ABSTRACT MJ: A Rational Module System for Java and its Applications

coherent module system and instead provides only packages (which are primarily a name space mechanism) and classloaders (which are very low-level). As a result, large Java applications suffer from unexpected interactions between independent components, require complex CLASSPATH definitions, and are often extremely complex to install and maintain. We have implemented a module system for Java called MJ that is implemented with class loaders, but provides a much higher-level interface. High-level properties can be specified in a module definition and are enforced by the module system as new modules are loaded. To experimentally validate the ability of MJ to properly handle the complex module interrelationships found in large Java server systems, we replaced the classloader mechanisms of Apache Tomcat 4.1.18 [27] with 30 MJ modules. The modified Tomcat is functionally identical to the original, but requires noCLASSPATH definitions, and will operate correctly even if user code loads a different version of a module used by Tomcat, such as the Xerces XML parser [31]. Furthermore, by making a small change to the Java core libraries enabled by MJ, we obtained a 30 % performance improvement in a servlet microbenchmark