Breaking Free from the Evil Singleton
TL;DR: Refactor singletons to reduce coupling
Problems Addressed
- High
- Multi-threading issues
Related Code Smells
Steps
- Identify the singleton
- Locate all references to its getInstance() method
- Refactor the singleton to a standard class
- Inject it as a dependency
Sample Code
Before
CODE
public class DatabaseConnection {
private static DatabaseConnection instance;
private DatabaseConnection() {}
public static DatabaseConnection getInstance() {
if (instance == null) {
instance = new DatabaseConnection();
}
return instance;
}
public void connect() {
}
}
public class Service {
public void performTask() {
DatabaseConnection connection = DatabaseConnection.getInstance();
connection.connect();
}
}
After
CODE
public class DatabaseConnection {
// 1. Identify the singleton
public void connect() {
}
}
public class Service {
// 2. Locate all references to its getInstance() method.
private DatabaseConnection connection;
// 3. Refactor the singleton to a standard class.
public Service(DatabaseConnection connection) {
// 4. Inject it as a dependency.
this.connection = connection;
}
public void performTask() {
connection.connect();
}
}
DatabaseConnection connection = new DatabaseConnection();
// You can also mock the connection in your tests
Service service = new Service(connection);
service.performTask();
Type
[X] Semi-Automatic
Safety
This refactoring is safe when you update all references to the singleton and handle its dependencies correctly.
Testing each step ensures that no references to the singleton are missed.
Why the code is better?
Refactoring away from a singleton makes the code more modular, testable, and less prone to issues caused by the global state.
Injecting dependencies allows you to easily replace DatabaseConnection with a mock or different implementation in testing and other contexts.
Tags
- Coupling
Related Refactorings
See also
Credits
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This article is part of the Refactoring Series.
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