TBEL Overview

ThingsBoard Expression Language (TBEL) is a lightweight scripting language purpose-built for IoT data transformation. It is a fork of MVEL with additional security constraints and built-in memory management.

TBEL is the default scripting engine for Script calculated fields, rule engine filter and transformation nodes, and other server-side scripting contexts.

Prior to TBEL, ThingsBoard used Nashorn (the JDK-bundled JavaScript engine) for server-side scripting. Nashorn was deprecated in JDK 11 and removed in JDK 15 — leaving two alternatives:

• GraalVM JavaScript — excellent but carries a heavy runtime footprint and licensing constraints that make it impractical for most self-hosted deployments.
• Remote JS Executors — Node.js processes that receive scripts over the message queue. They work, but add inter-process communication latency and consume extra system resources.

TBEL was created to provide a fast, secure, zero-dependency scripting option that runs inside the JVM without external processes.

TBEL delivers roughly 1 000× better performance for typical transformation scripts.

Metric	Nashorn	TBEL
1 000 iterations of return msg.temperature > 20	~16 000 ms	~12 ms
Startup cost per script	High (JIT compilation)	Negligible
Memory isolation	Weak	Strong (per-execution memory limit)
Security sandbox	Manual, error-prone	Built-in (restricted class access)

Aspect	Remote JS Executors	TBEL
Additional process	Yes (Node.js)	No
Network/queue overhead	Yes	No
Language	Full ES6+ JavaScript	MVEL-based (Java-like)
Use case	Complex scripts that need full JS ecosystem	Typical IoT transformations

• Language Guide — syntax, data types, collections, and control flow.
• Helper Functions — encoding, byte parsing, hex conversion, date/time, geofencing, and more.