add geteventType expresion

Signed-off-by: Shenoy Pratik <sgguruda@amazon.com>

Author: ps48

data-prepper-expression/src/main/java/org/opensearch/dataprepper/expression/GetEventTypeExpressionFunction.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright OpenSearch Contributors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package org.opensearch.dataprepper.expression;
+
+import org.opensearch.dataprepper.model.event.Event;
+import javax.inject.Named;
+import java.util.function.Function;
+import java.util.List;
+
+@Named
+public class GetEventTypeExpressionFunction implements ExpressionFunction {
+
+    @Override
+    public String getFunctionName() {
+        return "getEventType";
+    }
+
+    @Override
+    public Object evaluate(final List<Object> args, Event event, Function<Object, Object> convertLiteralType) {
+        if (!args.isEmpty()) {
+            throw new RuntimeException("getEventType() does not take any arguments");
+        }
+        return event.getMetadata().getEventType();
+    }
+}

data-prepper-expression/src/test/java/org/opensearch/dataprepper/expression/GetEventTypeExpressionFunctionTest.java

@@ -0,0 +1,54 @@
+package org.opensearch.dataprepper.expression;
+
+import org.opensearch.dataprepper.model.event.Event;
+import org.opensearch.dataprepper.model.event.JacksonEvent;
+import org.junit.jupiter.api.Test;
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+
+class GetEventTypeExpressionFunctionTest {
+
+    private GetEventTypeExpressionFunction createObjectUnderTest() {
+        return new GetEventTypeExpressionFunction();
+    }
+
+    private Event createTestEvent(final String eventType) {
+        return JacksonEvent.builder()
+                .withEventType(eventType)
+                .withData(Map.of())
+                .build();
+    }
+
+    @Test
+    void testGetEventTypeReturnsCorrectType() {
+        GetEventTypeExpressionFunction function = createObjectUnderTest();
+        Event testEvent = createTestEvent("LOG");
+
+        Object result = function.evaluate(List.of(), testEvent, Function.identity());
+
+        assertThat(result, equalTo("LOG"));
+    }
+
+    @Test
+    void testGetEventTypeThrowsExceptionWhenArgumentsProvided() {
+        GetEventTypeExpressionFunction function = createObjectUnderTest();
+        Event testEvent = createTestEvent("LOG");
+
+        assertThrows(RuntimeException.class, () -> function.evaluate(List.of("arg1"), testEvent, Function.identity()));
+    }
+
+    @Test
+    void testGetEventTypeReturnsNullForNullEventType() {
+        GetEventTypeExpressionFunction function = createObjectUnderTest();
+        Event testEvent = createTestEvent(null);
+
+        Object result = function.evaluate(List.of(), testEvent, Function.identity());
+
+        assertThat(result, equalTo(null));
+    }
+}