Java20的新特性

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    Java语言特性系列

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    本文主要讲述一下Java20的新特性

    版本号

    java -version
openjdk version "20" 2023-03-21
OpenJDK Runtime Environment (build 20+36-2344)
OpenJDK 64-Bit Server VM (build 20+36-2344, mixed mode, sharing)
    从version信息可以看出是build 20+36

    特性列表

    JEP 429: Scoped Values (Incubator)

    ScopedValue是一种类似ThreadLocal的线程内/父子线程传递变量的更优方案。ThreadLocal提供了一种无需在方法参数上传递通用变量的方法，InheritableThreadLocal使得子线程可以拷贝继承父线程的变量。但是ThreadLocal提供了set方法，变量是可变的，另外remove方法很容易被忽略，导致在线程池场景下很容易造成内存泄露。ScopedValue则提供了一种不可变、不拷贝的方案，即不提供set方法，子线程不需要拷贝就可以访问父线程的变量。具体使用如下：

    class Server {
  public final static ScopedValue<User> LOGGED_IN_USER = ScopedValue.newInstance();
 
  private void serve(Request request) {
    // ...
    User loggedInUser = authenticateUser(request);
    ScopedValue.where(LOGGED_IN_USER, loggedInUser)
               .run(() -> restAdapter.processRequest(request));
    // ...
  }
}
    通过ScopedValue.where可以绑定ScopedValue的值，然后在run方法里可以使用，方法执行完毕自行释放，可以被垃圾收集器回收

    JEP 432: Record Patterns (Second Preview)

    JDK19的JEP 405: Record Patterns (Preview)将Record的模式匹配作为第一次preview
    JDK20则作为第二次preview

    • 针对嵌套record的推断，可以这样

      record Point(int x, int y) {}
enum Color { RED, GREEN, BLUE }
record ColoredPoint(Point p, Color c) {}
record Rectangle(ColoredPoint upperLeft, ColoredPoint lowerRight) {}

static void printColorOfUpperLeftPoint(Rectangle r) {
  if (r instanceof Rectangle(ColoredPoint(Point p, Color c),
                             ColoredPoint lr)) {
      System.out.println(c);
  }
}

    • 整体而言，模式匹配有如下几种：

      Pattern:
TypePattern
ParenthesizedPattern
RecordPattern

TypePattern:
LocalVariableDeclaration

ParenthesizedPattern:
( Pattern )

RecordPattern:
ReferenceType RecordStructurePattern

RecordStructurePattern:
( [ RecordComponentPatternList ] )

RecordComponentPatternList : 
Pattern { , Pattern }

    • 针对泛型推断

      record Box<T>(T t) {}

static void test1(Box<String> bo) {
  if (bo instanceof Box<String>(var s)) {
      System.out.println("String " + s);
  }
}

      也支持嵌套

      static void test3(Box<Box<String>> bo) {
  if (bo instanceof Box<Box<String>>(Box(var s))) {    
      System.out.println("String " + s);
  }
}

    JEP 433: Pattern Matching for switch (Fourth Preview)

    在JDK14JEP 305: Pattern Matching for instanceof (Preview)作为preview
    在JDK15JEP 375: Pattern Matching for instanceof (Second Preview)作为第二轮的preview
    在JDK16JEP 394: Pattern Matching for instanceof转正
    JDK17引入JEP 406: Pattern Matching for switch (Preview)
    JDK18的JEP 420: Pattern Matching for switch (Second Preview)则作为第二轮preview
    JDK19的JEP 427: Pattern Matching for switch (Third Preview)作为第三轮preview
    JDK20作为第四轮preview
    自第三次预览以来的主要变化是：
    • 针对枚举类型出现无法匹配的时候抛出MatchException而不是IncompatibleClassChangeError
    • 开关标签的语法更简单
    • switch现在支持record泛型的推断

    以前针对null值switch会抛出异常，需要特殊处理

    static void testFooBar(String s) {
    if (s == null) {
        System.out.println("Oops!");
        return;
    }
    switch (s) {
        case "Foo", "Bar" -> System.out.println("Great");
        default           -> System.out.println("Ok");
    }
}

    现在可以直接switch

    static void testFooBar(String s) {
    switch (s) {
        case null         -> System.out.println("Oops");
        case "Foo", "Bar" -> System.out.println("Great");
        default           -> System.out.println("Ok");
    }
}

    case when的支持，以前这么写

    class Shape {}
class Rectangle extends Shape {}
class Triangle  extends Shape { int calculateArea() { ... } }

static void testTriangle(Shape s) {
    switch (s) {
        case null:
            break;
        case Triangle t:
            if (t.calculateArea() > 100) {
                System.out.println("Large triangle");
                break;
            }
        default:
            System.out.println("A shape, possibly a small triangle");
    }
}

    现在可以这么写

    static void testTriangle(Shape s) {
    switch (s) {
        case null -> 
            { break; }
        case Triangle t
        when t.calculateArea() > 100 ->
            System.out.println("Large triangle");
        case Triangle t ->
            System.out.println("Small triangle");
        default ->
            System.out.println("Non-triangle");
    }
}

    针对record泛型的类型推断：

    record MyPair<S,T>(S fst, T snd){};

static void recordInference(MyPair<String, Integer> pair){
    switch (pair) {
        case MyPair(var f, var s) -> 
            ... // Inferred record Pattern MyPair<String,Integer>(var f, var s)
        ...
    }
}

    JEP 434: Foreign Function & Memory API (Second Preview)

    Foreign Function & Memory (FFM) API包含了两个incubating API
    JDK14的JEP 370: Foreign-Memory Access API (Incubator)引入了Foreign-Memory Access API作为incubator
    JDK15的JEP 383: Foreign-Memory Access API (Second Incubator)Foreign-Memory Access API作为第二轮incubator
    JDK16的JEP 393: Foreign-Memory Access API (Third Incubator)作为第三轮，它引入了Foreign Linker API (JEP 389)
    FFM API在JDK 17的JEP 412: Foreign Function & Memory API (Incubator)作为incubator引入
    FFM API在JDK 18的JEP 419: Foreign Function & Memory API (Second Incubator)作为第二轮incubator
    JDK19的JEP 424: Foreign Function & Memory API (Preview)则将FFM API作为preview API
    JDK20作为第二轮preview

    使用示例

     javac --release 20 --enable-preview ... and java --enable-preview ....

// 1. Find foreign function on the C library path
Linker linker          = Linker.nativeLinker();
SymbolLookup stdlib    = linker.defaultLookup();
MethodHandle radixsort = linker.downcallHandle(stdlib.find("radixsort"), ...);
// 2. Allocate on-heap memory to store four strings
String[] javaStrings = { "mouse", "cat", "dog", "car" };
// 3. Use try-with-resources to manage the lifetime of off-heap memory
try (Arena offHeap = Arena.openConfined()) {
    // 4. Allocate a region of off-heap memory to store four pointers
    MemorySegment pointers = offHeap.allocateArray(ValueLayout.ADDRESS, javaStrings.length);
    // 5. Copy the strings from on-heap to off-heap
    for (int i = 0; i < javaStrings.length; i++) {
        MemorySegment cString = offHeap.allocateUtf8String(javaStrings[i]);
        pointers.setAtIndex(ValueLayout.ADDRESS, i, cString);
    }
    // 6. Sort the off-heap data by calling the foreign function
    radixsort.invoke(pointers, javaStrings.length, MemorySegment.NULL, '\0');
    // 7. Copy the (reordered) strings from off-heap to on-heap
    for (int i = 0; i < javaStrings.length; i++) {
        MemorySegment cString = pointers.getAtIndex(ValueLayout.ADDRESS, i);
        javaStrings[i] = cString.getUtf8String(0);
    }
} // 8. All off-heap memory is deallocated here
assert Arrays.equals(javaStrings, new String[] {"car", "cat", "dog", "mouse"});  // true

    JEP 436: Virtual Threads (Second Preview)

    在JDK19https://openjdk.org/jeps/425)作为第一次preview
    在JDK20作为第二次preview，此版本java.lang.ThreadGroup被永久废弃

    使用示例

    void handle(Request request, Response response) {
    var url1 = ...
    var url2 = ...
 
    try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
        var future1 = executor.submit(() -> fetchURL(url1));
        var future2 = executor.submit(() -> fetchURL(url2));
        response.send(future1.get() + future2.get());
    } catch (ExecutionException | InterruptedException e) {
        response.fail(e);
    }
}
 
String fetchURL(URL url) throws IOException {
    try (var in = url.openStream()) {
        return new String(in.readAllBytes(), StandardCharsets.UTF_8);
    }
}

    JEP 437: Structured Concurrency (Second Incubator)

    在JDK19JEP 428: Structured Concurrency (Incubator)作为第一次incubator
    在JDK20作为第二次incubator

    JEP 438: Vector API (Fifth Incubator)

    JDK16引入了JEP 338: Vector API (Incubator)提供了jdk.incubator.vector来用于矢量计算
    JDK17进行改进并作为第二轮的incubatorJEP 414: Vector API (Second Incubator)
    JDK18的JEP 417: Vector API (Third Incubator)进行改进并作为第三轮的incubator
    JDK19JEP 426:Vector API (Fourth Incubator)作为第四轮的incubator
    JDK20作为第五轮的incubator

    细项解读

    上面列出的是大方面的特性，除此之外还有一些api的更新及废弃，主要见JDK 20 Release Notes，这里举几个例子。

    添加项

    移除项

    • Thread.suspend/resume Changed to Throw UnsupportedOperationException JDK-8249627
    • Thread.Stop Changed to Throw UnsupportedOperationException JDK-8289610

    • Improved Control of G1 Concurrent Refinement Threads JDK-8137022

      以下这些参数未来版本移除
      -XX:-G1UseAdaptiveConcRefinement

-XX:G1ConcRefinementGreenZone=buffer-count

-XX:G1ConcRefinementYellowZone=buffer-count

-XX:G1ConcRefinementRedZone=buffer-count

-XX:G1ConcRefinementThresholdStep=buffer-count

-XX:G1ConcRefinementServiceIntervalMillis=msec

    废弃项

    完整列表见Java SE 20 deprecated-list

    • java.net.URL Constructors Are Deprecated JDK-8294241

      URL的构造器被废弃，可以使用URL::of(URI, URLStreamHandler)工厂方法替代

    已知问题

    • java.lang.Float.floatToFloat16 and java.lang.Float.float16ToFloat May Return Different NaN Results when Optimized by the JIT Compiler (JDK-8302976, JDK-8289551, JDK-8289552)

      JDK20引入了java.lang.Float.floatToFloat16及java.lang.Float.float16ToFloat方法，在JIT编译优化时可能会返回不同的Nan结果，可以使用如下参数禁用JIT对此的优化
      -XX:+UnlockDiagnosticVMOptions -XX:DisableIntrinsic=_floatToFloat16,_float16ToFloat

    其他事项

    • Disabled TLS_ECDH_* Cipher Suites (JDK-8279164)

      TLS_ECDH_* cipher suites默认被禁用了
    • HTTP Response Input Streams Will Throw an IOException on Interrupt (JDK-8294047)

    • HttpClient Default Keep Alive Time is 30 Seconds (JDK-8297030)

      http1.1及http2的空闲连接的超时时间从1200秒改为30秒

    小结

    Java20主要有如下几个特性

    doc

    javajava20jdk20
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