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<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: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 twhen 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 pointersMemorySegment pointers = offHeap.allocateArray(ValueLayout.ADDRESS, javaStrings.length);// 5. Copy the strings from on-heap to off-heapfor (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 functionradixsort.invoke(pointers, javaStrings.length, MemorySegment.NULL, '\\0');// 7. Copy the (reordered) strings from off-heap to on-heapfor (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