對 HttpUrlConnection 你還有印象嗎？JDK 11為 HttpUrlConnection 重新設計了 HTTP Client API。HTTP Client API 使用簡單，支持 HTTP/2（默認）和 HTTP/1.1。為了向后兼容，當服務器不支持 HTTP/2時，HTTP Client API 會自動從 HTTP/2 降到 HTTP1.1。
此外，HTTP Client API 支持同步和異步編程模型，並依靠 stream 傳輸數據（reactive stream）。它還支持 WebSocket 協議，用於實時 Web 應用程序，降低客戶端與服務器間通信開銷。
除了多路復用（Multiplexing），HTTP/2 另一個強大的功能是 服務器推送 。傳統方法（HTTP/1.1）中，主要通過瀏覽器發起請求 HTML 頁面，解析接收的標記（Markup）並標識引用的資源（例如JS、CSS、圖像等）。
為了獲取資源，瀏覽器會繼續發送資源請求（每個資源一個請求）。相反，HTTP/2 會發送 HTML 頁面和引用的資源，不需要瀏覽器主動請求。因此，瀏覽器請求 HTML 頁面后，就能收到頁面以及顯示所需的所有其他信息。HTTP Client API 通過 PushPromiseHandler 接口支持 HTTP/2 功能。
接口實現必須作為 send() 或 sendAsync() 方法的第三個參數填入。PushPromiseHandler 依賴下面三項協同：
-
客戶端發起的 send request（initiatingRequest）
-
合成 push request（pushPromiseRequest）
-
acceptor 函數，必須成功調用該函數才能接受 push promise（acceptor）
調用特定 acceptor 函數接受 push promise。acceptor 函數必須傳入一個 BodyHandler（不能為 null）用來處理 Promise 的 request body。acceptor 函數會返回一個 CompletableFuture 實例，完成 promise response。
基於以上信息，看一下 PushPromiseHandler 實現：
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());
};
}現在，觸發一個 request 把 PushPromiseHandler 傳給 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");完整源代碼可在 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 匯總到指定的 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(){ //外匯跟單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);
}完整源代碼可在 GitHub 上找到。
github.com/PacktPublishing/Java-Coding-Problems/tree/master/Chapter13/P268_ServerPushToMap
前面兩個解決方案中 BodyHandler 都用到了 String 類型的 ofString()。如果服務器還需要推送二進制數據（比如圖像），就不是很適用。因此，如果要處理二進制數據，則需要用 ofByteArray() 切換到byte[] 類型的 BodyHandler。也可以用 ofFile() 把 push 資源保存到磁盤，下面的解決方案是之前方案的改進版
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);
}上面的代碼把 push 資源保存到應用程序 classpath 中， 完整源代碼可在 GitHub 上找到。
github.com/PacktPublishing/Java-Coding-Problems/tree/master/Chapter13/P268_ServerPushToDisk