Run-time code generation is a well-known technique for improving the efficiency of programs by exploiting dynamic information. Unfortunately, the difficulty of constructing run-time code-generators has hampered their widespread use. We describe Jumbo, a tool for easily creating run-time code generators for Java. Jumbo is a compiler for a two-level version of Java, where programs can contain quoted code fragments. The Jumbo API allows the code fragments to be combined at run-time and then executed. We illustrate Jumbo with several examples that show significant speed-ups over simila code written in plain Java, and argue further that Jumbo is a generalized software component system.