How to implement a thread-safe singleton pattern in Java?
Jul 13, 2025 am 01:12 AM
There are three main ways to implement thread-safe singleton mode: First, use double check lock and volatile keywords, enter the synchronization block after the first check that the instance is empty, and confirm again whether it is empty, ensuring that only one instance is created; second, use static inner class (Bill Pugh implementation) to ensure thread safety during class loading through JVM, delay loading and no explicit synchronization is required; third, use enumeration to implement singleton, which is naturally thread-safe and can prevent reflection and serialization attacks, but may not be suitable for complex initialization or inheritance. In addition, the simple lazy style affects performance because each call needs to be synchronized, and is not recommended to use in a multi-threaded environment. Choose different implementation methods according to your needs to take into account security, performance and simplicity.
Implementing a thread-safe singleton pattern in Java is important when you want to ensure that only one instance of a class is created, even in a multi-threaded environment. Here's how to do it properly without overcomplicating things.
Double-Checked Locking with volatile
This method ensures that the singleton instance is created only once and is safe for use in multi-threaded contexts.
You check if the instance exists twice — once without locking, and again inside the synchronized block. This minimizes the performance hit from synchronization after the instance is already created.
public class Singleton {
private static volatile Singleton instance;
private Singleton() {}
public static Singleton getInstance() {
if (instance == null) {
synchronized (Singleton.class) {
if (instance == null) {
instance = new Singleton();
}
}
}
return instance;
}
}Key points:
- The
volatilekeyword ensures visibility across threads. - Two checks (
if (instance == null)) prevent unnecessary synchronization after the first creation. - This approach is efficient and widely used in real applications.
Use Static Inner Class (Bill Pugh Singleton Implementation)
This method uses a nested class to hold the singleton instance. The JVM takes care of thread safety during class initialization.
public class Singleton {
private Singleton() {}
private static class SingletonHelper {
private static final Singleton INSTANCE = new Singleton();
}
public static Singleton getInstance() {
return SingletonHelper.INSTANCE;
}
}Why this works well:
- The inner class isn't loaded until
getInstance()is called. - Class loading in Java is guaranteed to be thread-safe.
- No need for explicit synchronization or
volatile.
This is often preferred because it's clean, thread-safe, and doesn't require special handling for performance.
Enum-Based Singleton
Enums are inherently thread-safe and provide a simple and effective way to implement singletons.
public enum Singleton {
INSTANCE;
// You can add methods here
public void doSomething() {
// ...
}
}To use it:
Singleton.INSTANCE.doSomething();
Advantages:
- Prevents against reflection attacks (constructors can't be called more than once).
- Handles serialization automatically.
- Very concise and safe.
However, some developers avoid enums for more complex initializations or when they need inheritance.
Avoid Simple Lazy Initialization
A basic implementation like this is not thread-safe:
public class Singleton {
private static Singleton instance;
private Singleton() {}
public static synchronized Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
} The problem? Every call to getInstance() will go through synchronization, which affects performance after the first time. That's why double-checked locking or other approaches are better.
Also, if you skip the synchronized keyword entirely, multiple threads could create multiple instances.
So depending on your needs — whether it's simplicity, performance, or protection against edge cases — you can choose between double-checked locking, static inner classes, or enums.
Basically that's it.
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