Java21新特性

public class StringTest {public static void main(String[] args) {String sport="basketball";String msg=STR."I like \{sport}";System.out.println(msg);}
}

预览功能(java文件不要加package包信息)

$>javac --enable-preview -source 21 StringTest.java
注: StringTest.java 使用 Java SE 21 的预览功能。
注: 有关详细信息，请使用 -Xlint:preview 重新编译。
$>java --enable-preview  StringTest
I like basketball

上面使用的STR是java中定义的模版处理器，它可以将变量的值取出，完成字符串的拼接。在每个java源文件中都引入了一个public static final修饰的STR属性，STR通过打印STR可以知道它是java.lang.StringTemplate,是一个接口。

Processor<String, RuntimeException> STR = StringTemplate::interpolate;default String interpolate() {return StringTemplate.interpolate(fragments(), values());}static String interpolate(List<String> fragments, List<?> values) {Objects.requireNonNull(fragments, "fragments must not be null");Objects.requireNonNull(values, "values must not be null");int fragmentsSize = fragments.size();int valuesSize = values.size();if (fragmentsSize != valuesSize + 1) {throw new IllegalArgumentException("fragments must have one more element than values");}JavaTemplateAccess JTA = SharedSecrets.getJavaTemplateAccess();return JTA.interpolate(fragments, values);}public static JavaTemplateAccess getJavaTemplateAccess() {var access = javaTemplateAccess;if (access == null) {try {Class.forName("java.lang.runtime.TemplateSupport", true, null);access = javaTemplateAccess;} catch (ClassNotFoundException e) {}}return access;}final class TemplateSupport implements JavaTemplateAccess {
...@Overridepublic String interpolate(List<String> fragments, List<?> values) {int fragmentsSize = fragments.size();int valuesSize = values.size();if (fragmentsSize == 1) {return fragments.get(0);}int size = fragmentsSize + valuesSize;String[] strings = new String[size];int i = 0, j = 0;for (; j < valuesSize; j++) {strings[i++] = fragments.get(j);strings[i++] = String.valueOf(values.get(j));}strings[i] = fragments.get(j);return JLA.join("", "", "", strings, size);}

其他使用示例，在STR中可以进行基本的运算（支持三元运算）

int x=10,y=20;
String result=STR."\{x} + \{y} = \{x+y}";
System.out.println(result);//10 + 20 = 30

调用方法

String res=STR."获取一个随机数:\{Math.random()}";
System.out.println(res);

获取属性

String res1=STR."int的最大值是:\{Integer.MAX_VALUE}";
System.out.println(res1);

查看时间

String res2=STR."现在时间:\{new SimpleDateFormat("yyyy-MM-dd").format(new Date()) }";
System.out.println(res2);

计数操作

int index=0;
String result=STR."\{index++},\{index++},\{index++}";
System.out.println(result);

获取数组数据

String[] cars ={"bmw","ben","audi"};
String result = STR. "\{ cars[0] },\{ cars[1] },\{ cars[2] }" ;
System.out.println(result);

拼接多行数据

String[] cars ={"bmw","ben","audi"};
String result = STR. """\{cars[0] }\{ cars[1] }\{ cars[2] }""" ;
System.out.println(result);

自定义模版

    public static void main(String[] args) {var INTER = StringTemplate.Processor.of((StringTemplate st) -> {StringBuilder sb = new StringBuilder();Iterator<String> iterator = st.fragments().iterator();for (Object value : st.values()) {sb.append(iterator.next());sb.append(value);}sb.append(iterator.next());return sb.toString();});int x = 10, y = 20;String result = INTER. "\{ x } + \{ y } = \{ x + y }" ;System.out.println(result);}

2、scoped values

scoped values是一个隐藏的方法参数，只有方法可以访问scoped values，它可以让两个方法之间传递参数时无需声明形参。例如在UserDao类中编写savaUser方法，LogDao类中编写了saveLog方法，那么在保存用户的时候需要保证事务，此时就需要在service层获取Connection对象，然后将该对象分别传入到两个Dao的方法中，但对于savaUser方法来说并不是直接使用Connection对象，却又不得不在方法的形参中写上该对象，其实仅从业务上来看，该方法中只要传入User对象就可以了。

public class ScopeedValueTest {private static final ScopedValue<String> GIFT=ScopedValue.newInstance();public static void main(String[] args) {ScopeedValueTest scopeedValueTest=new ScopeedValueTest();scopeedValueTest.giveGift();}public void giveGift(){ScopedValue.where(GIFT,"手机").run(()->recieveGift());}public void recieveGift(){System.out.println(GIFT.get());}
}

多线程环境是否会出问题

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;public class ScopeedValueTest {private static final ScopedValue<String> GIFT = ScopedValue.newInstance();public static void main(String[] args) {ScopeedValueTest t = new ScopeedValueTest();ExecutorService pool = Executors.newCachedThreadPool();for (int i = 0; i < 10; i++) {pool.submit(() -> {t.giveGift();});}pool.shutdown();}public void giveGift() {ScopedValue.where(GIFT, Thread.currentThread().getName()).run(() -> recieveGift());}public void recieveGift() {System.out.println(GIFT.get());}
}

变更中间数值，和当前线程绑定关系

public class ScopeedValueTest {private static final ScopedValue<String> GIFT = ScopedValue.newInstance();public static void main(String[] args) {ScopeedValueTest t = new ScopeedValueTest();t.giveGift();}public void giveGift() {ScopedValue.where(GIFT, "500").run(() -> recieveMiddleMan());}//中间人public void recieveMiddleMan() {System.out.println(GIFT.get());//500ScopedValue.where(GIFT, "200").run(() -> recieveGift());//中间人抽成300System.out.println("recieveMiddleMan=" + GIFT.get());//500}public void recieveGift() {System.out.println(GIFT.get());//200}
}

3、record pattern

通过该特性可以解构record类型中的值，例如：

public class RecordTest {public static void main(String[] args) {Student s = new Student(1, "小米");print(s);}static void print(Object obj) {if (obj instanceof Student(int a, String b)) {System.out.println("a=" + a);System.out.println("b=" + b);}}
}record Student(int id, String name) {}

4、switch格式匹配

public class SwitchTest {public static void main(String[] args) {String str="hello";String result = getObjInstance(str);System.out.println(result);}public static String getObjInstance(Object obj) {return switch (obj) {case null -> "空对象";case Integer i -> "Integer对象" + i;case String s -> "String对象" + s;default -> obj.toString();};}
}

5、可以在switch中使用when

public class Switch02Test {public static void main(String[] args) {yesOrNo("yes");}public static void yesOrNo(String str) {switch (str) {case null -> {System.out.println("空对象");}case String swhen s.equalsIgnoreCase("yes") -> {System.out.println("确定");}case String swhen s.equalsIgnoreCase("no") -> {System.out.println("取消");}case String s -> {System.out.println("请输入yes或no");}}}
}

6、Unnamed Classes and Instance Main Methods

对于初学者来说，写第一个HelloWorld代码有太多的概念，为了方便初学者快速编写第一段java代码，这里提出了无名类和实例main方法，下面代码可以直接运行编译，相当于是少了类的定义，main方法的修饰符和形参也省略掉了。

void main() {System.out.println("Hello,World!");
}

7、Structured Concurrency

结构化并发：该特性主要作用是在使用虚拟线程时，可以使任务和子任务的代码编写起来可读性更强，维护性更高，更加可靠。

import java.util.concurrent.ExecutionException;
import java.util.concurrent.StructuredTaskScope;
import java.util.function.Supplier;public class Test {public static void main(String[] args) {Food f = new Test().handle();System.out.println(f);}Food handle() {try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {Supplier<String> fork1 = scope.fork(() -> "奶茶做好了");//任务1：线程1Supplier<String> fork2 = scope.fork(() -> "烤串烤好了");//任务2：线程2scope.join().throwIfFailed();//失败传播//当两个任务都成功后，最终才能吃饭return new Food(fork1.get(), fork2.get());} catch (ExecutionException e) {throw new RuntimeException(e);} catch (InterruptedException e) {throw new RuntimeException(e);}}
}record Food(String f1, String f2) {}