MJ: a rational module system for Java and its applications

ABSTRACT

While Java provides many software engineering benefits, it lacks a 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 inter-relationships 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 no CLASSPATH 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.

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INDEX TERMS

Primary Classification:
  D. Software
  D.2 SOFTWARE ENGINEERING
      D.2.3 Coding Tools and Techniques

Additional Classification:
  D. Software
  D.3 PROGRAMMING LANGUAGES
      D.3.2 Language Classifications
      D.3.3 Language Constructs and Features
          Subjects: Modules, packages

General Terms:
Design, Experimentation, Languages, Performance

Keywords:
Java, components, language design, modularity