Johns Hopkins University is freezing hiring for faculty and staff and cutting executives’ salaries after its investments fell about 20 percent in the second half of 2008 because of the global financial crisis. The university is projecting a $100 million budget shortfall for fiscal years 2010 and 2011 combined, William Brody, Johns Hopkins’s president, said in a letter e-mailed today to faculty, staff and students. Twenty administrators, including the president, vice presidents, and divisional deans and directors, agreed to 5 percent cuts in their salaries, said Dennis O’Shea, a school spokesman.