idapython_rename_JNI_first_arg.py

import idaapi
import idautils
import ida_hexrays

jnienv_tinfo = idaapi.tinfo_t()
idaapi.parse_decl(jnienv_tinfo, idaapi.cvar.idati, "JNIEnv *env;", 0)

for func_ea in idautils.Functions():
    name = idaapi.get_name(func_ea)
    if not name.startswith("Java_"):
        continue

    print(f"{name}: {idaapi.print_type(func_ea, idaapi.PRTYPE_1LINE)}", end="...")
    ida_hexrays.open_pseudocode(func_ea, idaapi.OPF_REUSE)
    
    tif = idaapi.tinfo_t()
    if not idaapi.get_tinfo(tif, func_ea):
        print("KO! (couldn't get tinfo)")
        continue
        
    funcdata = idaapi.func_type_data_t()
    if not tif.get_func_details(funcdata):
        print("KO! (couldn't get func details)")
        continue
        
    if not len(funcdata):
        print("KO! (func has no parameters)")
        continue

    funcdata[0].type = jnienv_tinfo
    funcdata[0].name = "env"

    new_tif = idaapi.tinfo_t()
    new_tif.create_func(funcdata)

    idaapi.apply_tinfo(func_ea, new_tif, idaapi.TINFO_DEFINITE)
    print("OK!")