Why Does a JDBC Connection Pool Exhaust Connections When Context Reload Is Enabled in Tomcat?

What causes a JDBC connection pool to run out of available connections when using a context reload in Tomcat?

// Example code for obtaining a JDBC connection
Connection conn = null;
try {
    conn = dataSource.getConnection();
    // Perform database operations
} catch (SQLException e) {
    e.printStackTrace();
} finally {
    if (conn != null) conn.close();
}

When Tomcat's context has the attribute reload set to true, it causes the web application to reload and potentially disrupts the lifecycle of JDBC connections. This can lead to connection leaks or exhaustion, where the connection pool runs out of available connections, causing database operations to fail.

<Context reload="true">
    <Resource name="jdbc/MyDataSource"
              auth="Container"
              type="javax.sql.DataSource"
              maxActive="100"
              maxWait="10000"
              username="user"
              password="password"
              driverClassName="com.mysql.cj.jdbc.Driver"
              url="jdbc:mysql://localhost:3306/mydb"/>
</Context>

Causes

• Tomcat reloads the context, resulting in all existing connections being terminated improperly.
• The connection pool's resources are not released properly during reloads, leading to a leak.
• New requests attempt to acquire connections while old ones are still terminating.

Solutions

• Set the context reload attribute to false in production environments.
• Implement proper resource management code to ensure connections are closed after use.
• Increase the maximum connection pool size to accommodate burst loads during reloads.

Mistake: Not closing database connections after use.

Solution: Always close ResultSet, Statement, and Connection objects in a finally block.

Mistake: Ignoring the connection pool configuration limits.

Solution: Review and set the maximum and minimum pool sizes according to application needs.