OpenFeign/feign · error · IllegalArgumentException

formatted errorMessageTemplate with errorMessageArgs

Error message

formatted errorMessageTemplate with errorMessageArgs

What it means

Util.checkArgument is Feign's Guava-style precondition: it throws IllegalArgumentException with a formatted message when the given boolean expression is false. The message template uses %s placeholders filled by errorMessageArgs. This is a guard against invalid caller-supplied arguments.

Solutions

1. Read the formatted message to see which precondition failed
2. Correct the invalid argument before calling the API (non-empty, in-range values)
3. Validate inputs in your own code before invoking Feign's builder/target APIs

Example fix

// before
Feign.builder().target("", ""); // checkArgument fails
// after
Feign.builder().target("my-service", "https://api.example.com");

if (name == null || name.isEmpty()) throw new IllegalArgumentException("target name must be non-empty");

boolean validTarget(String name, String url) { return name != null && !name.isEmpty() && url != null && !url.isEmpty(); }

try { builder.target(name, url); } catch (IllegalArgumentException e) { throw new ConfigException("Invalid Feign target: " + e.getMessage(), e); }

When it happens

Trigger: Any Feign API validated with Util.checkArgument receiving an invalid argument, e.g. empty strings for target names/URLs, negative values, or null/empty collections passed to builders and RequestTemplate methods.

Common situations: Feign.builder().target(...) with blank name/url; RequestTemplate with invalid inputs; misuse of internal Util-validated helpers when composing templates or encoders.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Expected a Class, ParameterizedType, or GenericArrayType…
• an expression is required.
• Status Code [ ] has already been declared to throw [ ] and…
• Cannot generate exception - check constructor parameter…
• Too many constructors marked with @FeignExceptionConstructor

AI-assisted analysis of OpenFeign/feign@e2a1e27560 (2026-09-10). Data as JSON: /api/errors/3d96e01e05ec4ce7. Report an issue: GitHub.

Appendix: source

Thrown at core/src/main/java/feign/Util.java:117

  private static final Pattern XML_CONTENT_TYPE =
      Pattern.compile("(?i)\\w+/(?:[\\w._-]+\\+)?xml.*");

  /** Type literal for {@code Map<String, ?>}. */
  public static final Type MAP_STRING_WILDCARD =
      new Types.ParameterizedTypeImpl(
          null,
          Map.class,
          String.class,
          new Types.WildcardTypeImpl(new Type[] {Object.class}, new Type[0]));

  private Util() { // no instances
  }

  /** Copy of {@code com.google.common.base.Preconditions#checkArgument}. */
  public static void checkArgument(
      boolean expression, String errorMessageTemplate, Object... errorMessageArgs) {
    if (!expression) {
      throw new IllegalArgumentException(format(errorMessageTemplate, errorMessageArgs));
    }
  }

  /** Copy of {@code com.google.common.base.Preconditions#checkNotNull}. */
  public static <T> T checkNotNull(
      T reference, String errorMessageTemplate, Object... errorMessageArgs) {
    if (reference == null) {
      // If either of these parameters is null, the right thing happens anyway
      throw new NullPointerException(format(errorMessageTemplate, errorMessageArgs));
    }
    return reference;
  }

  /** Copy of {@code com.google.common.base.Preconditions#checkState}. */
  public static void checkState(
      boolean expression, String errorMessageTemplate, Object... errorMessageArgs) {
    if (!expression) {
      throw new IllegalStateException(format(errorMessageTemplate, errorMessageArgs));

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