What causes "DecisionRequirementsDiagramGenerator already closed" in flowable/flowable-engine?

Call generateBufferedImage before close().

flowable/flowable-engine · error · FlowableImageException

DecisionRequirementsDiagramGenerator already closed

Error message

DecisionRequirementsDiagramGenerator already closed

What it means

DefaultDecisionRequirementsDiagramCanvas.generateBufferedImage throws FlowableImageException when the canvas has been closed, mirroring generateImage. Once closed, no BufferedImage can be produced from the canvas.

Solutions

Call generateBufferedImage before close().
Regenerate the diagram into a fresh canvas if a buffered image is needed later.
Restructure code so all image extraction happens inside the canvas's open lifecycle.

Example fix

// before
canvas.close();
BufferedImage img = canvas.generateBufferedImage("png"); // throws
// after
BufferedImage img = canvas.generateBufferedImage("png");
canvas.close();

Defensive patterns

Strategy: type-guard

Validate before calling

if (!canvasOpen) throw new IllegalStateException("Canvas already closed; regenerate the diagram");

Type guard

boolean canRender(DefaultDecisionRequirementsDiagramCanvas c) { return c != null && !c.isClosed(); }

Try / catch

try {
    BufferedImage img = canvas.generateBufferedImage("png");
} catch (FlowableImageException e) {
    logger.error("Canvas closed before buffered image generation", e);
}

Prevention

Extract BufferedImage before close()
Never reuse a closed canvas
Centralize canvas creation/closing in one code path

When it happens

Trigger: Calling generateBufferedImage(imageType) after close() was called on the same canvas instance.

Common situations: Requesting a BufferedImage after already exporting an InputStream via generateImage followed by close(); reusing a cached canvas after its lifecycle ended.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/ce7e38d09753f466. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-dmn-image-generator/src/main/java/org/flowable/dmn/image/impl/DefaultDecisionRequirementsDiagramCanvas.java:212

            throw new FlowableImageException("CaseDiagramGenerator already closed");
        }

        try (ByteArrayOutputStream out = new ByteArrayOutputStream()) {
            ImageIO.write(decisionRequirementsDiagram, imageType, out);
            return new ByteArrayInputStream(out.toByteArray());
        } catch (IOException e) {
            throw new FlowableImageException("Error while generating case image", e);
        }
    }

    /**
     * Generates an image of what currently is drawn on the canvas.
     *
     * Throws an {@link FlowableImageException} when {@link #close()} is already called.
     */
    public BufferedImage generateBufferedImage(String imageType) {
        if (closed) {
            throw new FlowableImageException("DecisionRequirementsDiagramGenerator already closed");
        }

        // Try to remove white space
        minX = (minX <= 5) ? 5 : minX;
        minY = (minY <= 5) ? 5 : minY;
        BufferedImage imageToSerialize = decisionRequirementsDiagram;
        imageToSerialize = decisionRequirementsDiagram.getSubimage(minX - 5, minY - 5, canvasWidth - minX + 5, canvasHeight - minY + 5);
        return imageToSerialize;
    }

    /**
     * Closes the canvas which disallows further drawing and releases graphical resources.
     */
    public void close() {
        g.dispose();
        closed = true;
    }

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