使用HttpURLConnection下载网络图片并显示

效果图如下：

xml中代码：

<LinearLayout 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"
    android:orientation="vertical"
    tools:context="com.example.administrator.myapplu.MainActivity">
<ProgressBar
    android:id="@+id/pro"
    android:layout_width="match_parent"
    style="@style/Widget.AppCompat.ProgressBar.Horizontal"
    android:layout_height="5dp" />
    <Button
        android:id="@+id/btn"
        android:text="start"
        android:layout_width="match_parent"
        android:layout_height="50dp" />
    <ImageView
        android:id="@+id/imageview"
        android:scaleType="fitCenter"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
LinearLayout>

AsyncTask代码如下：

package com.example.administrator.myapplu;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Environment;
import android.widget.ImageView;
import android.widget.Toast;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.MalformedURLException;
import java.net.URL;

import javax.net.ssl.HttpsURLConnection;

/**
 * Created by Administrator on 2018/3/14/014.
 */

public class DownLoadTask extends AsyncTask<String, Integer, Integer> {
    private String dirPath;
    private String FilePath;

    private Context context;
    private ImageView imageView;

    public DownLoadTask(Context context, ImageView imageView) {
        this.context=context;
        this.imageView=imageView;
    }

    @Override
    protected Integer doInBackground(String... strings) {
        dirPath= Environment.getExternalStorageDirectory()+"/download_pics";
        File dir=new File(dirPath);
        if (!dir.exists()){
            dir.mkdir();
        }
        FilePath=dirPath+strings[1];
        File file=new File(FilePath);
        if (file.exists()){
            return -1;
        }else {
            try {
                file.createNewFile();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        InputStream inputStream=null;
        OutputStream outputStream=null;

        try {
            URL url=new URL(strings[0]);
            HttpsURLConnection connection= (HttpsURLConnection) url.openConnection();
            if (connection.getResponseCode()==200){
                inputStream=connection.getInputStream();
            }else {
                return -2;
            }
            outputStream=new FileOutputStream(file);
            int length=0;
            byte[] buffer=new byte[4*1024];
            while ((length=inputStream.read(buffer))!=-1){
                outputStream.write(buffer,0,length);
            }
            inputStream.close();
            outputStream.close();

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        return 1;
    }

    @Override
    protected void onPostExecute(Integer integer) {
        super.onPostExecute(integer);
        switch (integer){
            case 1:
                Toast.makeText(context,"文件下载完成",Toast.LENGTH_SHORT).show();
                Bitmap bm= BitmapFactory.decodeFile(FilePath);
                imageView.setImageBitmap(bm);
                break;
            case -1:
                Toast.makeText(context,"文件已存在",Toast.LENGTH_SHORT).show();
                break;
            case -2:
                Toast.makeText(context,"网络异常",Toast.LENGTH_SHORT).show();
                break;
        }
    }
}

MainActivity中代码：

package com.example.administrator.myapplu;

import android.icu.text.UnicodeSetSpanner;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.ProgressBar;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
private ProgressBar progressBar;
private Button button;
private ImageView imageView;
private String picUrl="https://ss2.bdstatic.com/70cFvnSh_Q1YnxGkpoWK1HF6hhy/it/u=2239146502,165013516&fm=27&gp=0.jpg";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        bindID();
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                new DownLoadTask(MainActivity.this,imageView).execute(picUrl,"jy.jpg");
            }
        });
    }

    private void bindID() {
        progressBar=findViewById(R.id.pro);
        button=findViewById(R.id.btn);
        imageView=findViewById(R.id.imageview);
    }
}