China's strong response to Japanese PM's remarks

On November 7, during a parliamentary debate, Japan's Prime Minister Sanae Takaichi said that the Chinese mainland's "use of force on Taiwan" could constitute a "survival-threatening situation" for Japan, implying the possibility of Japan's armed intervention in the Taiwan Strait. This has sparked a wave of unprecedented reactions from China, including stern diplomatic protests and live-fire drills at sea. The situation shows no sign of easing as Takaichi has refused to retract her remarks so far. What exactly did she say? Why is China so angry? Has China overreacted, as some suggested?