What causes "Connection is closed" in brettwooldridge/HikariCP?

Use try-with-resources so connections cannot be used after close

brettwooldridge/HikariCP · error · SQLException

Connection is closed

Error message

Connection is closed

What it means

When a HikariCP proxy connection has been closed (or its entry evicted), the delegate is replaced with a synthetic ClosedConnection dynamic proxy. Any method call other than close()/isClosed()/isValid()/abort()/toString() on it throws SQLException 'Connection is closed'. This makes use-after-close fail fast instead of touching a recycled real connection.

Source

Thrown at src/main/java/com/zaxxer/hikari/pool/ProxyConnection.java:559

      private static Connection getClosedConnection()
      {
         InvocationHandler handler = (proxy, method, args) -> {
            final String methodName = method.getName();
            switch (methodName) {
               case "isClosed":
                  return Boolean.TRUE;
               case "isValid":
                  return Boolean.FALSE;
               case "abort":
                  return Void.TYPE;
               case "close":
                  return Void.TYPE;
               case "toString":
                  return ClosedConnection.class.getCanonicalName();
            }

            throw new SQLException("Connection is closed");
         };

         return (Connection) Proxy.newProxyInstance(Connection.class.getClassLoader(), new Class[] { Connection.class }, handler);
      }
   }
}

View on GitHub (pinned to a4d93f4f85)

Solutions

Use try-with-resources so connections cannot be used after close
Do not cache connections across requests/threads; borrow per unit of work
Ensure async work completes before the owning request returns the connection
Check isClosed() defensively in code paths that may outlive the borrow scope

Example fix

// before
Connection c = ds.getConnection();
use(c); c.close();
useLater(c); // SQLException: Connection is closed

// after
try (Connection c = ds.getConnection()) {
   use(c);
   useLater(c); // inside scope
}

Defensive patterns

Strategy: try-catch

Validate before calling

if (!conn.isClosed()) { conn.prepareStatement(...); }

Try / catch

try { stmt = conn.prepareStatement(sql); }
catch (SQLException e) {
    if ("Connection is closed".equals(e.getMessage())) { /* re-borrow a connection and retry the unit of work */ }
    else throw e;
}

Prevention

Always use try-with-resources for connections/statements
Never share a connection across threads or requests
Complete async work before the owner returns the connection

When it happens

Trigger: Using a Connection after close() (explicit or try-with-resources exit); holding a connection past connectionTimeout/maxLifetime and continuing after HikariCP evicts it; async code capturing the connection and running after the request returned it to the pool.

Common situations: Forgotten close ordering, connection leaked then recycled to another thread, background jobs using a request-scoped connection, frameworks returning connections early (e.g. after transaction commit in some setups).

Related errors

AI-assisted analysis of brettwooldridge/HikariCP@a4d93f4f85 (2026-08-14). Data as JSON: /api/errors/4a788059cef3e85f. Report an issue: GitHub.