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ReadResolve must have Object return type, not void

ID: java/wrong-readresolve-signature
Kind: problem
Security severity: 
Severity: warning
Precision: medium
Tags:
   - reliability
   - maintainability
   - language-features
Query suites:
   - java-security-and-quality.qls

Click to see the query in the CodeQL repository

If a class uses the readResolve method to specify a replacement object instance when the object is read from a stream, ensure that the signature of readResolve is exactly what the Java serialization mechanism expects.

Recommendation

Ensure that the signature of the readResolve method in the class matches the expected signature:

ANY-ACCESS-MODIFIER Object readResolve() throws ObjectStreamException;

Note that the method must return a java.lang.Object.

If readResolve is used for instance control of a serializable singleton, (that is, to make sure that deserializing a singleton class does not result in another instance of the singleton) it may be possible to use an enum with a single element instead. The Java serialization specification explicitly ensures that deserializing an enum does not create a new instance. (For details about this technique, see [Bloch].)

Example

In the following example, FalseSingleton.readResolve has the wrong signature, which causes deserialization to create a new instance of the singleton. However, Singleton.readResolve has the correct signature, which means that deserialization does not result in another instance of the singleton.

class FalseSingleton implements Serializable {
	private static final long serialVersionUID = -7480651116825504381L;
	private static FalseSingleton instance;
	
	private FalseSingleton() {}
	
	public static FalseSingleton getInstance() {
		if (instance == null) {
			instance = new FalseSingleton();
		}
		return instance;
	}
	
	// BAD: Signature of 'readResolve' does not match the exact signature that is expected
	// (that is, it does not return 'java.lang.Object').
	public FalseSingleton readResolve() throws ObjectStreamException {
		return FalseSingleton.getInstance();
	}
}

class Singleton implements Serializable {
	private static final long serialVersionUID = -7480651116825504381L;
	private static Singleton instance;
	
	private Singleton() {}
	
	public static Singleton getInstance() {
		if (instance == null) {
			instance = new Singleton();
		}
		return instance;
	}
	
	// GOOD: Signature of 'readResolve' matches the exact signature that is expected.
	// It replaces the singleton that is read from a stream with an instance of 'Singleton',
	// instead of creating a new singleton.
	private Object readResolve() throws ObjectStreamException {
		return Singleton.getInstance();
	}
}

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