---
task: Write and load a Java PROCESS Service class
version: R1.03
source: IM 36K10P11-01EN §1–§5, §10; IM 36K10L10-01EN §1; practiced with CI-Eng-Sample class CIENG_JFLOW 2026-09-30
status: verified
---

# Java class

Worked example: class `CIENG_JFLOW`, object `CI-Eng-Sample.TANK1.JFLOW`
(`examples/CI-Eng-Sample/06_class_df.qli`, `07_object_df.qli`). Language reference:
`knowledge/manuals/IM36K10P11-01EN.md`. Loading and removing: `load-class-object.md`.

Choose Java when you need loops, arrays of signals, inheritance, libraries, or signals that may
stay unattached; otherwise the Traditional language is simpler.

## 1. Class record

Same as a Traditional class but `PF_LANGUAGE` = `"Java"` (cannot be changed later) and Java
source in `METHOD`. The Java class name must equal the `CLASS_DF` name exactly.

```
@FIELDS
NAME,DESCRIPTION,PF_LANGUAGE,TRIGGER_1,COMPILE,LISTING_WANTED,METHOD

@CLASS_DF
"CIENG_JFLOW","Inflow per minute (Java)","Java","",1,0,@"
import fasttools.processfast.*;

public class CIENG_JFLOW extends ProcessClass {

    Signal sInflow = defineDoubleSignal(Signal.Trigger.VALUE);
    Signal sPerMin = defineDoubleSignal();

    public void create() {
        attachItemToSignal(sInflow, getSectionsPath() + ".INFLOW");
        createItemForSignal(sPerMin, "CI-Eng-Sample.TEMPLATES.TPL_REAL",
            "DESCRIPTION", "Tank inflow (m3/min), class CIENG_JFLOW");
    }

    public void signalEvent(Signal signal) {
        double mInflow = sInflow.getValueAsDouble();
        sPerMin.setValue(mInflow / 60.0);
        systemMessage("CIENG_JFLOW " + getTag() + ": inflow " + mInflow);
    }
}
"@
```

`OPCMAN` starts the Java compiler `OPCJCOM` on demand; allow ~15 s, then export the class and
check `VALID_METHOD` = 1 (as in `load-class-object.md`).

## 2. Object

```
@FIELDS
NAME,DESCRIPTION,CLASS

@OBJECT_DF
"CI-Eng-Sample.TANK1.JFLOW","Tank 1 inflow per minute (Java)","CIENG_JFLOW"
```

Prompts: `Attribute aX = defineStringAttributeForPrompt("…");` → answer in `ATTR_VALUE_n`;
refuse bad answers with `cancel("text")` in `create()`.

## 3. Results on the test system

- Signals: `sInflow` → `CI-Eng-Sample.TANK1.INFLOW`; `sPerMin` → created item
  **`CI-Eng-Sample.TANK1.JFLOW.sPerMin`** — automatic names keep the identifier's case, and
  `itmval` needs that exact case (`…SPERMIN` is not found). Give a `"NAME"` if you want
  upper-case item names.
- `INFLOW` 120 → `sPerMin` 2; 90 → 1.5.
- `systemMessage()` appears in `umhlog.log` from `OPCJEXE` as
  `UMH-I-INFO, (<object>: <text>)`.
- The first object ran without resetting the new class, although the manual asks for a reset
  (ISS-029). After changing a Java class that has objects, compile **and reset** it
  (`change-reset-class.md`).
- Deleting the object deleted `…JFLOW.sPerMin` and section `…TANK1.JFLOW`; `INFLOW` stayed.

## 4. Pitfalls (manual)

- Definitions (`define…()`) only at class level, without `public`/`private`.
- `create()`/`destroy()` run in a different process than the trigger methods: only attributes
  keep values between them; no timers or signal values in `create()`.
- Never sleep or loop long in a trigger method: it blocks all objects of the same priority.
- No anonymous listener classes (fail at object start with `NoClassDefFoundError`).
- Dataset access: get the session with `getDssSession()` and never close it (release notes).