Json Handling
Handling JSON data with SwimOS
Overview
We’re going to look how to handle JSON data with SwimOS. We’ll look at three use cases:
- parsing JSON data – swim.json.Json.parse(String)
- converting to JSON – swim.json.Json.toString(Vallue)
- Using the structure parser – swim.json.Json.structureParser().valueParser()
We will demonstrate parsing a JSON string and sending it to a lane using a Swim Client to send the command and subscribe to notifications.
Defining a Web Agent
So we’ll have something to interact with, We’ll create a Web Agent with two end-points:
- ValueLane for storing state and feeding subscriptions
- CommandLane for processing mutations to update state
public class JsonTestAgent extends AbstractAgent {
// ValueLane<Value> state, whose didSet(newValue, oldValue) callback prints newValue
@SwimLane("state")
ValueLane<Value> state = this.<Value>valueLane()
.didSet((newValue, oldValue) -> {
System.out.println("state: " + newValue);
});
// CommandLane<Value> addState, whose onCommand(value) callback invokes state.set(value)
@SwimLane("addState")
CommandLane<Value> addState = this.<Value>commandLane()
.onCommand((Value value) -> {
// we use Value::get() to read a field's value
int currentIntValue = value.get("intValue").intValue();
// we use Value::updated() to update a field
this.state.set(value.updated("intValue", currentIntValue + 1));
});
}
Now Let’s Handle the JSON
Within our client application’s main, we’ll instantiate and start SwimOS runtime, and then specify host and node URIs for our Web Agent.
final ClientRuntime swimClient = new ClientRuntime();
swimClient.start();
final String hostUri = "warp://localhost:9001"; // we specify the warp protocol instead of ws
final String nodeUri = "/json-test/default"; // 'default' is the node ID
Before we go ahead and send data, let’s register for notifications:
swimClient.downlink()
.hostUri(hostUri)
.nodeUri(nodeUri)
.laneUri("state")
.onEvent((Value event) -> {
System.out.println("addState output (Recon) ->> " + Recon.toString(event));
System.out.println("addState output (JSON) ->> " + Json.toString(event));
})
.open(); // lane must be opened to receive updates
A common mistake when opening downlinks is to forget to invoke open() to open the stream. As you see, we use Json.toString to convert from a Value to JSON.
Finally, let’s come up with some random data and toss it to the Web Agent:
String stateJson =
"{"
+ "\"@state\":null,"
+ "\"intValue\":5,"
+ "\"strValue\":\"five\","
+ "\"boolValue\":true,"
+ "\"listValue\":[1,2,3],"
+ "\"objectValue\":{\"a\":1,\"b\":2,\"c\":3}"
+ "}";
Value stateValue = Json.parse(stateJson);
System.out.println("addState input (toString) ->> " + stateValue);
swimClient.command(hostUri, nodeUri, "addState", stateValue);
Note the use of Json.parse to convert from JSON to Value.
Parsing JSON from a Resource File
We can parse JSON from application resources with the following function:
public static Value loadJsonResource(String jsonResource) {
Value jsonValue = null;
InputStream jsonInput = null;
try {
jsonInput = MainPlane.class.getClassLoader().getResourceAsStream(jsonResource);
if (jsonInput != null) {
jsonValue = Utf8.read(Json.structureParser().valueParser(), jsonInput);
}
} catch (IOException cause) {
throw new ParserException(cause);
} finally {
try {
if (jsonInput != null) {
jsonInput.close();
}
} catch (IOException swallow) {
}
}
return jsonValue;
}