Singleton Pattern

Definition

The Singleton Pattern ensures that a class has only one instance and provides a global point of access to it.

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Real-World Analogy

Imagine you have a database that every part of your application needs to access. Creating a new database connection each time can be expensive and requires repetitive configuration connection strings, usernames, passwords, and so on. Instead, you can use a single shared database connector that’s created once and used everywhere. This is exactly where the Singleton Pattern is useful.

Implementation in Java

First, create a DatabaseConnector.java class with a private constructor so that no one outside the class can directly create an instance. Then define a private static field to hold the single instance of the class. Provide a public static method, getInstance(), which returns the instance. If the instance doesn’t exist yet, it’s created; otherwise, the existing instance is returned.

DatabaseConnector.java

public class DatabaseConnector {
    // Holds the single instance
    private static DatabaseConnector instance;
 
    private String connectionString;
 
    // Private constructor prevents external instantiation
    private DatabaseConnector(String connectionString) {
        this.connectionString = connectionString;
    }
 
    @Override
    public String toString() {
        return "DatabaseConnector[connectionString=" + connectionString + "]";
    }
 
    // Synchronized to avoid race conditions in multithreaded environments
    public static synchronized DatabaseConnector getInstance() {
        if (instance == null) {
            instance = new DatabaseConnector("PostgresSQL");
        }
        return instance;
    }
}

Notice that getInstance() is synchronized to prevent multiple threads from creating separate instances simultaneously.

To use the connector:

SingletonPatternExample.java

public class SingletonPatternExample {
    public static void main(String[] args) {
        DatabaseConnector conn1 = DatabaseConnector.getInstance();
        System.out.println(conn1);
 
        DatabaseConnector conn2 = DatabaseConnector.getInstance();
        System.out.println(conn2);
    }
}

Output:

DatabaseConnector[connectionString=PostgresSQL]
DatabaseConnector[connectionString=PostgresSQL]

Both calls return the same instance.

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Design Diagram

classDiagram
    class DatabaseConnector {
        - static DatabaseConnector instance
        - String connectionString
        - DatabaseConnector(String connectionString)
        + static DatabaseConnector getInstance()
        + String toString()
    }

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Real-World Example in Java

The Runtime class in Java also uses the Singleton Pattern. There is only one JVM per machine, so Java provides a single Runtime instance:

RuntimeExample.java

public class RuntimeExample {
    public static void main(String[] args) {
        Runtime rt = Runtime.getRuntime();
        System.out.println("Available processors: " + rt.availableProcessors());
    }
}

Output:

Available processors: 8