build(examples): convert Gradle example projects to Maven

Convert the three remaining Gradle-based example projects
(client-v2-apache-arrow, demo-service, demo-kotlin-service) to Maven so
every project under examples/ builds with a single, consistent toolchain.

- Add a standalone pom.xml to each project, mirroring the existing Maven
  examples: the CI-rewritten <clickhouse-java.version> property and the
  <clickhouse-packages.classifier>all</> shaded artifacts.
  * client-v2-apache-arrow: plain Java 17, Arrow BOM, exec:java run (the
    required --add-opens flags live in .mvn/jvm.config).
  * demo-service: spring-boot-starter-parent 3.4.4 + repackage.
  * demo-kotlin-service: kotlin-maven-plugin 2.0.20 + Ktor BOM 2.3.12,
    maven-shade-plugin fat jar (Main-Class io.ktor.server.netty.EngineMain).
- Remove the Gradle wrapper, build scripts, gradle.properties, version
  catalog and the gradle-wrapper .gitattributes; switch each .gitignore to
  target/ and update the arrow/demo-service READMEs to mvn commands.
- Drop the now-dead gradlew branch from .github/workflows/build.yml and
  build_examples.sh, and the gradle.properties rewrite from release.sh
  (release.sh already rewrites the example poms' <clickhouse-java.version>).
- run_examples.yml has no Gradle references, so it needs no change.

Implements: #2915

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: polyglotAI-bot

.github/workflows/build.yml

@@ -65,7 +65,6 @@ jobs:
           find `pwd`/examples -type f -name pom.xml -exec sed -i -e "s|\(<clickhouse-java.version>\).*\(<\)|\1$LIB_VER\2|g" {} \;
           for d in $(ls -d `pwd`/examples/*/); do \
             if [ -e $d/pom.xml ]; then cd $d && mvn  --batch-mode --no-transfer-progress clean compile; fi;
-            if [ -e $d/gradlew ]; then cd $d && ./gradlew clean build; fi;
           done
       - name: Save clickhouse-jdbc-all for tests 
         uses: actions/upload-artifact@v4

CHANGELOG.md

@@ -60,6 +60,13 @@
   backtick-quoted `INSERT` column-name component before the by-name server-schema lookup, matching how the
   table and database identifiers are already handled. (https://github.com/ClickHouse/clickhouse-java/issues/2896)
 
+### Docs & Examples
+
+- **[examples]** Converted the remaining Gradle-based example projects (`client-v2-apache-arrow`, `demo-service`,
+  `demo-kotlin-service`) to Maven so that every project under `examples/` builds with a single, consistent toolchain.
+  The Gradle wrapper and build scripts were removed and each project now has a standalone `pom.xml`.
+  (https://github.com/ClickHouse/clickhouse-java/issues/2915)
+
 ## 0.10.0-rc1,  
 
 [Release Migration Guide](docs/releases/0_10_0.md)

build_examples.sh

@@ -3,6 +3,5 @@ LIB_VER=$(grep '<revision>' pom.xml | sed -e 's|[[:space:]]*<[/]*revision>[[:spa
 find `pwd`/examples -type f -name pom.xml -exec sed -i -e "s|\(<clickhouse-java.version>\).*\(<\)|\1$LIB_VER\2|g" {} \;
 for d in $(ls -d `pwd`/examples/*/); do \
     if [ -e $d/pom.xml ]; then cd $d && mvn  --batch-mode --no-transfer-progress clean compile; fi;
-    if [ -e $d/gradlew ]; then cd $d && ./gradlew clean build; fi;
 done
 

docs/client-v2-json-support.md

@@ -616,7 +616,7 @@ For these types, callers should obtain the parsed value through
 
 ## Examples
 
-Two runnable Gradle examples are provided under `examples/`:
+Two runnable Maven examples are provided under `examples/`:
 
 - `examples/client-v2-json-processors` exercises the `client-v2` API
   directly, switching between Jackson and Gson factories against a shared sample

examples/client-v2-apache-arrow/.gitattributes

@@ -1,5 +1,2 @@
-# Ignore Gradle project-specific cache directory
-.gradle
-
-# Ignore Gradle build output directory
-build
+# Maven build output
+target/

examples/client-v2-apache-arrow/.gitignore

@@ -0,0 +1,3 @@
+--add-opens=java.base/java.nio=ALL-UNNAMED
+--add-opens=java.base/sun.nio.ch=ALL-UNNAMED
+--add-opens=java.base/jdk.internal.misc=ALL-UNNAMED

examples/client-v2-apache-arrow/.mvn/jvm.config

@@ -14,17 +14,18 @@ The example shows:
   into another table by streaming the same `VectorSchemaRoot` straight back to
   `client.insert(...)`.
 
-Unlike the other examples in this repository, this one is built with **Gradle**
-(JDK 17 toolchain) and is not part of the Maven multi-module build.
+Like the other examples in this repository, this one is built with **Maven**
+(targeting JDK 17). It is a standalone project and is not part of the Maven
+multi-module build.
 
 ## Requirements
 
-- JDK 17 or newer (the Gradle toolchain will fetch one if it is missing).
+- JDK 17 or newer.
 - A running ClickHouse server reachable from the machine running the example.
 
 Apache Arrow needs access to direct memory and a few internal JDK APIs. The
-required `--add-opens` flags are already wired into `applicationDefaultJvmArgs`
-in `build.gradle.kts`, so running the example through Gradle just works:
+required `--add-opens` flags are wired into `.mvn/jvm.config`, so running the
+example through `mvn ... exec:java` (which runs in the Maven JVM) just works:
 
 ```text
 --add-opens=java.base/java.nio=ALL-UNNAMED
@@ -39,7 +40,7 @@ If you run the produced jar manually, pass these flags to the JVM yourself.
 From this directory:
 
 ```shell
-./gradlew run
+mvn compile exec:java
 ```
 
 Connection properties can be supplied as system properties:
@@ -52,7 +53,7 @@ Connection properties can be supplied as system properties:
 Example with custom connection properties:
 
 ```shell
-./gradlew run \
+mvn compile exec:java \
   -DchEndpoint=http://localhost:8123 \
   -DchUser=default \
   -DchPassword= \
@@ -62,7 +63,7 @@ Example with custom connection properties:
 To see the wire-level data flow, raise the SLF4J log level:
 
 ```shell
-./gradlew run -Dorg.slf4j.simpleLogger.defaultLogLevel=DEBUG
+mvn compile exec:java -Dorg.slf4j.simpleLogger.defaultLogLevel=DEBUG
 ```
 
 ## Executable Example