In SpEL, create a new inline list using a previously-constructed list

user_16750357 0
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In spring-integration I'm wanting to "update" a list of values that are kept within a message header, using a header-enricher.

In my example, I've created an inline list {'x', 'y'} and I want to add 'z' to that list. I haven't found an example of how to do this in SpEL. It seems like it should be a common, trivial thing to do.

<si:header-enricher>
    <si:header name="myList" expression="{ 'x', 'y' }" />
</si:header-enricher>

...

<si:header-enricher default-overwrite="true" >
    <!-- CLEARLY THIS DOESN'T WORK BUT USING TO DEMONSTRATE THE DESIRED RESULT -->
    <si:header name="myList" expression="{ headers.myList, 'z' }" />
</si:header-enricher>

How does one indicate that you want to create a new inline list that is the result of adding a value to an existing list?

In the demonstration code, the outcome would be [ [ 'x', 'y' ], 'z' ] rather than the desired ['x', 'y', 'z' ].

thanks for any pointers

spring-el spring-integration spring
2 Answers

This uses the Java DSL, but the expressions will work in XML too

@Bean
IntegrationFlow flow() {
    return IntegrationFlows.fromSupplier(() -> "foo", e -> e.poller(Pollers.fixedDelay(Duration.ofSeconds(5))))
            .enrichHeaders(headers -> headers
                    .headerExpression("list", "{ 'x', 'y' }"))
            .enrichHeaders(headers -> headers
                    .headerExpression("list", "T(java.util.stream.Stream).concat(headers.list.stream(), "
                            + "{ 'z' }.stream()).collect(T(java.util.stream.Collectors).toList())")
                    .defaultOverwrite(true))
            .log()
            .get();
}

Result:

GenericMessage [payload=foo, headers={list=[x, y, z], ...

EDIT

Here's another alternative:

public class MyList<T> extends ArrayList<T> {

    public MyList(List<T> original, T... elements) {
        super(original);
        for (T element : elements) {
            add(element);
        }
    }

}

Then "new com.example.demo.MyList(headers.list, 'z')"

user_12407630

For completeness, and in case it's interesting for someone else, I'm posting a solution using custom SpEL functions, recommended by @Artem.

Adding a static method into an abstract class for list concatenation:

package me.myself;

...

public abstract class MySpelFunctions {

    public static <T> List<T> listConcat(Iterable<? extends T> a, Iterable<? extends T> b) {
        return unmodifiableList(Lists.newArrayList(Iterables.concat(a, b)));
    }

}

Then using the namespace support to add this custom function to the evaluation context:

<int:spel-function id="listConcat" 
                   class="me.myself.MySpelFunctions" 
                   method="listConcat(java.lang.Iterable, java.lang.Iterable)" />

We can now use the custom function within the SpEL expression for the header-enricher:

<si:header-enricher>
    <si:header name="myList" expression="{ 'x', 'y' }" />
</si:header-enricher>

...

<si:header-enricher default-overwrite="true" >
    <si:header name="myList" expression="#listConcat( headers.myList, { 'z' } )" />
</si:header-enricher>

Thanks again for your help in arriving at a readable solution.

user_167503570
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