Managing MySQL Connection Overhead: Understanding the CJ Abandoned Connection Cleanup Thread

In the context of MySQL and its Connector/J (the JDBC driver for MySQL), the “CJ Abandoned Connection Cleanup” thread is a background thread responsible for cleaning up abandoned JDBC connections. The application does not properly close these connections, and if left open, they can cause resource leaks.

Here’s a brief explanation of how this thread works and some considerations:

What is the Abandoned Connection Cleanup Thread?

When a JDBC connection to MySQL is established but not properly closed by the application, it can remain open indefinitely, consuming server resources. The cleanup thread periodically checks for these abandoned connections and closes them to free up resources.

Reasons for Seeing This Running Thread:

  1. Application Code: The cleanup thread will step in if the code does not consistently close database connections using the close() method.
  2. Unexpected Termination: Connections may not be appropriately closed if the application crashes or is forcefully terminated.
  3. Connection Pooling: Connection pools that manage a set of JDBC connections may interact with this thread when closing idle or stale connections.

Potential Issues:

  1. Resource Leaks: Abandoned connections consume resources unnecessarily until they are cleaned up.
  2. Performance Impact: Having too many open connections can degrade the performance of both the application and the database server.
  3. Unexpected Behavior: If the cleanup thread is too aggressive or not well-configured, it might close connections that are still in use, leading to errors in the application.

Best Practices for Management:

  1. Proper Closure: Always ensure that your application code properly closes database connections. Use a finallyblock or a try-with-resources statement to achieve this.

try (Connection conn = dataSource.getConnection()) {
// Use the connection
} catch (SQLException e) {
// Handle exception
// The connection is automatically closed here

2. Utilize Connection Pools: Implement a connection pool that efficiently manages connection lifecycles and handles abandoned connections. Examples of connection pool libraries include HikariCP, c3p0, or Apache DBCP.

3. Configure Cleanup: If you are using Connector/J, you can configure the behavior of the abandoned connection cleanup thread.

4. Keep Libraries Updated: Make sure you are using the latest version of MySQL Connector/J. Newer versions often include improvements and bug fixes related to connection management.

If you are reading this thread and have concerns about its operation, first ensure that your application is correctly managing database connections. If the issue persists, consult the JDBC driver’s documentation for any specific configuration options that may control the behavior of the abandoned connection cleanup thread.


Managing MySQL connection overhead is vital for maintaining optimal performance and resource utilization. Understanding and effectively utilizing the CJ Abandoned Connection Cleanup Thread in MySQL Connector/J can help in automatically closing abandoned JDBC connections, thereby preventing resource leaks and potential performance degradation.

By implementing proper connection closure in the application code, utilizing connection pooling, and keeping the JDBC driver updated, you can ensure efficient management of database connections and enhance the overall performance and reliability of your MySQL database system.

About Shiv Iyer 455 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.