Question

我是Android应用程序开发和Java编程的新手，目前，我正在开发一个项目，用于从我的数据库中检索服务的状态。但是，每当我尝试提交跟踪ID时，应用程序都将停止工作。

MainActivity.java

package com.example.trackstatus;

import com.example.testapp.R;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends Activity {

    Button btnViewStatus;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

     // Buttons
        btnViewStatus = (Button) findViewById(R.id.btnViewStatus);

        // view products click event
        btnViewStatus.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View view) {
                // Launching View status activity
                Intent i = new Intent(getApplicationContext(), ViewTrackingStatusActivity.class);
                startActivity(i);
            }
        });
    }}

TrackStatus清单

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package = "com.example.trackstatus"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="20" />
    <!--  Internet Permissions -->
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >

        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

         <!-- View Tracking Status Activity -->
        <activity
            android:name=".ViewTrackingStatusActivity"
            android:label="View Tracking Status" >
        </activity>
    </application>


</manifest>

ViewTrackingStatusActivity.java

package com.example.trackstatus;

import java.util.ArrayList;
import java.util.List;

import org.apache.http.NameValuePair;
import org.apache.http.message.BasicNameValuePair;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import com.example.testapp.R;

import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class ViewTrackingStatusActivity extends Activity{

    TextView textView1;
    EditText trackingNumberText;
    Button btnViewStatus;

    String tid;
    String tracking; 

    // Progress Dialog
    private ProgressDialog pDialog;

    // JSON parser class
    JSONParser jsonParser = new JSONParser();

  // url to get service status
    private static String url = "http://10.0.2.2/1201716f/android%20connect/get_svc.php?tid=0";

 // JSON Node names
    private static final String TAG_SUCCESS = "success";
    private static final String TAG_TRACKING = "tracking";
    private static final String TAG_TID = "tid";
    public String trackingno;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textView1 = (TextView) findViewById(R.id.textView1);
        trackingNumberText = (EditText) findViewById(R.id.trackingNumberText);
        trackingno = trackingNumberText.getText().toString();
        btnViewStatus = (Button) findViewById(R.id.btnViewStatus);

        Intent intent = getIntent();

        tid = intent.getStringExtra(TAG_TID);

        // Getting tracking status details in background thread
        new GetStatusDetails().execute();

       //Toast.makeText(getApplicationContext(),"Hello", 
       //  Toast.LENGTH_LONG).show();
    }
        class GetStatusDetails extends AsyncTask<String, String, String> {
            @Override
            protected void onPreExecute() {
                super.onPreExecute();
                pDialog = new ProgressDialog(ViewTrackingStatusActivity.this);
                pDialog.setMessage("Loading details");
                pDialog.setIndeterminate(false);
                pDialog.setCancelable(true);
                pDialog.show();
        }
            protected String doInBackground(String... params) {

                // updating UI from Background Thread
                runOnUiThread(new Runnable() {
                    public void run() {
                        // Check for success tag
                        int success;
                        try {
                            // Building Parameters
                            List<NameValuePair> params = new ArrayList<NameValuePair>();
                            params.add(new BasicNameValuePair("tid", trackingno));

                            // getting product details by making HTTP request
                            // Note that product details url will use GET request
                            JSONObject json = jsonParser.makeHttpRequest(url, "GET", params);

                            // check your log for json response
                            Log.d("Tracking Status Details", json.toString());

                            // json success tag
                            success = json.getInt(TAG_SUCCESS);
                            if (success == 1) {
                                // successfully received product details
                                JSONArray trackingObj = json.getJSONArray(TAG_TRACKING); // JSON Array

                                // get first product object from JSON Array
                                JSONObject tracking = trackingObj.getJSONObject(0);

                                // display tracking status in ToastBox
                               Toast.makeText(ViewTrackingStatusActivity.this,json.getString(TAG_TRACKING),Toast.LENGTH_LONG).show();

                            }
                            else
                            {
                                //service with tid not found
                                Toast.makeText(getApplicationContext(),"Service not found!",Toast.LENGTH_LONG).show();
                            }
                        }
                        catch (JSONException e)
                        {
                            e.printStackTrace();
                        }
                    }
                });

                return null;
            }

        }
    }

JSONParser.java

package com.example.trackstatus;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.UnsupportedEncodingException;
import java.util.List;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.client.utils.URLEncodedUtils;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONException;
import org.json.JSONObject;

import android.util.Log;

public class JSONParser {

    static InputStream is = null;
    static JSONObject jObj = null;
    static String json = "";

    // constructor
    public JSONParser() {

    }

    // function get json from url
    // by making HTTP POST or GET mehtod
    public JSONObject makeHttpRequest(String url, String method,
            List<NameValuePair> params) {

        // Making HTTP request
        try {

            // check for request method
            if(method == "POST"){
                // request method is POST
                // defaultHttpClient
                DefaultHttpClient httpClient = new DefaultHttpClient();
                HttpPost httpPost = new HttpPost(url);
                httpPost.setEntity(new UrlEncodedFormEntity(params));

                HttpResponse httpResponse = httpClient.execute(httpPost);
                HttpEntity httpEntity = httpResponse.getEntity();
                is = httpEntity.getContent();

            }
            else if(method == "GET"){
                // request method is GET
                DefaultHttpClient httpClient = new DefaultHttpClient();
                String paramString = URLEncodedUtils.format(params, "utf-8");
                url += "?" + paramString;
                HttpGet httpGet = new HttpGet(url);

                HttpResponse httpResponse = httpClient.execute(httpGet);
                HttpEntity httpEntity = httpResponse.getEntity();
                is = httpEntity.getContent();
            }           

        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        } catch (ClientProtocolException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(
                    is, "iso-8859-1"), 8);
            StringBuilder sb = new StringBuilder();
            String line = null;
            while ((line = reader.readLine()) != null) {
                sb.append(line + "\n");
            }
            is.close();
            json = sb.toString();
        } catch (Exception e) {
            Log.e("Buffer Error", "Error converting result " + e.toString());
        }

        // try parse the string to a JSON object
        try {
            jObj = new JSONObject(json);
        } catch (JSONException e) {
            Log.e("JSON Parser", "Error parsing data " + e.toString());
        }

        // return JSON String
        return jObj;

    }
}

logcat的

08-19 23:52:46.644: D/AndroidRuntime(3809): Shutting down VM
08-19 23:52:46.644: W/dalvikvm(3809): threadid=1: thread exiting with uncaught exception (group=0x41b37700)
08-19 23:52:46.654: E/AndroidRuntime(3809): FATAL EXCEPTION: main
08-19 23:52:46.654: E/AndroidRuntime(3809): android.os.NetworkOnMainThreadException
08-19 23:52:46.654: E/AndroidRuntime(3809):     at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1133)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at libcore.io.BlockGuardOs.connect(BlockGuardOs.java:84)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at libcore.io.IoBridge.connectErrno(IoBridge.java:127)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at libcore.io.IoBridge.connect(IoBridge.java:112)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:192)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:459)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at java.net.Socket.connect(Socket.java:842)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at org.apache.http.conn.scheme.PlainSocketFactory.connectSocket(PlainSocketFactory.java:119)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at org.apache.http.impl.conn.DefaultClientConnectionOperator.openConnection(DefaultClientConnectionOperator.java:144)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at org.apache.http.impl.conn.AbstractPoolEntry.open(AbstractPoolEntry.java:164)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at org.apache.http.impl.conn.AbstractPooledConnAdapter.open(AbstractPooledConnAdapter.java:119)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at org.apache.http.impl.client.DefaultRequestDirector.execute(DefaultRequestDirector.java:360)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:555)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:487)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:465)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at com.example.trackstatus.JSONParser.makeHttpRequest(JSONParser.java:63)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at com.example.trackstatus.ViewTrackingStatusActivity$GetStatusDetails$1.run(ViewTrackingStatusActivity.java:92)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at android.os.Handler.handleCallback(Handler.java:730)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at android.os.Handler.dispatchMessage(Handler.java:92)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at android.os.Looper.loop(Looper.java:137)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at android.app.ActivityThread.main(ActivityThread.java:5293)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at java.lang.reflect.Method.invokeNative(Native Method)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at java.lang.reflect.Method.invoke(Method.java:525)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:738)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:554)
08-19 23:52:46.654: E/AndroidRuntime(3809):     at dalvik.system.NativeStart.main(Native Method)
08-19 23:52:49.086: I/Process(3809): Sending signal. PID: 3809 SIG: 9

Answer 1

根据您的LogCat消息：

Caused by: java.lang.NullPointerException at com.example.trackstatus.ViewTrackingStatusActivity.onCreate(ViewTrackingStatusActivity.java:55)

你错过了：

setContentView(R.layout.your_layout);

必须声明您的EditText trackingNumberText。

这就是trackingNumberText为空的原因

 trackingNumberText = (EditText) findViewById(R.id.trackingNumberText);

更多信息：

<强> setContentView() method

更新

由于您有以下新例外： NetworkOnMainThreadException

 android.os.NetworkOnMainThreadException  at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1133)

您必须实施 AsyncTask

的使用

嘿！你不需要在Asynctask的runOnUiThread方法中使用doInBackground()：P：

 protected String doInBackground(String... params) {

                // updating UI from Background Thread
                runOnUiThread(new Runnable() {
                    public void run() {

在UI线程上移动暗示操作的行，如

Toast.makeText(getApplicationContext(),"Service not found!",Toast.LENGTH_LONG).show();

到onPostExecute()

的Asynctask方法

Answer 2

错误在Line55

String trackingno = trackingNumberText.getText().toString();

trackingNumberText NULL

而是尝试进行NULL检查

String trackingno = "";
if(trackingNumberText.getText().length() > 0)
{
    trackingno = trackingNumberText.getText().toString();
} else  {
    Toast.makeText(getApplicationContext(), "Enter values", Toast.LENGTH_LONG).show();
 return;
}

修改
您必须在后台线程中将所有请求发送到服务器，使用AsyncTask

以下是performing Network Operation.
的详细示例

Android应用程序强制关闭（检索数据）

2 个答案:

更新