ThreadLocal to implement a per-thread Singleton

Posted in: ThreadLocal in Java Thread-local variables are commonly used to render stateful Singleton or shared objects thread-safe, either by encapsulating the entire unsafe object in a ThreadLocal or by encapsulating the object's thread-specific state in a ThreadLocal. For example, in an application that is tightly tied to a database, many methods may need to access the database. It could be inconvenient to include a Connection as an argument to every method in the system -- a sloppier, but significantly more convenient technique would be to access the connection with a Singleton. However, multiple threads cannot safely share a JDBC Connection. By using a ThreadLocal in our Singleton, as shown in Listing 3, we can allow any class in our program to easily acquire a reference to a per-thread Connection. In this way, we can think of a ThreadLocal as allowing us to create a per-thread-singleton.

public class ConnectionDispenser {    private static class ThreadLocalConnection extends ThreadLocal {     public Object initialValue() {    return DriverManager.getConnection(ConfigurationSingleton.getDbUrl());     }   }  private static ThreadLocalConnection conn = new ThreadLocalConnection();    public static Connection getConnection() {     return (Connection) conn.get();   } }