Java SE 11 新增特性

作者：Grey

原文地址：Java SE 11 新增特性

源码

源仓库: Github：java_new_features

镜像仓库: GitCode：java_new_features

Lambda 类型推断

JDK 10引入了局部变量类型推理（ JEP 286 ）。这简化了代码，因为你不再需要明确说明局部变量的类型，而是可以使用 var 。 JEP 323 将这种语法的使用扩展到Lambda表达式的参数。这里有一个简单的例子。

<pre class="prettyprint hljs swift" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">package git.snippets.jdk11;import java.util.HashMap;import java.util.Map;/** * lambda类型推断 * * @author <a href="mailto:410486047@qq.com">Grey</a> * @date 2021/11/29 * @since 11 */public class LambdaInfer {    public static void main(String[] args) {        Map<String, Integer> map = new HashMap<>(2);        map.put("hello", 1);        map.put("world", 2);        map.forEach((var s, var i) -> System.out.println(s + " " + i));    }}</pre>

原先需要 javac ,然后 java

<pre class="hljs nginx" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 0.75em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">javac Hello.java && java Hello</pre>

现在合成一个 java 命令搞定

<pre class="hljs nginx" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 0.75em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">java Hello.java</pre>

同样的

<pre class="hljs gradle" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 0.75em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">java -classpath /home/foo/java Hello.java Bonjour</pre>

等同于如下两条

<pre class="prettyprint hljs groovy" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">javac -classpath /home/foo/java Hello.javajava -classpath /home/foo/java Hello Bonjour</pre>

该JEP还提供“shebang”支持。为了减少在命令行上提及 Java 启动器的需要，包含在源文件的第一行。例如：

<pre class="hljs php" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 0.75em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;"># !/usr/bin/java --source 11   public class HelloWorld {   ...   }</pre>

注： --source 11 是必要的，指定了 JDK 的版本。

HTTP 客户端

JDK 9 引入了一个新的 API 来提供支持 HTTP 客户端协议，该API被作为孵化器模块包含在内。孵化器模块旨在提供新的 API，而不会使它们成为Java SE标准的一部分。开发人员可以试用 API 并提供反馈。

HTTP Client API 现在是Java SE 11标准的一部分。JDK 引入了一个新的模块和包: java.net.http 。该 API 可以同步或异步使用。

示例代码如下

<pre class="prettyprint hljs swift" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">package git.snippets.jdk11;import java.io.IOException;import java.net.URI;import java.net.http.HttpClient;import java.net.http.HttpRequest;import java.net.http.HttpResponse;/** * jdk9 中httpclient还在jdk.incubator.httpclient模块中 * * @author <a href="mailto:410486047@qq.com">Grey</a> * @date 2021/11/29 * @since 11 */public class HttpClientTest {    public static void main(String[] args) throws IOException, InterruptedException {        HttpClient client = HttpClient.newHttpClient();        URI uri = URI.create(";);        HttpRequest req = HttpRequest.newBuilder(uri).header("accept", "application/json").GET().build();        HttpResponse<String> resp = client.send(req, HttpResponse.BodyHandlers.ofString());        String body = resp.body();        System.out.println(body);    }}</pre>

执行后控制台输出

<pre class="prettyprint hljs json" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">{  "args": {},   "headers": {    "Accept": "application/json",     "Content-Length": "0",     "Host": "httpbin.org",     "Http2-Settings": "AAEAAEAAAAIAAAABAAMAAABkAAQBAAAAAAUAAEAA",     "Upgrade": "h2c",     "User-Agent": "Java-http-client/18.0.1",     "X-Amzn-Trace-Id": "Root=1-62fddb2a-7813af3246da12d203c31825"  },   "origin": "211.136.194.3",   "url": ";}</pre>

随着 JDK 9 中 JPMS 的引入，可以将整体式 rt.jar 文件划分为多个模块。JPMS 的另一个优点是，现在可以创建一个仅包含应用程序所需模块的 Java 运行时，从而大大减小了依赖包的大小。通过明确定义的模块边界，现在可以更轻松地删除 Java API 中过时的部分。

java.se.ee 元模块包括六个模块，这些模块将不再是 Java SE 11 标准的一部分，也不再包含在 JDK 中。受影响的模块包括：

corbatransactionactivationxml.bindxml.wsxml.ws.annotation

自 JDK 9 以来，这些模块已被弃用，并且默认情况下未包含在编译或运行时中。如果您尝试编译或运行一个应用程序，该应用程序在 JDK 9 或 JDK 10 上使用这些模块中的 API 的时候会报错。如果在代码中使用这些模块中的 API，则需要将它们作为单独的模块或库提供。

jdeprscan

JDK提供了 jdeprscan 来检查你的代码是否使用了deprecated的方法.

JDK 10 到 JDK 11 所有涉及调整的API清单

举几个例子， String 类中增加了很多有用的 API。

<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">package git.snippets.jdk11;/** * String 新增API * * @author <a href="mailto:410486047@qq.com">Grey</a> * @date 2021/11/29 * @see String * @since 11 */public class StringNewAPI {    public static void main(String[] args) {        isBlankTest();        linesTest();        repeatTest();        stripTest();    }    // 按行打印    private static void linesTest() {        String abcd = "\na\nb\tc\td";        abcd.lines().forEach(System.out::println);    }    // String.isBlank()    private static void isBlankTest() {        String blank = "   ";        System.out.println(blank.isBlank());    }    static void repeatTest() {        String repeat = "abcd";        // repeat(n) 表示复制几次        String repeat3 = repeat.repeat(3);        System.out.println(repeat3);    }    //trim 只能去除半角空格，而 strip 是去除各种空白符。    static void stripTest() {        String a = "   bbbd a  ";        String b = a.strip();        System.out.println(b);        System.out.println(a);        String strip = "    我是       ";        // 去前面空白字符，如全角空格，TAB        System.out.println("==" + strip.stripLeading() + "==");// 去后面空白字符，如全角空格，TAB        System.out.println("==" + strip.stripTrailing() + "==");    }}</pre>

File 中增加了一些 API，比如 Files.writeString 和 Files.readString

<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">package git.snippets.jdk11;import java.io.IOException;import java.nio.file.Files;import java.nio.file.Path;/** * File增强API * * @author <a href="mailto:410486047@qq.com">Grey</a> * @date 2021/11/29 * @since */public class FileNewAPI {    public static void main(String[] args) throws IOException {        testFile();    }    static void testFile() throws IOException {        // 创建临时文件        Path path = Files.writeString(Files.createTempFile("test", ".txt"), "hello world,你好，世界");        System.out.println(path);// 读取文件// String ss = Files.readString(Path.of("file.json"));        String s = Files.readString(path);        System.out.println(s);    }}</pre>

Optional 和 Predicate 相关 API 增强

<pre class="prettyprint hljs swift" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">package git.snippets.jdk11;import java.util.Arrays;import java.util.List;import java.util.Objects;import java.util.Optional;import java.util.function.Predicate;import java.util.stream.Collectors;/** * Optional用法 * * @author <a href="mailto:410486047@qq.com">Grey</a> * @date 2021/11/19 * @since 11 */public class OptionalAndPredicateTest {    public static void main(String[] args) {        Optional<String> helloOptional = Optional.of("Hello");        Optional<String> emptyOptional = Optional.empty();        helloOptional.ifPresent(s -> System.out.println(s.length()));        emptyOptional.ifPresent(s -> System.out.println(s.length()));        List<String> lines = Arrays.asList("a", "  ", "", null, "b");        List<String> ans = lines.stream().filter(Objects::nonNull).filter(Predicate.not(String::isBlank)).collect(Collectors.toList());        System.out.println(ans);    }}</pre>

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