使用 Java 11 HTTP Client API 实现 HTTP/2 服务器推送
对 HttpUrlConnection 你还有印象吗？JDK 11为 HttpUrlConnection 重新设计了 HTTP Client API。HTTP Client API 使用简单，支持 HTTP/2（默认）和 HTTP/1.1。为了向后兼容，当服务器不支持 HTTP/2时，HTTP Client API 会自动从 HTTP/2 降到 HTTP1.1。
此外，HTTP Client API 支持同步和异步编程模型，并依靠 stream 传输数据（reactive stream）。它还支持 WebSocket 协议，用于实时 Web 应用程序，降低客户端与服务器间通信开销。
除了多路复用（Multiplexing），HTTP/2 另一个强大的功能是 服务器推送 。传统方法（HTTP/1.1）中，主要通过浏览器发起请求 HTML 页面，解析接收的标记（Markup）并标识引用的资源（例如JS、CSS、图像等）。
为了获取资源，浏览器会继续发送资源请求（每个资源一个请求）。相反，HTTP/2 会发送 HTML 页面和引用的资源，不需要浏览器主动请求。因此，浏览器请求 HTML 页面后，就能收到页面以及显示所需的所有其他信息。HTTP Client API 通过 PushPromiseHandler 接口支持 HTTP/2 功能。
接口实现必须作为 send() 或 sendAsync() 方法的第三个参数填入。PushPromiseHandler 依赖下面三项协同：
-
客户端发起的 send request（initiatingRequest）
-
合成 push request（pushPromiseRequest）
-
acceptor 函数，必须成功调用该函数才能接受 push promise（acceptor）
调用特定 acceptor 函数接受 push promise。acceptor 函数必须传入一个 BodyHandler（不能为 null）用来处理 Promise 的 request body。acceptor 函数会返回一个 CompletableFuture 实例，完成 promise response。
基于以上信息，看一下 PushPromiseHandler 实现：
private
static final List<CompletableFuture<Void>>
asyncPushRequests =
new CopyOnWriteArrayList<>();
...
private
static HttpResponse.PushPromiseHandler<String> pushPromiseHandler() {
return (HttpRequest initiatingRequest,
HttpRequest pushPromiseRequest,
Function<HttpResponse.BodyHandler<String> ,
CompletableFuture<HttpResponse<String>>> acceptor) -> {
CompletableFuture<Void> pushcf =
acceptor.apply(HttpResponse.BodyHandlers.ofString())
.thenApply(HttpResponse::body)
.thenAccept((b) -> System.out.println(
"\nPushed resource body:\n " + b));
asyncPushRequests.add(pushcf);
System.out.println(
"\nJust got promise push number: " +
asyncPushRequests.size());
System.out.println(
"\nInitial push request: " +
initiatingRequest.uri());
System.out.println(
"Initial push headers: " +
initiatingRequest.headers());
System.out.println(
"Promise push request: " +
pushPromiseRequest.uri());
System.out.println(
"Promise push headers: " +
pushPromiseRequest.headers());
};
}现在，触发一个 request 把 PushPromiseHandler 传给 sendAsync()：
HttpClient client = HttpClient.newHttpClient();
HttpRequest request = HttpRequest.newBuilder()
.uri(URI.create(
"https://http2.golang.org/serverpush"))
.build();
client.sendAsync(request,
HttpResponse.BodyHandlers.ofString(), pushPromiseHandler())
.thenApply(HttpResponse::body)
.thenAccept((b) -> System.out.println(
"\nMain resource:\n" + b))
.join();
asyncPushRequests.forEach(CompletableFuture::join);
System.out.println(
"\nFetched a total of " +
asyncPushRequests.size() +
" push requests");完整源代码可在 GitHub 上找到。
github.com/PacktPublishing/Java-Coding-Problems/tree/master/Chapter13/P268_ServerPush
如果要把所有 push promise 及 response 汇总到指定的 map 中，可以使用 PushPromiseHandler.of() 方法，如下所示：
private
static
final ConcurrentMap<HttpRequest,
CompletableFuture<HttpResponse<String>>> promisesMap
=
new
ConcurrentHashMap<>();
private
static
final Function<HttpRequest,
HttpResponse.BodyHandler<String>> promiseHandler
= (HttpRequest req) -> HttpResponse.BodyHandlers.ofString();
public
static
void
main
(String[] args)
throws IOException, InterruptedException
{
HttpClient client = HttpClient.newHttpClient();
HttpRequest request = HttpRequest.newBuilder()
.uri(URI.create(
"https://http2.golang.org/serverpush"
))
.build();
client.sendAsync(request,
HttpResponse.BodyHandlers.ofString(), pushPromiseHandler())
.thenApply(HttpResponse::body)
.thenAccept((b) -> System.out.println(
"\nMain resource:\n"
+ b))
.join();
function(){ //外汇跟单www.gendan5.com
System.out.println(
"\nPush promises map size: "
+
promisesMap.size() +
"\n"
);
promisesMap.entrySet().forEach((entry) -> {
System.out.println(
"Request = "
+ entry.getKey() +
", \nResponse = "
+ entry.getValue().join().body());
});
}
private
static
HttpResponse.PushPromiseHandler<String> pushPromiseHandler() {
return
HttpResponse.PushPromiseHandler.of(promiseHandler, promisesMap);
}完整源代码可在 GitHub 上找到。
github.com/PacktPublishing/Java-Coding-Problems/tree/master/Chapter13/P268_ServerPushToMap
前面两个解决方案中 BodyHandler 都用到了 String 类型的 ofString()。如果服务器还需要推送二进制数据（比如图像），就不是很适用。因此，如果要处理二进制数据，则需要用 ofByteArray() 切换到byte[] 类型的 BodyHandler。也可以用 ofFile() 把 push 资源保存到磁盘，下面的解决方案是之前方案的改进版
private
static final ConcurrentMap<HttpRequest,
CompletableFuture<HttpResponse<Path>>>
promisesMap =
new ConcurrentHashMap<>();
private
static final Function<HttpRequest,
HttpResponse.BodyHandler<Path>> promiseHandler
= (HttpRequest req) -> HttpResponse.BodyHandlers.ofFile(
Paths.get(req.uri().getPath()).getFileName());
public
static
void
main
(String[] args)
throws IOException, InterruptedException {
HttpClient client = HttpClient.newHttpClient();
HttpRequest request = HttpRequest.newBuilder()
.uri(URI.create(
"https://http2.golang.org/serverpush"))
.build();
client.sendAsync(request, HttpResponse.BodyHandlers.ofFile(
Path.of(
"index.html")), pushPromiseHandler())
.thenApply(HttpResponse::body)
.thenAccept((b) -> System.out.println(
"\nMain resource:\n" + b))
.join();
System.out.println(
"\nPush promises map size: " +
promisesMap.size() +
"\n");
promisesMap.entrySet().forEach((entry) -> {
System.out.println(
"Request = " + entry.getKey() +
", \nResponse = " + entry.getValue().join().body());
});
}
private
static HttpResponse.PushPromiseHandler<Path> pushPromiseHandler() {
return HttpResponse.PushPromiseHandler.of(promiseHandler, promisesMap);
}上面的代码把 push 资源保存到应用程序 classpath 中， 完整源代码可在 GitHub 上找到。
github.com/PacktPublishing/Java-Coding-Problems/tree/master/Chapter13/P268_ServerPushToDisk
