GraalVM native support guide sample

Author: fugerit79

README.md

@@ -250,3 +250,130 @@ Refer to the [GraalVM documentation](https://www.graalvm.org/latest/docs/) for m
 - [Accessing Resources in Native Image](https://www.graalvm.org/jdk21/reference-manual/native-image/dynamic-features/Resources/)
 
 **TIP:** You can find many configuration samples in the [graalvm-reachability-metadata](https://github.com/oracle/graalvm-reachability-metadata) repository.
+
+Here is a sample usage guide for ApacheFreeMarker + GraalVM.
+
+To run the sample in classic Just In Time Way, we only need :
+
+* FreeMarkerGraalVMSample.java
+* sample.ftl
+
+But for the Ahead Of Time application with GraalVM some additional configuration is required :
+
+* custom-reflect-config.json
+
+#### FreeMarkerGraalVMSample.java sample class
+
+```java
+import freemarker.log.Logger;
+import freemarker.template.Configuration;
+import freemarker.template.Template;
+import freemarker.template.TemplateException;
+
+import java.io.IOException;
+import java.io.StringWriter;
+import java.io.Writer;
+import java.util.HashMap;
+import java.util.Map;
+
+public class FreeMarkerGraalVMSample {
+
+    private final static Logger LOG = Logger.getLogger(FreeMarkerGraalVMSample.class.getName());
+
+    /* data model */
+    public class Data {
+        private String description;
+        public String getDescription() {
+            return description;
+        }
+        public void setDescription(String description) {
+            this.description = description;
+        }
+    }
+
+    private void handleTemplate(Writer writer, String templatePath, Map<String, Object> dataModel) throws IOException, TemplateException {
+        Configuration cfg = new Configuration( Configuration.VERSION_2_3_34 );
+        cfg.setClassForTemplateLoading( FreeMarkerGraalVMSample.class, "/templates" );
+        Template template = cfg.getTemplate( templatePath );
+        template.process( dataModel, writer );
+    }
+
+    public void runSample() {
+        try ( StringWriter writer = new StringWriter() ) {
+            Map<String, Object> dataModel = new HashMap<>();
+            Data data = new Data();
+            data.setDescription( "FreeMarkerGraalVMSample" );
+            dataModel.put("data", data);
+            handleTemplate( writer, "sample.ftl", dataModel );
+            LOG.info( writer.toString() );
+        } catch (Exception e) {
+            LOG.error( e.getMessage(), e );
+        }
+    }
+
+    public static void main(String[] args) {
+        FreeMarkerGraalVMSample sample = new FreeMarkerGraalVMSample();
+        sample.runSample();
+    }
+
+}
+```
+
+#### Apache FreeMarker template
+
+```ftl
+<freemarker-graalvm-sample>
+    <freemarker-version>${.version}</freemarker-version>
+    <description>${data.description}</description>
+</freemarker-graalvm-sample>
+```
+
+#### Reflection configuration, custom-reflect-config.json
+
+Refers to [Reflection in Native Image](https://www.graalvm.org/jdk21/reference-manual/native-image/dynamic-features/Reflection/) guide
+
+```json
+[{
+  "name" : "FreeMarkerGraalVMSample$Data",
+  "methods" : [ {
+    "name" : "<init>",
+    "parameterTypes" : [ ]
+  },{
+    "name" : "getDescription",
+    "parameterTypes" : [ ]
+  } ]
+}]
+```
+
+#### Build the native image
+
+```shell
+#!/bin/bash
+
+# setting up environment
+export BASEDIR=.
+export CP=./lib/freemarker-gae-2.3.35-SNAPSHOT.jar:.
+
+# just in time application build
+javac -cp ${CP} -d build ./src/FreeMarkerGraalVMSample.java
+
+# ahead of time application build
+#
+# -H:IncludeResources=^templates/.* 
+#      will make the templates available to the native-image
+#
+# -H:ReflectionConfigurationFiles=./config/custom-reflect-config.json
+#      will setup reflection custom configuration
+native-image \
+  -cp "${CP}:build" \
+  -H:Path=build \
+  -H:Class=FreeMarkerGraalVMSample \
+  -H:IncludeResources=^templates/.* \
+  -H:+UnlockExperimentalVMOptions \
+  -H:ReflectionConfigurationFiles=./config/custom-reflect-config.json \
+  --no-fallback \
+  --report-unsupported-elements-at-runtime
+
+# running the application
+./build/freemarkergraalvmsample
+```