Value + EspHomeHandler + SmartMeterHandler

application.properties

@@ -1,11 +1,11 @@
-logging.level.de.ph87=DEBUG
+logging.level.de.ph87.data.series=DEBUG
 #-
 spring.datasource.url=jdbc:h2:./database;AUTO_SERVER=TRUE
 spring.datasource.driverClassName=org.h2.Driver
 spring.datasource.username=sa
 spring.datasource.password=password
 #-
-#spring.jpa.hibernate.ddl-auto=create
+spring.jpa.hibernate.ddl-auto=create
 #-
 de.ph87.data.message.receive.mqtt.host=10.0.0.50
 de.ph87.data.message.receive.mqtt.topic=#

src/main/java/de/ph87/data/Action.java

@@ -1,4 +1,4 @@
-package de.ph87.data.series;
+package de.ph87.data;
 
 public enum Action {
   CREATED, CHANGED, DELETED

src/main/java/de/ph87/data/Backend.java

@@ -1,13 +1,13 @@
 package de.ph87.data;
 
-import org.springframework.boot.SpringApplication;
+import org.springframework.boot.*;
-import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.boot.autoconfigure.*;
 
 @SpringBootApplication
 public class Backend {
-
+  
   public static void main(String[] args) {
     SpringApplication.run(Backend.class, args);
   }
-
+  
 }

src/main/java/de/ph87/data/message/IMessageHandler.java

@@ -1,9 +1,9 @@
 package de.ph87.data.message;
 
-import lombok.NonNull;
+import lombok.*;
 
 public interface IMessageHandler {
-
+  
-  void handle(@NonNull final Message message);
+  void handle(@NonNull final Message message) throws Exception;
-
+  
 }

src/main/java/de/ph87/data/message/Message.java

@@ -1,26 +1,22 @@
 package de.ph87.data.message;
 
-import lombok.Getter;
+import lombok.*;
-import lombok.ToString;
-
-import java.time.ZonedDateTime;
 
 @Getter
 @ToString
 public class Message {
-
+  
-  public final ZonedDateTime date = ZonedDateTime.now();
-
   public final String topic;
-
+  
+  @ToString.Exclude
   public final String payload;
-
+  
   public final String payloadLoggable;
-
+  
   public Message(final String topic, final String payload) {
     this.topic = topic;
     this.payload = payload;
     this.payloadLoggable = payload.replace("\n", "\\n").replace("\r", "\\r");
   }
-
+  
 }

src/main/java/de/ph87/data/message/MessageService.java

@@ -1,27 +1,26 @@
 package de.ph87.data.message;
 
-import lombok.NonNull;
+import lombok.*;
-import lombok.RequiredArgsConstructor;
+import lombok.extern.slf4j.*;
-import lombok.extern.slf4j.Slf4j;
+import org.springframework.stereotype.*;
-import org.springframework.stereotype.Service;
 
-import java.util.List;
+import java.util.*;
 
 @Slf4j
 @Service
 @RequiredArgsConstructor
 public class MessageService {
-
+  
   private final List<IMessageHandler> messageHandlers;
-
+  
-  public void onMessage(@NonNull final Message message) {
+  public void handle(@NonNull final Message message) {
     messageHandlers.forEach(handler -> {
       try {
         handler.handle(message);
       } catch (Exception e) {
-        log.warn("Message handler error: message={}", e.getMessage(), e);
+        log.error("Failed handling message: message={}, error={}", message, e.toString());
       }
     });
   }
-
+  
 }

src/main/java/de/ph87/data/message/handler/EspHomeHandler.java

@@ -0,0 +1,92 @@
+package de.ph87.data.message.handler;
+
+import com.fasterxml.jackson.annotation.*;
+import com.fasterxml.jackson.databind.*;
+import com.fasterxml.jackson.databind.annotation.*;
+import de.ph87.data.message.*;
+import de.ph87.data.series.*;
+import de.ph87.data.unit.Value;
+import de.ph87.data.unit.*;
+import lombok.*;
+import lombok.extern.slf4j.*;
+import org.springframework.stereotype.*;
+
+import java.time.*;
+import java.util.*;
+import java.util.regex.*;
+import java.util.stream.*;
+
+@Slf4j
+@Service
+@RequiredArgsConstructor
+public class EspHomeHandler implements IMessageHandler {
+  
+  private static final Pattern REGEX = Pattern.compile("^(?<area>\\w+)/sensor/(?<property>\\w+)$");
+  
+  private final ObjectMapper objectMapper;
+  
+  private final SeriesService seriesService;
+  
+  @Override
+  public void handle(@NonNull final Message message) throws Exception {
+    final Matcher matcher = REGEX.matcher(message.topic);
+    if (!matcher.find()) {
+      return;
+    }
+    final String area = matcher.group("area");
+    final String property = propertyWorkaround(matcher.group("property"));
+    final String name = "%s/%s".formatted(area, property.replace("_", "/"));
+    final Unit targetUnit = switch (property) {
+      case "iaq" -> Unit.IAQ;
+      case "iaq_co2_equivalent" -> Unit.IAQ_CO2_EQUIVALENT;
+      case "iaq_voc_equivalent" -> Unit.IAQ_VOC_EQUIVALENT;
+      case "pressure" -> Unit.PRESSURE_HPA;
+      case "temperature" -> Unit.TEMPERATURE_C;
+      case "humidity_relative" -> Unit.HUMIDITY_RELATIVE_PERCENT;
+      case "humidity_absolute" -> Unit.HUMIDITY_ABSOLUTE_GM3;
+      case "sun" -> Unit.SUN_DC;
+      default -> null;
+    };
+    if (targetUnit == null) {
+      log.debug("Skipping property: {}", name);
+      return;
+    }
+    final Inbound inbound = objectMapper.readValue(message.payload, Inbound.class);
+    final Unit unitFromPayload = inbound.units.stream().filter(ufp -> ufp.base == targetUnit.base).findFirst().orElse(null);
+    if (unitFromPayload == null) {
+      log.error("Unit mismatch: fromTopic={}, fromPayload=[{}]", targetUnit, inbound.getUnits().stream().map(Enum::name).collect(Collectors.joining(",")));
+      return;
+    }
+    final Value value = new Value(inbound.value, unitFromPayload);
+    seriesService.receive(new SeriesInbound(name, inbound.date, value.as(targetUnit)));
+  }
+  
+  private String propertyWorkaround(final String property) {
+    if ("iaq_equivalent_voc".equals(property)) {
+      return "iaq_voc_equivalent";
+    }
+    return property;
+  }
+  
+  @Getter
+  @ToString
+  private static class Inbound {
+    
+    @NonNull
+    public final ZonedDateTime date;
+    
+    public final double value;
+    
+    @NonNull
+    @JsonDeserialize(using = UnitListDeserializer.class)
+    public final List<Unit> units;
+    
+    public Inbound(final long timestamp, final double value, @JsonProperty("unit") @NonNull final List<Unit> units) {
+      this.date = ZonedDateTime.ofInstant(Instant.ofEpochSecond(timestamp), ZoneId.systemDefault());
+      this.value = value;
+      this.units = units;
+    }
+    
+  }
+  
+}

src/main/java/de/ph87/data/message/handler/SimpleJsonHandler.java

@@ -1,58 +1,55 @@
 package de.ph87.data.message.handler;
 
-import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.*;
-import com.fasterxml.jackson.databind.ObjectMapper;
+import de.ph87.data.message.*;
-import de.ph87.data.message.IMessageHandler;
+import de.ph87.data.series.*;
-import de.ph87.data.message.Message;
+import de.ph87.data.unit.*;
-import de.ph87.data.series.SeriesInbound;
+import lombok.*;
-import de.ph87.data.series.SeriesService;
+import lombok.extern.slf4j.*;
-import de.ph87.data.unit.Unit;
+import org.springframework.stereotype.*;
-import lombok.Getter;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-import lombok.ToString;
-import lombok.extern.slf4j.Slf4j;
-import org.springframework.stereotype.Service;
 
-import java.time.Instant;
+import java.time.*;
-import java.time.ZoneId;
-import java.time.ZonedDateTime;
 
 @Slf4j
 @Service
 @RequiredArgsConstructor
 public class SimpleJsonHandler implements IMessageHandler {
-
+  
   private final ObjectMapper objectMapper;
-
+  
   private final SeriesService seriesService;
-
+  
   @Override
-  public void handle(@NonNull final Message message) {
+  public void handle(@NonNull final Message message) throws Exception {
-    try {
+    if (!message.topic.endsWith("/SimpleJson")) {
-      final Inbound inbound = objectMapper.readValue(message.payload, Inbound.class);
+      return;
-      seriesService.receive(new SeriesInbound(message.topic, inbound.date, inbound.value, inbound.unit));
-    } catch (JsonProcessingException e) {
-      log.debug("Failed to parse inbound message: topic={}, message={}, error={}", message.topic, message, e.toString());
     }
+    final Inbound inbound = objectMapper.readValue(message.payload, Inbound.class);
+    seriesService.receive(new SeriesInbound(inbound.name, inbound.date, inbound.value, inbound.unit));
   }
-
+  
   @Getter
   @ToString
   private static class Inbound {
-
+    
+    @NonNull
+    public final String name;
+    
+    @NonNull
     public final ZonedDateTime date;
-
+    
     public final double value;
-
+    
+    @NonNull
     public final Unit unit;
-
+    
-    public Inbound(final long timestamp, final double value, final Unit unit) {
+    public Inbound(@NonNull final String name, final long timestamp, final double value, @NonNull final Unit unit) {
+      this.name = name;
       this.date = ZonedDateTime.ofInstant(Instant.ofEpochSecond(timestamp), ZoneId.systemDefault());
       this.value = value;
       this.unit = unit;
     }
-
+    
   }
-
+  
 }

src/main/java/de/ph87/data/message/handler/SmartMeterHandler.java

@@ -0,0 +1,56 @@
+package de.ph87.data.message.handler;
+
+import com.fasterxml.jackson.databind.*;
+import de.ph87.data.message.*;
+import de.ph87.data.series.*;
+import de.ph87.data.unit.Value;
+import de.ph87.data.unit.*;
+import lombok.*;
+import lombok.extern.slf4j.*;
+import org.springframework.stereotype.*;
+
+import java.time.*;
+
+@Slf4j
+@Service
+@RequiredArgsConstructor
+public class SmartMeterHandler implements IMessageHandler {
+  
+  private final ObjectMapper objectMapper;
+  
+  private final SeriesService seriesService;
+  
+  @Override
+  public void handle(@NonNull final Message message) throws Exception {
+    if (!"electricity/grid/json".equals(message.topic)) {
+      return;
+    }
+    final Inbound inbound = objectMapper.readValue(message.payload, Inbound.class);
+    seriesService.receive(new SeriesInbound("energy/purchased", inbound.date, inbound.energyPurchased));
+    seriesService.receive(new SeriesInbound("energy/delivered", inbound.date, inbound.energyDelivered));
+    seriesService.receive(new SeriesInbound("power/difference", inbound.date, inbound.powerDifference));
+  }
+  
+  @Getter
+  @ToString
+  private static class Inbound {
+    
+    @NonNull
+    public final ZonedDateTime date;
+    
+    public final Value energyPurchased;
+    
+    public final Value energyDelivered;
+    
+    public final Value powerDifference;
+    
+    public Inbound(final long timestamp, final double purchaseWh, final double deliveryWh, final double powerW) {
+      this.date = ZonedDateTime.ofInstant(Instant.ofEpochSecond(timestamp), ZoneId.systemDefault());
+      this.energyPurchased = new Value(purchaseWh, Unit.ENERGY_WH);
+      this.energyDelivered = new Value(deliveryWh, Unit.ENERGY_WH);
+      this.powerDifference = new Value(powerW, Unit.POWER_W);
+    }
+    
+  }
+  
+}

src/main/java/de/ph87/data/message/receive/mqtt/MqttReceiver.java

@@ -1,5 +1,6 @@
 package de.ph87.data.message.receive.mqtt;
 
+import de.ph87.data.message.*;
 import jakarta.annotation.*;
 import lombok.*;
 import lombok.extern.slf4j.*;
@@ -19,6 +20,8 @@ public class MqttReceiver {
   
   private final MqttConfig config;
   
+  private final MessageService messageService;
+  
   @Nullable
   private MqttClient client;
   
@@ -108,8 +111,9 @@ public class MqttReceiver {
   private void _receive(final String topic, final MqttMessage mqttMessage) {
     Thread.currentThread().setName("MQTT-RECEIVE");
     final String payload = new String(mqttMessage.getPayload(), StandardCharsets.UTF_8);
-    final String payloadLoggable = payload.replace("\n", "\\n").replace("\r", "\\r");
+    final Message message = new Message(topic, payload);
-    log.debug("received: topic={}, message={}", topic, payloadLoggable);
+    log.debug("received: {}", message);
+    messageService.handle(message);
   }
   
 }

src/main/java/de/ph87/data/series/Series.java

@@ -1,34 +1,39 @@
 package de.ph87.data.series;
 
-import de.ph87.data.unit.Unit;
+import de.ph87.data.unit.*;
 import jakarta.persistence.*;
 import lombok.*;
 
-import java.util.UUID;
-
 @Entity
 @Getter
 @ToString
 @NoArgsConstructor
 public class Series {
-
+  
   @Id
-  private String uuid = UUID.randomUUID().toString();
+  @GeneratedValue(strategy = GenerationType.IDENTITY)
-
+  private long id;
+  
+  @Setter
+  @NonNull
+  @Column(nullable = false, unique = true)
+  private String name;
+  
   @Setter
   @NonNull
   @Column(nullable = false, unique = true)
   private String title;
-
+  
   @Setter
   @NonNull
   @Column(nullable = false)
   @Enumerated(EnumType.STRING)
   private Unit unit;
-
+  
-  @Setter
+  public Series(@NonNull final String name, @NonNull final Unit unit) {
-  @NonNull
+    this.name = name;
-  @Column(nullable = false, unique = true)
+    this.title = name;
-  private String source;
+    this.unit = unit;
-
+  }
+  
 }

src/main/java/de/ph87/data/series/SeriesDto.java

@@ -1,27 +1,25 @@
 package de.ph87.data.series;
 
-import de.ph87.data.unit.Unit;
+import de.ph87.data.unit.*;
-import lombok.Getter;
+import lombok.*;
-import lombok.NonNull;
-import lombok.ToString;
 
 @Getter
 @ToString
 public class SeriesDto {
-
+  
-  public final String uuid;
+  public final long id;
-
+  
+  public final String name;
+  
   public final String title;
-
+  
   public final Unit unit;
-
+  
-  public final String source;
-
   public SeriesDto(@NonNull final Series series) {
-    this.uuid = series.getUuid();
+    this.id = series.getId();
+    this.name = series.getName();
     this.title = series.getTitle();
     this.unit = series.getUnit();
-    this.source = series.getSource();
   }
-
+  
 }

src/main/java/de/ph87/data/series/SeriesInbound.java

@@ -1,32 +1,33 @@
 package de.ph87.data.series;
 
-import de.ph87.data.unit.Unit;
+import de.ph87.data.unit.Value;
-import lombok.Getter;
+import de.ph87.data.unit.*;
-import lombok.NonNull;
+import lombok.*;
-import lombok.ToString;
 
-import java.time.ZonedDateTime;
+import java.time.*;
 
 @Getter
 @ToString
 public class SeriesInbound {
-
+  
   @NonNull
   public final String name;
-
+  
   @NonNull
   public final ZonedDateTime date;
-
+  
-  public final double value;
+  public final Value value;
-
+  
-  @NonNull
+  public SeriesInbound(@NonNull final String name, @NonNull final ZonedDateTime date, final Value value) {
-  public final Unit unit;
-
-  public SeriesInbound(@NonNull final String name, @NonNull final ZonedDateTime date, final double value, @NonNull final Unit unit) {
     this.name = name;
     this.date = date;
     this.value = value;
-    this.unit = unit;
   }
-
+  
+  public SeriesInbound(@NonNull final String name, @NonNull final ZonedDateTime date, final double value, @NonNull final Unit unit) {
+    this.name = name;
+    this.date = date;
+    this.value = new Value(value, unit);
+  }
+  
 }

src/main/java/de/ph87/data/series/SeriesRepository.java

@@ -1,7 +1,12 @@
 package de.ph87.data.series;
 
-import org.springframework.data.repository.ListCrudRepository;
+import lombok.*;
+import org.springframework.data.repository.*;
 
-public interface SeriesRepository extends ListCrudRepository<Series, String> {
+import java.util.*;
 
+public interface SeriesRepository extends ListCrudRepository<Series, Long> {
+  
+  Optional<Series> findByName(@NonNull String name);
+  
 }

src/main/java/de/ph87/data/series/SeriesService.java

@@ -1,81 +1,62 @@
 package de.ph87.data.series;
 
-import de.ph87.data.series.entry.EntryService;
+import de.ph87.data.*;
-import jakarta.annotation.PostConstruct;
+import de.ph87.data.series.entry.*;
-import lombok.NonNull;
+import de.ph87.data.unit.*;
-import lombok.RequiredArgsConstructor;
+import lombok.*;
-import lombok.extern.slf4j.Slf4j;
+import lombok.extern.slf4j.*;
-import org.springframework.stereotype.Service;
+import org.springframework.stereotype.*;
-import org.springframework.transaction.annotation.Transactional;
+import org.springframework.transaction.annotation.*;
 
-import java.util.HashMap;
+import java.util.function.*;
-import java.util.Map;
-import java.util.function.Consumer;
 
 @Slf4j
 @Service
 @Transactional
 @RequiredArgsConstructor
 public class SeriesService {
-
+  
-  private final Map<String, String> sources = new HashMap<>();
-
   private final SeriesRepository seriesRepository;
-
+  
   private final EntryService entryService;
-
+  
-  @PostConstruct
+  public SeriesDto modify(@NonNull final long id, @NonNull final Consumer<Series> modifier) {
-  public void cacheInit() {
+    final Series series = getById(id);
-    synchronized (sources) {
-      seriesRepository.findAll().forEach(this::cachePut);
-    }
-  }
-
-  private void cachePut(@NonNull final Series series) {
-    synchronized (sources) {
-      sources.put(series.getUuid(), series.getSource());
-    }
-  }
-
-  private void cacheRemove(final Series series) {
-    synchronized (sources) {
-      sources.remove(series.getUuid());
-    }
-  }
-
-  public SeriesDto modify(@NonNull final String uuid, @NonNull final Consumer<Series> modifier) {
-    final Series series = getByUuid(uuid);
     modifier.accept(series);
-    cachePut(series);
     return publish(series, Action.CHANGED);
   }
-
+  
-  public void delete(@NonNull final String uuid) {
+  public void delete(@NonNull final long id) {
-    final Series series = getByUuid(uuid);
+    final Series series = getById(id);
-    cacheRemove(series);
   }
-
+  
-  private Series getByUuid(@NonNull final String uuid) {
+  private Series getById(@NonNull final long id) {
-    return seriesRepository.findById(uuid).orElseThrow();
+    return seriesRepository.findById(id).orElseThrow();
   }
-
+  
   private SeriesDto publish(@NonNull final Series series, @NonNull final Action action) {
     final SeriesDto dto = toDto(series);
     log.info("Series {}: {}", action, series);
     return dto;
   }
-
+  
   private SeriesDto toDto(@NonNull final Series series) {
     return new SeriesDto(series);
   }
-
+  
-  public void receive(@NonNull final SeriesInbound measure) {
+  public void receive(@NonNull final SeriesInbound measure) throws Unit.NotConvertible {
-    final String uuid = sources.get(measure.name);
+    final Series series = getOrCreate(measure.name, measure.value.unit);
-    if (uuid == null) {
-      return;
-    }
-    final Series series = getByUuid(uuid);
     entryService.write(series, measure);
   }
-
+  
+  private Series getOrCreate(@NonNull final String name, @NonNull final Unit unit) {
+    return seriesRepository
+               .findByName(name)
+               .orElseGet(() -> {
+                 final Series series = seriesRepository.save(new Series(name, unit));
+                 publish(series, Action.CREATED);
+                 return series;
+               });
+  }
+  
 }

src/main/java/de/ph87/data/series/entry/Entry.java

@@ -1,40 +1,45 @@
 package de.ph87.data.series.entry;
 
-import de.ph87.data.series.Series;
+import de.ph87.data.series.*;
+import de.ph87.data.unit.Value;
+import de.ph87.data.unit.*;
 import jakarta.persistence.*;
-import lombok.Getter;
+import lombok.*;
-import lombok.NoArgsConstructor;
-import lombok.NonNull;
-import lombok.ToString;
 
-import java.time.ZonedDateTime;
+import java.time.*;
-import java.time.temporal.ChronoUnit;
+import java.time.temporal.*;
 
 @Entity
 @Getter
 @ToString
 @NoArgsConstructor
+@Table(
+    uniqueConstraints = {
+        @UniqueConstraint(columnNames = {"series_id", "date"})
+    }
+)
 public class Entry {
-
+  
   @Id
   @GeneratedValue(strategy = GenerationType.IDENTITY)
   private long id;
-
+  
   @NonNull
   @ManyToOne
   private Series series;
-
+  
   @NonNull
   @Column(nullable = false)
   private ZonedDateTime date;
   
-  @Column(nullable = false,name = "`value`")
+  @Setter
+  @Column(nullable = false, name = "`value`")
   private double value;
-
+  
-  public Entry(@NonNull final Series series, @NonNull final ZonedDateTime date, final double value) {
+  public Entry(@NonNull final Series series, @NonNull final ZonedDateTime date, final Value value) throws Unit.NotConvertible {
     this.series = series;
     this.date = date.truncatedTo(ChronoUnit.MINUTES);
-    this.value = value;
+    this.value = value.as(series.getUnit()).value;
   }
-
+  
 }

src/main/java/de/ph87/data/series/entry/EntryRepository.java

@@ -1,7 +1,14 @@
 package de.ph87.data.series.entry;
 
-import org.springframework.data.repository.ListCrudRepository;
+import de.ph87.data.series.*;
+import lombok.*;
+import org.springframework.data.repository.*;
+
+import java.time.*;
+import java.util.*;
 
 public interface EntryRepository extends ListCrudRepository<Entry, Long> {
-
+  
+  Optional<Entry> findBySeriesAndDate(@NonNull Series series, @NonNull ZonedDateTime truncated);
+  
 }

src/main/java/de/ph87/data/series/entry/EntryService.java

@@ -1,23 +1,29 @@
 package de.ph87.data.series.entry;
 
-import de.ph87.data.series.Series;
+import de.ph87.data.series.*;
-import de.ph87.data.series.SeriesInbound;
+import de.ph87.data.unit.*;
-import lombok.NonNull;
+import lombok.*;
-import lombok.RequiredArgsConstructor;
+import lombok.extern.slf4j.*;
-import lombok.extern.slf4j.Slf4j;
+import org.springframework.stereotype.*;
-import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.*;
-import org.springframework.transaction.annotation.Transactional;
+
+import java.time.*;
+import java.time.temporal.*;
 
 @Slf4j
 @Service
 @Transactional
 @RequiredArgsConstructor
 public class EntryService {
-
+  
   private final EntryRepository entryRepository;
-
+  
-  public void write(@NonNull final Series series, @NonNull final SeriesInbound measure) {
+  public void write(@NonNull final Series series, @NonNull final SeriesInbound measure) throws Unit.NotConvertible {
-    // TODO
+    final ZonedDateTime truncated = measure.date.truncatedTo(ChronoUnit.MINUTES);
+    if (entryRepository.findBySeriesAndDate(series, truncated).isEmpty()) {
+      final Entry created = entryRepository.save(new Entry(series, truncated, measure.value));
+      log.debug("Created: {}", created);
+    }
   }
-
+  
 }

src/main/java/de/ph87/data/unit/Unit.java

@@ -1,6 +1,6 @@
 package de.ph87.data.unit;
 
-import lombok.NonNull;
+import lombok.*;
 
 public enum Unit {
   TEMPERATURE_C("°C"),
@@ -15,24 +15,36 @@ public enum Unit {
   POWER_KW("kW", 1000, POWER_W),
   ENERGY_WH("W"),
   ENERGY_KWH("kWh", 1000, ENERGY_WH),
+  IAQ("IAQ"),
+  IAQ_CO2_EQUIVALENT("ppm"),
+  IAQ_VOC_EQUIVALENT("ppm"),
+  SUN_DC("Δ°C"),
   ;
-
+  
   public final String unit;
-
+  
   public final double factor;
-
+  
   public final Unit base;
-
+  
   Unit(@NonNull final String unit) {
     this.unit = unit;
     this.factor = 1.0;
     this.base = this;
   }
-
+  
-  Unit(@NonNull final String unit, double factor, @NonNull Unit base) {
+  Unit(@NonNull final String unit, final double factor, @NonNull final Unit base) {
     this.unit = unit;
     this.factor = factor;
     this.base = base;
   }
-
+  
+  public static class NotConvertible extends Exception {
+    
+    public NotConvertible(final @NonNull Unit source, final Unit target) {
+      super("Cannot convert Units: source=%s, target=%s".formatted(source, target));
+    }
+    
+  }
+  
 }

src/main/java/de/ph87/data/unit/UnitListDeserializer.java

@@ -0,0 +1,17 @@
+package de.ph87.data.unit;
+
+import com.fasterxml.jackson.core.*;
+import com.fasterxml.jackson.databind.*;
+
+import java.io.*;
+import java.util.*;
+
+public class UnitListDeserializer extends JsonDeserializer<List<Unit>> {
+  
+  @Override
+  public List<Unit> deserialize(final JsonParser jsonParser, final DeserializationContext deserializationContext) throws IOException {
+    final String name = jsonParser.getValueAsString();
+    return Arrays.stream(Unit.values()).filter(unit -> unit.unit.equals(name)).toList();
+  }
+  
+}

src/main/java/de/ph87/data/unit/Value.java

@@ -0,0 +1,23 @@
+package de.ph87.data.unit;
+
+import lombok.*;
+
+public class Value {
+  
+  public final double value;
+  
+  public final Unit unit;
+  
+  public Value(final double value, @NonNull final Unit unit) {
+    this.value = value;
+    this.unit = unit;
+  }
+  
+  public Value as(@NonNull final Unit target) throws Unit.NotConvertible {
+    if (this.unit.base != target.base) {
+      throw new Unit.NotConvertible(this.unit, target);
+    }
+    return new Value(value * this.unit.factor / target.factor, target);
+  }
+  
+}