Java语言特性系列
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本文主要讲述一下Java19的新特性
版本号
java -version
openjdk version "19" 2022-09-20
OpenJDK Runtime Environment (build 19+36-2238)
OpenJDK 64-Bit Server VM (build 19+36-2238, mixed mode, sharing)从version信息可以看出是build 19+36
特性列表
JEP 405: Record Patterns (Preview)
instanceof的模式匹配在JDK14作为preview，在JDK15作为第二轮的preview，在JDK16JEP 394: Pattern Matching for instanceof转正 switch模式匹配在JDK17的JEP 406: Pattern Matching for switch (Preview)引入作为preview版本，在JDK18的JEP 420: Pattern Matching for switch (Second Preview)作为第二轮的preview
针对record类型，引入instance of可以这么写
record Point(int x, int y) {}
static void printSum(Object o) {
if (o instanceof Point p) {
int x = p.x();
int y = p.y();
System.out.println(x+y);
}
}现在可以这么写
record Point(int x, int y) {}
void printSum(Object o) {
if (o instanceof Point(int x, int y)) {
System.out.println(x+y);
}
}比较复杂的例子：
record Point(int x, int y) {}
enum Color { RED, GREEN, BLUE }
record ColoredPoint(Point p, Color c) {}
record Rectangle(ColoredPoint upperLeft, ColoredPoint lowerRight) {}
Rectangle r = new Rectangle(new ColoredPoint(new Point(x1, y1), c1),
new ColoredPoint(new Point(x2, y2), c2));
static void printXCoordOfUpperLeftPointWithPatterns(Rectangle r) {
if (r instanceof Rectangle(ColoredPoint(Point(var x, var y), var c),
var lr)) {
System.out.println("Upper-left corner: " + x);
}
}如果是泛型record的话：
record Box<T>(T t) {}
static void test1(Box<Object> bo) {
if (bo instanceof Box<Object>(String s)) {
System.out.println("String " + s);
}
}
static void test2(Box<Object> bo) {
if (bo instanceof Box<String>(var s)) {
System.out.println("String " + s);
}
}JEP 422: Linux/RISC-V Port
RISC-V是一个基于精简指令集（RISC）原则的开源指令集架构（ISA）,这个JEP则移植JDK到RISC-V上
JEP 424: Foreign Function & Memory API (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的这个JEP则将FFM API作为preview API
使用示例
// 1. Find foreign function on the C library path
Linker linker = Linker.nativeLinker();
SymbolLookup stdlib = linker.defaultLookup();
MethodHandle radixSort = linker.downcallHandle(
stdlib.lookup("radixsort"), ...);
// 2. Allocate on-heap memory to store four strings
String[] javaStrings = { "mouse", "cat", "dog", "car" };
// 3. Allocate off-heap memory to store four pointers
SegmentAllocator allocator = SegmentAllocator.implicitAllocator();
MemorySegment offHeap = allocator.allocateArray(ValueLayout.ADDRESS, javaStrings.length);
// 4. Copy the strings from on-heap to off-heap
for (int i = 0; i < javaStrings.length; i++) {
// Allocate a string off-heap, then store a pointer to it
MemorySegment cString = allocator.allocateUtf8String(javaStrings[i]);
offHeap.setAtIndex(ValueLayout.ADDRESS, i, cString);
}
// 5. Sort the off-heap data by calling the foreign function
radixSort.invoke(offHeap, javaStrings.length, MemoryAddress.NULL, '\0');
// 6. Copy the (reordered) strings from off-heap to on-heap
for (int i = 0; i < javaStrings.length; i++) {
MemoryAddress cStringPtr = offHeap.getAtIndex(ValueLayout.ADDRESS, i);
javaStrings[i] = cStringPtr.getUtf8String(0);
}
assert Arrays.equals(javaStrings, new String[] {"car", "cat", "dog", "mouse"}); // trueJEP 425: Virtual Threads (Preview)
虚拟线程应该是JDK19最重磅的特性，首次集成进来即为preview的API 虚拟线程是由JDK提供的用户态线程，类似golang的goroutine，erlang的processes 虚拟线程采用的是M:N的调度模式，即M数量的虚拟线程运行在N个Thread上，使用的是ForkJoinPool以FIFO模式来进行调度，N默认是为Runtime.availableProcessors()，可以通过 jdk.virtualThreadScheduler.parallelism来修改
使用示例
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
IntStream.range(0, 10_000).forEach(i -> {
executor.submit(() -> {
Thread.sleep(Duration.ofSeconds(1));
return i;
});
});
} // executor.close() is called implicitly, and waits如上使用了少数几个OS线程来运行10000个虚拟线程 虚拟线程在超过上千个非CPU密集并发任务场景可以显著提升系统的吞吐率
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);
}
}像这种场景虽然是block的代码，但是因为引入的是虚拟线程，系统可以很好地伸缩；当虚拟线程block在IO或者其他操作( BlockingQueue.take())时，虚拟线程会从Thread unmount，当操作完成才重新mount上继续执行。不过有些操作不会unmount虚拟线程，会一同thread和底层的OS线程一起block住(比如进入synchronized代码块/方法，比如执行一个native方法或者foreign function)。 虚拟线程开销不大，因而不需要使用池化技术 使用 jcmd<pid>Thread.dump_to_file-format=json<file>可以以json格式来dump虚拟线程，实例如下
Thread.Builder, Thread.ofVirtual(), Thread.ofPlatform() 可以用来创建虚拟线程或者是平台线程，比如
Thread thread = Thread.ofVirtual().name("duke").unstarted(runnable);Thread.startVirtualThread(Runnable)等同于创建和启动虚拟线程 Thread.threadId() 作为final方法会返回线程标识，而非final的Thread.getId()则被废弃 Thread.getAllStackTraces()现在返回的是平台线程而非所有线程
JEP 426:Vector API (Fourth 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，而JDK19则作为第四轮的incubator
JEP 427: Pattern Matching for switch (Third Preview)
instanceof的模式匹配在JDK14作为preview，在JDK15作为第二轮的preview，在JDK16转正 JDK17引入JEP 406: Pattern Matching for switch (Preview) JDK18的JEP 420: Pattern Matching for switch (Second Preview)则作为第二轮的preview，JDK19作为第三轮preview
JEP 428: Structured Concurrency (Incubator)
结构化并发也是JDK19的一个重要特性。JDK5引入的ExecutorService可以用于并行处理任务，比如
Response handle() throws ExecutionException, InterruptedException {
Future<String> user = esvc.submit(() -> findUser());
Future<Integer> order = esvc.submit(() -> fetchOrder());
String theUser = user.get(); // Join findUser
int theOrder = order.get(); // Join fetchOrder
return new Response(theUser, theOrder);
}但是当findUser抛出异常时，fetchOrder还是会在自己的线程继续运行，或者findUser需要运行很长时间，而当fetchOrder异常时，整个handle方法还是需要浪费时间等待findUser执行完，它阻塞在user.get()。为了更好地处理这种在异常场景下取消其他子任务，引入结构化并发来解决此问题，其主要是StructuredTaskScope这个类，它可以fork子任务，然后一起join或者一起cancel。
Response handle() throws ExecutionException, InterruptedException {
try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
Future<String> user = scope.fork(() -> findUser());
Future<Integer> order = scope.fork(() -> fetchOrder());
scope.join(); // Join both forks
scope.throwIfFailed(); // ... and propagate errors
// Here, both forks have succeeded, so compose their results
return new Response(user.resultNow(), order.resultNow());
}
}如果其中一个子任务失败了，则会取消另外一个在运行的任务。在scope.join()之后，可以使用resultNow()或者exceptionNow()来获取结果
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 19 Release Notes，这里举几个例子。
添加项
- System Properties for System.out and System.err (JDK-8283620)
新增stdout.encoding这两个系统属性stderr.encoding
- Support Unicode 14.0 (JDK-8268081)
新增支持unicode 14.0版本
- Additional Date-Time Formats (JDK-8176706)
以前java.time.format.DateTimeFormatter/DateTimeFormatterBuilder只支持FormatStyle.FULL/LONG/MEDIUM/SHORT这四种，现在可以自定义，比如 DateTimeFormatter.ofLocalizedPattern("yMMM")
- Automatic Generation of the CDS Archive (JDK-8261455)
新增 -XX:+AutoCreateSharedArchive参数可以自动创建CDS archive，比如 java-XX:+AutoCreateSharedArchive-XX:SharedArchiveFile=app.jsa-cp app.jarApp
移除项
- Remove Finalizer Implementation in SSLSocketImpl (JDK-8212136)
移除SSLSocket的finalizer实现
- Removal of Diagnostic Flag GCParallelVerificationEnabled (JDK-8286304)
移除GCParallelVerificationEnabled参数
废弃项
完整列表见Java SE 19 deprecated-list
- java.lang.ThreadGroup Is degraded (JDK-8284161)
ThreadGroup不能再被显示destroy了，它现在不再与其subgroup保持强引用
- Deprecation of Locale Class Constructors (JDK-8282819)
Locale的构造器被废弃了，可用Locale.of()这个工厂方法替代
已知问题
- ForkJoinPool and ThreadPoolExecutor do not use Thread::start to Start Worker Threads (JDK-8284161)
ForkJoinPool及ThreadPoolExecutor在这个版本不再使用`Thread::start来启动线程了，因而那些override无参start的工作线程可能会受影响，不过ForkJoinWorkerThread.onStart()不受影响
- InflaterInputStream.read Throws EOFException (JDK-8292327)
InflaterInputStream在这个版本可能抛出EOFException
- G1 Remembered set memory footprint regression after JDK-8286115 (JDK-8292654)
JDK-8286115这个变动了G1的RSet的ergonomic size，会造成本地内存使用增加，可以通过增加G1RemSetArrayOfCardsEntries值来缓解，比如
-XX:+UnlockExperimentalVMOptions -XX:G1RemSetArrayOfCardsEntries=128其他事项
- JNI GetVersion Returns JNIVERSION19 (JDK-8286176)
GetVersion这个jni方法返回JNIVERSION19
- Double.toString(double) and Float.toString(float) may Return Slightly Different Results (JDK-4511638)
Double.toString(2e23), 现在会返回"2.0E23", 而之前的版本会返回"1.9999999999999998E23"
- Make HttpURLConnection Default Keep Alive Timeout Configurable (JDK-8278067)
新增了http.keepAlive.time.server及http.keepAlive.time.proxy系统属性可以用于修改默认的Keep Alive Timeout
- JVM TI Changes to Support Virtual Threads (JDK-8284161)
The JVM Tool Interface (JVM TI)已经更新，现可支持虚拟线程
- -Xss may be Rounded up to a Multiple of the System Page Size (JDK-8236569)
实际的java线程堆栈大小可能与-Xss命令行选项指定的值不同；它可能四舍五入为系统所需的页面大小的倍数。
小结
Java18主要有如下几个特性
- JEP 405: Record Patterns (Preview)
- JEP 422: Linux/RISC-V Port
- JEP 424: Foreign Function & Memory API (Preview)
- JEP 425: Virtual Threads (Preview)
- JEP 426: Vector API (Fourth Incubator)
- JEP 427: Pattern Matching for switch (Third Preview)
- JEP 428: Structured Concurrency (Incubator)
doc
- JDK 19 Features
- JDK 19 Release Notes
- Consolidated JDK 19 Release Notes
- Java SE 19 deprecated-list
- The Arrival of Java 19
- JDK 19 G1/Parallel/Serial GC changes
- Java 19 Delivers Features for Projects Loom, Panama and Amber
- JDK 19 and JDK 20: What We Know So Far
