协程、AsyncTask、new Thread() 访问网络的对比

使用Kotlin 从网络获取网络图片，需要学习 HttpURLConnection的使用， 多线程(AsyncTask)的使用等 。

先说总结，获取网络图片有几种方式：

1. 直接创建一个线程获取， 会导致显示的图片错乱。

2. 使用AsyncTask , 确保正常显示图片

3. 使用Kotlin 的协程， 用看似同步的代码写异步的操作。

一、 创建根据URL 获取图片的类

第一种方式为直接创建一个线程获取，但是这种方式是明显不可行的。

// 获取网络图片实现类
class NetworkUtils {private var picture : Bitmap ?= nullprivate var context: Contextcompanion object{const val TAG = "NetworkUtils"}constructor(context: Context) {this.context = context}// 获取网络图片fun loadPicture(url: URL): Bitmap? {// 开启一个单独线程进行网络读取Thread(Runnable {var bitmap: Bitmap ? = nulltry {// 根据URL 实例， 获取HttpURLConnection 实例var httpURLConnection: HttpURLConnection = url.openConnection() as HttpURLConnection// 设置读取 和 连接 time out 时间httpURLConnection.readTimeout = 2000httpURLConnection.connectTimeout = 2000// 获取图片输入流var inputStream = httpURLConnection.inputStream// 获取网络响应结果var responseCode = httpURLConnection.responseCode// 获取正常if (responseCode == HttpURLConnection.HTTP_OK) {// 解析图片bitmap = BitmapFactory.decodeStream(inputStream)}} catch(e: IOException) { // 捕获异常 (例如网络异常)Log.d(TAG, "loadPicture - error: ${e?.toString()}")}this.picture = bitmap}).start()// 返回的图片可能为空- 多线程 - 上面的线程还没跑完，已经返回 结果了return picture}
}

第二种是使用AsyncTask.

// 三个泛型参数， 第一个为执行，第二个进度，第三个返回
class NetworkUtilsAsyncTask : AsyncTask {private var resultPicture: Bitmap? = nullprivate lateinit var context: Contextcompanion object {const val TAG = "NetworkUtilsAsyncTask"}constructor(context: Context) {this.context = context}override fun doInBackground(vararg params: URL?): Bitmap? {return loadPicture(params[0])}// 获取网络图片private fun loadPicture(url: URL?): Bitmap? {// 开启一个单独线程进行网络读取var bitmapFromNetwork: Bitmap? = nullurl?.let {try {// 根据URL 实例， 获取HttpURLConnection 实例var httpURLConnection: HttpURLConnection = url.openConnection() as HttpURLConnection// 设置读取 和 连接 time out 时间httpURLConnection.readTimeout = 2000httpURLConnection.connectTimeout = 2000// 获取图片输入流var inputStream = httpURLConnection.inputStream// 获取网络响应结果var responseCode = httpURLConnection.responseCodeLog.d(TAG, "loadPicture - responseCode: $responseCode")// 获取正常if (responseCode == HttpURLConnection.HTTP_OK) {// 解析图片bitmapFromNetwork = BitmapFactory.decodeStream(inputStream)}} catch (e: IOException) { // 捕获异常 (例如网络异常)Log.d(TAG, "loadPicture - error: ${e?.toString()}")//printErrorMessage(e?.toString())}Log.d(TAG, "loadPicture - bitmapFromNetwork: $bitmapFromNetwork")this.resultPicture = bitmapFromNetwork// 返回的图片可能为空}Log.d(TAG, "loadPicture - resultPicture: $resultPicture")return resultPicture}// 调用UI线程的更新UI操作override fun onPostExecute(result: Bitmap?) {super.onPostExecute(result)Log.d(TAG, "onPostExecute - result: $result")if (context is MainActivity) {(context as MainActivity).setResult(result)}}
}

使用AsyncTask需要注意几点：
1. 三个泛型参数  AsyncTask

Params： 为你在UI线程启动执行该任务时，需要传递进来的参数

Result： 为你在想在执行任务后，返回什么类型的结果

Progress: 进度条， 一般为Int

2. 每个任务仅能被执行一次，执行多次会报错，记得cancel

AndroidRuntime:Caused by: java.lang.IllegalStateException:Cannot execute task: the task has already been executed (a task can be executed only once)

3. 任务执行完成后，可以在 onPostExecute 调用UI 逻辑 进行更新UI

第三种是使用Kotlin的协程，其实从网络获取图片的逻辑是一样，区别是怎样调用这个逻辑

class NetworkUtilsCoroutines {private var resultPicture: Bitmap? = nullprivate var context: Contextcompanion object {const val TAG = "NetworkUtilsCoroutines"}constructor(context: Context) {this.context = context}// 获取网络图片fun loadPicture(url: URL): Bitmap? {// 开启一个单独线程进行网络读取var bitmapFromNetwork: Bitmap? = nulltry {// 根据URL 实例， 获取HttpURLConnection 实例var httpURLConnection: HttpURLConnection = url.openConnection() as HttpURLConnection// 设置读取 和 连接 time out 时间httpURLConnection.readTimeout = 2000httpURLConnection.connectTimeout = 2000// 获取图片输入流var inputStream = httpURLConnection.inputStream// 获取网络响应结果var responseCode = httpURLConnection.responseCode// 获取正常if (responseCode == HttpURLConnection.HTTP_OK) {// 解析图片bitmapFromNetwork = BitmapFactory.decodeStream(inputStream)}} catch (e: IOException) { // 捕获异常 (例如网络异常)Log.d(TAG, "loadPicture - error: ${e?.toString()}")//printErrorMessage(e?.toString())}this.resultPicture = bitmapFromNetworkreturn resultPicture}}

需要注意的是，要在gradle文件配置Kotlin 协程库：

项目根目录的build.gradle文件：

ext.kotlin_coroutines = '1.3.1'

APP目录的build.gradle：   

 implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:$kotlin_coroutines"implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:$kotlin_coroutines"

其中 kotlinx-coroutines-core 为核心库， kotlinx-coroutines-android 为平台库

版本参考： 

Kotlin releases | Kotlin
https://kotlinlang.org/docs/releases.html#release-details

二、定义网络图片的地址 （单独创建一个常量类，方便管理而已，也可以在用到的地方定义）

object CommonConstants {const val Address1 ="http://e.hiphotos.baidu.com/zhidao/pic/item/8cb1cb1349540923f12939199458d109b3de4910.jpg"const val Address2 ="http://e.hiphotos.baidu.com/zhidao/pic/item/aec379310a55b31907d3ba3c41a98226cffc1754.jpg"
}

三、定义布局， 获取图片的布局

其中，两个TextView 只是方便调试用的，例如显示当前点击的Button等。

两个Button 分别对应获取两个不同的图片资源。

ImageView 为把从网络获取到的图片显示出来

四、MainActivity 主要调用操作

class MainActivity : AppCompatActivity(), View.OnClickListener {private lateinit var context: Contextprivate lateinit var networkUtils: NetworkUtilsprivate lateinit var networkUtilsAsyncTask: NetworkUtilsAsyncTaskprivate lateinit var networkUtilsCoroutines: NetworkUtilsCoroutinescompanion object {const val TAG = "MainActivity"}//可以变数组， 添加图片URLprivate val urlList = mutableListOf(URL(CommonConstants.Address1), URL(CommonConstants.Address2))//根据Button Id 获取对应的图片URLprivate var urlMap = mutableMapOf()// 根据Button Id 获取对应是第几个Buttonprivate var buttonIndexMap = mutableMapOf()override fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)setContentView(R.layout.activity_main)context = thisnetworkUtils = NetworkUtils(this)networkUtilsAsyncTask = NetworkUtilsAsyncTask(this)networkUtilsCoroutines = NetworkUtilsCoroutines(this)//button1, button2 ...buttonIndexMap[get_picture_button1.id] = 1buttonIndexMap[get_picture_button2.id] = 2urlMap[get_picture_button1.id] = urlList[0]urlMap[get_picture_button2.id] = urlList[1]}override fun onClick(v: View?) {when (v?.id) {get_picture_button1.id, get_picture_button2.id -> {text1.text = "Button : " + buttonIndexMap[v.id] + " is clicked!!!"//loadPictureDirectly(v.id)//loadPictureAsyncTask(v.id)loadPictureCoroutines(v.id)}}}fun setResult(bitmap: Bitmap?) {if (bitmap != null) {Toast.makeText(context, "Load picture success!!!", Toast.LENGTH_SHORT).show()image_view.setImageBitmap(bitmap)} else {Toast.makeText(context, "Can not load picture !!!", Toast.LENGTH_SHORT).show()}}// 1. 使用Thread - 此方法获取的图片存在错误可能，// 例如第一次点击，获取不到图片； 第二次点击，显示的却是第一次点击的获取的图片?// --> 多线程问题private fun loadPictureDirectly(id: Int) {var bitmap = urlMap[id]?.let { networkUtils.loadPicture(it) }setResult(bitmap)}//2. 使用AsyncTask -  一个AsyncTask 仅能被执行一次//AndroidRuntime:              Caused by: java.lang.IllegalStateException:// Cannot execute task: the task has already been executed (a task can be executed only once)private fun loadPictureAsyncTask(id: Int) {if (networkUtilsAsyncTask != null) {networkUtilsAsyncTask.cancel(true)networkUtilsAsyncTask = NetworkUtilsAsyncTask(this)}urlMap[id]?.let { networkUtilsAsyncTask.execute(it) }}//3. 使用协程 - 看似同步的代码实现异步效果，private fun loadPictureCoroutines(id: Int) {// 在主线程开启一个协程CoroutineScope(Dispatchers.Main).launch {// 切换到IO 线程 - withContext 能在指定IO 线程执行完成后，切换原来的线程var bitmap = withContext(Dispatchers.IO) {text2.text = Thread.currentThread().name.toString()urlMap[id]?.let { networkUtilsCoroutines.loadPicture(it) }}// 切换了UI 线程，更新UItext2.text = Thread.currentThread().name.toString()setResult(bitmap)}}/*    private suspend fun loadPictureCoroutinesInner(id: Int): Bitmap? {return withContext(Dispatchers.IO) {urlMap[id]?.let { networkUtilsCoroutines.loadPicture(it) }}}*/
}

其中，两个图片地址定义为：

object CommonConstants {const val Address1 ="http://e.hiphotos.baidu.com/zhidao/pic/item/8cb1cb1349540923f12939199458d109b3de4910.jpg"const val Address2 ="http://e.hiphotos.baidu.com/zhidao/pic/item/aec379310a55b31907d3ba3c41a98226cffc1754.jpg"
}

五、需要在AndroidManifest.xml中添加网络权限，否则会报错(缺少网络权限)

AndroidRuntime: java.lang.SecurityException: Permission denied (missing INTERNET permission?)

六、使用http 网址，还需要在配置文件中设置 usesCleartextTraffic ，否则会报错

AndroidRuntime: java.io.IOException: Cleartext HTTP traffic to XXX not permitted   

七， 若需要使用旧版的HttpClient ，则需在官网下载jar 包，然后导入项目中 ----》 这种方法不好....

Apache HttpComponents – HttpComponents Downloads

若有如下错误，可能是因为使用导入的HttpClient相关的，和手机系统F/W上的上冲突了

09-02 10:09:10.888 E 21378    26514    AndroidRuntime:                                                         java.lang.NoClassDefFoundError: Failed resolution of: Lorg/apache/http/message/LineFormatter; 
09-02 10:09:10.888 E 21378    26514    AndroidRuntime:                                                             at org.apache.http.impl.conn.ManagedHttpClientConnectionFactory.(ManagedHttpClientConnectionFactory.java:66) 
09-02 10:09:10.888 E 21378    26514    AndroidRuntime:                                                             at org.apache.http.impl.conn.ManagedHttpClientConnectionFactory.(ManagedHttpClientConnectionFactory.java:78) 
09-02 10:09:10.888 E 21378    26514    AndroidRuntime:                                                             at org.apache.http.impl.conn.ManagedHttpClientConnectionFactory.(ManagedHttpClientConnectionFactory.java:57) 
09-02 10:09:10.888 E 21378    26514    AndroidRuntime:                                                             at org.apache.http.impl.conn.PoolingHttpClientConnectionManager$InternalConnectionFactory.(PoolingHttpClientConnectionManager.java:499)  09-02 09:44:31.569 E 20772    21629    AndroidRuntime:                                                         java.lang.NoSuchFieldError: No static field INSTANCE of type Lorg/apache/http/conn/ssl/AllowAllHostnameVerifier; in class Lorg/apache/http/conn/ssl/AllowAllHostnameVerifier; or its superclasses (declaration of org.apache.http.conn.ssl.AllowAllHostnameVerifier appears in /system/framework/framework.jar!classes4.dex) 
09-02 09:44:31.569 E 20772    21629    AndroidRuntime:                                                             at org.apache.http.conn.ssl.SSLConnectionSocketFactory.(SSLConnectionSocketFactory.java:151) 
09-02 09:44:31.569 E 20772    21629    AndroidRuntime:                                                             at org.apache.http.conn.ssl.SSLConnectionSocketFactory.getSocketFactory(SSLConnectionSocketFactory.java:194) 
09-02 09:44:31.569 E 20772    21629    AndroidRuntime:                                                             at org.apache.http.impl.conn.PoolingHttpClientConnectionManager.getDefaultRegistry(PoolingHttpClientConnectionMa

可以使用以下方法解决(删掉导入的所有Http 包)

1. AndroidManifest.xml 

2. 所在模块 的build.gradle

android {useLibrary 'org.apache.http.legacy'
}dependencies {...implementation group: 'org.apache.httpcomponents', name:'httpclient-android', version: '4.3.5'
}