在Java中创建一个静态字段final

Question

Looking at the following code snippet, the scenario given is about fetching a database connection from a database connection pool in the Tomcat Server.

public final class DatabaseConnection {

    private static DataSource dataSource;

    static {
        try {
            Context initContext = new InitialContext();
            Context context = (Context) initContext.lookup("java:comp/env");
            dataSource = (DataSource) context.lookup("jdbc/assignment_db");
        } catch (NamingException e) {
            Logger.getLogger(DatabaseConnection.class.getName()).log(Level.SEVERE, null, e);
        }
    }

    public static Connection getConnection() throws SQLException {
        return dataSource.getConnection();
    }
}

I would like making the only field in the class final . If I make it a final field then, the compiler complains,

variable dataSource might not have been initialized.

After making it final , if it is initialized to null at the declaration place then, further initialization of this field in the try block is severely forbidden causing a compilation error,

cannot assign a value to final variable dataSource.

NamingException is a checked exception. Hence, it must be caught or declared to be thrown.

---

Earlier, there was a different scenario. The strategy was to use Apache DBCP as follows.

public final class DatabaseConnection {

    private static final BasicDataSource basicDataSource;

    static {
        basicDataSource = new BasicDataSource();
        basicDataSource.setDriverClassName("com.mysql.jdbc.Driver");
        basicDataSource.setUrl("jdbc:mysql://localhost:3306/assignment_db");
        basicDataSource.setUsername("Tiny");
        basicDataSource.setPassword("root");
    }

    public static Connection getConnection() throws SQLException {
        return basicDataSource.getConnection();
    }
}

In which, the field could be declared final because there is no checked exception and try...catch is unnecessary.

Can I somehow make dataSource final as in the first case?

Answer 1

This has got me stuck many times, until I found out a workaround. You can use a temporary variable in the static block, and assign the dataSource after try-catch ends using that variable:

private static final DataSource dataSource;

static
{
    DataSource tempDataSource = null;
    try
    {                
        Context initContext = new InitialContext();
        Context context = (Context) initContext.lookup("java:comp/env");
        tempDataSource = (DataSource) context.lookup("jdbc/assignment_db");
    } catch (NamingException ex) {
        Logger.getLogger(DatabaseConnection.class.getName()).log(Level.SEVERE, null, ex);
    }
    dataSource = tempDataSource;
}

However, I would rather not follow normally, and throw the exception from the catch block only if dataSource is not initialized, in which case, you won't need a local variable:

static
{
    try
    {                
        Context initContext = new InitialContext();
        Context context = (Context) initContext.lookup("java:comp/env");
        dataSource = (DataSource) context.lookup("jdbc/assignment_db");
    } catch (NamingException ex) {
        Logger.getLogger(DatabaseConnection.class.getName()).log(Level.SEVERE, null, ex);
        throw new ExceptionInInitializerError("dataSource not initialized");
    }
}