how is connection injected into the inline function of jdbctemplate.query?

Question

I saw code as the following,

List<UploadedDocument> docs = jdbcTemplate.query(
                connection -> {
                PreparedStatement stmt = connection.prepareStatement(sql);
                stmt.setString(1, userName);
                return stmt;
            },
            this::getDocumentsFromResultSet
    );

I knew connection should database connection but could not find how it's created. It's not declared in the method or anywhere in the class.

Answer 1

It's basically no different from other JdbcTemplate methods where a query is executed without you explicitly needing to create or acquire a connection: Spring handles the creation/acquisition of the connection for you; it's made available to JdbcTemplate by Spring.

(The connection may have been created as part of a DB connection pool, when your app starts. But either way, all you have to provide are the connection properties.)

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Looking more closely at your specific example, there is actually an explicit mention of a connection instance.

So, why is that?

It's because of the lambda expression used in the example.

The specific query method in the question is:

query(PreparedStatementCreator psc,  ResultSetExtractor<T> rse)

Reference.

(At least, I think that's a ResultSetExtractor - I can't see the implementation of this::getDocumentsFromResultSet. But that is not relevant here.)

Ignoring the ResultSetExtractor, if we look at PreparedStatementCreator, we see that is a functional interface - which is why we can use the lambda expression shown in the question.

The specific method is for this functional interface is:

createPreparedStatement(Connection con)

That is where the connection object is provided (by Spring):

The Javadoc states:

This interface creates a PreparedStatement given a connection, provided by the JdbcTemplate class.

You get to see that reference - but you don't create it. You just make use of it to build your PreparedStatement.

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It's possible the JdbcTemplate was constructed using a DataSource "to obtain connections from".

But again, you would not see any code in the JdbcTemplate relating to the creation of any connections, or acquiring a connection from the pool (if you are using a pool)>

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Extra Note

Looking in the source code for JdbcTemplate, you can see where instances of that class acquire their database connection. Here is one example:

Connection con = DataSourceUtils.getConnection(obtainDataSource());

See here.