Java20的新特性

Java语言特性系列

• Java5的新特性
• Java6的新特性
• Java7的新特性
• Java8的新特性
• Java9的新特性
• Java10的新特性
• Java11的新特性
• Java12的新特性
• Java13的新特性
• Java14的新特性
• Java15的新特性
• Java16的新特性
• Java17的新特性
• Java18的新特性
• Java19的新特性
• Java20的新特性
• Java21的新特性

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 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) {}

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

也支持嵌套

static void test3(Box> bo) {
    if (bo instanceof Box>(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 fst, T snd){};

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

• 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)

doc

• JDK 20 Features
• JDK 20 Release Notes
• Consolidated JDK 20 Release Notes
• Java SE 20 deprecated-list
• The Arrival of Java 20
• JDK 20 G1/Parallel/Serial GC changes
• Java 20 Delivers Features for Projects Amber, Loom and Panama
• Java 20: a faster future is Looming
• JDK 20 Security Enhancements
• Project Loom’s Scoped Values: The Most Interesting New Java 20 Feature