How to create a singleton class in Java? (Full Example)
The Java singleton pattern is implemented through private constructors, static instance fields and public static get methods; it is recommended to use enumerations because of their natural thread safety, serialization safety and simplicity.
To create a singleton class in Java, ensure only one instance of the class exists throughout the application and provide global access to it. The core idea is to control instantiation — prevent external instantiation via constructors and expose a static method to retrieve the sole instance.
Make the constructor private
This prevents other classes from using new to create instances.
- Declare the constructor with private access modifier.
- No public or protected constructors allowed.
Hold a static reference to the single instance
Store the one-and-only instance as a private static field inside the class.
- Initialize it either at declaration (eager) or on first access (lazy).
- Eager initialization is simpler and thread-safe by default.
Provide a public static method to get the instance
Typically named getInstance() , this method returns the stored instance.
- For eager initialization: just return the already-created instance.
- For lazy initialization with thread safety: use synchronized or the double-checked locking pattern (with volatile ).
Full working example (eager initialization)
public class DatabaseConnection {
// Private static instance — created when class loads
private static final DatabaseConnection INSTANCE = new DatabaseConnection();
// Private constructor — prevents instantiation from outside
private DatabaseConnection() {
// Optional: add logic like initializing a connection
}
// Public static method to access the instance
public static DatabaseConnection getInstance() {
return INSTANCE;
}
// Example method to demonstrate usage
public void connect() {
System.out.println("Connected to database.");
}
}Usage:
DatabaseConnection db1 = DatabaseConnection.getInstance(); DatabaseConnection db2 = DatabaseConnection.getInstance(); System.out.println(db1 == db2); // true — same object
Why not use enum? (Bonus robust alternative)
An enum singleton is inherently thread-safe, serialization-safe, and concise:
public enum DatabaseConnection {
INSTANCE;
public void connect() {
System.out.println("Connected to database.");
}
}Access via DatabaseConnection.INSTANCE.connect() . This is often the most reliable approach in modern Java.
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