Explain connection pooling in Java
💡 Concept: Connection Pooling
Connection pooling is a technique used to reuse and manage database connections efficiently instead of creating a new connection for every request.
📘 Quick Intro
Establishing database connections is expensive. Connection pooling helps by keeping a pool of open connections and reusing them instead of opening new ones repeatedly.
🧠 Analogy
Think of connection pooling like a cab stand. Instead of calling a new cab every time, you just take the next available one. Once you're done, the cab returns to the stand for others to use.
🔧 Technical Explanation
- A pool is created with a fixed number of database connections.
- When an app requests a connection, it is assigned from the pool.
- After use, the connection is returned to the pool rather than being closed.
- Reduces connection creation overhead and improves performance.
- Popular libraries: HikariCP, Apache DBCP, C3P0.
🎯 Use Cases
- ✅ Web applications with high user traffic
- ✅ APIs making frequent database calls
- ✅ Enterprise Java apps needing scalable database access
💻 Example: HikariCP Connection Pool
import com.zaxxer.hikari.HikariConfig;
import com.zaxxer.hikari.HikariDataSource;
import java.sql.Connection;
public class HikariExample {
public static void main(String[] args) throws Exception {
HikariConfig config = new HikariConfig();
config.setJdbcUrl("jdbc:mysql://localhost:3306/testdb");
config.setUsername("root");
config.setPassword("root123");
HikariDataSource ds = new HikariDataSource(config);
try (Connection conn = ds.getConnection()) {
System.out.println("Connected using pool!");
}
}
}
❓ Interview Q&A
Q1: What is connection pooling?
A: Reusing a pool of database connections to reduce connection overhead.
Q2: Why is it important?
A: It improves performance and scalability by reducing connection creation time.
Q3: What happens when the pool is exhausted?
A: Requests wait or fail, depending on configuration.
Q4: Can pooled connections be closed?
A: They are returned to the pool, not actually closed.
Q5: Name some pooling libraries.
A: HikariCP, Apache DBCP, C3P0.
Q6: Is pooling suitable for multi-threaded apps?
A: Yes, most pooling libraries are thread-safe.
Q7: Can JDBC work without pooling?
A: Yes, but it's inefficient.
Q8: What config is typically set in pool?
A: Max pool size, timeout, idle time.
Q9: Is pooling done at JVM level?
A: No, by libraries or containers.
Q10: What is the default size of HikariCP pool?
A: 10 connections.
📝 MCQs
Q1. What is connection pooling?
- Creating new connections
- Reusing database connections
- Batching SQL
- Caching queries
Q2. Which library is known for pooling?
- Log4j
- Hibernate
- HikariCP
- Tomcat
Q3. Is pooling faster than creating new connections?
- No
- Yes
- Equal
- Depends on DB
Q4. What happens when all pool connections are used?
- New pool is created
- App restarts
- Requests wait or fail
- Nothing
Q5. Which config limits pool size?
- Idle timeout
- Query limit
- Maximum pool size
- Cache size
Q6. Do you need to close pooled connections?
- No
- Yes
- They are returned to pool
- They expire automatically
Q7. What does HikariCP stand for?
- Java Config Pool
- High-performance JDBC pool
- Lightweight Connection Provider
- None
Q8. Which Java interface is used to acquire connections?
- Connector
- Driver
- DataSource
- ResultSet
Q9. Is connection pooling part of JDBC spec?
- Yes
- No
- Partially
- Depends on DB
Q10. When was HikariCP made default in Spring Boot?
- 1.0
- 1.5
- 2.0
- 3.0
💡 Bonus Insight
Modern frameworks like Spring Boot use HikariCP as the default connection pool because of its speed and low latency under load.
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