Try-with-resources - 它会自动关闭连接吗？ Java

Question

I've been working on a SQL utility and I am trying to set the parameters inside a prepared statement in multiple functions.

To lessen the code, I have a function that returns a prepared statement where all the params are set.

My question is:
Does the connection reference in the configureStatement() get closed using the try with resources in the query() ? If not how can the code be refactored to close the PreparedStatement and the Connection every time?

    public void query(String queryString, List<String> queryParams, Consumer<ResultSet> sqlConsumer)
    {
        try (PreparedStatement preparedStatement = this.configureStatement(queryString, queryParams))
        {
            sqlConsumer.accept(preparedStatement.executeQuery());
        } catch(SQLException exception)
        {
            exception.printStackTrace();
        }
    }
    
    private PreparedStatement configureStatement(String query, List<String> queryParams) throws SQLException
    {
        PreparedStatement preparedStatement = this.getConnection().prepareStatement(query);
        for (int i = 0; i < queryParams.size(); ++i)
            preparedStatement.setString(i, queryParams.get(i));

        return preparedStatement;
    }

Answer 1

No, the try with resources does not close the Connection that is used inside the PreparedStatement . Only the PreparedStatement and its ResultSet are closed.

When a Statement object is closed, its current ResultSet object, if one exists, is also closed.

It is possible to reuse a connection to execute many PreparedStatements . Each of which is closed after usage. When the connection is no longer needed it can be closed as well.

You could perhaps check it like this:

public void query(String queryString, List<String> queryParams, Consumer<ResultSet> sqlConsumer)
{
    Connection connection;
    try (PreparedStatement preparedStatement = this.configureStatement(queryString, queryParams))
    {
        connection=preparedStatement.getConnection();
        sqlConsumer.accept(preparedStatement.executeQuery());
    } catch(SQLException exception)
    {
        exception.printStackTrace();
    }
    if(connection!=null){
        System.out.println("Is Connection closed:"+connection.isClosed());
    }
}

private PreparedStatement configureStatement(String query, List<String> queryParams) throws SQLException
{
    PreparedStatement preparedStatement = this.getConnection().prepareStatement(query);
    for (int i = 0; i < queryParams.size(); ++i)
        preparedStatement.setString(i, queryParams.get(i));

    return preparedStatement;
}

A refactoring that closes connections by using the try-with-resources with multiple statements:

public void query(String queryString, List<String> queryParams, Consumer<ResultSet> sqlConsumer)
    {

        try ( Connection connection=this.getConnection(); 
              PreparedStatement preparedStatement = this.configureStatement(connection, queryString, queryParams);)
        {
            sqlConsumer.accept(preparedStatement.executeQuery());
        } catch(SQLException exception)
        {
            exception.printStackTrace();
        }
        if(connection!=null){
           connection.close();
        }
    }

private PreparedStatement configureStatement( Connection connection,String query, List<String> queryParams) throws SQLException
    {
        PreparedStatement preparedStatement = connection.prepareStatement(query);
        for (int i = 0; i < queryParams.size(); ++i)
            preparedStatement.setString(i, queryParams.get(i));

        return preparedStatement;
    }

Answer 2

Q: What makes you think returning an object from one of your own methods won't allow the object to be "closed" in a Java try with resources?

From the Java documentation:

https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html

Any object that implements java.lang.AutoCloseable, which includes all objects which implement java.io.Closeable, can be used as a resource.

The key issue: the object returned from your method must implement java.lang.AutoCloseable . In your case, it does:

https://docs.oracle.com/javase/8/docs/api/java/sql/PreparedStatement.html

Interface PreparedStatement

• All Superinterfaces:

  AutoCloseable, Statement, Wrapper