How to Avoid Exhausted Connection Pools in JDBC MySQL Applications?

JDBC MySQL Connection Pooling Practices to Avoid Exhausted Connection Pool

In connection pooling, maintaining an active pool of established database connections is crucial to improve performance and resource utilization. However, improper handling of connections can lead to an exhausted connection pool and subsequent application failures.

Problem Overview

When working with a Java-JSF Web Application on GlassFish, you may encounter an error indicating that the connection pool was exhausted due to the number of in-use connections exceeding the max-pool-size and expired max-wait-time. This occurs when the application acquires connections faster than they can be closed and returned to the pool.

Root Cause

The error typically arises when database connections are not properly closed after use. This can happen if connections are acquired outside a try-with-resources block or a try-finally block with proper closure handling.

Solution: Ensuring Proper Connection Closure

To resolve this issue and prevent connection leaks, it is essential to acquire and close all database connections (Connection, Statement, and ResultSet) within the same method block. This can be achieved using the try-with-resources block in Java 7 or in a try-finally block in earlier versions of Java.

Correct Practices

Try-with-resources Block (Java 7 ):

public void create(Entity entity) throws SQLException {
    try (
        Connection connection = dataSource.getConnection();
        PreparedStatement statement = connection.prepareStatement(SQL_CREATE);
    ) { 
        statement.setSomeObject(1, entity.getSomeProperty());
        // ...
        statement.executeUpdate();
    }
}

Try-finally Block (Java 6 and Earlier):

public void create(Entity entity) throws SQLException {
    Connection connection = null;
    PreparedStatement statement = null;

    try { 
        connection = dataSource.getConnection();
        statement = connection.prepareStatement(SQL_CREATE);
        statement.setSomeObject(1, entity.getSomeProperty());
        // ...
        statement.executeUpdate();
    } finally {
        if (statement != null) try { statement.close(); } catch (SQLException logOrIgnore) {}
        if (connection != null) try { connection.close(); } catch (SQLException logOrIgnore) {}
    }
}

Additional Considerations

Connection Pool Behavior:

Even when using connection pooling, it is the developer's responsibility to close connections properly. Connection pooling mechanisms do not automatically close connections, but rather return them to a pool for reuse.

Resource Handling:

Connection leaks can also occur when resources such as ResultSets or Statements are not closed properly. It is equally important to close these resources within the same try-with-resources or try-finally block to prevent resource exhaustion.

Troubleshooting:

If connection pooling issues persist, consider checking the following:

• Check the connection pool configuration, including max-pool-size, max-wait-time, and other settings.
• Monitor the number of in-use connections using GlassFish monitoring tools or similar methods.
• Use JDBC profiling tools to identify potential connection leaks or performance bottlenecks.

By implementing proper connection pooling practices and ensuring that connections are closed appropriately, you can prevent the exhaustion of connection pools and maintain stable performance and reliability in your web applications.

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