The Bank of Japan announced an extra dose of monetary stimulus Friday, joining fresh efforts by Prime Minister Shinzo Abe to reboot the economy. The central bank said it would buy ¥6 trillion worth of exchange-traded funds annually, up from ¥3.3 trillion previously, in an attempt to stoke inflation and growth by pumping money into the economy. It said it would leave its asset-purchase target at ¥80 trillion a year. The monetary policy board voted approved the expansion of ETF purchases by a vote of 7-2. The BOJ’s choices risk disappointing investors who expected more-aggressive action to counter faltering inflation and a stronger currency. Pressure on the BOJ to expand its stimulus had been rising for months, and speculation in recent weeks about what it might do went as far as “helicopter money,” a radical policy involving direct underwriting of government spending.