Android 利用 AsyncTask 异步读取网络图片

1.新建Android工程AsyncLoadPicture

新建布局文件activity_main.xml主界面为一个ListView

2.功能主界面MainActivity.java,主代码如下

package com.bwie.httpclienthomework1;

import android.graphics.Bitmap;
import android.os.AsyncTask;
import android.os.Handler;
import android.os.Message;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;

import com.bwie.com.bwie.beans.Book;
import com.bwie.com.bwie.beans.Data;
import com.bwie.com.bwie.beans.SuperClass;
import com.bwie.utils.LoadImageUtil;
import com.bwie.utils.NetWorkUtils;
import com.bwie.utils.URLDB;
import com.google.gson.Gson;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {
private Button btn1,btn2;
private ListView lv;
private NetWorkUtils netWorkUtils;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

init();

}

private void init() {
netWorkUtils = new NetWorkUtils();
btn1 = (Button) findViewById(R.id.btn1);
btn2 = (Button) findViewById(R.id.btn2);
lv = (ListView) findViewById(R.id.lv);
btn2.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new AsyncTask<String,Integer,String>(){

@Override
protected String doInBackground(String... params) {
String url = params[0];//得到url
String json = netWorkUtils.getJsonByHttpUrlConnectionGet(url);
return json;
}

@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
ShopClass shopClass = new Gson().fromJson(s,ShopClass.class);
final ArrayList<ArrayList<String>> rs = shopClass.getResult();

lv.setAdapter(new BaseAdapter() {

@Override
public int getCount() {
return rs.size();
}

@Override
public Object getItem(int position) {
return rs.get(position);
}

@Override
public long getItemId(int position) {
return position;
}

4000
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHold hold = null;
if(convertView == null){
hold = new ViewHold();
convertView = View.inflate(MainActivity.this,R.layout.listview_item,null);
hold.img = (ImageView) convertView.findViewById(R.id.imageView);
hold.tv1 = (TextView) convertView.findViewById(R.id.textView1);
hold.tv2 = (TextView) convertView.findViewById(R.id.textView2);

convertView.setTag(hold);
}else{
hold = (ViewHold) convertView.getTag();
}
//获得当前要显示的数据对象
ArrayList<String> strs = rs.get(position);
hold.tv1.setText(strs.get(0));
hold.tv2.setText(strs.get(1));
//加载图片

return convertView;
}

class ViewHold{
TextView tv1,tv2;
ImageView img;
}
});

}
}.execute(URLDB.JSON_URL2);
}
});

btn1.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
new AsyncTask<String,Integer,String>(){

@Override
protected String doInBackground(String... params) {
String url = params[0];//得到url
String json = netWorkUtils.getJsonByHttpUrlConnectionGet(url);
return json;
}

@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
//解析json串
SuperClass superClass = new Gson().fromJson(s,SuperClass.class);
Data data = superClass.getData();
final ArrayList<Book> topics = data.getTopics();
lv.setAdapter(new BaseAdapter() {
//实例化加载和缓存图片的类
LoadImageUtil loadImageUtil = new LoadImageUtil(MainActivity.this);
@Override
public int getCount() {
return topics.size();
}

@Override
public Object getItem(int position) {
return topics.get(position);
}

@Override
public long getItemId(int position) {
return position;
}

@Override
public View getView(int position, View convertView, final ViewGroup parent) {
ViewHold hold = null;

if(convertView == null){
hold = new ViewHold();
convertView = View.inflate(MainActivity.this,R.layout.listview_item,null);
hold.img = (ImageView) convertView.findViewById(R.id.imageView);
hold.tv1 = (TextView) convertView.findViewById(R.id.textView1);
hold.tv2 = (TextView) convertView.findViewById(R.id.textView2);

convertView.setTag(hold);
}else{
hold = (ViewHold) convertView.getTag();
}
//获得当前要显示的数据对象
Book book = topics.get(position);
hold.tv1.setText(book.getId()+"");
if(book.getDescription().length() > 10){
hold.tv2.setText(book.getDescription().substring(0,10)+"...");
}else{
hold.tv2.setText(book.getDescription());
}
//把imgview做好标记
hold.img.setTag(book.getCover_image_url());
//处理图片，通过缓存
//得到图片
Bitmap bitmap = loadImageUtil.loadBitmapByImgUrl(book.getCover_image_url(), new LoadImageUtil.CallBack() {
@Override
public void callBack(String imgUrl, Bitmap bitmap) {
if(bitmap != null){
ImageView img = (ImageView) parent.findViewWithTag(imgUrl);
img.setImageBitmap(bitmap);
}

}
});
if(bitmap != null){//如果图片在缓存中，直接获取展示
hold.img.setImageBitmap(bitmap);
}

return convertView;
}

class ViewHold{
TextView tv1,tv2;
ImageView img;
}
});

}
}.execute(URLDB.JSON_URL1);
}
});
}
}

下面来写一下 ImageLoader的原理

package com.bwie.utils;

import android.content.Context;
import android.graphics.Bitmap;
import android.os.Handler;
import android.os.Message;

import java.lang.ref.SoftReference;
import java.util.HashMap;

/**
* Created by Administrator on 2017/9/5.
*/

public class LoadImageUtil {
private Context context;
private HashMap<String,SoftReference<Bitmap>> bitMaps;

public LoadImageUtil(Context context) {
this.context = context;
this.bitMaps = new HashMap<String,SoftReference<Bitmap>>();//实例化缓存容器
}
//回调的接口
public interface CallBack{
public void callBack(String imgUrl,Bitmap bitmap);
}
//加载图片，进行缓存
public Bitmap loadBitmapByImgUrl(final String imgUrl, final CallBack callBack){
//先判断缓存中有没有图片，得到图片
SoftReference<Bitmap> soft = bitMaps.get(imgUrl);
if(soft != null){
Bitmap bitmap = soft.get();//从软引用中获得bitmap
if(bitmap != null){//缓存中有次图片，直接用
return bitmap;
}else{
bitMaps.remove(imgUrl);//key(图片的url)对应的bitmap消耗了，key也没有意义
}
}
//代码走到此处，说明缓存中没有图片，需要到第三级缓存：网络缓存去取
//通过handler实现子线程和ui主线程的通信
final Handler handler = new Handler(){
@Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
Bitmap bitmap = (Bitmap) msg.obj;
callBack.callBack(imgUrl,bitmap);
}
};
//启动线程访问图片
new Thread(){
@Override
public void run() {
super.run();
//加载图片
Bitmap bitmap = new NetWorkUtils().getBitmapByHttpUrlConnectionGet(imgUrl);
//压缩图片

//把bitmap缓存到缓存中,通过软引用
bitMaps.put(imgUrl,new SoftReference<Bitmap>(bitmap));

handler.sendMessage(handler.obtainMessage(1,bitmap));//发给handler
}
}.start();

// //内存中没有图片，通过次线程去加载，然后放在缓存中
// bitMaps.put(imgUrl,bitmap);

return null;
}
}
3、网络请求的工具类
package com.bwie.utils;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;

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

/**
* 网络请求的工具类
* Created by Administrator on 2017/9/1.
*/

public class NetWorkUtils {

//获得json串的方法，通过HttpClient get
public String getJsonByHttpClientGet(String url){

return "";
}
//获得json串的方法，通过HttpClient get
public String getJsonByHttpUrlConnectionGet(String jsonUrl){

URL url = null;
HttpURLConnection httpURLConnection = null;
String strJson = "";
InputStream inputStream = null;
try {
url = new URL(jsonUrl);
httpURLConnection = (HttpURLConnection) url.openConnection();

int responseCode = httpURLConnection.getResponseCode();
if(responseCode == 200){
inputStream = httpURLConnection.getInputStream();
byte[] b = new byte[1024];
int length = 0;

while((length = inputStream.read(b)) != -1){//循环读取json串，拼装
strJson += new String(b,0,length);
}
}

} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}finally {
if(inputStream != null){
try {
inputStream.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
return strJson;//返回最终的json数据
}

//获得json串的方法，通过HttpClient get
public Bitmap getBitmapByHttpUrlConnectionGet(String imageUrl){
URL url = null;
HttpURLConnection httpURLConnection = null;
Bitmap bitmap = null;
InputStream inputStream = null;
try {
url = new URL(imageUrl);
httpURLConnection = (HttpURLConnection) url.openConnection();

int responseCode = httpURLConnection.getResponseCode();
if(responseCode == 200){
inputStream = httpURLConnection.getInputStream();
bitmap = BitmapFactory.decodeStream(inputStream);//把图片信息（输入流）
}

} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}finally {
if(inputStream != null){
try {
inputStream.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
return bitmap;//返回最终的json数据
}
}
4、一定要记得加网络权限
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.bwie.httpclienthomework1">
<uses-permission android:name="android.permission.INTERNET"></uses-permission>
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />

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

</manifest>
今天写的网络请求图片，是ImageLoader的原理，所以用的不多，但是原理一定要懂，不然在工作中遇到实际的问题，会让你懵逼，对，懵逼！！