need to install Java separately.
Database integrity refers to the accuracy, consistency, and reliability of the data stored within a database. It ensures that data remains uncorrupted, and adheres to defined rules. In SQL databases, integrity is supported through constraints like primary keys, foreign keys, unique constraints, and check constraints, alongside physical and logical data …
Declare a typed array of objects with an interface or type alias, then add, find, filter, sort, group and update its objects without surprises.
TypeScript has no Vector class. Model each vector as a tuple such as [x: number, y: number, z: number], store them in an array, and do vector math with typed functions.
Generate PDF from HTML template in Java with OpenHTMLtoPDF 1.1.97, Thymeleaf or FreeMarker, plus jsoup for broken HTML, page numbers, fonts and safety.
Since version 3.4, Spring Boot will provide native support for structured logging in the most common and popular formats, such as JSON and XML.
Creating REST APIs is a better approach for building one-time token-based authentication combined with Jwt tokens in Spring Security.
Read a file line by line in Python with a for loop, readlines(), readline(), generators, islice() and mmap. Handle large files, encodings and line numbers.
orphanRemoval = true deletes a child entity as soon as it leaves its parent’s association. See the SQL Hibernate 7 runs, how it differs from CascadeType.REMOVE, and the common mistakes.
Save a parent and its child entities with one persist() call in Hibernate 7. See cascade PERSIST and MERGE, the helper method that sets both sides, managed and detached parents, getReference(), and the real SQL and errors.
This short Logback tutorial lists examples for configuring logback.xml to log messages to both the console and/or file appenders. 1. Logback Dependencies To include Logback, include the latest version of logback-classic dependency, which transitively includes all the necessary dependencies, including SLF4J. Spring Boot comes with SLF4J and Logback as the …
1. Problem When writing tests that depend on H2 database in a Spring boot application, we may encounter the CommandAcceptanceException: Error executing DDL. The root cause exception will be org.h2.jdbc.JdbcSQLSyntaxErrorException in the logs. The exception stack trace will look something like this: If you are trying to run unit tests …
Learn to configure log4j2.yaml file to output the log statements to the console, rolling files, etc. Also, learn to configure log4j2 appenders, levels, and patterns.
Learn to make HTTP GET requests (sync and async) using Spring Boot WebClient, pass URI and query params, handle responses, and handle errors.
Learn to compare Java objects without getters or equals() using EqualsBuilder.reflectionEquals(), excluded fields, recursive mode and AssertJ.
In Spring Security, a One-Time Token (OTT) is a server-side generated string that can be used only once for user login authentication purpose.
Spring AI pgvector example: run PostgreSQL with pgvector in Docker, configure PgVectorStore, load and search documents, and fix the dimension errors.
Build a Spring AI RAG chatbot that lets users upload PDF files and ask questions about each file, with Tika, a vector store and a safe metadata filter.
Spring AI tool calling with Ollama: describe Java methods with @Tool, pass them to ChatClient, and handle errors, loops and tools that change data safely.
Learn to download, install, and run an LLM model using Ollama. Also learn to configure Spring AI Ollama module to access the model’s chat API.
Spring AI tutorial: a learning path of articles in reading order, from the first ChatClient call to RAG, tools, agents, testing and other model providers.
Spring AI text to SQL: turn a question into a SELECT with a ChatClient, check it, run it read-only, explain the rows and fix the model’s common SQL errors.
For creating an ETL pipeline for Spring AI data ingestion microservice, Spring cloud functions are an excellent choice for serverless providers.
Spring AI ETL pipeline example: read Markdown and PDF files, add metadata, split with TokenTextSplitter and load a vector store without duplicates.
Spring AI vector store example: load documents into SimpleVectorStore, search by meaning with filters, then move the same code to ChromaDB in Docker.
Spring AI embedding model example with Ollama: turn text into a 768-number vector and compare a question with policy sentences by cosine similarity.
Spring AI speech to text with OpenAI whisper-1: TranscriptionModel, language and prompt options, a voice-note endpoint, and answering it with ChatClient.
Spring AI text to speech with OpenAI: TextToSpeechModel, voices, speed and tone per request, streaming audio, and the traps of length and cost.
Spring AI prompt template examples: system prompt and user template from files, few-shot examples as message pairs, and measured results on a small model.
Serialize object to String in Java with ObjectOutputStream and Base64 or with Jackson JSON, read it back, and avoid the deserialization security risk.
Learn to read or write the records in a JSON file using Spring Batch JsonItemReader and JsonFileItemWriter implementations.
HowToDoInJava provides tutorials and how-to guides on Java and related technologies.
It also shares the best practices, algorithms & solutions and frequently asked interview questions.
