用HTTPURLConnecttion 加载网络图片

首先要分清思路，大概的流程是什么？

package com.example.lenovo.myapplication;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;

public class MainActivity extends AppCompatActivity {
private Button button;
private ImageView imageView;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        button=findViewById(R.id.main_button);
        imageView=findViewById(R.id.main_ImageView);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Demo task=new Demo(MainActivity.this,button,imageView);
                 task.execute("http://img31.mtime.cn/mg/2012/10/30/201631.37192876.jpg");
            }
        });
    }
}

我们要创建一个外部类Demo.java 更清晰更方便的 使用这个方法

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

import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.BitSet;

/**
 * Created by lenovo on 2018/3/13.
 */

public  class Demo  extends AsyncTask<String,Integer,Integer>{
    private ImageView imageView;
    private Bitmap bitmap;

    public  Demo(MainActivity mainActivity, Button button, ImageView imageView){
        this.imageView=imageView;
    }
    @Override
    protected Integer doInBackground(String... strings) {
        try{
            URL url=new URL(strings[0]);
            HttpURLConnection httpURLConnection= (HttpURLConnection) url.openConnection();
            InputStream inputStream=httpURLConnection.getInputStream();
            bitmap= BitmapFactory.decodeStream(inputStream);//将输入流转换成bitmap类型对象

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return 1;
    }
    @Override
    protected void onPostExecute(Integer result) {
        super.onPostExecute(result);
        switch (result) {
            case 1:
                imageView.setImageBitmap(bitmap);
                break;

            default:
                break;
        }
    }
}

XML里 我们写简单的控件：

<?xml version="1.0" encoding="utf-8"?>
<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.lenovo.myapplication.MainActivity">

   <Button
       android:textSize="25dp"
       android:text="加载网络图片"
       android:layout_width="match_parent"
       android:layout_height="80dp"
       android:id="@+id/main_button"
       />
    <ImageView
        android:id="@+id/main_ImageView"
        android:layout_marginTop="15dp"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:scaleType="fitXY"
        />
</LinearLayout>

效果图：