jContractor is a 100% pure Java implementation of Design By Contract for the Java language. Contracts are written as methods that follow a simple naming convention. jContractor provides runtime contract checking by instrumenting the bytecode of classes that define contracts. jContractor can either add contract checking code to class files to be executed later, or it can instrument classes at runtime as they are loaded. All contracts are written in standard Java, so there is no need to learn a special contract specification language. jContractor is purely library based, requires no preprocessing or modifications to the JVM.