Techniques for customization of Java runtime environments are disclosed. The techniques can be used to provide Java runtime environments that are specifically tailored for various Java applications. Accordingly, for a particular Java application, an optimized runtime environment can be created. One or more optional attributes which represent the desired runtime customizations are generated. As will be appreciated, the optional attributes can be generated in the attribute table in the class file. The optional attributes can then be parsed and appropriate features can be loaded into the virtual machine. In this way, Java runtime environments can be customized based on a particular Java application requirement. Moreover, customizations can be automated using a runtime performance manager that interacts with various other components that operate to first generate and then load optional attributes into the Java runtime environment.