How to access a try-with-resource variable outside the block?

Question

Why can't I assign a try-with-resource variable outside of the try block?

The following statement is invalid:

Connection con = null;
try (con = DatabaseService.getConnection()) { //this is invalid. why?
     con.execute(...);
} catch (Exception e) {
    con.rollback();
}

How can I get access to the con variable inside the catch block?

Answer 1

This is only with Java version 8. It has improved Since Java 9.

With Java 9 the Try-With-Resources has been improved by introducing a new syntax:

public void loadDataFromDB() throws SQLException {
    Connection dbCon = DriverManager.getConnection("url", "user", "password");
    try (dbCon; ResultSet rs = dbCon.createStatement().executeQuery("select * from emp")) {
        while (rs.next()) {
            System.out.println("In method loadDataFromDB()" + rs.getString(1));
        }
    } catch (SQLException e) {
        System.out.println("Exception occured while reading data from DB " + e.getMessage());
    }
}

The same when compiled with Java 8 or lesser version will result in a compilation error.

In Java 9 you can use the variables in the scope of try and catch blocks.

Answer 2

I'd design it like this:

try (Connection con = DatabaseService.getConnection()) {
    try {
        con.execute(...);
    } catch (Exception e) {
        con.rollback();
    }
}

Answer 3

Since try-with-resources calls close() on a AutoCloseable class, you could go back to manually closing the connection:

Connection con = DatabaseService.getConnection();
try {
    con.execute(...);
} catch (Exception e) {
    con.rollback();
} finally {
    con.close();
}

Answer 4

My answer is based on answer from @Luiggi Mendoza:

public void doWithConnection() throws SQLException {
    try (Connection con = DatabaseService.getConnection()) {
        execute(con);
    }
}

private void execute(Connection con) throws SQLException {
    try {
        // TODO do smth. here
    } catch(Exception e) {
        con.rollback();
    }
}

Answer 5

In fact, since Java 9 you can use try-with-resources with final or effective final variables. So you can initialize the variable outside the try block, and just indicate that you want to use it in the try-with-resources block. This way the variable is available in the scope of the catch block.

Connection con = DatabaseService.getConnection();
try (con) {
     con.execute(...);
} catch (Exception e) {
    con.rollback();
}

Note: Even though the variable will be available in the catch method, the call to the close() method will happen before it enters the catch or finally blocks. So in this case it won't be useful to call a rollback() in a transaction, but it is to gather additional information from the object state.