Java20的新特性

Java语言特性系列

序

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JDK19[https://openjdk.org/jeps/425](JEP 425: Virtual Threads (Preview))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

Support Unicode 15.0 Update Unicode Data Files to 15.0.0
Add GarbageCollectorMXBean for Remark and Cleanup Pause Time in G1 JDK-8297247

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

Java20的新特性

序

版本号

特性列表

JEP 429: Scoped Values (Incubator)

JEP 432: Record Patterns (Second Preview)

JEP 433: Pattern Matching for switch (Fourth Preview)

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

JEP 436: Virtual Threads (Second Preview)

JEP 437: Structured Concurrency (Second Incubator)

JEP 438: Vector API (Fifth Incubator)

细项解读

添加项

移除项

废弃项

已知问题

其他事项

小结

doc

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