Memory Leaks in Java

There is an excellent blog that describes classloaders and memory leaks in Java: Frank Kievet Blog. The next entry in the blog shows a new tool from Java 1.6 that can be used to isolate memory leaks.

The first memory leak can be removed by modifying XMLHelper. There is a static variable that is not used anywhere, except in Configuration. By removing the variable from XMLHelper and removing the reference to it (replace with null) in Configuration, the leak goes away.

The second memory leak is more entwined in cglib. The leak can be removed by having cglib loaded by a different classloader than the one for the web app. The solution is to move the cglib jar and the asm jar to the shared lib directory of Tomcat.

I have recompiled hibernate and created zip files for the jar files that still belong in the web app and the two that belong in the shared lib directory of Tomcat. If you are using a system and do not have access to the shared lib folder (or common lib folder), then place these jars in the web app with the understanding that you will run out of memory eventually.

I have updated the JAR files that are needed for the features in Chapter 6. Please download this copy of the ZIP file that contains the JAR files: JAR files for web app lib.
You can also unzip these directly in ocelot with this command
unzip ~downeyt/cs/public_html/cgs4854/hibernate-3.2.jars/webapp.jars.zip
If you are developing locally, you will need some additional JAR files. These files are already in a shared lib in ocelot, but you will need to add them to your lib dir if you develop locally: JAR files for local development.
After you upload the WAR file, please delete these JAR files from the lib dir on ocelot.
1. asm.jar
2. cglib-2.1.3.jar

Additionally, a minor change must be made to all beans that are saved to a database. To mark the id property as the id for the table, use the following code.

import org.hibernate.validator.NotNull;
import org.hibernate.validator.Pattern;
import javax.persistence.Entity;
import javax.persistence.Transient;
import javax.persistence.Id;
import javax.persistence.GeneratedValue;

@Entity
public class RequestDataPersistent
{

    protected Long id;

    @Id
    @GeneratedValue
    public Long getId() { return id; }

    private void setId(Long id) { this.id = id; }
    
    //Add additional properties for your bean
    ...

}

There are only a few JAR files that are needed at compile time. It is not necessary to have every JAR file in the lib dir in the CLASSPATH. Update the .cshrc file so that the following are the only CLASSPATH statements for JAR files in your lib dir. You still need the CLASSPATH statements for servlet-api and the classes dirs of your web apps.

setenv CLASSPATH {$CLASSPATH}:$HOME/cgs4854/webapps/book/WEB-INF/lib/ejb3-persistence.jar
setenv CLASSPATH {$CLASSPATH}:$HOME/cgs4854/webapps/book/WEB-INF/lib/hibernate-3.2.jar
setenv CLASSPATH {$CLASSPATH}:$HOME/cgs4854/webapps/book/WEB-INF/lib/hibernate-annotations-3.2.jar
setenv CLASSPATH {$CLASSPATH}:$HOME/cgs4854/webapps/book/WEB-INF/lib/log4j-1.2.11.jar
setenv CLASSPATH {$CLASSPATH}:$HOME/cgs4854/webapps/book/WEB-INF/lib/commons-beanutils.jar