Android 开发者从URL获取数据的Volley库

在本教程中，我们将使用Volley库从URL获取JSON数据。Volley是一个HTTP库，为我们的应用程序提供网络连接的功能。

使用Volley库的优点如下:

• 更舒适和更快的请求管理。
• 它提供高效的网络管理。

Android Volley从URL获取JSON数据示例

在这个示例中，我们将使用Volley库从URL加载JSON数据。JSON数据包含教程的字符串 “name” 、字符串 “imageurl” 和字符串 “description” 。在从URL获取数据后，它们将在ListView中显示。您可以在以下链接中了解更多关于JSON解析教程的信息： https://www.javatpoint.com/android-json-parsing-tutorial 。

让我们创建包含以下信息的JSON数据。

URL上的JSON数据（提供您的信息URL）= http://192.168.1.35:8080/jsondata/

{
"tutorials":[
{
"name":"Java",
"imageurl":"https://www.javatpoint.com/images/logo/javahome.png",
"description":"Java is a high level, robust, object-oriented and secure programming language."
},
{
"name":"Android",
"imageurl":"https://www.javatpoint.com/images/logo/androidhome.png",
"description":"Android is a complete set of software for mobile devices."
},
{
"name":"Python",
"imageurl":"https://www.javatpoint.com/images/logo/pythonhome.png",
"description":"Python is a general purpose, dynamic, high level and interpreted programming language."
},
{
"name":"C++",
"imageurl":"https://www.javatpoint.com/cpp/images/cpp-home.png",
"description":"C++ is an object-oriented programming language."
}
]
}

JSON数据的样式如下：

在layout中创建一个 activity_main.xml 文件，并添加以下代码。

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="example.javatpoint.com.volleyreadimagejson.MainActivity">

    <ListView
        android:id="@+id/listView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <ProgressBar
        android:id="@+id/progressBar"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:layout_marginBottom="292dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:visibility="gone"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>

现在，在布局目录中创建一个 list_item.xml 文件，其中包含ListView的行项目。这个文件包含一个ImageView用于显示图像，以及两个TextView用于显示文本。

list_item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="16dp"
    android:orientation="vertical">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="horizontal">
        <ImageView
            android:id="@+id/imageView"
            android:layout_width="50dp"
            android:layout_height="50dp"
            android:src="@drawable/placeholder"
            android:contentDescription="image view" />
        <TextView
            android:id="@+id/textViewName"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="title"
            android:textAppearance="@style/Base.TextAppearance.AppCompat.Large" />
    </LinearLayout>


    <TextView
        android:id="@+id/textViewImageUrl"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="url"
        android:autoLink="web"
        android:textColor="#08308e" />


</LinearLayout>

创建一个名为 Tutorial.java 的数据模型类，并包含字符串”name”、字符串”imageurl”和字符串”description”的信息。

Tutorial.java

package example.javatpoint.com.volleyreadimagejson;

public class Tutorial {
    String name, imageUrl, description;

    public Tutorial(String name, String imageUrl, String description) {
        this.name = name;
        this.imageUrl = imageUrl;
        this.description = description;
    }
    public String getName() {
        return name;
    }
    public String getImageUrl() {
        return imageUrl;
    }
    public String getDescription() {
        return description;
    }
}

创建一个名为 MyAdapter.java 的自定义适配器类，并扩展ArrayAdapter以处理自定义ListView。在这个类中，我们调用一个匿名类 new ImageDownloaderTask(holder.imageView).execute(imageUrl) 来从URL下载图片。

MyAdapter.java

package example.javatpoint.com.volleyreadimagejson;

import android.app.ProgressDialog;
import android.content.Context;
import android.graphics.Bitmap;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.TextView;
import java.util.List;


public class MyAdapter extends ArrayAdapter {
    //the tutorial list that will be displayed
    private List tutorialList;
    private Bitmap bitmap;
    private Context mCtx;
    //here we are getting the tutoriallist and context
    //so while creating the object of this adapter class we need to give tutoriallist and context
    public MyAdapter(List tutorialList, Context mCtx) {
        super(mCtx, R.layout.list_item, tutorialList);
        this.tutorialList = tutorialList;
        this.mCtx = mCtx;
    }

    //this method will return the list item
    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        //getting the layoutinflater
        ViewHolder holder;
        LayoutInflater inflater = LayoutInflater.from(mCtx);
        convertView = inflater.inflate(R.layout.list_item, null, true);
        holder = new ViewHolder();
        //getting text views
        holder.textViewName = convertView.findViewById(R.id.textViewName);
        holder.textDescription = convertView.findViewById(R.id.textViewImageUrl);
        holder.imageView = convertView.findViewById(R.id.imageView);

        convertView.setTag(holder);
        //Getting the tutorial for the specified position
        Tutorial tutorial = tutorialList.get(position);
        String imageUrl = tutorial.getImageUrl();
        String tutorialDescription = tutorial.getDescription();
        String tutorialTitle = tutorial.getName();

        holder.textViewName.setText(tutorialTitle);
        holder.textDescription.setText(tutorialDescription);

        if (holder.imageView != null) {
            /*-------------fatching image------------*/;
            new ImageDownloaderTask(holder.imageView).execute(imageUrl);
        }
        holder.imageView.setImageBitmap(bitmap);
        return convertView;
    }
    static class ViewHolder {
        TextView textViewName;
        TextView textDescription;
        ImageView imageView;
    }
}

在 build.gradle 文件中添加以下库文件。

build.gradle

android {
    useLibrary 'org.apache.http.legacy'
}

dependencies {
compile 'com.android.volley:volley:1.0.0'
compile 'org.apache.httpcomponents:httpcore:4.4.1'
compile 'org.apache.httpcomponents:httpclient:4.5'
}

创建一个匿名类 ImageDownloaderTask.java ，它继承自AsyncTask<>。该类在执行doInBackground()方法时从URL下载（或提取）图像，并将结果（位图）返回给onPostExecute()方法。在onPostExecute()方法中，将位图设置到ImageView中。

ImageDownloaderTask.java

package example.javatpoint.com.volleyreadimagejson;

import java.io.InputStream;
import java.lang.ref.WeakReference;
import java.net.HttpURLConnection;
import java.net.URL;
import org.apache.http.HttpStatus;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.drawable.Drawable;
import android.os.AsyncTask;
import android.util.Log;
import android.widget.ImageView;

public class ImageDownloaderTask extends AsyncTask {

    private final WeakReference imageViewReference;
    public ImageDownloaderTask(ImageView imageView) {
        imageViewReference = new WeakReference(imageView);
    }
    @Override
    protected Bitmap doInBackground(String... params) {
        return downloadBitmap(params[0]);
    }
    @Override
    protected void onPostExecute(Bitmap bitmap) {
        if (isCancelled()) {
            bitmap = null;
        }
        if (imageViewReference != null) {
            ImageView imageView = imageViewReference.get();
            if (imageView != null) {
                if (bitmap != null) {
                    imageView.setImageBitmap(bitmap);
                } else {
                    Drawable placeholder = imageView.getContext().getResources().getDrawable(R.drawable.placeholder);
                    imageView.setImageDrawable(placeholder);
                }
            }
        }
    }

    private Bitmap downloadBitmap(String imageUrl) {
        HttpURLConnection urlConnection = null;
        try {
            URL uri = new URL(imageUrl);
            urlConnection = (HttpURLConnection) uri.openConnection();

            int statusCode = urlConnection.getResponseCode();
            if (statusCode != HttpStatus.SC_OK) {
                return null;
            }
            InputStream inputStream = urlConnection.getInputStream();
            if (inputStream != null) {
                Bitmap bitmap = BitmapFactory.decodeStream(inputStream);
                return bitmap;
            }
        }
        catch (Exception e) {
            urlConnection.disconnect();
            Log.w("ImageDownloader", "Error downloading image from " + imageUrl);
        }
        finally {
            if (urlConnection != null) {
                urlConnection.disconnect();
            }
        }
        return null;
    }
}

在 MainActivity.java 类中，我们使用Volley从URL获取和解析JSON数据。

MainActivity.java

package example.javatpoint.com.volleyreadimagejson;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ListView;
import android.widget.ProgressBar;
import android.widget.Toast;
import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.StringRequest;
import com.android.volley.toolbox.Volley;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    ListView listView;
    private static final String JSON_URL = "http://192.168.1.35:8080/jsondata/";
    //the tutorial list where we will store all the tutorial objects after parsing json
    List tutorialList;

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

        listView = (ListView) findViewById(R.id.listView);
        tutorialList = new ArrayList<>();
        //this method will fetch and parse the data
        loadTutorialList();
    }

    private void loadTutorialList() {
        //getting the progressbar
        final ProgressBar progressBar = (ProgressBar) findViewById(R.id.progressBar);
        progressBar.setVisibility(View.VISIBLE);

        //creating a string request to send request to the url
        StringRequest stringRequest = new StringRequest(Request.Method.GET, JSON_URL,
                new Response.Listener() {
                    @Override
                    public void onResponse(String response) {
                        //hiding the progressbar after completion
                        progressBar.setVisibility(View.INVISIBLE);


                        try {
                            //getting the whole json object from the response
                            JSONObject obj = new JSONObject(response);

                            //we have the array named tutorial inside the object
                            //so here we are getting that json array
                            JSONArray tutorialsArray = obj.getJSONArray("tutorials");

                            //now looping through all the elements of the json array
                            for (int i = 0; i < tutorialsArray.length(); i++) {
                                //getting the json object of the particular index inside the array
                                JSONObject tutorialsObject = tutorialsArray.getJSONObject(i);

                                //creating a tutorial object and giving them the values from json object
                                Tutorial tutorial = new Tutorial(tutorialsObject.getString("name"), tutorialsObject.getString("imageurl"),tutorialsObject.getString("description"));

                                //adding the tutorial to tutoriallist
                                tutorialList.add(tutorial);
                            }

                            //creating custom adapter object
                            MyAdapter adapter = new MyAdapter(tutorialList, getApplicationContext());

                            //adding the adapter to listview
                            listView.setAdapter(adapter);

                        } catch (JSONException e) {
                            e.printStackTrace();
                        }
                    }
                },
                new Response.ErrorListener() {
                    @Override
                    public void onErrorResponse(VolleyError error) {
                        //displaying the error in toast if occur
                        Toast.makeText(getApplicationContext(), error.getMessage(), Toast.LENGTH_SHORT).show();
                    }
                });

        //creating a request queue
        RequestQueue requestQueue = Volley.newRequestQueue(this);

        //adding the string request to request queue
        requestQueue.add(stringRequest);
    }
}

在AndroidManifest.xml中添加Internet权限

AndroidManifest.xml

<uses-permission android:name="android.permission.INTERNET" />

输出：