Flowable怎么通过委托表达式获取到Micronaut的bean

flowable Unknown property used in expression: ${testListener}

众所周知在spring中工作流引擎activiti/flowable是可以通过委托表达式获取到spring bean的。但在micronaut中会报这个异常，因为activiti/flowable官方已经集成好了spring，而micronaut并没有人帮你集成过，获取不到容器里的bean是理所当然的。

由于micronaut是个新框架，网上翻了一圈都没找到对路的文章，以及在micronaut中怎么解决。

无奈之下自能自己出手了，翻了翻flowable的源码，找到跟spring集成部分的代码，理解了一下，模仿着写了一个针对micronaut的简单集成，打通flowable和micronaut容器，经过一番调试之后问题解决。

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在给任务节点设置监听器，运行到这个节点时会出现这个异常flowable Unknown property used in expression: ${testListener}，监听器的设置如下图所示，设置了委托表达式delegateExpression，本意是想获取一个bean作为监听器。

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对应的xml为：

<userTask id="task1" name="task1" flowable:formFieldValidation="true">
    <extensionElements>
        <flowable:taskListener event="complete" delegateExpression="${billGenerateListener}">
        </flowable:taskListener>
    </extensionElements>
</userTask>

监听器代码如下：

@Context
@Named("billGenerateListener")//监听器名称，跟委托表达式里的设置对应
public class BillGenerateTaskListener implements TaskListener {

    @Override
    public void notify(final DelegateTask delegateTask) {
        delegateTask.getTaskDefinitionKey();
        final Map<String, Object> variables = delegateTask.getVariables();
        final Object costPrice = variables.get("costPrice");
        log.warn("Generate Bill...");
        log.warn("costPrice:{}",costPrice);
    }
}

下面直接看怎么解决的吧，不扯什么工作原理、底层机制，我们只是为了解决问题，直接抄了能用最好！

我们知道flowable引擎对象ProcessEngine一般是通过ProcessEngineConfiguration#buildProcessEngine()创建的，这是使用引擎的入口。
那么需要在ProcessEngineConfiguration中设置一个自己扩展的ProcessExpressionManager

cfg.setExpressionManager(
                new MicronautExpressionManager(applicationContext, cfg.getBeans()));

MicronautExpressionManager:

public class MicronautExpressionManager extends ProcessExpressionManager {
    protected ApplicationContext applicationContext;//micronaut的context
    
    public MicronautExpressionManager(ApplicationContext applicationContext, Map<Object, Object> beans) {
        super(beans);
        this.applicationContext = applicationContext;
    }

    @Override
    protected ELResolver createElResolver(VariableContainer variableContainer) {
        CompositeELResolver compositeElResolver = new CompositeELResolver();
        compositeElResolver.add(createVariableElResolver(variableContainer));

        compositeElResolver.add(createMicronautElResolver());//主要是这里
        compositeElResolver.add(new ArrayELResolver());
        compositeElResolver.add(new ListELResolver());
        compositeElResolver.add(new MapELResolver());
        compositeElResolver.add(new JsonNodeELResolver());
        compositeElResolver.add(new BeanELResolver());
        compositeElResolver.add(new CouldNotResolvePropertyELResolver());
        return compositeElResolver;
    }

    protected ELResolver createMicronautElResolver() {
        if (beans != null) {
            return new ReadOnlyMapELResolver(beans);
        } else {
            // 还有这里，在表达式中暴露 application-context
            return new MicronautApplicationContextElResolver(applicationContext);
        }
    }
}

再看下MicronautApplicationContextElResolver：

public class MicronautApplicationContextElResolver extends ELResolver {
    protected ApplicationContext applicationContext;

    public MicronautApplicationContextElResolver(ApplicationContext applicationContext) {
        this.applicationContext = applicationContext;
    }

    //主要就是这里通过bean名称获取micronaut的bean
    @Override
    public Object getValue(ELContext context, Object base, Object property) {
        if (base == null) {
            String key = (String) property;

            final Object o = applicationContext.getBean( Object.class, Qualifiers.byName(key));
            if (o!=null) {
                context.setPropertyResolved(true);
                return o;
            }
        }
        return null;
    }

    @Override
    public boolean isReadOnly(ELContext context, Object base, Object property) {
        return true;
    }

    @Override
    public void setValue(ELContext context, Object base, Object property, Object value) {
        if (base == null) {
            String key = (String) property;
            final Object o = applicationContext.getBean( Object.class, Qualifiers.byName(key));
            if (o!=null) {
                throw new FlowableException("Cannot set value of '" + property + "', it resolves to a bean defined in the micronaut application-context.");
            }
        }
    }

    @Override
    public Class<?> getCommonPropertyType(ELContext context, Object arg) {
        return Object.class;
    }

    @Override
    public Iterator<FeatureDescriptor> getFeatureDescriptors(ELContext context, Object arg) {
        return null;
    }

    @Override
    public Class<?> getType(ELContext context, Object arg1, Object arg2) {
        return Object.class;
    }
}

好了，抄完作业就阔以了。

原文发表于jenwang的随想

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