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OkHttp是一个高效的HTTP库:
支持 SPDY ，共享同一个 Socket 来处理同一个服务器的所有请求
如果 SPDY 不可用，则通过连接池来减少请求延时
无缝的支持GZIP来减少数据流量
缓存响应数据来减少重复的网络请求
OkHttp 处理了很多网络疑难杂症：会从很多常用的连接问题中自动恢复。如果您的服务器配置了多个IP地址，当第一个IP连接失败的时候，OkHttp会自动尝试下一个IP。OkHttp还处理了代理服务器问题和SSL握手失败问题。
OkHttp是一个相对成熟的解决方案，据说Android4.4的源码中可以看到HttpURLConnection已经替换成OkHttp实现了。所以我们更有理由相信OkHttp的强大。
1、HTTP请求方法
同步GET请求
private final OkHttpClient client = new OkHttpClient();
public void run() throws Exception {
Request request = new Request.Builder()
.url("http://publicobject.com/helloworld.txt")
.build();
Response response = client.newCall(request).execute();
if (!response.isSuccessful())
throw new IOException("Unexpected code " + response);
Headers responseHeaders = response.headers();
for (int i = 0; i < responseHeaders.size(); i++) {
System.out.println(responseHeaders.name(i) + ": " + responseHeaders.value(i));
}
System.out.println(response.body().string());
}Response类的string()方法会把文档的所有内容加载到内存，适用于小文档，对应大于1M的文档，应 使用流()的方式获取。
response.body().byteStream()
异步GET请求
private final OkHttpClient client = new OkHttpClient();
public void run() throws Exception {
Request request = new Request.Builder()
.url("http://publicobject.com/helloworld.txt")
.build();
client.newCall(request).enqueue(new Callback() {
@Override
public void onFailure(Request request, IOException e) {
e.printStackTrace();
}
@Override
public void onResponse(Response response) throws IOException {
if (!response.isSuccessful()) {
throw new IOException("Unexpected code " + response);
}
Headers responseHeaders = response.headers();
for (int i = 0; i < responseHeaders.size(); i++) {
System.out.println(responseHeaders.name(i) + ": " + responseHeaders.value(i));
}
System.out.println(response.body().string());
}
});
}读取响应会阻塞当前线程，所以发起请求是在主线程，回调的内容在非主线程中。
POST方式提交字符串
private static final MediaType MEDIA_TYPE_MARKDOWN
= MediaType.parse("text/x-markdown; charset=utf-8");
private final OkHttpClient client = new OkHttpClient();
public void run() throws Exception {
String postBody = ""
+ "Releases\n"
+ "--------\n"
+ "\n"
+ " * _1.0_ May 6, 2013\n"
+ " * _1.1_ June 15, 2013\n"
+ " * _1.2_ August 11, 2013\n";
Request request = new Request.Builder()
.url("https://api.github.com/markdown/raw")
.post(RequestBody.create(MEDIA_TYPE_MARKDOWN, postBody))
.build();
Response response = client.newCall(request).execute();
if (!response.isSuccessful())
throw new IOException("Unexpected code " + response);
System.out.println(response.body().string());
}因为整个请求体都在内存中，应避免提交1M以上的文件。
POST方式提交流
private final OkHttpClient client = new OkHttpClient();
public void run() throws Exception {
RequestBody requestBody = new RequestBody() {
@Override
public MediaType contentType() {
return MEDIA_TYPE_MARKDOWN;
}
@Override
public void writeTo(BufferedSink sink) throws IOException {
sink.writeUtf8("Numbers\n");
sink.writeUtf8("-------\n");
for (int i = 2; i <= 997; i++) {
sink.writeUtf8(String.format(" * %s = %s\n", i, factor(i)));
}
}
private String factor(int n) {
for (int i = 2; i < n; i++) {
int x = n / i;
if (x * i == n) return factor(x) + " × " + i;
}
return Integer.toString(n);
}
};
Request request = new Request.Builder()
.url("https://api.github.com/markdown/raw")
.post(requestBody)
.build();
Response response = client.newCall(request).execute();
if (!response.isSuccessful())
throw new IOException("Unexpected code " + response);
System.out.println(response.body().string());
}使用Okio框架以流的形式将内容写入，这种方式不会出现内存溢出问题。
POST方式提交文件
public static final MediaType MEDIA_TYPE_MARKDOWN
= MediaType.parse("text/x-markdown; charset=utf-8");
private final OkHttpClient client = new OkHttpClient();
public void run() throws Exception {
File file = new File("README.md");
Request request = new Request.Builder()
.url("https://api.github.com/markdown/raw")
.post(RequestBody.create(MEDIA_TYPE_MARKDOWN, file))
.build();
Response response = client.newCall(request).execute();
if (!response.isSuccessful())
throw new IOException("Unexpected code " + response);
System.out.println(response.body().string());
}POST方式提交表单
private final OkHttpClient client = new OkHttpClient();
public void run() throws Exception {
RequestBody formBody = new FormEncodingBuilder()
.add("search", "Jurassic Park")
.build();
Request request = new Request.Builder()
.url("https://en.wikipedia.org/w/index.php")
.post(formBody)
.build();
Response response = client.newCall(request).execute();
if (!response.isSuccessful())
throw new IOException("Unexpected code " + response);
System.out.println(response.body().string());
}表单的每个Names-Values都进行了URL编码。如果服务器端接口未进行URL编码，可定制个 FormBuilder。
文件上传（兼容html文件上传）
private static final String IMGUR_CLIENT_ID = "...";
private static final MediaType MEDIA_TYPE_PNG = MediaType.parse("p_w_picpath/png");
private final OkHttpClient client = new OkHttpClient();
public void run() throws Exception {
// Use the imgur p_w_picpath upload API as documented at https://cache.yisu.com/upload/information/20200311/46/200136.jpg")))
.build();
Request request = new Request.Builder()
.header("Authorization", "Client-ID " + IMGUR_CLIENT_ID)
.url("https://api.imgur.com/3/p_w_picpath")
.post(requestBody)
.build();
Response response = client.newCall(request).execute();
if (!response.isSuccessful())
throw new IOException("Unexpected code " + response);
System.out.println(response.body().string());
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