Foreign exchange API: real-time foreign exchange quotation (free account, updated every minute, 1000 times a day, supporting https).
JSON API for foreign exchange rate and currency conversion (requires registration, provides free packages, supports https and pricing plan -Fixer).
YQL console (Yahoo's exchange rate interface, the probability of service unavailability in the last year is quite high)
Open exchange rate (click here to register a free account and make free calls at 1000 every month; Charging users can update for up to one hour)
Generally speaking, the exchange rate of legal tender is relatively stable, and data can be obtained once a day. If your application needs to rely on high real-time, spend some money or go to the foreign exchange to get real-time data.