$.ajax(
  type: 'POST,
  dataType:"json",
  url: 'test.com'
  data: { mail: 'bob@test.com' }
)

HttpServletRequest request;
request.getParameter('mail');

fetch('test.com', {
  method: 'POST',
  headers: {
    'Content-Type': 'text/plain',
  },
  body: 'mail=bob@test.com',
})

fetch('test.com', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({ mail: 'bob@test.com' }),
})