Add InstantEpochTimeSerializer

src/main/java/org/kiwiproject/jackson/ser/InstantEpochTimeSerializer.java

@@ -0,0 +1,29 @@
+package org.kiwiproject.jackson.ser;
+
+import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.databind.SerializerProvider;
+import com.fasterxml.jackson.databind.ser.std.StdSerializer;
+
+import java.io.IOException;
+import java.time.Instant;
+
+/**
+ * Jackson serializer that converts an {@link Instant} into milliseconds since the epoch.
+ */
+public class InstantEpochTimeSerializer extends StdSerializer<Instant> {
+
+    /**
+     * Create a new instance that serializes {@link Instant} to epoch millis.
+     */
+    public InstantEpochTimeSerializer() {
+        super(Instant.class);
+    }
+
+    @Override
+    public void serialize(Instant value,
+                          JsonGenerator jsonGenerator,
+                          SerializerProvider provider) throws IOException {
+
+        jsonGenerator.writeNumber(value.toEpochMilli());
+    }
+}

src/test/java/org/kiwiproject/jackson/ser/InstantEpochTimeSerializerTest.java

@@ -0,0 +1,104 @@
+package org.kiwiproject.jackson.ser;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import com.fasterxml.jackson.core.JsonFactory;
+import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
+import com.fasterxml.jackson.databind.module.SimpleModule;
+import lombok.Builder;
+import lombok.Value;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import java.io.IOException;
+import java.io.StringWriter;
+import java.time.Instant;
+import java.util.Map;
+
+@DisplayName("InstantEpochTimeSerializer")
+class InstantEpochTimeSerializerTest {
+
+    private InstantEpochTimeSerializer serializer;
+    private JsonGenerator jsonGenerator;
+    private StringWriter writer;
+    private Instant now;
+
+    @BeforeEach
+    void setUp() throws IOException {
+        serializer = new InstantEpochTimeSerializer();
+
+        writer = new StringWriter();
+        var jsonFactory = new JsonFactory();
+        jsonGenerator = jsonFactory.createGenerator(writer);
+
+        now = Instant.now();
+    }
+
+    @Test
+    void shouldSerializeToEpochMillis() throws IOException {
+        serializer.serialize(now, jsonGenerator, null);
+        jsonGenerator.close();
+
+        var serializedValue = Long.parseLong(writer.toString());
+        assertThat(serializedValue).isEqualTo(now.toEpochMilli());
+    }
+
+    @Test
+    void shouldSerializeWhenRegisteredOnObjectMapper() throws JsonProcessingException {
+        var entry = Entry.builder()
+                .name("Diane")
+                .text("Today we went sledding in the snow...")
+                .createdAt(now)
+                .build();
+
+        var module = new SimpleModule().addSerializer(serializer);
+        var objectMapper = new ObjectMapper().registerModule(module);
+        var json = objectMapper.writeValueAsString(entry);
+
+        assertSerializesCreatedAtToEpochMillis(json, objectMapper, now);
+    }
+
+    @Test
+    void shouldSerializeWhenUsingJsonSerializeAnnotationOnClass() throws IOException {
+        var post = Post.builder()
+                .name("Carlos")
+                .text("Yesterday we went fishing...")
+                .createdAt(now)
+                .build();
+
+        var objectMapper = new ObjectMapper();
+        var json = objectMapper.writeValueAsString(post);
+
+        assertSerializesCreatedAtToEpochMillis(json, objectMapper, now);
+    }
+
+    private static void assertSerializesCreatedAtToEpochMillis(String json, ObjectMapper mapper, Instant expected)
+            throws JsonProcessingException {
+
+        var map = mapper.readValue(json, Map.class);
+        var createdAt = (long) map.get("createdAt");
+        assertThat(createdAt).isEqualTo(expected.toEpochMilli());
+    }
+
+    @Value
+    @Builder
+    private static class Entry {
+        String name;
+        String text;
+        Instant createdAt;
+    }
+
+    @Value
+    @Builder
+    private static class Post {
+        String name;
+        String text;
+
+        @JsonSerialize(using = InstantEpochTimeSerializer.class)
+        Instant createdAt;
+    }
+}