Komatsu, 23, was drafted in the eighth round of the 2008 draft. At Class AA Huntsville this season, Komatsu has hit .294/.393/.416 with six home runs and 13 stolen bases.
Hairston filled a versatile, valuable role for the Nationals this season, becoming the regular third baseman as Ryan Zimmerman missed two months with a torn abdominal muscle. He also played left and center field and came off the bench as a right-handed pinch-hitter. Hairston hit .268/.342/.385 with four homers and two stolen bases in 75 games.
This winter, Hairston signed a $2 million, one-year deal to play for the Nationals. The Brewers will pick up the remainder of his salary, saving the Nationals about $650,000.
The Brewers and Nationals also completed a trade this spring, when the Nationals sent outfielder Nyjer Morgan to Milwaukee for minor league infielder Cutter Dykstra.