TemplatedJDBC

A zero dependency light-weight wrapper for JDBC, using String Templates and records.

Introduction

This framework uses the String Templates preview feature. In order to use it you must use Java 21 or higher with preview features enabled (--enable-preview). This is still very much an experimental framework to see what a convenient syntax would be for a light-weight JDBC wrapper, and as such is subject to change.

It leans heavily on immutable records, and is assuming there will be a future convenient syntax for reconstructing records with different values. Until that time, custom with-ers, or a framework like Record Builder can be used to alleviate some of these discomforts.

It is extensible enough to allow using types that are not records, although currently there is limited support to make this usage easy and convenient. A custom solution that generates the necessary meta data using reflection and annotations may be included at a later time; until that time it is possible to provide your own (or make a pull request).

Features

• Zero dependencies
• Use String Templates to execute SQL safe from injection attacks
• Use Rows, records or custom types to read and write data
• Lambda based transactions that can be retried on transitive failures

Maven

Add this dependency to your pom.xml.

<dependency>
    <groupId>org.int4.db</groupId>
    <artifactId>db-core</artifactId>
    <version>0.1.0</version>
</dependency>

You will also need a JDBC driver for your database, and something that can provide Connections on demand (you can roll your own Supplier<Connection> or, highly recommended, use a connection pool like HikariCP).

Activating Preview Features

For the compiler plugin, add:

<configuration>
    <enablePreview>true</enablePreview>
</configuration>

For the surefire and failsafe plugins add:

<configuration>
    <argLine>--enable-preview</argLine>
</configuration>

Quick Example

class Example {
    record Employee(Integer id, String name, double salary) {}

    // Create a Reflector to hold Java to Database name mapping information:
    static final Reflector ALL = Reflector.of(Employee.class);

    final Database database;

    // Obtain DataSource from HikariCP for example
    Example(DataSource dataSource) {
        // Database is reusable, and can be shared for a whole application
        // so this can also be provided as a constructor parameter instead
        // of creating it here:
        this.database = DatabaseBuilder.using(dataSource::getConnection).build();
    }

    // Generates: "INSERT INTO employee (id, name, salary) VALUES (?, ?, ?)"
    void insertExample() {
        Employee employee = new Employee(1, "Jane", 1000.0);

        db.accept(tx ->
            // Execute an insert:
            tx."INSERT INTO employee (\{ALL}) VALUES (\{employee})"
                .execute()
        );
    }

    // Generates: "SELECT e.id, e.name, e.salary FROM employee e"
    List<Employee> queryExample() {
        return db.query(tx -> 
            tx."SELECT e.\{ALL} FROM employee e"
                .map(ALL)  // Convert to Employee records
                .toList();  // Execute and return a List
        );
    }

    // Generates: "UPDATE employee SET id = ?, name = ?, salary = ? WHERE id = ?"
    long updateExample() {
        return db.apply(tx -> 
            tx."""
                UPDATE employee SET \{ALL.entries(employee)}
                    WHERE id = \{employee.id()}
            """.executeUpdate();  // Execute and return row count
        );
    }
}

For more examples see DatabaseIT.java.

The Basics

Creating a Database instance

The Database class is the starting point for all your SQL needs. To create one, all you need is a source of Connections that it can use when needed. It is highly recommended to use a connection pool for this, and the below examples shows how to do this with HikariCP:

HikariConfig config = new HikariConfig();

config.setJdbcUrl(url);
config.setUsername(user);
config.setPassword(password);

DataSource dataSource = new HikariDataSource(config);

Database db = DatabaseBuilder.using(dataSource::getConnection).build();

In further examples below, we'll assume that db holds an instance of Database.

Transactions

Transactions can either be obtained directly by calling Database::beginTransaction or can be provided via a functional approach using Database::apply or Database::query. Transactions are associated with a thread, and starting a nested transaction will create a save point in the top level transaction to which a nested transaction may be rolled back.

Provided Transactions

Transactions provided via a callback do not need to be closed. They will be automatically committed if the function completes normally. If the function completes exceptionally, the transaction is rolled back.

db.apply(tx -> {
    // Perform database modifications here, transaction
    // commits if the function completes normally.
});

Manual Transactions

Manually created transactions should be closed. They are AutoCloseable making it possible to use them with a try with resources construct.

Transactions when closed will by default perform a rollback, unless Transaction::commit is called.

try (Transaction tx = db.beginTransaction()) {
    // query or modify database here

    // when done commit (or do nothing for rollback)
    tx.commit();
}

In further examples, tx will hold an active Transaction.

SQL execution

Simple SQL statements which require no template parameters can be executed directly:

List<Row> employees = tx."SELECT * FROM employee".toList();
List<Double> salaries = tx."SELECT salary FROM employee".asDouble().toList();
int count = tx."SELECT COUNT(*) FROM employee".map(r -> r.getInteger(0)).get();
long affectedRows = tx."UPDATE employee SET salary = salary * 2".execute();

The template allows several customizations before the statement is executed. Depending on the statement type, the template can be executed immediately to get the number of affected rows, or it can be treated as a query in which case it will return Rows unless mapped to a more specific type. Only terminal operations will trigger the execution of the statement.

Method Signature	Operation Type	Description
map(Function)	Intermediate	Maps from the current type T to a new type U
mapGeneratedKeys	Intermediate	Switches to the Rows that contain the generated keys
asInt, asLong, ...	Intermediate	Assumes a single column, and maps the first column to the indicated type
execute	Terminal Statement	Executes the statement and returns the number of affected rows
get	Terminal Query	Executes the statement and returns the first and only result, or null if unavailable
getFirst	Terminal Query	Executes the statement and returns the first result, or null if unavailable
getOptional	Terminal Query	Executes the statement and returns the first and only result as an Optional<T>
toList	Terminal Query	Executes the statement and returns the results as a (possibly empty) List
consume(Consumer)	Terminal Query	Executes the statement, and calls the given consumer for each type T
consume(Consumer, long)	Terminal Query	Executes the statement, and calls the given consumer for each type T up to a given maximum, returns false if all rows were processed

Note: Rows are thin wrapper around a JDBC ResultSet but use zero based column indices to fit in better.

Using template parameters

String name = "John"

int count = tx."SELECT COUNT(*) FROM employee WHERE name = \{name}".asInt().get();

Resulting SQL: SELECT COUNT(*) FROM employee WHERE name = ?

Leveraging records

Handling data using untyped Rows is cumbersome and error prone. Instead, records can be used to handle groups of fields as a single unit.

In its most simple form, a record can be used as a list of values. Given:

record Employee(Integer id, String name, double salary);

Then the record can be leveraged as:

Record employee = new Employee(5, "Jane", 1000.0);

tx."INSERT INTO employee (id, name, salary) VALUES (\{employee})".execute();

SQL: INSERT INTO employee (id, name, salary) VALUES (?, ?, ?)

The above statement will take the required values directly form the record. Note that this is still error prone as the order, and number of the fields must match those of the record.

Introducing Reflectors, Mappers and Extractors

Type	Purpose
Mapper	Takes a Row from a database and converts it a Java object
Extractor	Provides fields or fields + values to INSERT, UPDATE or SELECT statements
Reflector	The combination of a Mapper and Extractor

A Reflector can be obtained by providing it with a suitable record definition. The record must use primitive types supported by the underlying database, but can include nested record types consisting of only supported types.

Reflectors can also be created for arbitrary types, although this involves providing both the means to create such a type, and a way to extract values from it. At the moment there is only limited support for such types, and some manual work is needed. In the future it's possible that Reflection + Annotations may make this task a bit easier. For now it is assumed we're dealing with record types.

By default, the component names of a record are translated by converting a camel case name to an underscored name. For example, creationDate would become creation_date. The names can be overridden by calling additional methods on the Reflector.

Reflectors and Extractors can be used as template parameters. Given the following static definitions:

static final Reflector<Employee> ALL = Reflector.of(Employee.class);
static final Extractor<Employee> EXCEPT_ID = ALL.excluding("id");

The table below shows the generated SQL for all possible usages of these types:

Template Parameter	Templated as	SQL
\{ALL}	Field List	id, name, salary
\{EXCEPT_ID}		name, salary
e.\{EXCEPT_ID} (with alias)		e.name, e.salary
\{ALL.values(employee)} or \{employee}	Value List	?, ?, ? (id, name, salary)
\{EXCEPT_ID.values(employee)}		?, ? (name, salary)
\{EXCEPT_ID.entries(employee)}	Key/Value List	name = ?, salary = ?

Note: records that are provided directly as template parameters must have their components defined in the exact same order as the supplied fields. No attempt is made to match fields by name.

Examples

// Reading records
List<Employee> = tx."SELECT \{ALL} FROM employee WHERE id > 2"
    .map(ALL)  // The ALL Reflector is also a Mapper
    .toList();

SQL: SELECT id, name, salary FROM employee WHERE id > 2

// Inserting records
Employee newEmployee = new Employee(null, "John", 42.42);

tx."INSERT INTO employee (\{EXCEPT_ID}) VALUES (\{newEmployee})".execute();

SQL: INSERT INTO employee (name, salary) VALUES (?, ?)

// Updating records
Employee updatedEmployee = newEmployee.withName("Jane");

tx."UPDATE employee SET \{EXCEPT_ID.entries(updatedEmployee)} WHERE id = 5".execute();

SQL: UPDATE employee SET name = ?, salary = ? WHERE id = 5

Nested Records

Reflectors support nested records amd cam flatten such records to a single set of fields. Given:

record Company(Integer id, String name, Address address) {}
record Address(String street, String city) {}

A Reflector can be created for Company which inlines the fields of Address:

Reflector<Company> COMPANY = Reflector.of(Company.class)
    .inline("address", Reflector.of(Address.class));

The fields will be: id, name, address_street and address_city

Company c = tx."SELECT \{COMPANY} FROM company c JOIN address a WHERE a.id = c.id"
    .map(COMPANY)
    .get();

Obtaining and using generated ID's

Tables are often defined with id's generated by the database. In order to obtain these and (re)construct the resulting record with the correct id, use the following form for an INSERT statement:

Employee employee = new Employee(null, "John", 42.42);

Employee employeeWithId = tx."INSERT INTO employee (\{ALL}) VALUES (\{employee})"
    .mapGeneratedKeys()  // switch to the generated keys result set
    .asInt()  // map first column to integers
    .map(employee::withId)  // reconstruct the record using a custom wither
    .get();  // executes the statement and returns the new record

Advanced Stuff

In JDBC, normally only parameters can be templated, which is very limited. In this framework, the templates recognize several types that it can integrate safely into an SQL statement. This is done by strictly limiting what these types can provide.

Type	Use
Any Record type	As JDBC placeholders
Extractor	As field names
Extractor::entries	A combination of names and JDBC placeholders suitable for UPDATE statements
Extractor::values	As JDBC placeholders, possibly filtered
Identifier	As an SQL identifier

Identifiers

Sometimes it is needed to be able to template more than just fields and values. A system that can create tables on demand, and query from them, may need to be able to specify its own table name. It is not possible to concatenate such information into a String Template, as templates do not allow concatenation for security reasons, nor does the template engine in this framework allow for inclusion of arbitrary Strings.

In order to support this use case, one can convert a String into an Identifier. This involves a strict validation check that ensures the String can only be an SQL identifier, and thus would be safe to include. This looks like this:

tx."CREATE TABLE \{Identifier.of("my_table")} ( ... )".execute();

Checked Exceptions, or not...

You choose. You can create either a normal Database type that will throw an unchecked DatabaseException which is a wrapper around an SQLException, or you can create a CheckedDatabase that will throw SQLExceptions.

Database db = DatabaseBuilder.using(dataSource::getConnection).build();

CheckedDatabase checkedDB = DatabaseBuilder.using(dataSource::getConnection).throwingSQLExceptions();

Future

• Batch support, by providing a Collection to appropriate statements
• More investigating how to support aliasing of fields