Before we can dynamicly modify the Minecraft, we need remap it first.

After the Minecraft 1.15 19w36a, every version of Minecraft has their own client mappings(except Combat Test 3).The download address can be found in every version of client.json, the key of it is called client_mappings. Our program needs it to remap the Minecraft.

Our program has a remapper in the JAR, the main class of it is RemapperMain. To remap the Minecraft, you can enter this command:

javaw MCDynamicExchanger.jar [--output:<destination>] [--tmpdir:<tmpDir>] <mapping_url> <minecraft_jar>

or

java-rmi -jar MCDynamicExchanger.jar [--output:<destination>] [--tmpdir:<tmpDir>] <mapping_url> <minecraft_jar>

Mapping file should be provided in URL, such as file:D:\\client.txt or https://github.com/Nickid2018/MCDynamicExchanger/a.txt. After remapping, Minecraft client can also run normally except its class names are changed.