alibaba/canal · error · IllegalStateException

Duplicate extension {} name {} on {} and {}

Error message

Duplicate extension {} name {} on {} and {}

What it means

Error "Duplicate extension {} name {} on {} and {}" thrown in alibaba/canal.

Source

Thrown at client-adapter/common/src/main/java/com/alibaba/otter/canal/client/adapter/support/ExtensionLoader.java:362

                                                                + "), class " + clazz.getName()
                                                                + "is not subtype of interface.");
                                            } else {
                                                try {
                                                    clazz.getConstructor(type);
                                                } catch (NoSuchMethodException e) {
                                                    clazz.getConstructor();
                                                    String[] names = NAME_SEPARATOR.split(name);
                                                    if (names != null && names.length > 0) {
                                                        for (String n : names) {
                                                            if (!cachedNames.containsKey(clazz)) {
                                                                cachedNames.put(clazz, n);
                                                            }
                                                            Class<?> c = extensionClasses.get(n);
                                                            if (c == null) {
                                                                extensionClasses.put(n, clazz);
                                                            } else if (c != clazz) {
                                                                cachedNames.remove(clazz);
                                                                throw new IllegalStateException(
                                                                        "Duplicate extension " + type.getName() + " name "
                                                                                + n + " on "
                                                                                + c.getName() + " and "
                                                                                + clazz.getName());
                                                            }
                                                        }
                                                    }
                                                }
                                            }
                                        }
                                    } catch (Throwable t) {
                                        IllegalStateException e = new IllegalStateException(
                                                "Failed to load extension class(interface: " + type + ", class line: "
                                                        + line + ") in " + url
                                                        + ", cause: "
                                                        + t.getMessage(),
                                                t);
                                        exceptions.put(line, e);

View on GitHub (pinned to 87be50e876)

Solutions

1. Rename one of the conflicting extension implementations so each name is unique per extension point.
2. Remove duplicate adapter jars from the classpath that register the same extension name.

When it happens

Trigger: Thrown at client-adapter/common/src/main/java/com/alibaba/otter/canal/client/adapter/support/ExtensionLoader.java:362 when the library encounters an invalid state.

Common situations: Duplicate extension name across two classes. Enforce uniqueness of extension names at load time; last-one-wins silently would make behavior order-dependent.

AI-assisted analysis of alibaba/canal@87be50e876 (2026-08-14). Data as JSON: /api/errors/2b41366af9577005. Report an issue: GitHub.