本文主要是介绍OKHTTP学习之高级特性，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
前言
上一篇我已经将OKHTTP的基础知识介绍了一番<<
OKHTTP学习之基础知识及运用 >>。这一篇我们一起探索一些复杂的功能。
在这之前我们将基础知识再回顾一下。
- Call
- 同步请求 execute
- 异步请求 enqueue
- 异步请求时的回调 Callback
- 服务器的回复 Response
- 服务的消息体 ResponseBody
- 网络访问的请求 Request
- Header
- 请求的消息体 RequestBody
- 消息体的数据类型MediaType
不熟悉的话大家也可以返回点击这里。
基础功能
我们已经知道了
同步请求
execute()
异步请求
call()方法和Callback回调。
get和post
复杂功能
这一部分，我讲解Okhttp能够帮助我们做的一些工作。
下载文件
我之前讲ResponseBody的时候讲了它的
- byte()
-
string()
-
bytesStream()
- charStream()
其中byte()和string()是一次读取，用来获取体积比较小的内容。但如果遇到大文件的话，就应该用流的方式。
所谓下载也就是将服务器返回的数据存储在本地。
- 当体积体积较小时，用byte()或者string()获取内容。
- 当体积很大时（超过1m），就应该用流的方式，用byteStream()或者charStream().
这里我用流的方式演示从网络上下载一张图片，然后保存在本地，然后显示出来。我是用bytesStream()方法。
private void testDownload(){String url = "http://img4.cache.netease.com/photo/0026/2015-05-19/APVC513454A40026.jpg";final File filePath = new File(getExternalCacheDir().toString(),"tmp.jpg");OkHttpClient client = new OkHttpClient();Request request = new Request.Builder().url(url).build();Call call = client.newCall(request);call.enqueue(new Callback() {@Overridepublic void onFailure(Call call, IOException e) {}@Overridepublic void onResponse(Call call, Response response) throws IOException {if(response.isSuccessful()){InputStream is = response.body().byteStream();FileOutputStream fos = new FileOutputStream(filePath);int b = 0;while((b = is.read()) != -1){fos.write(b);}fos.close();final Bitmap bitmap = BitmapFactory.decodeFile(filePath.getAbsolutePath());runOnUiThread(new Runnable() {@Overridepublic void run() {mImg.setImageBitmap(bitmap);}});}response.body().close();}});}
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效果如下图：
上传文件
说到下载功能就得说到上传功能，这样http访问才完整。
因为没有找到网络上现在的可以上传调用的API，所以这部分还是要在我自己的电脑上编写PHP服务来验证。
php代码编写
php代码的上传功能分为两个部分。
1. 客户端html发送表单数据。
2. 服务端php程序通过$_FILES这个域变量来接收传过来的文件，然后移动文件到指定目录，整个过程就完成了。
html代码
文件名testupload.html
<html>
<head><title>test upload</title>
</head>
<body>
<form action="upload_file.php" method="post" enctype="multipart/form-data">
<label>filename:</label><input type="file" name="file" id="file" /><br/><input type="submit" name="submit" value="submit">
</form>
</body>
</html>
重点在于这个标签。
action定义到表单发送的位置，这里是upload_file.php，说明表单将会发送到主机上的upload_file.php上。
method 中的方法是post。这个一定要写对，文件上传的内容必须放在实体中，不能添加在header中，所以不能用get，要用post.
enctype,这个定义内容是 multipart/form-data.
enctype有两个值的范围。
* 一个是application/x-www-form-urlencoded(默认值)，传输文本信息
* 一个是multipart/form-data.传输二进制信息
标签的 type=”file” 属性规定了应该把输入作为文件来处理。举例来说，当在浏览器中预览时，会看到输入框旁边有一个浏览按钮。
php代码
我直接就张贴代码了。我用的是Phpnow套件。解压后就放在E盘。
然后把Php代码放在解压的的目录的package\hotdoc目录下。
比如：E:\PHPnow-1.5.6.1428396605\Package\htdocs。前面的testupload.html也是放在这个目录。不熟悉php的朋友可以直接copy我的代码，然后放在里面。
如果熟悉Php或者j2ee的同学则自己进行模拟。
upload_file.php
<?phpif($_FILES['file']['error'] > 0){echo "Return code: ".$_FILES['franktest']['error']."<br/>";}else{echo "Upload: ".$_FILES['file']['name']."<br/>";if(file_exists($_FILES['file']['name'])){echo $_FILES['file']['name']." already exists.";}else{move_uploaded_file($_FILES['file']['tmp_name'],$_FILES['file']['name']);echo "upload successed";}}
?>
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为了更清楚的说明上传的原理，我用fiddler来抓包。不熟悉fiddler的同学可以自行上网查阅相关知识。
如下图所演示：
用fiddler抓取刚才的包可以得到下面消息：
POST http://localhost/upload_file.php HTTP/1.1
Host: localhost
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:27.0) Gecko/20100101 Firefox/27.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: zh-cn,zh;q=0.8,en-us;q=0.5,en;q=0.3
Accept-Encoding: gzip, deflate
Referer: http://localhost/upload.html
Connection: keep-alive
Content-Type: multipart/form-data; boundary=---------------------------178612565028255
Content-Length: 11185-----------------------------178612565028255
Content-Disposition: form-data; name="file"; filename="test.png"
Content-Type: image/pngPNGIHDR ) sRGB gAMA a pHYs o d *"IDATx^ a'p kf? Y#AJT 8 P V nd # Gl p D B^ +Z W $.- * B AG KI C; ~ [ G 7 t `z `z `z `z `z `z `z `z w E t s P IEND B`
-----------------------------178612565028255
Content-Disposition: form-data; name="submit"submit
-----------------------------178612565028255--
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提取我们关心的内容则是：
Content-Type: multipart/form-data; boundary=---------------------------178612565028255
Content-Length: 11185-----------------------------178612565028255
Content-Disposition: form-data; name="file"; filename="test.png"
Content-Type: image/pngPNGIHDR ) sRGB gAMA a pHYs o d *"IDATx^ a'p kf? Y#AJT 8 P V nd # Gl p D B^ +Z W $.- * B AG KI C; ~ [ G 7 t `z `z `z `z `z `z `z `z w E t s P IEND B`
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Content-Type: multipart/form-data;
在这里可以看到Content-Type果真是multipart/form-data,而后面的boundary=—————————178612565028255，boundary是分界线的意思，后面的数字是随机数，用来分割实体。比如我上传了一个文本一张图片，它们之间就用这个来分割区别开来。
Content-Disposition
disposition的英文单词是配置的意思，在这里用来区分表单的内容，因为一个表单中有许多项，这里为了说明这一段属于哪一项。name=”file”是因为之前我的标签中定义了name=”file”。filename=”test.png”代表我此次上传的文件名字为test.png.
Content-Type:在Content-Disposition:后面是为了说明表单中某一项传输的内容格式。比如，我此次上传的是一个图片文件test.png。所以它的值是image/png.如果我上传的是一个文本文件text.txt.则它的值是text/plain。
常见的Content-type值如下：
* .*(二进制文件，不知道格式的文件) application/octet-stream
* .txt -> text/plain
* .png -> image/png
* .jpg -> image/jpeg
* .html -> text/html
* .mp4 -> video/mpeg
* .apk -> application/vnd.Android.package-archive
更多的情况请访问这个链接：
常用的html content-type对照表
Android用Okhttp上传文件代码
前面用一大段介绍了html上传文件流程。接下来就要编写如何在Android上编写上传代码。
清楚了上传的原理与流程，我们就可以用okhttp来模拟表单发送消息，从而达到上传文件的目的。
我们再把思路捋一捋。
1. 用http协议。
2. 添加相应的header.这里指Content-type:multipart/form-data
3. 在表单项的实体中添加对应的内容描述。Content-Disposition:form-data; name=”file”; filename=”test.png”和Content-Type: image/png在这里Content-Type:application/octet-stream的话可以传输任何文件。
4. 添加具体的实体数据。
好了，现在假设我们要用Android手机上传一张图片到服务器。代码如下：
private void testUpload(){String url = "http://172.26.133.50//upload_file.php";File file = new File(Environment.getExternalStorageDirectory().toString(),"test.png");OkHttpClient client = new OkHttpClient();MediaType mediaType = MediaType.parse("application/octet-stream");RequestBody fileBody = RequestBody.create(mediaType,file);RequestBody requestBody = new MultipartBody.Builder().setType(MultipartBody.FORM).addPart(Headers.of("Content-Disposition","form-data;name=\"file\";" +"filename=\"test.png\""),fileBody).build();Request request = new Request.Builder().url(url).post(requestBody).build();final Call call = client.newCall(request);new Thread(new Runnable() {@Overridepublic void run() {try {Response response = call.execute();if(response.isSuccessful()){Log.d(TAG, "run: upload is successed.");}else{Log.d(TAG, "run: upload is failed ");}response.close();} catch (IOException e) {e.printStackTrace();Log.e(TAG, "run: "+e.getLocalizedMessage() );}}}).start();}
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大家对照我前面所讲的，细细体会一下。
这里运用了一个知识点MultiPartBody.
MultipartBody.Builder can build sophisticated request bodies compatible with HTML file upload forms. Each part of a multipart request body is itself a request body, and can define its own headers. If present, these headers should describe the part body, such as its Content-Disposition. The Content-Length and Content-Type headers are added automatically if they’re available.
这是官网上的说明。
MuiltipartBody.Builder可以构建一个html的文件上传表单这样的复杂的网络请求消息实体(request body).
注意它能够构造复杂的消息实体。复杂在于它包含的内容也可以由RequestBody构成，在Okhttp中称为Part.
如我可以同时发送一段文本、一张图片、一个Mp4文件给服务器，它们被MultipartBody封装在同一个表单，然后进行post请求。
每一个part,也就是每一个实体都可以定义自己的headers。目前，这些被包含在form-data中的消息实体应该有描述了Content-Disosition的header。当然，Okhttp会自动添加它的Content-Length属性。
下面是精简的MultipartBody源码
public final class MultipartBody extends RequestBody {public static final MediaType MIXED = MediaType.parse("multipart/mixed");public static final MediaType ALTERNATIVE = MediaType.parse("multipart/alternative");public static final MediaType DIGEST = MediaType.parse("multipart/digest");public static final MediaType PARALLEL = MediaType.parse("multipart/parallel");/*** The media-type multipart/form-data follows the rules of all multipart MIME data streams as* outlined in RFC 2046. In forms, there are a series of fields to be supplied by the user who* fills out the form. Each field has a name. Within a given form, the names are unique.*/public static final MediaType FORM = MediaType.parse("multipart/form-data");private final ByteString boundary;private final MediaType originalType;private final MediaType contentType;private final List<Part> parts;private long contentLength = -1L;MultipartBody(ByteString boundary, MediaType type, List<Part> parts) {this.boundary = boundary;this.originalType = type;this.contentType = MediaType.parse(type + "; boundary=" + boundary.utf8());this.parts = Util.immutableList(parts);}public List<Part> parts() {return parts;}public Part part(int index) {return parts.get(index);}/** A combination of {@link #type()} and {@link #boundary()}. */@Override public MediaType contentType() {return contentType;}@Override public long contentLength() throws IOException {long result = contentLength;if (result != -1L) return result;return contentLength = writeOrCountBytes(null, true);}public static final class Part {public static Part create(RequestBody body) {return create(null, body);}private final Headers headers;private final RequestBody body;private Part(Headers headers, RequestBody body) {this.headers = headers;this.body = body;}}public static final class Builder {private final ByteString boundary;private MediaType type = MIXED;private final List<Part> parts = new ArrayList<>();public Builder() {this(UUID.randomUUID().toString());}public Builder(String boundary) {this.boundary = ByteString.encodeUtf8(boundary);}/*** Set the MIME type. Expected values for {@code type} are {@link #MIXED} (the default), {@link* #ALTERNATIVE}, {@link #DIGEST}, {@link #PARALLEL} and {@link #FORM}.*/public Builder setType(MediaType type) {if (type == null) {throw new NullPointerException("type == null");}if (!type.type().equals("multipart")) {throw new IllegalArgumentException("multipart != " + type);}this.type = type;return this;}/** Add a part to the body. */public Builder addPart(RequestBody body) {return addPart(Part.create(body));}/** Add a part to the body. */public Builder addPart(Headers headers, RequestBody body) {return addPart(Part.create(headers, body));}/** Add a form data part to the body. */public Builder addFormDataPart(String name, String value) {return addPart(Part.createFormData(name, value));}/** Add a form data part to the body. */public Builder addFormDataPart(String name, String filename, RequestBody body) {return addPart(Part.createFormData(name, filename, body));}/** Add a part to the body. */public Builder addPart(Part part) {if (part == null) throw new NullPointerException("part == null");parts.add(part);return this;}/** Assemble the specified parts into a request body. */public MultipartBody build() {if (parts.isEmpty()) {throw new IllegalStateException("Multipart body must have at least one part.");}return new MultipartBody(boundary, type, parts);}}
}
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代码不多。主要使用步骤。
1. new MultiPartBody.Builder()创建Builder对象。
2. addPart()或者addFormDataPart()添加文件或者是表单数据。
3. 然后调用build()方法生成MultiPartBody对象。
4. 调用Requst对象的post()方法，访问远程服务。
拦截器（Interceptors）
拦截器是一个强大的机制，它能对Call进行监测、改写、重试连接。它能够对请求和回复进行二次加工。
OKHTTP中的拦截器是链式的这个跟MINA框架中的拦截器类似。
拦截器的作用之log
下面是官网的一个例子。
class LoggingInterceptor implements Interceptor {@Override public Response intercept(Interceptor.Chain chain) throws IOException {Request request = chain.request();long t1 = System.nanoTime();Log.i(TAG, "intercept: "+String.format("Sending request %s on %s%n%s",request.url(), chain.connection(), request.headers()));Response response = chain.proceed(request);long t2 = System.nanoTime();Log.i(TAG, "intercept: "+String.format("Received response for %s in %.1fms%n%s",response.request().url(), (t2 - t1) / 1e6d, response.headers()));return response;}}
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这段代码的主要功能是打印Request和Response的相关信息，比如url地址，比如路由，比如headers.
下面我们来编写代码测试一下。
在编写代码之前，先介绍两个概念就是应用拦截器（Application Interceptors）和网络拦截器(Network Interceptors).
Interceptors要么被当成Application Interceptors注册，要么被当成Network Interceptors注册。
应用拦截器(Application Interceptors)
如果当成应用拦截器添加的话，那么要在OkHttpClient.Builder中的addInterceptors()方法中添加。
private void testInterceptors(){OkHttpClient client = new OkHttpClient.Builder().addInterceptor(new LoggingInterceptor()).build();String url = "http://blog.csdn.net/briblue";Request request = new Request.Builder().url(url).addHeader("name","frank").build();final Call call = client.newCall(request);new Thread(new Runnable() {@Overridepublic void run() {try {Response response = call.execute();response.body().close();} catch (IOException e) {e.printStackTrace();}}}).start();}
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我在请求中人为中添加了一个头信息，这个信息只用来演示，没有实际意义。结果Log信息如下：
I/SeniorActivity: intercept: Sending request http://blog.csdn.net/briblue on null
name: frankI/SeniorActivity: intercept: Received response for http://blog.csdn.net/briblue in 164.0msServer: openresty
Date: Mon, 24 Oct 2016 04:03:57 GMT
Content-Type: text/html; charset=utf-8
Transfer-Encoding: chunked
Connection: keep-alive
Keep-Alive: timeout=20
Vary: Accept-Encoding
Cache-Control: private
Set-Cookie: uuid=1ffd7e7e-6bed-41c3-b48c-f61f915f02f8;
expires=Tue, 25-Oct-2016 04:03:57 GMT; path=/
X-Powered-By: PHP 5.4.28
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可以看到它详细打印了Request和Response的的一些信息。
那么我们再看NetworkInterceptors
网络拦截器（Network Interceptors）
上面讲了一个拦截器被当成Application Interceptors注册到了Okhttpclient.同时，一个拦截器也可以当成Netowork Interceptors注册到Okhttpclient.调用的是它的Builder对象的addNetworkInterceptor.代码区别不大。
private void testNetworkInterceptors(){OkHttpClient client = new OkHttpClient.Builder().addNetworkInterceptor(new LoggingInterceptor()).build();String url = "http://blog.csdn.net/briblue";Request request = new Request.Builder().url(url).addHeader("name","frank").build();final Call call = client.newCall(request);new Thread(new Runnable() {@Overridepublic void run() {try {Response response = call.execute();response.body().close();} catch (IOException e) {e.printStackTrace();}}}).start();}
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而log信息，也是完全一样的。说到这里大家可能有些迷惑。其实是这样的，NetworkInterceptor比Application打印更详尽的信息。我举的例子中http://blog.csdn.net/briblue没有进行重定向。如果我把上面例子中的url换成是http:www.github.com来进行测试的话。情况大有不同。
I/SeniorActivity: intercept: Sending request http://www.github.com/ on Connection{www.github.com:80, proxy=DIRECT hostAddress=www.github.com/192.30.253.113:80 cipherSuite=none protocol=http/1.1}
name: frank
host: www.github.com
Connection: Keep-Alive
Accept-Encoding: gzip
User-Agent: okhttp/3.4.1I/SeniorActivity: intercept: Received response for http://www.github.com/ in 1040.4ms
Content-length: 0
Location: https://www.github.com/
Connection: closeI/SeniorActivity: intercept: Sending request https://www.github.com/ on Connection{www.github.com:443, proxy=DIRECT hostAddress=www.github.com/192.30.253.113:443 cipherSuite=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 protocol=http/1.1}
name: frank
Host: www.github.com
Connection: Keep-Alive
Accept-Encoding: gzip
User-Agent: okhttp/3.4.1I/SeniorActivity: intercept: Received response for https://www.github.com/ in 703.6ms
Content-length: 0
Location: https://github.com/
Connection: closeI/SeniorActivity: intercept: Sending request https://github.com/ on Connection{github.com:443, proxy=DIRECT hostAddress=github.com/192.30.253.113:443 cipherSuite=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 protocol=http/1.1}
name: frank
Host: github.com
Connection: Keep-Alive
Accept-Encoding: gzip
User-Agent: okhttp/3.4.1I/SeniorActivity: intercept: Received response for https: Server: GitHub.com Date: Mon, 24 Oct 2016 06:17:32 GMT Content-Type: text/html; charset=utf-8 Transfer-Encoding: chunked Status: 200 OK Cache-Control: no-cache Vary: X-PJAX X-UA-Compatible: IE=Edge,chrome=1 Set-Cookie: logged_in=no; domain=.github.com; path=/; expires=Fri, 24 Oct 2036 06:17:32 -0000; secure; HttpOnly Set-Cookie: _gh_sess=eyJzZXNzaW9uX2lkIjoiNWJhMWZkYTUyYjFhMmVmZWM1OTc5ZTUzNGE0ZTQ4OTMiLCJfY3NyZl90b2tlbiI6IlZXNGVTTVhSRHhiOVlWL2E5anEwSHFxYXBOcFd3OHFvcHJtZjI5c05FOVk9In0%3D--64b03a16a3f1bf8b15208b101702a02a0419d9ec; path=/; secure; HttpOnly X-Request-Id: 9eaae01ca7d4e264de21da3d1a1c36e7 X-Runtime: 0.010049 Content-Security-Policy: default-src 'none'; base-uri 'self'; block-all-mixed-content; child-src render.githubusercontent.com; connect-src 'self' uploads.github.com status.github.com api.github.com www.google-analytics.com github-cloud.s3.amazonaws.com wss: Strict-Transport-Security: max-age=31536000; includeSubdomains; preload Public-Key-Pins: max-age=5184000; X-Content-Type-Options: nosniff X-Served-By: a22dbcbd09a98eacdd14ac7804a635dd Content-Encoding: gzip X-GitHub-Request-Id: 0E17AF3E:6C7B:67BF0A4:580DA77B
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可以看到我们原本是访问http://www.github.com，但是它内部重定向到https://www.github.com.
但是添加NetworkInterceptor追踪到了它的状态。并且NetworkInterceptror能够打印的信息更多。比如Content-Encoding:gzip。
Application Interceptor还是NetworkInterceptor?
它们各有优点。
Application interceptor特点
- 不必关心url的重定向和重连。
- 只执行一次，即使Resopnse是来自于缓存。
- 只关心request的原始意图，而不用关心额外添加的Header信息如If-None-Match
NetworkInterceptor的特点
- 能够详尽地追踪访问链接的重定向。
- 短时间内的网络访问，它将不执行缓存过来的回应。
- 监测整个网络访问过程中的数据流向。
实际开发中，大家可以根据自己的需求添加相应的Interceptor.
拦截器作用之压缩数据
因为拦截器可以拿到请求的数据，和回应的数据，所以基本上它能做任何事。比如我们可以在这里拦截一些不符合特定场景的请求。比如我们可以在回应中校验数据的完整性。比如为了节省带宽，我们可以将数据进行gzip压缩进行数据发送，然后在Response中解压，一切都神不知鬼不觉的。下面的例子来自官网，讲得是一个如何定义一个压缩数据功能的拦截器。
/** This interceptor compresses the HTTP request body. Many webservers can't handle this! */
final class GzipRequestInterceptor implements Interceptor {@Override public Response intercept(Interceptor.Chain chain) throws IOException {Request originalRequest = chain.request();if (originalRequest.body() == null || originalRequest.header("Content-Encoding") != null) {return chain.proceed(originalRequest);}Request compressedRequest = originalRequest.newBuilder().header("Content-Encoding", "gzip").method(originalRequest.method(), gzip(originalRequest.body())).build();return chain.proceed(compressedRequest);}private RequestBody gzip(final RequestBody body) {return new RequestBody() {@Override public MediaType contentType() {return body.contentType();}@Override public long contentLength() {return -1; }@Override public void writeTo(BufferedSink sink) throws IOException {BufferedSink gzipSink = Okio.buffer(new GzipSink(sink));body.writeTo(gzipSink);gzipSink.close();}};}
}
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接下来，我们要讲OKHttp开发中一个很重要和实用的功能，缓存。但因为篇幅有限，我新开一篇文章来讲解。
这篇关于OKHTTP学习之高级特性的文章就介绍到这儿，希望我们推荐的文章对编程师们有所帮助！
