The default singleton pattern which is often propagated (given in the Singleton class) is simply not thread safe which means it does not fulfil the basic idea of the Singleton pattern.

There are two working solutions which are either SingletonEnum version or one which contains a synchronized locking via separate object (SingletonWithSynchronizedLock).

you can build the project with the following requirements:

• JDK 17+
• Apache Maven 3.8.6+

Just run:

$ mvn package -DskipTests

The following will execute the Main.main method which uses the Singleton (classical wrong implementation):

$ java -cp target/example-minimal-1.0-SNAPSHOT.jar com.soebes.kata.singleton.Main 

The above call will produce an output similar like this:

k: e8a47975-690c-4f20-802d-643c1b45f99b v:30
k: fc972e8b-0080-48c1-b93a-d2c06b971003 v:1
k: 65edd7d1-5e9e-4263-bcd0-524a02871438 v:302
k: a4ff3d6a-8341-46b4-8825-87f076cce1af v:49
k: 6160bddf-b1af-46cf-b32c-d94e0ff449de v:1
k: 129ddc35-d378-41ae-9479-c635e4461103 v:1
k: 23ba1321-c764-4b4f-8964-1cb14ca9cef4 v:999400
k: eee78336-5be4-4826-bb13-54937ed3b2ea v:3
k: 5e1b3f8f-235f-4b46-a935-dc9090eb9b7f v:1
k: 3f3cfdf7-176b-49a4-89b2-d8946003717e v:212

This indicated that there existing multiple instances of the Singleton class based on the different uuid.

You can also run the execution with the SingletonEnum pattern which uses an Enum class as a singleton (There might be cases where you can't use an Enum because sometimes that will not work with an enum.)

$ java -cp target/example-minimal-1.0-SNAPSHOT.jar com.soebes.kata.singleton.MainEnum 

and the output will look like this:

k: f3dfb9d6-34e2-44e1-88a7-c9f24f2ed4db v:1000000

This correctly shows there exists only a single instance of the SingletonEnum class which is not really astonishing because enums are handled by the compiler (and the JVM).

And in cases where you can't use the previous enum pattern you have to implement like the SingletonWithLock to make sure having only a single instance within the whole JVM.

This example can be executed via:

$ java -cp target/example-minimal-1.0-SNAPSHOT.jar com.soebes.kata.singleton.MainSynchronizedLock 

which will result in the following output:

k: c620ab52-bd67-45fe-a833-43554da79f98 v:1000000