build a java function -- 3

Case 1 :
source json will be

{
"a":"val_a",
"b":
{
"b1":"val_b1",
"b2":"val_b2"
},
"c":
{
"c1":"val_c1",
"c2":
{
"c21":"val_c21",
"c22":"val_c22"
}
}
}

mapping json will be

{
"oa" :"a" ,
"ob" : "b.b1",
"oc" : "c.c1",
"oc1": "c.c2.c21"
}


output should be

{
"oa":"val_a",
"ob":"val_b1",
"oc":"val_c1",
"oc1" : "val_c21"
}