关于java:Java20的新特性

Java语言个性系列

Java5的新个性
Java6的新个性
Java7的新个性
Java8的新个性
Java9的新个性
Java10的新个性
Java11的新个性
Java12的新个性
Java13的新个性
Java14的新个性
Java15的新个性
Java16的新个性
Java17的新个性
Java18的新个性
Java19的新个性
Java20的新个性
Java21的新个性

序

本文次要讲述一下Java20的新个性

版本号

java -versionopenjdk version "20" 2023-03-21OpenJDK Runtime Environment (build 20+36-2344)OpenJDK 64-Bit Server VM (build 20+36-2344, mixed mode, sharing)

从version信息能够看出是build 20+36

个性列表

JEP 429: Scoped Values (Incubator)

ScopedValue是一种相似ThreadLocal的线程内/父子线程传递变量的更优计划。ThreadLocal提供了一种无需在办法参数上传递通用变量的办法，InheritableThreadLocal使得子线程能够拷贝继承父线程的变量。然而ThreadLocal提供了set办法，变量是可变的，另外remove办法很容易被疏忽，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、不拷贝的计划，即不提供set办法，子线程不须要拷贝就能够拜访父线程的变量。具体应用如下：

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能够绑定ScopedValue的值，而后在run办法里能够应用，办法执行结束自行开释，能够被垃圾收集器回收

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，能够这样

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:TypePatternParenthesizedPatternRecordPatternTypePattern:LocalVariableDeclarationParenthesizedPattern:( Pattern )RecordPattern:ReferenceType RecordStructurePatternRecordStructurePattern:( [ 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);  }}

也反对嵌套

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次要变动是：

针对枚举类型呈现无奈匹配的时候抛出MatchException而不是IncompatibleClassChangeError
开关标签的语法更简略
switch当初反对record泛型的推断

以前针对null值switch会抛出异样，须要非凡解决

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");    }}

当初能够间接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的反对，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");    }}

当初能够这么写

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蕴含了两个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

应用示例

 javac --release 20 --enable-preview ... and java --enable-preview ....// 1. Find foreign function on the C library pathLinker linker          = Linker.nativeLinker();SymbolLookup stdlib    = linker.defaultLookup();MethodHandle radixsort = linker.downcallHandle(stdlib.find("radixsort"), ...);// 2. Allocate on-heap memory to store four stringsString[] javaStrings = { "mouse", "cat", "dog", "car" };// 3. Use try-with-resources to manage the lifetime of off-heap memorytry (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 hereassert 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被永恒废除

应用示例

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进行改良并作为第二轮的incubatorJEP 414: Vector API (Second Incubator)
JDK18的JEP 417: Vector API (Third Incubator)进行改良并作为第三轮的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

下面列出的是大方面的个性，除此之外还有一些api的更新及废除，次要见JDK 20 Release Notes，这里举几个例子。

增加项

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

以下这些参数将来版本移除

-XX:-G1UseAdaptiveConcRefinement-XX:G1ConcRefinementGreenZone=buffer-count-XX:G1ConcRefinementYellowZone=buffer-count-XX:G1ConcRefinementRedZone=buffer-count-XX:G1ConcRefinementThresholdStep=buffer-count-XX:G1ConcRefinementServiceIntervalMillis=msec

废除项

残缺列表见Java SE 20 deprecated-list

java.net.URL Constructors Are Deprecated JDK-8294241
URL的结构器被废除，能够应用URL::of(URI, URLStreamHandler)工厂办法代替

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办法，在JIT编译优化时可能会返回不同的Nan后果，能够应用如下参数禁用JIT对此的优化
```
-XX:+UnlockDiagnosticVMOptions -XX:DisableIntrinsic=_floatToFloat16,_float16ToFloat
```

其余事项

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的闲暇连贯的超时工夫从1200秒改为30秒

小结

Java20次要有如下几个个性

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