JDK17初体验

Posted on 2023-03-20 Symbols count in article: 5.1k Reading time ≈ 5 mins.

从刚工作时使用的JDK8到现在的JDK17，Java的版本一直在飞速迭代，每个版本都会引入一些新特性和优化改进，比如模块化系统、var类型推断、switch表达式增强、record类、instanceof增强、Sealed类/接口、新的GC等，下面就让我们来体验一下这些新特性吧。

==对于某些不太好测试的特性，这里就忽略了，如模块化系统、ZGC等==

var类型推断

代码

public static void main(String[] args) {
    var greeting = "Hello, JDK17";
    var num1 = 10;
    long num2 = 1000L;
    var sum = num1 + num2;
    var result = sum % 2 != 0;

    System.out.println("greeting: " + greeting);
    System.out.println("" + num1 + " + " + num2 + " = " + sum);
    System.out.println("result: " + result);
}

输出

1
2
3

greeting: HELLO, JDK17
10 + 1000 = 1010
result: false

class文件

可以看到，var只是一种语法糖，编译时会自动转成对应的类型

public static void main(String[] args) {
    String greeting = "Hello, JDK17";
        int num1 = 10;
        long num2 = 1000L;
        long sum = (long)num1 + num2;
        boolean result = sum % 2L != 0L;
        System.out.println("greeting: " + (String)greeting.transform(String::toUpperCase));
        System.out.println("" + num1 + " + " + num2 + " = " + sum);
        System.out.println("result: " + result);
}

Http请求客户端

代码

public static void main(String[] args) throws IOException, InterruptedException {
    HttpClient httpClient = HttpClient.newHttpClient();
    HttpRequest httpRequest = HttpRequest
        .newBuilder()
        .uri(URI.create("http://localhost:9090/user/get/202000"))
        .build();
    HttpResponse<String> httpResponse = httpClient
        .send(httpRequest, HttpResponse.BodyHandlers.ofString());
    System.out.println("response: " + httpResponse.body());
}

输出

1	response: {"data":{"id":1001,"userNo":"10010","nickname":"codecho","mobilePhone":"18755667788","createTime":"2023-03-08 12:04:20","updateTime":"2023-03-08 12:04:20"},"success":true}

switch表达式增强

代码

public static void main(String[] args) {
    int month = 4;
    System.out.println("month: " + month + ", season: " + getSeason(4));
}    

public static String getSeason(int month) {
    String season = switch (month) {
        case 3, 4, 5 -> "Spring";
        case 6, 7, 8 -> "Summer";
        case 9, 10, 11 -> "Autumn";
        case 12, 1, 2 -> {
            System.out.println("month: " + month);
            yield  "Winter";
        }
        default -> "illegal month!";
    };

    return season;
}

输出
1
month: 4, season: Spring

class文件

可以看到，switch增强也是语法糖，和普通的switch没什么区别

public static void main(String[] args) {
    int month = 4;
    System.out.println("month: " + month + ", season: " + getSeason(4));
}

public static String getSeason(int month) {
    String var10000;
    switch (month) {
        case 1:
        case 2:
        case 12:
            System.out.println("month: " + month);
            var10000 = "Winter";
            break;
        case 3:
        case 4:
        case 5:
            var10000 = "Spring";
            break;
        case 6:
        case 7:
        case 8:
            var10000 = "Summer";
            break;
        case 9:
        case 10:
        case 11:
            var10000 = "Autumn";
            break;
        default:
            var10000 = "illegal month!";
    }

    String season = var10000;
    return season;
}

文本块

代码

public static void main(String[] args) {

    var text = """
                {
                    "name": "codecho",
                    "age": 28,
                    "gender": "male"
                }
                """;
	System.out.println("text block: ");
    System.out.println(text);
}

输出

text block: 
{
    "name": "codecho",
    "age": 28,
    "gender": "male"
}

class文件

实际还是使用的转换符

public static void main(String[] args) {
    String text = "{\n    \"name\": \"codecho\",\n    \"age\": 28,\n    \"gender\": \"male\"\n}\n";
    System.out.println("text block: ");
    System.out.println(text);
}

record类

代码

public record Boy(String name, int age) {
    
}

public static void main(String[] args) {
    Boy boy = new Boy("Gary", 12);
    System.out.println("the boy's name is " + boy.name() + ", he is " + boy.age() + " years old");
}

输出

1	the boy's name is Gary, he is 12 years old

反编译class文件

实际继承了Record类（==注意：当前类是final的，不可继承==），编译器自动添加了各个属性方法，和equals、hashCode、toString方法

public abstract class Record {
    
    protected Record() {}

    @Override
    public abstract boolean equals(Object obj);
    
    @Override
    public abstract int hashCode();

    @Override
    public abstract String toString();
}

instanceof增强

增强前

public static void main(String[] args) {
    Object o = "instanceof";
    if (o instanceof String) {
        String str = (String) o;
        System.out.println("str: " + str);
    }
}

增强后

public static void main(String[] args) {
    Object o = "instanceof";
    if (o instanceof String str) {
        System.out.println("str: " + str);
    }
}

sealed密封类/接口

代码

permit：指定可以继承的类/接口
sealed`：密封类/接口
no-sealed：非密封类/接口
final：不可被继承类/接口
子类继承被sealed标记的超类后，同样要求指定密封性，使用sealed、no-sealed、final其中之一指定

// MotoVehicle只能由Bus、Car继承
public sealed class MotoVehicle permits Bus, Car {
    
}

// Bus不能再被继承
public final class Bus extends MotoVehicle {
    
}

// Car非密封，可以被继承
public non-sealed class Car extends MotoVehicle {
    
}

// Suv继承Car
public class Suv extends Car {
    
}

作用

使用密封类/接口可以更好地限制类/接口的继承范围，提高代码的安全性和可维护性。