Camunda custom form handling example

1. ExampleProcessEnginePlugin.java

class ExampleProcessEnginePlugin extends AbstractProcessEnginePlugin {
	@Override
	public void preInit(ProcessEngineConfigurationImpl processEngineConfiguration) {
		super.preInit(processEngineConfiguration);
		ExampleFormEngine formEngine = new ExampleFormEngine();
		processEngineConfiguration.setCustomFormEngines(singletonList(formEngine));
		processEngineConfiguration.setCustomPreBPMNParseListeners(singletonList(new ExampleParseListener(formEngine)));
	}
}

2. ExampleParseListener.java

class ExampleParseListener extends AbstractBpmnParseListener {
	private final ExampleFormEngine formEngine;

	public ExampleParseListener(ExampleFormEngine formEngine) {
		this.formEngine = formEngine;
	}

	@Override
	public void parseUserTask(Element userTaskElement, ScopeImpl scope, ActivityImpl activity) {
		TaskDefinition taskDefinition = ((UserTaskActivityBehavior) activity.getActivityBehavior()).getTaskDefinition();
		taskDefinition.setTaskFormHandler(new ExampleFormHandler(formEngine));
	}
}

3. ExampleFormHandler.java

class ExampleFormHandler implements TaskFormHandler, StartFormHandler {
	private final ExampleFormEngine formEngine;

	private String deploymentId;
	private String formKey; // fully qualified name of form class

	public ExampleFormHandler(ExampleFormEngine formEngine) {
		this.formEngine = formEngine;
	}

	@Override
	public void parseConfiguration(Element activityElement, DeploymentEntity deployment, ProcessDefinitionEntity processDefinition, BpmnParse bpmnParse) {
		this.deploymentId = deployment.getId();
		this.formKey = activityElement.attributeNS(BpmnParse.CAMUNDA_BPMN_EXTENSIONS_NS, "formKey");
	}

	@Override
	public StartFormData createStartFormData(ProcessDefinitionEntity processDefinition) {
		checkState(deploymentId != null, "deploymentId must be set");
		checkState(formKey != null, "formKey must be set");

		StartFormDataImpl startFormData = new StartFormDataImpl();
		startFormData.setDeploymentId(deploymentId);
		startFormData.setProcessDefinition(processDefinition);
		startFormData.setFormKey(formKey);
		return startFormData;
	}

	@Override
	public TaskFormData createTaskForm(TaskEntity task) {
		checkState(deploymentId != null, "deploymentId must be set");
		checkState(formKey != null, "formKey must be set");

		TaskFormDataImpl taskFormData = new TaskFormDataImpl();
		taskFormData.setDeploymentId(deploymentId);
		taskFormData.setTask(task);
		taskFormData.setFormKey(formKey);
		return taskFormData;
	}

	@Override
	public void submitFormVariables(VariableMap properties, VariableScope variableScope) {
		Object result = formEngine.submitForm(deploymentId, formKey, properties);
		variableScope.setVariable(formKey, result);
	}
}

4. ExampleFormEngine.java

public class ExampleFormEngine implements FormEngine {
	private final ObjectMapper objectMapper = new ObjectMapper();
	// and some kind of cache for the form infos

	@Override
	public String getName() {
		return "example";
	}

	@Override
	public Object renderStartForm(StartFormData startForm) {
		String deploymentId = startForm.getDeploymentId();
		String formKey = startForm.getFormKey();
		return renderForm(deploymentId, formKey, null);
	}

	@Override
	public Object renderTaskForm(TaskFormData taskForm) {
		String deploymentId = taskForm.getDeploymentId();
		String formKey = taskForm.getFormKey();
		VariableMap vars = ((VariableScope) taskForm.getTask()).getVariablesLocalTyped();
		return renderForm(deploymentId, formKey, vars);
	}

	ObjectNode renderForm(String deploymentId, String formKey, VariableMap vars) {
		checkNotNull(deploymentId, "deploymentId required");
		checkNotNull(formKey, "formKey required");
		FormInfo formInfo = createFormInfo(getFormClass(deploymentId, formKey)); // can be cached
		return renderForm(formInfo, vars);
	}

	@SuppressWarnings("unchecked")
	ObjectNode renderForm(FormInfo formInfo, VariableMap vars) {
		ArrayNode fields = objectMapper.createArrayNode();
		for (FieldInfo field : formInfo.getFields()) {
			ObjectNode fieldNode = objectMapper.createObjectNode();
			fieldNode.set("name", TextNode.valueOf(field.getName()));
			fieldNode.set("type", TextNode.valueOf(field.getType().name()));
			fieldNode.set("label", TextNode.valueOf(field.getLabel()));

			String valueStr = vars.getValue(field.getName(), String.class);
			fieldNode.set("value", TextNode.valueOf(valueStr));

			fields.add(fieldNode);
		}

		ObjectNode formNode = objectMapper.createObjectNode();
		formNode.set("fields", fields);

		return formNode;
	}

	Form submitForm(String deploymentId, String formKey, VariableMap vars) {
		checkNotNull(deploymentId, "deploymentId required");
		checkNotNull(formKey, "formKey required");
		FormInfo formInfo = createFormInfo(getFormClass(deploymentId, formKey)); // can be cached
		return submitForm(formInfo, vars);
	}

	Form submitForm(FormInfo formInfo, VariableMap vars) {
		Form form = formInfo.newInstance();
		for (FieldInfo field : formInfo.getFields()) {
			String valueStr = vars.getValue(field.getName(), String.class);
			Object value = Converters.get(field.getType()).fromString(valueStr);
			field.setValue(form, value);
		}
		return form;
	}


	FormInfo getFormInfo(String deploymentId, String formKey) {
		// we can apply caching here using (deploymentId, formKey) as key
		return createFormInfo(getFormClass(deploymentId, formKey));
	}

	@SuppressWarnings("unchecked")
	Class<? extends Form> getFormClass(String deploymentId, String formKey) {
		try {
			ProcessApplicationInterface pa = ProcessApplicationContextUtil.getTargetProcessApplication(deploymentId).getProcessApplication();
			return (Class<? extends Form>) pa.getProcessApplicationClassloader().loadClass(formKey);
		}
		catch (Exception e) {
			throw new RuntimeException("Unable to get form class for deployment '" + deploymentId + "' and form key '" + formKey + "'", e);
		}
	}

	FormInfo createFormInfo(Class<? extends Form> formClass) {
	  // ...
	}
}