Java SE 20 新增特性
原文地址：
CSDN：Java SE 20 新增特性
源码
镜像仓库: GitCode：java_new_features
Switch类型匹配(第四次预览
注：执行如下代码需要基于 Java SE 17 +，且增加--enable-preview参数。
package git.snippets.jdk20;
/**
* switch类型匹配(二次预览
*
* @author <a href="mailto:410486047@qq.com">Grey</a>
* @date 2023/05/03
* @since 20
*/
public class SwitchMatchTest {
public static void main(String[] args {
switchMatch(3;
switchMatch("HELLO";
switchMatch("hello world";
switchMatch(null;
}
static void switchMatch(Object obj {
switch (obj {
case String s when s.length( > 5 -> System.out.println(s.toUpperCase(;
case String s -> System.out.println(s.toLowerCase(;
case Integer i -> System.out.println(i * i;
case null -> System.out.println("null obj";
default -> {
}
}
}
}
范围值（Scoped Value，孵化阶段）
JEP 429 在 Java SE 20 的孵化阶段引入了范围值（ScopedValue）, 范围值可以与虚拟线程很好地结合。它允许在有限的时间内存储一个值，而且只有写入该值的线程可以读取它。类似ThreadLocal对于线程的作用。详见：SCOPED VALUES IN JAVA
record 的匹配增强（第二次预览）
package git.snippets.jdk20;
/**
* record 模式匹配增强（二次预览）
* 需要增加 --enable-preview参数
*
* @author <a href="mailto:410486047@qq.com">Grey</a>
* @date 2022/9/22
* @since 19
*/
public class RecordTest {
public static void main(String[] args {
Points points = new Points(1, 2;
Line line = new Line(new Points(1, 2, new Points(3, 4;
printPoints(points;
printLine(line;
}
private static void printPoints(Object object {
if (object instanceof Points(int x, int y {
System.out.println("jdk 19 object is a position, x = " + x + ", y = " + y;
}
if (object instanceof Points points {
System.out.println("pre jdk 19 object is a position, x = " + points.x(
+ ", y = " + points.y(;
}
switch (object {
case Points position -> System.out.println("pre jdk 19 object is a position, x = " + position.x(
+ ", y = " + position.y(;
default -> throw new IllegalStateException("Unexpected value: " + object;
}
switch (object {
case Points(int x, int y -> System.out.println(" jdk 19 object is a position, x = " + x
+ ", y = " + y;
default -> throw new IllegalStateException("Unexpected value: " + object;
}
}
public static void printLine(Object object {
if (object instanceof Line(Points(int x1, int y1, Points(int x2, int y2 {
System.out.println("object is a path, x1 = " + x1 + ", y1 = " + y1
+ ", x2 = " + x2 + ", y2 = " + y2;
}
switch (object {
case Line(Points(int x1, int y1, Points(int x2, int y2 ->
System.out.println("object is a path, x1 = " + x1 + ", y1 = " + y1
+ ", x2 = " + x2 + ", y2 = " + y2;
// other cases ...
default -> throw new IllegalStateException("Unexpected value: " + object;
}
}
}
record Points(int x, int y {
}
record Line(Points from, Points to {
}
此外，在 JEP 432 中，Java SE 20 的 record 支持类型推断，例如，定义了如下数据结构
interface Multi<T> {}
record Tuple<T>(T t1, T t2 implements Multi<T> {}
record Triple<T>(T t1, T t2, T t3 implements Multi<T> {}
在 Java SE 20 之前，需要这样做
// 需要指定类型
static void preJDK20(Multi<String> multi {
if (multi instanceof Tuple<String>(var s1, var s2 {
System.out.println("Tuple: " + s1 + ", " + s2;
} else if (multi instanceof Triple<String>(var s1, var s2, var s3 {
System.out.println("Triple: " + s1 + ", " + s2 + ", " + s3;
}
}
需要指定类型，例如：本实例需要指定 String 类型。
static void JDK20(Multi<String> multi {
if (multi instanceof Tuple(var s1, var s2 {
System.out.println("Tuple: " + s1 + ", " + s2;
} else if (multi instanceof Triple(var s1, var s2, var s3 {
System.out.println("Triple: " + s1 + ", " + s2 + ", " + s3;
}
}
在循环中也可以支持类似的用法，示例如下:
record Position(int x, int y {
}
static void preJDK20Loop(List<Position> positions {
for (Position p : positions {
System.out.printf("(%d, %d%n", p.x(, p.y(;
}
}
在 Java SE 20 版本中，可直接写成如下形式
static void JDK20Loop(List<Position> positions {
for (Position(int x, int y : positions {
System.out.printf("(%d, %d%n", x, y;
}
}
此外，在 Java SE 20 中，移除了对 record 命名模式的支持，在 Java SE 19 中，如下写法是对的
if (object instanceof Points(int x, int y points {
System.out.println("pre jdk 19 object is a position, x = " + points.x(
+ ", y = " + points.y(;
}
但是到了 Java SE 20，已经将上述写法废弃，Java SE 20 只支持如下两种写法
if (object instanceof Points(int x, int y {
System.out.println("jdk 19 object is a position, x = " + x + ", y = " + y;
}
if (object instanceof Points points {
System.out.println("pre jdk 19 object is a position, x = " + points.x(
+ ", y = " + points.y(;
}
废弃 java.net.URL 的构造方法
java.net.URL的构造函数已被标记为"废弃"。应该使用URI.create(..和URI.toURL(方法。下面是一个例子：package git.snippets.jdk20; import java.net.URI; import java.net.URL; /** * URL的构造方法被彻底废弃 * * @author <a href="mailto:410486047@qq.com">Grey</a> * @date 2023/05/03 * @since 20 */ public class URLConstructorTest { public static void main(String[] args throws Exception { // 以下构造方法在 Java SE 20 被彻底废弃 // URL url = new URL("xxx"; // Java SE 20 用如下方法构造 URL URL url = URI.create("xxx".toURL(; } }更多
Java SE 7及以后各版本新增特性，持续更新中..。
参考资料
JDK 20 Release Notes
JAVA 20 FEATURES(WITH EXAMPLES