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Java Servlet - @WebListener Examples

Java Servlet JAVA EE 

This example demonstrate how to use ServletContextListener along with Servlet 3.0 annotation @WebListener. This examples assumes a requirement where a global cache is needed by multiple servlets and other components during client requests. Guava Cache is used and initialized during the web application startup time in a ServletContextListener implementation. 

import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
import java.util.concurrent.TimeUnit;

@WebListener
public class AppContextListener implements ServletContextListener {

    @Override
    public void contextInitialized (ServletContextEvent event) {
        System.out.println("context initialized " + this);

        LoadingCache<String, Object> cache =
                            CacheBuilder.newBuilder()
                                        .maximumSize(100)
                                        .expireAfterAccess(30, TimeUnit.MINUTES)
                                        .build(new CacheLoader<String, Object>() {
                                            @Override
                                            public Object load (String s) throws Exception {
                                                System.out.println("getting data for " + s);
                                                /**
                                                 * In real situations will get data from database
                                                 * etc.
                                                 */
                                                return "Cached data for " + s;
                                            }
                                        });

        event.getServletContext()
             .setAttribute("GLOBAL_CACHE", cache);
    }

    @Override
    public void contextDestroyed (ServletContextEvent sce) {

    }
}
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This example shows the use of ServletContext#setSessionTrackingModes. 

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.SessionTrackingMode;
import javax.servlet.annotation.WebListener;
import java.util.EnumSet;

@WebListener
public class SessionTrackingModeSetter implements ServletContextListener {

    @Override
    public void contextInitialized (ServletContextEvent event) {
        System.out.println("--------\ncontext initialized\n------\n " + this);
        System.out.println("The default session tracking modes: " +
                            event.getServletContext()
                                 .getDefaultSessionTrackingModes());

        event.getServletContext()
             .setSessionTrackingModes(EnumSet.of(SessionTrackingMode.URL));
    }

    @Override
    public void contextDestroyed (ServletContextEvent sce) {
    }
}
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See Also