Java9、10、11特性总结

一、Java9

1、接⼝私有⽅法

JDK8新增了静态⽅法和默认⽅法，但是不⽀持私有⽅法

JDK9中新增了私有⽅法

public interface OrderPay {
    void pay();

    default void defaultPay() {
        privateMethod();
    }

    //接⼝的私有⽅法可以在JDK9中使⽤
    private void privateMethod() {
        System.out.println("调⽤接⼝的私有⽅法");
    }
}

public class OrderPayImpl implements OrderPay {
    @Override
    public void pay() {
        System.out.println("OrderPayImpl实现了pay()方法");
    }
}

        OrderPayImpl orderPay = new OrderPayImpl();
        orderPay.defaultPay();
        orderPay.pay();

面试题：

• 接⼝中的静态⽅法不能被实现类继承和⼦接⼝继承，但是接⼝中的⾮静态的默认⽅法可以被实现类继承
• 类的静态⽅法可以被继承

2、增强try-with-resource

1）、在JDK7中，新增了try-with-resources语句，可以在try后的括号中初始化资源，可以实现资源⾃动关闭

        String path = "/Users/hanxiantao/Desktop/hello.txt";
        try (OutputStream out = new FileOutputStream(path);) {
            out.write("hello world".getBytes());
        } catch (IOException e) {
            e.printStackTrace();
        }

2）、在JDK9中，改进了try-with-resources语句，在try外进⾏初始化，在括号内引⽤，即可实现资源⾃动关闭，多个变量则⽤分号进⾏分割

        String path = "/Users/hanxiantao/Desktop/hello.txt";
        OutputStream out = new FileOutputStream(path);
        try (out) {
            out.write("hello world".getBytes());
        } catch (IOException e) {
            e.printStackTrace();
        }

3、快速创建只读集合

只读集合：集合只能读取，不能增加或者删除⾥⾯的元素

1）、JDK9之前创建只读集合

        List<String> list = new ArrayList<>();
        list.add("SpringBoot");
        list.add("SpringCloud");
        list.add("Redis");
        //设置为只读list
        list = Collections.unmodifiableList(list);
        System.out.println(list);
        list.add("Docker");

执行add操作时会抛出java.lang.UnsupportedOperationException的异常

其他方法：

unmodifiableSet()：设置为只读set

unmodifiableMap()：设置为只读map

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2）、JDK9创建只读集合

        List<String> list = List.of("SpringBoot", "SpringCloud", "Redis");
        System.out.println(list);
        Set<String> set = Set.of("SpringBoot", "SpringCloud", "Redis");
        System.out.println(set);
        Map<String, String> map = Map.of("key1", "SpringBoot", "key2", "SpringCloud");
        System.out.println(map);

4、新增的Stream API

takeWhile()方法：从Stream中获取⼀部分数据，返回从头开始的尽可能多的元素，直到遇到第⼀个false结果（不包含该元素），如果第⼀个值不满⾜断⾔条件，将返回⼀个空的Stream

        List<String> list = List.of("SpringBoot", "", "SpringCloud", "Redis").stream()
                .takeWhile(obj -> !obj.isEmpty()).collect(Collectors.toList());
        System.out.println(list);

执行结果：

[SpringBoot]

dropWhile()方法：与takeWhile()相反，返回第一个false结果和其之后的元素，和takeWhile()⽅法形成互补

        List<String> list = List.of("SpringBoot", "", "SpringCloud", "Redis").stream()
                .dropWhile(obj -> !obj.isEmpty()).collect(Collectors.toList());
        System.out.println(list);

执行结果：

[, SpringCloud, Redis]

二、Java10

1、局部变量类型推断var

JDK10可以使⽤var作为局部变量类型推断标识符

• 仅适⽤于局部变量，如增强for循环的索引，传统for循环局部变量
• 不能使⽤于⽅法形参、构造函数形参、⽅法返回类型或任何其他类型的变量声明
• 标识符var不是关键字，⽽是⼀个保留类型名称，⽽且不⽀持类或接⼝叫var，也不符合命名规范

        var strVar = "SpringBoot";
        System.out.println(strVar instanceof String);//true

        //repeat(int count)方法:⽤于字符串循环输出
        System.out.println(strVar.repeat(2));

        var flagVar = Boolean.valueOf(true);
        System.out.println(flagVar instanceof Boolean);//true

        for (var i = 0; i < 10; ++i) {
            System.out.println(i);
        }

三、Java11

1、新增Http客户端

1）、常见类和接口

HttpClient.Builder：HttpClient构建⼯具类

HttpRequest.Builder：HttpRequest构建⼯具类

HttpRequest.BodyPublisher：将Java对象转换为可发送的HTTP Request Body字节流，如form表单提交

HttpResponse.BodyHandler：处理接收到的Response Body

2）、创建HttpClient

        HttpClient httpClient = HttpClient.newBuilder().build();

或者：

        HttpClient httpClient = HttpClient.newHttpClient();

3）、创建Get请求

        URI uri = URI.create("http://localhost:8080/api/hello");
        //设置建立连接的超时时间5秒
        var httpClient = HttpClient.newBuilder().connectTimeout(Duration.ofMillis(5000)).build();

        //设置读取数据超时read timeout
        var httpRequest = HttpRequest.newBuilder().timeout(Duration.ofMillis(3000))
                .header("key1", "v1")
                .header("key2", "v2")
                .uri(uri).build();
        
        try {
            HttpResponse<String> response = httpClient.send(httpRequest, HttpResponse.BodyHandlers.ofString());
            System.out.println(response.body());
        } catch (Exception e) {
            e.printStackTrace();
        }

4）、提交Post请求

    @Test
    public void testPost() {
        URI uri = URI.create("http://localhost:8080/api/insertStudent");
        //设置建立连接的超时时间5秒
        var httpClient = HttpClient.newBuilder().connectTimeout(Duration.ofMillis(5000)).build();

        Student student = new Student("小明", 13);

        //使用的是fastjson
        //设置读取数据超时read timeout
        var httpRequest = HttpRequest.newBuilder().timeout(Duration.ofMillis(3000))
                .uri(uri)
                .header("Content-Type", "application/json")
                .POST(HttpRequest.BodyPublishers.ofString(JSONObject.toJSONString(student)))
                .build();

        try {
            HttpResponse<String> response = httpClient.send(httpRequest, HttpResponse.BodyHandlers.ofString());
            System.out.println(response.body());
            JSONObject jsonObject = JSONObject.parseObject(response.body());
            Student result = JSONObject.toJavaObject(jsonObject, Student.class);
            System.out.println(result);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

5）、提交异步请求

    @Test
    public void testAsync() {
        URI uri = URI.create("http://localhost:8080/api/insertStudent");
        //设置建立连接的超时时间5秒
        var httpClient = HttpClient.newBuilder().connectTimeout(Duration.ofMillis(5000)).build();

        Student student = new Student("小明", 13);

        //使用的是fastjson
        //设置读取数据超时read timeout
        var httpRequest = HttpRequest.newBuilder().timeout(Duration.ofMillis(3000))
                .uri(uri)
                .header("Content-Type", "application/json")
                .POST(HttpRequest.BodyPublishers.ofString(JSONObject.toJSONString(student)))
                .build();

        try {
            CompletableFuture<String> response = httpClient.sendAsync(httpRequest, HttpResponse.BodyHandlers.ofString())
                    .thenApply(HttpResponse::body);
            System.out.println(response.get());
            JSONObject jsonObject = JSONObject.parseObject(response.get());
            Student result = JSONObject.toJavaObject(jsonObject, Student.class);
            System.out.println(result);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

6）、提交HTTP2请求

        URI uri = URI.create("https://http2.akamai.com/demo");
        //设置建立连接的超时时间5秒
        var httpClient = HttpClient.newBuilder()
                .connectTimeout(Duration.ofMillis(5000))
                //指定HTTP2
                .version(HttpClient.Version.HTTP_2)
                .build();

        //设置读取数据超时read timeout
        var httpRequest = HttpRequest.newBuilder().timeout(Duration.ofMillis(3000))
                .uri(uri)
                .build();

        try {
            HttpResponse<String> response = httpClient.send(httpRequest, HttpResponse.BodyHandlers.ofString());
            System.out.println(response.body());
            System.out.println(response.version());
        } catch (Exception e) {
            e.printStackTrace();
        }

2、javac和java命令优化

JDK11前运⾏java程序：

编译：javac xxx.java 
运⾏：java xxx 

JDK11后运⾏java程序（本地不会⽣成class⽂件）：

java xxx.java