我有一个asp.net-web-api使用以下GET方法的项目：
public string Get(string id)
{
List<dummy> dummies = new List<dummy>();
string con = "user id=sa;" +
"password=1234" +
"server=foo\\bar;" +
"database=game; " +
"connection timeout=30";
//SqlConnection sqlConn = new SqlConnection(con);
using (SqlConnection sqlconn = new SqlConnection(con))
{
sqlconn.Open();
StringBuilder sb = new StringBuilder();
sb.Append("SELECT PART.PARTNAME,PART.PARTDES, PARTPARAMA.LOCATION ");
sb.Append("FROM PART LEFT JOIN PARTPARAMA ");
sb.Append("ON PART.PART = PARTPARAMA.PARTPARAM ");
sb.Append("WHERE PART.PARTNAME = @part");
using (SqlCommand cmd = new SqlCommand(sb.ToString(), sqlconn))
{
cmd.Parameters.AddWithValue("part", id);
SqlDataReader sdr = cmd.ExecuteReader();
while (sdr.Read())
{
dummies.Add(new dummy
{
PartName = sdr.GetString(0),
PartDes = sdr.GetString(1),
PartLocation = sdr.GetString(2)
});
}
}
}
if (dummies.Count() > 0)
{
string json = JsonConvert.SerializeObject(dummies[0]);
return json;
}
else
{
string json = JsonConvert.SerializeObject(null);
return json;
}
}
（我json.net用于序列化）。
然后Android Activity使用它（删除不相关的部分）：
public class MainActivity extends Activity {
String tempResult = "";
JSONObject tempJo;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
try {
tempJo = mc.execute(someString).get();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (ExecutionException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
//Tried to check for null with the following
//if(aaa == null)
//{
// Toast.makeText(context, "clear null", Toast.LENGTH_LONG).show();
//}
//if(aaa == "null")
//{
// Toast.makeText(context, "null", Toast.LENGTH_LONG).show();
//}
//if(aaa == "\"null\"")
//{
// Toast.makeText(context, "special null", Toast.LENGTH_LONG).show();
//}
//if(aaa=="{}")
//{
// Toast.makeText(context, "empty", Toast.LENGTH_LONG).show();
//}
if(tempJo.toString() == null)
{
Toast.makeText(context, "tempJo is null", Toast.LENGTH_LONG).show();
}
else
{
PartDetails pd = getPart(tempResult);
txtPart.setText(pd.partName);
txtDescription.setText(pd.partDescription);
txtLocation.setText(pd.partLocation);
}
}
private PartDetails getPart(String json){
Toast.makeText(this, "*** the json value is: ***: " + json, Toast.LENGTH_LONG).show();
PartDetails pd = new PartDetails();
try {
Toast.makeText(context, "got here!!!", Toast.LENGTH_LONG).show();
JSONObject jo = new JSONObject(json);
pd.partName = jo.getString("PartName");
pd.partDescription = jo.getString("PartDes");
pd.partLocation = jo.getString("PartLocation");
} catch (JSONException e1) {
Toast.makeText(this, "*** JSONException ***: " + e1.getMessage(), Toast.LENGTH_LONG).show(); <=> Exception is here
}
return pd;
}
public class myClass extends AsyncTask<String, Void, JSONObject>{
protected JSONObject doInBackground(String... passing) {
String url = "http://1.2.3.4/api1/api/values/" + passing[0];
HttpClient httpclient = new DefaultHttpClient();
HttpGet httpget = new HttpGet(url);
HttpResponse response;
JSONObject json = new JSONObject();
try{
response = httpclient.execute(httpget);
HttpEntity entity = response.getEntity();
if(entity != null){
InputStream instream = entity.getContent();
String result= convertStreamToString(instream);
json=new JSONObject(result);
instream.close();
}
}
catch (ClientProtocolException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return json;
}
public String convertStreamToString(InputStream is) {
BufferedReader reader = new BufferedReader(new InputStreamReader(is));
StringBuilder sb = new StringBuilder();
String line = null;
try {
while ((line = reader.readLine()) != null) {
sb.append(line + "\n");
}
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
is.close();
} catch (IOException e) {
e.printStackTrace();
}
}
return sb.toString();
}
}
当使用 firefox 浏览api地址时，我得到这个“null”的空值和Json正确的查询。
我得到的例外是org.json.JSONException: Value null of type org.json.JSONObject$1 cannot be converted to JSONObject。我怎样才能让If语句捕获空 json？