jwtk/jjwt · error · UnsupportedOperationException

Registries are immutable and cannot be modified.

Error message

Registries are immutable and cannot be modified.

What it means

DefaultRegistry is a read-only lookup structure; all mutating Map methods (put, remove, putAll, clear) delegate to immutable(), which unconditionally throws UnsupportedOperationException. Registries are populated once at library initialization and are intentionally unmodifiable.

Solutions

1. Don't mutate the registry - use the library's extension/SPI mechanisms for custom algorithms or codecs.
2. Keep your own Map for custom entries and consult it as a fallback when the registry lookup fails.
3. Copy needed entries into your own mutable map if you need a modified lookup table.

Example fix

// before
registries.getAlgorithms().put("MYALG", myAlg); // UnsupportedOperationException
// after
Map<String, MyAlg> custom = new HashMap<>();
custom.put("MYALG", myAlg);
MyAlg alg = custom.containsKey(id) ? custom.get(id) : (MyAlg) registries.getAlgorithms().forKey(id);

try {
    registry.put(k, v);
} catch (UnsupportedOperationException e) {
    // use the library's extension API or your own map instead
}

When it happens

Trigger: Calling put/remove/putAll/clear on a DefaultRegistry instance obtained from the library, e.g. trying to register a custom algorithm or codec by mutating the shared registry.

Common situations: Attempting runtime plugin-style registration of a custom JWA algorithm; test code trying to seed a registry; refactoring code that previously used a mutable HashMap into a registry.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• instance is immutable and may not be modified.
• JWKs are immutable and may not be modified.
• Not intended to be called.
• Not supported
• Unrecognized

AI-assisted analysis of jwtk/jjwt@fb71496164 (2026-09-09). Data as JSON: /api/errors/4513852036dbaccf. Report an issue: GitHub.

Appendix: source

Thrown at impl/src/main/java/io/jsonwebtoken/impl/lang/DefaultRegistry.java:66

    }

    @Override
    public V apply(K k) {
        return get(k);
    }

    @Override
    public V forKey(K key) {
        V value = get(key);
        if (value == null) {
            String msg = "Unrecognized " + this.qualifiedKeyName + ": " + key;
            throw new IllegalArgumentException(msg);
        }
        return value;
    }

    static <T> T immutable() {
        throw new UnsupportedOperationException("Registries are immutable and cannot be modified.");
    }

    @Override
    public V put(K key, V value) {
        return immutable();
    }

    @Override
    public V remove(Object key) {
        return immutable();
    }

    @Override
    public void putAll(Map<? extends K, ? extends V> m) {
        immutable();
    }

    @Override

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