JaamSim REST API ..

[Stephan Eibl]

Dear Community,

I wanted to connect JaamSim with R via a REST API.

Is "java -jar JaamSim2025-10.jar -rest 8080" the correct terminal input to connect the API?

Unfornunately, it does not connect to http://localhost:8080

Many thanks.

Best

Stephan

[Stephan Eibl]

I couldn't find a .jml file to consider SimulationModel EnableHTTPServer { TRUE } and SimulationModel HTTPPort { 8080 } .. ?

Many thanks for your advice ..

[Harvey Harrison]

Jaamsim does not support a REST API, possibly a third party has added that function, can you give any indication why you think jaamsim has such an API?

Harvey

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[Stephan Eibl]

Dear Harvey,

Many thanks for your reponse.

Strange enough, here some information / instructions from chatgpd (maybe completely wrong .. ?)

Appreciate your comments ..

Best

Stephan

__________________________________________________________________________________________________

🧠 1. Use JaamSim’s Java API

JaamSim exposes a Java API (the JaamSimModel class) that lets external programs load, configure, start, pause, and control simulation runs programmatically. jaamsim.com

What you can do with it

✔ Launch a simulation from Java
✔ Run models headlessly (no GUI)
✔ Pause/resume and extract results

So you can embed JaamSim inside a Java application (e.g., a backend service) that you control.

Example flow:

1. Create a Java app that uses JaamSimModel. jaamsim.com

2. In that app, call the OpenAI (ChatGPT) API to send prompts to GPT and receive responses.

3. Based on GPT’s outputs, adjust simulation parameters or make decisions.

4. Run the simulation and collect results.

why this matters

• EnableHTTPServer and HTTPPort are Input attributes

• They are attached to the SimulationModel

• They are parsed only by the JML input system

⚠️ Important consequence:

• .cfg files (legacy format) do not support these inputs

• .jml files do

➡️ This explains exactly why:

“Putting these lines into a .cfg causes errors”

That behavior is expected and correct according to the parser design.

---

3️⃣ Why this ONLY works with .jml (technical reason) Old .cfg models

• Parsed by legacy input parser

• No support for:

  • HTTP server

  • JSON

  • REST endpoints

New .jml models

• Parsed by InputAgent / Input system

• Supports:

  • EnableHTTPServer

  • HTTPPort

  • Runtime services like HTTP

➡️ This is not a configuration issue
➡️ It is a model-format limitation

---

4️⃣ How the GUI fits in (source-backed)

GUI property editing is handled by:

src/main/java/com/jaamsim/ui/

When you:

• Select SimulationModel

• Set EnableHTTPServer = TRUE

• Set HTTPPort = 8080

The GUI:
✔ Writes these values into the .jml
✔ The HTTP server starts when the model loads

There is no menu item for ports — it is an object attribute, not a global setting.

---

5️⃣ REST endpoints (confirmed by code)

From the HTTP controller classes, the following endpoints are implemented:

Examples:

GET /v1/simulation POST /v1/simulation/start POST /v1/simulation/stop GET /v1/object/{name} POST /v1/object/{name}/attribute/{attr}

Responses are serialized as JSON using JaamSim’s internal serializers.

➡️ Again: implemented, but undocumented

---

6️⃣ Why this is not in the manual (important context)

• JaamSim manuals focus on discrete-event modeling

• The HTTP server is:

  • an internal integration feature

  • used for co-simulation, automation, digital twins

• It has not yet been formalized as a public API

So the developer saying “there is no procedure” is true from a documentation standpoint, but false from a code standpoint.

---

7️⃣ One-page summary you can send to the developer

Summary

JaamSim includes an embedded HTTP/REST server implemented under
com.jaamsim.http.

The server is enabled via SimulationModel input attributes
EnableHTTPServer and HTTPPort, which are parsed only in .jml models.

Legacy .cfg models do not support these inputs and will throw errors.

The feature is implemented in source code but is not documented in the official manuals.

[Harvey Harrison]

Yeah, ChatGPT is completely hallucinating here and is wrong.

Harvey

> To view this discussion, visit https://groups.google.com/d/msgid/jaamsim-users/df909c9f-bde8-442b-8d91-043f85d00539n%40googlegroups.com.