As a much-respected young guitarist, John Mayer doesn’t rely on his guitar solos to get him through. To his credit, Mayer plays in service of the song. For his fourth studio album, 2009’s Battle Studies, Mayer settles into an adult-contemporary atmosphere where his youth is given a serious growing up. His sense of humor can he hunted down on the acoustic stoner shuffle “Who Says,” where he slyly jokes, “I don’t remember you looking any better / But then again, I don’t remember you.” But mostly, Mayer prefers the solemn tones of the U2 chime of “Heartbreak Warfare,” the Peter Gabriel ethereal moan of “All We Ever Do Is Say Goodbye,” the Don Henley pop of “Perfectly Lonely” and the Elton John melody of “Assassin.”  His duet with country sweetheart Taylor Swift on “Half of My Heart” portrays him as a guy keeping an eye on his next stepping stone. “Crossroads” brings him back to the blues in time to turn it into near techno. “Edge of Desire,” “Do You Know Me” and “Friends, Lovers or Nothing” end things with a tender soft rock touch.