Java JDBC Interview Questions

Call connection.close() to release JDBC connection back to pool; use try-with-resources for automatic closure.

Use CallableStatement: CallableStatement cs = connection.prepareCall("{call procName(?, ?)}"); set parameters with setString(); execute with executeQuery(); retrieve results with getters.

JDBC uses DriverManager to establish connections, executes SQL via Statement/PreparedStatement objects, and processes results through ResultSet which provides methods like next(), getInt(), getString() to iterate and extract data.

Database requests are formed using SQL strings passed to Statement.executeQuery(), executeUpdate(), or PreparedStatement with parameterized queries to prevent SQL injection.

JDBC supports four isolation levels: READ_UNCOMMITTED, READ_COMMITTED, REPEATABLE_READ, and SERIALIZABLE, set via Connection.setTransactionIsolation().

Register JDBC driver via Class.forName("driver.class.name") which loads the driver class, or rely on automatic registration via ServiceLoader in Java 6+.

JDBC workflow: load driver → create connection → create statement → execute query → process ResultSet → close ResultSet/Statement/Connection.

JDBC consists of two layers: JDBC API (java.sql package with Connection, Statement, ResultSet) and JDBC Driver (vendor-specific implementation managing database communication).

JDBC types (VARCHAR, INTEGER, TIMESTAMP, BLOB) map to Java types (String, int/Integer, java.util.Date/LocalDateTime, byte[]/Blob) through JDBC type conversion.

JDBC URL is a connection string format like 'jdbc:drivertype://hostname:port/databasename' that specifies the database location and connection parameters.

JDBC advantages include database independence through drivers, direct SQL execution, connection pooling support, and integration with enterprise applications.

JDBC: Register driver → create Connection → execute queries using Statement/PreparedStatement → process ResultSet → close resources in finally/try-with-resources.

Database requests are formed using SQL statements: SELECT, INSERT, UPDATE, DELETE; executed through Statement, PreparedStatement, or CallableStatement objects.

JDBC supports four isolation levels: READ_UNCOMMITTED, READ_COMMITTED, REPEATABLE_READ, SERIALIZABLE, mapped from Connection constants.

Register JDBC driver via Class.forName("driver.class.name") or through DriverManager.registerDriver(); modern versions auto-load via SPI.

Main JDBC workflow: load driver → establish connection → create statement → execute query → process ResultSet → close resources.

JDBC comprises four components: DriverManager, Connection, Statement, ResultSet; plus the driver implementations for specific databases.

JDBC types: CHAR/VARCHAR map to String, INTEGER/BIGINT to int/long, DOUBLE/FLOAT to double/float, TIMESTAMP to java.sql.Timestamp, BLOB/CLOB to byte[]/String.

JDBC URL is a connection string format: jdbc:subprotocol:subname (e.g., jdbc:mysql://localhost:3306/dbname) specifying protocol, host, and database.

JDBC advantages: direct SQL control, minimal overhead, broad database support, suitable for performance-critical applications, no ORM complexity.

JdbcTemplate is Spring's wrapper around JDBC reducing boilerplate code; handles connection management, exception translation, result mapping, and SQL execution elegantly.

Use DriverManager.getConnection(url, username, password) or DataSource to establish JDBC connections with proper driver registration.

Statement is recompiled per execution; PreparedStatement is precompiled and parameterized, providing better performance and SQL injection protection.

ResultSet is a JDBC interface representing query results; provides cursor-based row access via next(), getters for columns by index/name.

execute() returns boolean (true if ResultSet), executeQuery() returns ResultSet, executeUpdate() returns affected row count; use based on expected output.

ResultSet maintains a cursor position; next() advances to next row, getters fetch column data from current row; internally uses database connection for lazy data loading.

JdbcTemplate is a Spring utility that simplifies JDBC operations by handling resource management, exception translation, and repetitive code, providing callback-based access.

Database connection is established using DriverManager.getConnection(url, user, password) or DataSource, after loading the JDBC driver.

Statement executes static SQL queries, while PreparedStatement uses parameterized queries preventing SQL injection and improving performance through precompilation.

ResultSet is a table-like object returned by database queries, containing rows and columns; accessed using cursor-based navigation and type-specific getters.

execute() returns boolean for any SQL, executeQuery() returns ResultSet for SELECT, executeUpdate() returns row count for INSERT/UPDATE/DELETE.

ResultSet maintains a cursor pointing to current row, retrieved from database in batches determined by fetch size, and can be scrollable or forward-only.

ResultSet provides tabular access to query results, allows row iteration, type-safe data retrieval, and supports various cursor types and concurrency modes.

execute() runs any SQL statement; executeQuery() returns a ResultSet for SELECT queries; executeUpdate() returns row count for INSERT/UPDATE/DELETE operations.