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的新特性序

本文主要讲述一下Java14的新特性

版本号

java -versionopenjdk version "14" 2020-03-17OpenJDK Runtime Environment (build 14+36-1461)OpenJDK 64-Bit Server VM (build 14+36-1461, mixed mode, sharing)

从version信息可以看出是build 14+36

特性列表305:Pattern Matching for instanceof (Preview)

JDK14引入了preview版本的针对instanceof的模式匹配，其用法示例如下

    public boolean isBadRequestError(Exception ex) {        return (ex instanceof HttpClientErrorException rce) &&                HttpStatus.BAD_REQUEST == rce.getStatusCode();    }

无需在自己进行类型强转

343:Packaging Tool (Incubator)

JDK14引入了jdk.incubator.jpackage.jmod，它基于JavaFX javapackager tool构建，目的在于创建一个简单的打包工具，可以用于构建exe、pkg、dmg、deb、rpm格式的安装文件；非模块化的app的构建示例如下

jpackage --name myapp --input lib --main-jar main.jar

345:NUMA-Aware Memory Allocation for G1

实现了NUMA-aware的内存分配，以提升G1在大型机器上的性能

349:JFR Event Streaming

JDK11引入了JFR，使用的时候先dump到磁盘上然后再分析；而在JDK14则支持stream方式来进行持续性的监控，示例如下

public class AgentMain implements Runnable {​    public static void premain(String agentArgs, Instrumentation inst) {        try {            Logger.getLogger("AgentMain").log(                    Level.INFO, "Attaching JFR Monitor");            new Thread(new AgentMain()).start();        } catch (Throwable t) {            Logger.getLogger("AgentMain").log(                    Level.SEVERE,"Unable to attach JFR Monitor", t);        }    }​    @Override    public void run() {        var sender = new JfrStreamEventSender();        try (var rs = new RecordingStream()) {            rs.enable("jdk.CPULoad")                    .withPeriod(Duration.ofSeconds(1));            rs.enable("jdk.JavaMonitorEnter")                    .withThreshold(Duration.ofMillis(10));            rs.onEvent("jdk.CPULoad", sender);            rs.onEvent("jdk.JavaMonitorEnter", sender);            rs.start();        }    }}

352:Non-Volatile Mapped Byte Buffers

该特性新增了java.base/jdk/internal/misc/ExtendedMapMode.java以支持MappedByteBuffer访问non-volatile memory (NVM)

358:Helpful NullPointerExceptions

该特性可以更好地提示哪个地方出现的空指针，需要通过-XX:+ShowCodeDetailsInExceptionMessages开启，示例如下

public class NullPointerExample {​    public record City(String name){​    }​    public record Location(City city) {​    }​    public record User(String name, Location location) {​    }​    public static void main(String[] args){        User user = new User("hello", new Location(null));        System.out.println(user.location().city().name());    }}

输出如下

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "com.example.NullPointerExample$City.name()" because the return value of "com.example.NullPointerExample$Location.city()" is null    at com.example.NullPointerExample.main(NullPointerExample.java:25)

359:Records (Preview)

JDK14引入了preview版本的record类型，示例如下

public record Point(int x, int y) {​    public Point {        System.out.println(String.format("(%d,%d)", x, y));    }​    public Point(int value) {        this(value, value);    }​    public static Point of(int value) {        return new Point(value, value);    }}

javap反编译如下

javac --enable-preview -source 14 Point.javajavap -verbose Point.classCompiled from "Point.java"public final class com.example.domain.Point extends java.lang.Record {  public com.example.domain.Point(int, int);  public static com.example.domain.Point of(int);  public java.lang.String toString();  public final int hashCode();  public final boolean equals(java.lang.Object);  public int x();  public int y();}

可以看见Point继承了java.lang.Record，而且通过invokedynamic提供了final的hashCode及equals

361:Switch Expressions (Standard)

JDK12引入的switch在JDK14变为正式版本，正式版主要是用->来替代以前的:+break；另外就是提供了yield来在block中返回值，示例如下

    public void testSwitchWithArrowBlockAndYield() {        int n = 3;        String quantityString = switch (n) {            case 1 -> "one";            case 2 -> "two";            default -> {                System.out.println("default");                yield "many";            }        };        System.out.println(quantityString);    }

362:Deprecate the Solaris and SPARC Ports

废弃了 Solaris/SPARC, Solaris/x64, and Linux/SPARC ports，以在未来的版本中移除

363:Remove the Concurrent Mark Sweep (CMS) Garbage Collector

移除了CMS垃圾收集器，如果在JDK14中使用-XX:+UseConcMarkSweepGC的话，会出现warning，但是不会exit而是以默认的垃圾收集器运行，如下

OpenJDK 64-Bit Server VM warning: Ignoring option UseConcMarkSweepGC; support was removed in 14.0

364:ZGC on macOS

之前的ZGC只能在linux上使用，现在mac上也能使用ZGC了，示例如下

-XX:+UnlockExperimentalVMOptions -XX:+UseZGC

365:ZGC on Windows

之前的ZGC只能在linux上使用，现在windows(不能低于1803版本)上也能使用ZGC了，示例如下

-XX:+UnlockExperimentalVMOptions -XX:+UseZGC

366:Deprecate the ParallelScavenge + SerialOld GC Combination

废弃了parallel young generation GC与SerialOld GC的组合(-XX:+UseParallelGC与-XX:-UseParallelOldGC配合开启)，现在使用-XX:+UseParallelGC -XX:-UseParallelOldGC或者-XX:-UseParallelOldGC都会出现告警如下

OpenJDK 64-Bit Server VM warning: Option UseParallelOldGC was deprecated in version 14.0 and will likely be removed in a future release.

单独使用-XX:+UseParallelGC则开启parallel young and old generation GC algorithms

367:Remove the Pack200 Tools and API

移除了Pack200 API

368:Text Blocks (Second Preview)

JDK13引入的text blocks进行第二轮preview，JDK14的版本主要增加了两个escape sequences，分别是\<line-terminator>与\s escape sequence，示例如下

    @Test    public void testTextBlockWithTwoNewEscapeSequences() {        String inOneLine = """                Lorem ipsum dolor sit amet, consectetur adipiscing \                elit, sed do eiusmod tempor incididunt ut labore \                et dolore magna aliqua.\                """;        System.out.println(inOneLine);​        String singleSpace = """                red  \s                green\s                blue \s                """;        System.out.println(singleSpace);    }

370:Foreign-Memory Access API (Incubator)

提供了incubator版本的API用于操纵堆外内存，使用示例如下

    @Test    public void testForeignMemoryAccessAPI() {        SequenceLayout intArrayLayout                = MemoryLayout.ofSequence(25,                MemoryLayout.ofValueBits(32,                        ByteOrder.nativeOrder()));​        VarHandle intElemHandle                = intArrayLayout.varHandle(int.class,                MemoryLayout.PathElement.sequenceElement());​        try (MemorySegment segment = MemorySegment.allocateNative(intArrayLayout)) {            MemoryAddress base = segment.baseAddress();            for (int i = 0; i < intArrayLayout.elementCount().getAsLong(); i++) {                intElemHandle.set(base, (long) i, i);            }        }    }

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 14 Release Notes，这里举几个例子。

添加项New Method to SAX ContentHandler for Handling XML Declaration (JDK-8230814)

给SAX ContentHandler新增了方法，如下

    default void declaration(String version, String encoding, String standalone)        throws SAXException    {        //no op    }

移除项Removal of sun.nio.cs.map System Property (JDK-8229960)

移除了sun.nio.cs.map属性

Removed Deprecated java.security.acl APIs (JDK-8191138)

移除了java.security.acl

Removal of the Default keytool -keyalg Value (JDK-8214024)

移除了默认的keytool -keyalg

废弃项Thread Suspend/Resume Are Deprecated for Removal (JDK-8231602)

废弃了Thread的如下方法

Thread.suspend()Thread.resume()ThreadGroup.suspend()ThreadGroup.resume()ThreadGroup.allowThreadSuspension(boolean)

已知问题Text Visibility Issues in macOS Dark Mode (JDK-8228555)

已知在macOS上的Dark Mode有Text Visibility Issues

其他事项Thread.countStackFrames Changed to Unconditionally Throw UnsupportedOperationException (JDK-8205132)

Thread.countStackFrames开启转为无条件地抛出UnsupportedOperationException

小结

Java14主要有如下几个特性

305:Pattern Matching for instanceof (Preview)349:JFR Event Streaming352:Non-Volatile Mapped Byte Buffers358:Helpful NullPointerExceptions359:Records (Preview)361:Switch Expressions (Standard)363:Remove the Concurrent Mark Sweep (CMS) Garbage Collector364:ZGC on macOS366:Deprecate the ParallelScavenge + SerialOld GC Combination368:Text Blocks (Second Preview)370:Foreign-Memory Access API (Incubator)docJDK 14 FeaturesJDK 14 Release NotesOracle JDK 14 Release NotesJava SE deprecated-listThe Arrival of Java 14!Java Flight Recorder and JFR Event Streaming in Java 14