Adicionei a biblioteca apache httpclient, mas ocorrendo erro na importação do StringEntity.
public String post(String json){
try{
HttpPost post = new HttpPost(url);
post.setEntity(new **StringEntity**(json));
post.setHeader("Content-type", "application/json");
post.setHeader("Accept", "application/json");
HttpClient client = new DefaultHttpClient();
HttpResponse resposta = client.execute(post);
return EntityUtils.toString(resposta.getEntity());
} catch(IOException e){
}