Creating a Spring Boot based AIS message decoder

(Posted 2018-09-13)

To demonstrate how easy it is to parse AIS messages (what is an AIS message?) with my open source library AISmessages, this post shows how to create a Spring Boot based microservice which can receive NMEA strings via HTTP and respond with the decoded AIS messages in JSON format.

So – for an HTTP request with a JSON array of NMEA strings like this:

POST http://localhost:8080/decode
Content-Type: application/json

[ 
  "!AIVDM,1,1,,A,18UG;P0012G?Uq4EdHa=c;7@051@,0*53",
  "!AIVDM,2,1,0,B,539S:k40000000c3G04PPh63<00000000080000o1PVG2uGD:00000000000,0*34"
  "!AIVDM,2,2,0,B,00000000000,2*27"
]

… we would like a response like this:

[
  {
    "repeatIndicator":0,
    "sourceMmsi": { "mmsi":576048000 },
    "navigationStatus":"UnderwayUsingEngine",
    "rateOfTurn":0,
    "speedOverGround":6.6,
    "positionAccuracy":false,
    "latitude":37.912167,
    "longitude":-122.42299,
    "courseOverGround":350.0
    "trueHeading":355,
    "second":40,
    "specialManeuverIndicator":"NotAvailable",
    "raimFlag":false,
    "communicationState": {
      "syncState":"UTCDirect",
      "slotTimeout":1,
      "numberOfReceivedStations":null,
      "slotNumber":null,
      "utcHour":8,
      "utcMinute":20,
      "slotOffset":null
    },
    "messageType":"PositionReportClassAScheduled",
    "transponderClass":"A",
    "valid":true
  }
]

Initializing a new Spring Boot project

A quick way to build such a service is to use Spring MVC. So, first we need to initialize a new Spring Boot project. An easy way to do this is to visit https://start.spring.io and fill in the form like this:

Generate and download the resulting project. Then move it to a suitable directory on your machine and unzip it like this:

$ mv ~/Downloads/aisdecoder.zip .
$ unzip aisdecoder.zip 
Archive:  aisdecoder.zip
   creating: aisdecoder/
  inflating: aisdecoder/gradlew      
   creating: aisdecoder/gradle/
   creating: aisdecoder/gradle/wrapper/
   creating: aisdecoder/src/
   creating: aisdecoder/src/main/
   creating: aisdecoder/src/main/java/
   creating: aisdecoder/src/main/java/dk/
   creating: aisdecoder/src/main/java/dk/tbsalling/
   creating: aisdecoder/src/main/java/dk/tbsalling/ais/
   creating: aisdecoder/src/main/java/dk/tbsalling/ais/decoder/
   creating: aisdecoder/src/main/java/dk/tbsalling/ais/decoder/aisdecoder/
   creating: aisdecoder/src/main/resources/
   creating: aisdecoder/src/main/resources/static/
   creating: aisdecoder/src/main/resources/templates/
   creating: aisdecoder/src/test/
   creating: aisdecoder/src/test/java/
   creating: aisdecoder/src/test/java/dk/
   creating: aisdecoder/src/test/java/dk/tbsalling/
   creating: aisdecoder/src/test/java/dk/tbsalling/ais/
   creating: aisdecoder/src/test/java/dk/tbsalling/ais/decoder/
   creating: aisdecoder/src/test/java/dk/tbsalling/ais/decoder/aisdecoder/
  inflating: aisdecoder/.gitignore   
  inflating: aisdecoder/build.gradle  
  inflating: aisdecoder/gradle/wrapper/gradle-wrapper.jar  
  inflating: aisdecoder/gradle/wrapper/gradle-wrapper.properties  
  inflating: aisdecoder/gradlew.bat  
  inflating: aisdecoder/settings.gradle  
  inflating: aisdecoder/src/main/java/dk/tbsalling/ais/decoder/aisdecoder/AisdecoderApplication.java  
  inflating: aisdecoder/src/main/resources/application.properties  
  inflating: aisdecoder/src/test/java/dk/tbsalling/ais/decoder/aisdecoder/AisdecoderApplicationTests.java 

As a smoke test we will first build new freshly, unmodified project - this is done with Gradle like this:

$ cd aisdecoder
$ ./gradlew build
... <a lot of build information>
BUILD SUCCESSFUL in 15s
5 actionable tasks: 5 executed

With the boiler plate project just built, we should see that it runs:

$ ./gradlew bootRun
> Task :bootRun
  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.0.5.RELEASE)
... <a lot of log output omitted>
o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2018-09-13 07:59:01.475  INFO 10989 --- [           main] d.t.a.d.a.AisdecoderApplication          : Started AisdecoderApplication in 1.7 seconds (JVM running for 2.008)
<=========----> 75% EXECUTING [21s]
> :bootRun

All seems well. The Spring MVC web application is running - but not doing much useful yet.

Adding custom code

Adding AISmessages as a dependency

The first thing we will do, is to add AISmessages as a dependency. This is done by adding this line into build.gradle:

...
dependencies {
  compile group: 'dk.tbsalling', name: 'aismessages', version: '2.2.3'   
  ...
}
...

Adding Spring MVC Controller

Next we will add the Spring MVC controller which handle incoming HTTP requests. This controller should be able to receive a JSON array of NMEA strings and output a JSON array of AIS messages.

So, in folder src/main/java/dk/tbsalling/ais/decoder/ we add file AisdecoderController.java like this:

package dk.tbsalling.ais.decoder.aisdecoder;

import dk.tbsalling.aismessages.ais.messages.AISMessage;
import org.springframework.http.MediaType;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;
import java.util.Collections;
import java.util.List;

@RestController
public class AisdecoderController {
    @RequestMapping(
        value = "/decode",
        method = RequestMethod.POST,
        consumes = MediaType.APPLICATION_JSON_VALUE,
        produces = MediaType.APPLICATION_JSON_VALUE
    )
    public List<AISMessage> decode(@RequestBody List<String> nmea) {
        return Collections.emptyList();
    }
}

This class is discovered by Spring through classpath scanning at startup, and handles incoming HTTP POST requests headed for URI /decode - such as http://localhost:8080/decode. The current implementation is mostly boiler plate and does nothing useful.

What we want it to do, is to call a service class which can convert the received NMEA strings into AIS messages. Like this:

@RestController
public class AisdecoderController {
    @Autowired
    private AisdecoderService aisdecoderService;
    ...
    public List<AISMessage> decode(@RequestBody List<String> nmea) {
        return aisdecoderService.decode(nmea);
    }
}

Adding AIS decode service

Then we need need the AisdecoderService. This is the most custom part of the code and where the real work happens. It should receive a list of n NMEA messages, convert and return these as a list of m AIS messages.

We start by adding to src/main/java/dk/tbsalling/ais/decoder/ the class AisdecoderService.java:

package dk.tbsalling.ais.decoder.aisdecoder;

import dk.tbsalling.aismessages.ais.messages.AISMessage;
import java.util.List;

@Service
@RequestScope
public class AisdecoderService {
    
    public List<AISMessage> decode(List<String> nmea) {
    }
    
}

So - how do we implement the decode method? The key here is class NMEAMessageHandler from AISmessages. NMEAMessageHandler is a class which can keep consuming NMEA messages and perform a callback whenever the received messages result in the successful decoding of a complete AIS message. Sometimes NMEA messages and AIS messages correspond 1:1 - other times it takes 2 NMEA messages to decode 1 AIS message.

So - we will extend AisdecoderService like this:

...
public class AisdecoderService implements Consumer<AISMessage> {
    
    public List<AISMessage> decode(List<String> nmeaMessagesAsStrings) {
        NMEAMessageHandler nmeaMessageHandler = new NMEAMessageHandler("SRC1", this);
    }
    
    @Override
    public void accept(AISMessage aisMessage) {
        aisMessages.add(aisMessage);
    }
}

Now the decode()-method initializes a NMEAMessageHandler. This handler is handed this (the decoder itself) to that it can make callbacks whenever an AIS message is fully constructed. To be used for callbacks, the AisdecoderService needs to implement the Consumer<AISMessage> interface.

Next, we need to start feeding the NMEA messages to the NMEAMessageHandler. One way to do that is this loop which iterates over all the NMEA strings and passes each one to the NMEAMessageHandler:

...
public class AisdecoderService implements Consumer<AISMessage> {
    ...
    public List<AISMessage> decode(List<String> nmeaMessagesAsStrings) {
       ...
        // Decode all received messages
        nmeaMessagesAsStrings.forEach(nmeaMessageAsString -> {
            try {
                NMEAMessage nmeaMessage = NMEAMessage.fromString(nmeaMessageAsString);
                nmeaMessageHandler.accept(nmeaMessage);
            } catch(NMEAParseException e) {
                System.err.printf(e.getMessage());
            }
        });
    }
    ...
}

Everytime the NMEA message handler can put the NMEA pieces together a complete AIS message, a callback is made to the AisdecoderService#accept(AISMessage msg) method. This method needs to store i AIS messages in a list, so that they can be returned by the decoder later:

public class AisdecoderService implements Consumer<AISMessage> {
    ...
    private final List<AISMessage> aisMessages = new LinkedList<>();
    @Override
    public void accept(AISMessage aisMessage) {
        aisMessages.add(aisMessage);
    }
    ...
}

Finally, in the decode() method, we must deal with the situation, where there are no more NMEA messages. This calls for a flush of the NMEAMessageHandler and return of the collected AIS messages:

...
public class AisdecoderService implements Consumer<AISMessage> {
    ...
    public List<AISMessage> decode(List<String> nmeaMessagesAsStrings) {
         ...
        // Flush receiver for unparsed message fragments
        List unparsedMessages = nmeaMessageHandler.flush();
        unparsedMessages.forEach(unparsedMessage -> {
            System.err.println("NMEA message not used: " + unparsedMessage);
        });
        // Return result
        return aisMessages;
    }
    ...
}

Complete code

The complete code resulting from the above can be viewed on Github: https://github.com/tbsalling/aisdecoder/tree/ready/spring-boot-webservice

Run it yourself

The code can then be cloned, compiled, run and invoked like this:

$ git clone https://github.com/tbsalling/aisdecoder.git
Cloning into 'aisdecoder'...
remote: Counting objects: 32, done.
remote: Compressing objects: 100% (18/18), done.
remote: Total 32 (delta 0), reused 32 (delta 0), pack-reused 0
Unpacking objects: 100% (32/32), done.
$ cd aisdecoder/
$ git checkout 7c02cbcef2ff273ab157e41fa71b193ae3304a93
$ ./gradlew build
...
BUILD SUCCESSFUL in 5s
$ ./gradlew bootRun
> Task :bootRun
  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.0.5.RELEASE)
...
<=========----> 75% EXECUTING [42s]
> :bootRun
Then, in a separate terminal window the web service can be invoked with e.g. Postman or curl (on Linux or MacOS), like this:

$ curl -X POST http://localhost:8080/decode -H 'Content-Type: application/json' -d '[ "!AIVDM,1,1,,A,18UG;P0012G?Uq4EdHa=c;7@051@,0*53" ]'
[{"nmeaMessages":[{"rawMessage":"!AIVDM,1,1,,A,18UG;P0012G?Uq4EdHa=c;7@051@,0*53","valid":true,"sequenceNumber":null,"numberOfFragments":1,"fragmentNumber":1,"radioChannelCode":"A","checksum":83,"messageType":"AIVDM","encodedPayload":"18UG;P0012G?Uq4EdHa=c;7@051@","fillBits":0}],"metadata":{"source":"SRC1","received":"2018-09-13T10:28:19.661343Z","decoderVersion":"2.2.2","category":"AIS"},"repeatIndicator":0,"sourceMmsi":{"mmsi":576048000},"navigationStatus":"UnderwayUsingEngine","rateOfTurn":0,"speedOverGround":6.6,"positionAccuracy":false,"latitude":37.912167,"longitude":-122.42299,"courseOverGround":350.0,"trueHeading":355,"second":40,"specialManeuverIndicator":"NotAvailable","raimFlag":false,"communicationState":{"syncState":"UTCDirect","slotTimeout":1,"numberOfReceivedStations":null,"slotNumber":null,"utcHour":8,"utcMinute":20,"slotOffset":null},"messageType":"PositionReportClassAScheduled","transponderClass":"A","valid":true}]

That’s it 🙂