singleton.md

The Singleton Pattern

The pattern belongs to the creational category of the design patterns.

Idea

Ensure a class only has one instance, and provide a global point of access to it.

Explanation

Wikipedia says:

In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system.

In plain words:

Ensures that only one object of a particular class is ever created.

Class Diagram

The class diagram will be:

Example

The task:

Let's consider to create a logger and runtime objects which must be instantiated only once.

Let's create a logger:

public enum Logger {
    INSTANCE;

    private final AtomicInteger level = new AtomicInteger(0);

    public int getLevel() {
        return level.get();
    }

    public void setLevel(int level) {
        this.level.set(level);
    }

    @Override
    public String toString() {
        return "Logger{" +
                "level=" + level +
                '}';
    }
}

And then it can be used as:

final var logger = Logger.INSTANCE;

Let's create a Runtime:

final class Runtime {

    private Runtime() {
    }

    public static Runtime getInstance() {
        return RuntimeHolder.INSTANCE;
    }

    private static class RuntimeHolder {
        private static final Runtime INSTANCE = new Runtime();
    }
}

And then it can be used as:

final var logger = Logger.INSTANCE;
// Check test results
assertNotNull(logger);
assertEquals(0, logger.getLevel());
logger.setLevel(1);
assertEquals(logger, Logger.INSTANCE);
assertEquals(1, Logger.INSTANCE.getLevel());
// Additional code
final var runtime = Runtime.getInstance();
// Check test results
assertNotNull(runtime);
assertEquals(runtime, Runtime.getInstance());

More Examples

• java.lang.Runtime#getRuntime()
• java.awt.Desktop#getDesktop()
• java.lang.System#getSecurityManager()

Links

• Singleton Pattern