Question

I have spring mvc application and Tomcat 7.0.21 as a servlet container.
And when I’m trying to use jdbcTemplate in my application it doesn’t redeploy correctly –
it prevents jvm from cleaning PermGen memory.
The simple example of bad code is the following
(of course breaking mvc concept for the sake of simplicity):

@Controller
class MainController {
    private JdbcTemplate jdbcTemplate;
    @Autowired
    public void setDataSource(DataSource  dataSource) {
        this.jdbcTemplate = new JdbcTemplate(dataSource);
    }
    @RequestMapping("/")
    public String mainPage() {  
        jdbcTemplate.queryForObject("SELECT val FROM tbl WHERE id=1",
            String.class);
        return "main";
    }
}

without jdbcTemplate.queryForObject(...) it works perfectly, but
with it when deploying to tomcat it says that:

Sep 21, 2011 1:54:38 PM org.apache.catalina.loader.WebappClassLoader clearReferencesJdbc
SEVERE: The web application [/my] registered the JDBC driver [com.mysql.jdbc.Driver] but failed to unregister it when the web application was stopped. To prevent a memory leak, the JDBC Driver has been forcibly unregistered.
Sep 21, 2011 1:54:38 PM org.apache.catalina.loader.WebappClassLoader clearReferencesThreads
SEVERE: The web application [/my] appears to have started a thread named [Timer-0] but has failed to stop it. This is very likely to create a memory leak.

And after that I can see in VisualVM a growth in PermGen each time the app is redeployed.

Of course I can just restart my production server each time when I want
to redeploy my app, but I want to figure out what’s wrong.

P.S.: datasource implementation:

@Configuration
public class ApplicationConfig {
    @Bean
    DataSource dataSource(@Value("${jdbc.driver}") String driver, 
        @Value("${jdbc.url}") String url, @Value("${jdbc.user}") String user,
        @Value("${jdbc.password}") String password) {

        BasicDataSource ds = new BasicDataSource();
        ds.setDriverClassName(driver);
        ds.setUrl(url);
        ds.setUsername(user);
        ds.setPassword(password);
        return ds;
    }
}

Answer 1

Unregistering JDBC drivers should be handled for you by the data source. See this related question. So check for a newer version of your data source implementation. BasicDataSource is not the best option for a datasource. Try using c3p0 and see if the problem is reproduced.

Even if it isn’t, Tomcat clears that for you. Btw, upgrade your tomcat as well – it has an improvement for DriverManager leaks in the latest version.

So, the PermGen issue lies somewhere else. It has been a long-known issue with tomcat, but they claim it it the application code that leaks. And this is mostly true, but it is hard to track what remains unregistered. Tomcat tries to clear common reasons for the PermGen issue, but can’t cover everything.