好的http。 无法保存加载的数据

Question

I can't save data loaded from okhttp. What is wrong in my code. When I try to load data on Log app crashes. Please Help.

public class MainActivity extends AppCompatActivity {
private static final String URL = "https://chroniclingamerica.loc.gov/search/titles/results/?terms=michigan&format=json&page=5";
private String data;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    loadData();
    Log.d("DATA", data);
}

private void loadData() {
    OkHttpClient client = new OkHttpClient();
    Request request = new Request.Builder()
            .url(URL)
            .build();
    Call call = client.newCall(request);
    call.enqueue(new Callback() {
        @Override
        public void onFailure(Call call, IOException e) {

        }

        @Override
        public void onResponse(Call call, Response response) throws IOException {
            data = response.body().string();
        }
    });
}

}

Answer 1

Explanation

The problem is around how your code is being executed (ie sync vs async).

You call the method loadData() which will not set the class attribute data as you're expecting - it will set the attribute data once it gets a response however that function loadData will not wait for the response (ie it doesn't block the current thread or execution code.) . Which means on the next line LOG.d("Data", data); , data has not been set, thus is null (and will crash your app).

Solution

If you just want to LOG data, then just move your log statement after you assign it on your onResponse callback.

private void loadData() {
    OkHttpClient client = new OkHttpClient();
    Request request = new Request.Builder()
            .url(URL)
            .build();
    Call call = client.newCall(request);
    call.enqueue(new Callback() {
        @Override
        public void onFailure(Call call, IOException e) {
            // Some error has occurred
        }

        @Override
        public void onResponse(Call call, Response response) throws IOException {   
            processData(response.body().string());
        }
    });
}

private void processData(String data) {
    Log.d("DATA", data);
    // To other stuff here given your value data. 
}