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CoAP Driver — Technical Reference

The CoAP driver in PlantPulse Edge is a Java implementation wrapping the Eclipse Californium 3.13.0 library. It accesses resource URIs on low-power IoT devices (RFC 7252) via GET / PUT.

Source: plantpulse.driver.protocol.coap.*

ClassResponsibility
CoAPDriverProtocolDriver implementation — connect/read(GET)/write(PUT)/close, per-resource CoapClient cache

Libraries:

  • lib/californium-core-3.13.0.jar
  • lib/element-connector-3.13.0.jar

Operation Flow​

[connect]   CoapClient(baseUri) 생성 → setConnected(true)
                ↓
[read]   resource path → resourceUri = baseUri + "/" + path
            → resourceClients.computeIfAbsent(uri, CoapClient::new)
            → CoapClient.get() (synchronous)
            → resp.isSuccess() ? resp.getResponseText() : ""
                ↓
[write]   resourceClients.computeIfAbsent(uri, CoapClient::new)
            → CoapClient.put(value, MediaTypeRegistry.TEXT_PLAIN /* 0 */)
            → resp.isSuccess()
                ↓
[close]   모든 CoapClient.shutdown() + resourceClients.clear()

Since CoAP runs over UDP, it is connectionless — simply creating the client object is treated as "connected." Actual device availability is confirmed at the first read.

Options (BasicDriverSource.options)​

This driver has no options. The only values used for the connection are host and port.

Setting dtls · psk-identity · psk-key has no effect

The driver always builds the connection URI as coap://. Even if you set dtls=true in the options or specify the DTLS standard port 5684, traffic goes out as plaintext CoAP — and since no error is raised, it is easy to mistakenly assume the traffic is encrypted.

For devices that require DTLS, place a DTLS-terminating gateway in front of the device and have this driver read the plaintext side of the gateway.

Request timeouts cannot be specified either. The Californium defaults apply as-is. For devices with slow responses, increase the collection interval (timecycle) instead of adjusting the timeout.

Address (resource path) Format​

FormatConversion Result
/sensors/temperaturecoap://host:5683/sensors/temperature
sensors/temperaturecoap://host:5683/sensors/temperature (leading / added automatically)
/.well-known/coreDevice resource discovery — list of registered resources (CoRE Link Format)

A CoapClient is cached per resource so sockets are reused. All are shut down on close().

Response Handling​

Californium ResponseDriver Handling
resp == nullread = "", write = false
resp.isSuccess() (2.xx)read = resp.getResponseText(), write = true
resp.getCode() error (4.xx / 5.xx)warn log + read = "", write = false

CoAP response text is returned as a String as-is regardless of mediaType — if the device responds with JSON, the caller (PlantPulse's JsonExtract) decodes it. CBOR responses are shown as text, so a separate decoder is required (not currently supported).

Unit Tests​

test/java/plantpulse/driver/protocol/coap/CoAPDriverTest.java — 13 methods:

  • Class load + initial state
  • Meta (isWriteSupported / isExternal / getDriverSource)
  • Safe read / write while not connected (no NPE)
  • close() safety
  • baseUri() — explicit / default port
  • resourceUri() — leading / correction
  • getDebugBaseUrl() — coap://host:port

Build-isolated tests with no network calls (./gradlew test green).

References​