apache/dubbo · error · IllegalArgumentException

Arguments' types are different

Error message

Arguments' types are different

What it means

Thrown by ArrayMerger.merge() when the arrays passed in have different component types. ArrayMerger is the Merger<Object[]> used by Dubbo's group/merger feature to combine array results returned by multiple group providers into one array; all arrays must share the same component type to be concatenated.

Source

Thrown at dubbo-cluster/src/main/java/org/apache/dubbo/rpc/cluster/merger/ArrayMerger.java:52

        int i = 0;
        while (i < items.length && items[i] == null) {
            i++;
        }

        if (i == items.length) {
            return new Object[0];
        }

        Class<?> type = items[i].getClass().getComponentType();

        int totalLen = 0;
        for (; i < items.length; i++) {
            if (items[i] == null) {
                continue;
            }
            Class<?> itemType = items[i].getClass().getComponentType();
            if (itemType != type) {
                throw new IllegalArgumentException("Arguments' types are different");
            }
            totalLen += items[i].length;
        }

        if (totalLen == 0) {
            return new Object[0];
        }

        Object result = Array.newInstance(type, totalLen);

        int index = 0;
        for (Object[] array : items) {
            if (array != null) {
                System.arraycopy(array, 0, result, index, array.length);
                index += array.length;
            }
        }
        return (Object[]) result;

View on GitHub (pinned to 3a3043227f)

Solutions

1. Ensure all provider groups return arrays of the same component type for the merged method.
2. If heterogeneous results are expected, merge at the application layer instead of using ArrayMerger, or normalize results to a common supertype array.
3. Validate return-type consistency across group implementations before enabling the merger.

Example fix

// before: providers return different array types -> ArrayMerger throws
// groupA returns String[], groupB returns Object[] -> mismatch

// after: all groups return the same component type
public interface Foo { String[] list(); } // every group returns String[]

// Before merging group array results, validate component types agree
Set<Class<?>> types = Arrays.stream(items)
    .filter(Objects::nonNull)
    .map(a -> a.getClass().getComponentType())
    .collect(Collectors.toSet());
if (types.size() > 1) {
    throw new IllegalArgumentException("Refusing to merge arrays of differing types: " + types);
}
return ArrayMerger.INSTANCE.merge(items);

// Guard: all non-null arrays share the same component type
static boolean sameComponentType(Object[]... items) {
    Class<?> t = null;
    for (Object[] a : items) {
        if (a == null) continue;
        Class<?> c = a.getClass().getComponentType();
        if (t == null) t = c;
        else if (c != t) return false;
    }
    return true;
}

try {
    return ArrayMerger.INSTANCE.merge(parts);
} catch (IllegalArgumentException e) {
    if (e.getMessage().contains("types are different")) {
        // normalize to a common array type or merge manually
        return mergeAsObjectList(parts);
    }
    throw e;
}

When it happens

Trigger: A service method declared with an array return type and @DubboReference(merger=...) / group merging enabled, where different provider groups return arrays of incompatible component types (e.g., one returns String[] and another Integer[]).

Common situations: Mixed provider implementations returning different array element types for the same interface method; a provider returning null arrays plus typed arrays in a way that resolves to differing component types; misuse of the merger feature on heterogeneous group results.

Related errors

• Failed to invoke service {entry.getKey()}: {e.getMessage()}
• Can not merge result because missing method [ {merger} ] in
• There is no merger to merge result.
• INTERNAL_ERROR
• Invalid configurator rule, please specify at least one param

AI-assisted analysis of apache/dubbo@3a3043227f (2026-08-14). Data as JSON: /api/errors/e1f623f2feed62d9. Report an issue: GitHub.