如何使用 SPEL 引用@GatewayHeader 中参数的属性

问题描述

这应该是一个简单的问题，但我在网上找不到合适的文档.我想这样做:

@MessagingGateway(name = "redemptionGateway", defaultRequestChannel = Channels.GATEWAY_OUTPUT, defaultHeaders = @GatewayHeader(name = "orderId", expression = "#redemption.orderId"))公共接口 RedemptionGateway {无效创建(TrivialRedemption 赎回)；}

我显然使用了错误的语句来引用 redemption 的 orderId 成员:

org.springframework.expression.spel.SpelEvaluationException: EL1007E:(pos 0): 在 null 上找不到属性或字段orderId"

解决方案

好的.看

@GatewayHeader(name = "orderId", expression = "#redemption.orderId")

此 SpEL 在运行时根据实际参数进行评估，以构建 MessageHeaders 以供目标 Message 发送.

这就是它的样子:

private StandardEvaluationContext createMethodInvocationEvaluationContext(Object[] arguments) {StandardEvaluationContext context = ExpressionUtils.createStandardEvaluationContext(this.beanFactory);context.setVariable("args", 参数);context.setVariable("gatewayMethod", this.method);返回上下文；}

因此，EvaluationContext 增加了两个变量 args 和 gatewayMethod.

如您所见，名称中没有任何参数.无论如何，这对所有 JVM 都有效吗?

您可以使用 args 中的参数索引来实现您的目标:

expression = "#args[0].orderId"

This should be a simple question, but I cannot find the proper doc online. I want to do this:

@MessagingGateway(name = "redemptionGateway", defaultRequestChannel = Channels.GATEWAY_OUTPUT, defaultHeaders = @GatewayHeader(name = "orderId", expression = "#redemption.orderId"))
public interface RedemptionGateway {
    void create(TrivialRedemption redemption);
}

I am obviously using the wrong statement to reference the orderId member of redemption:

org.springframework.expression.spel.SpelEvaluationException: EL1007E:(pos 0): Property or field 'orderId' cannot be found on null

解决方案

OK. Look

@GatewayHeader(name = "orderId", expression = "#redemption.orderId")

This SpEL is evaluated at runtime against actual arguments to build MessageHeaders for target Message to send.

And that is how it looks:

private StandardEvaluationContext createMethodInvocationEvaluationContext(Object[] arguments) {
    StandardEvaluationContext context = ExpressionUtils.createStandardEvaluationContext(this.beanFactory);
    context.setVariable("args", arguments);

    context.setVariable("gatewayMethod", this.method);
    return context;
}

So, EvaluationContext is enriched with two variables args and gatewayMethod.

As you see, there is no any arguments population by their name. Will that work anyway for all the JVMs?

You can achieve your goal using parameter index from the args:

expression = "#args[0].orderId"

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