TIP
本文主要是介绍 Java 14语言新特性 。
- 版本号
- 特性列表
- 305:Pattern Matching for instanceof (Preview)
- 343:Packaging Tool (Incubator)
- 345:NUMA-Aware Memory Allocation for G1
- 349:JFR Event Streaming
- 352:Non-Volatile Mapped Byte Buffers
- 358:Helpful NullPointerExceptions
- 359:Records (Preview)
- 361:Switch Expressions (Standard)
- 362:Deprecate the Solaris and SPARC Ports
- 363:Remove the Concurrent Mark Sweep (CMS) Garbage Collector
- 364:ZGC on macOS
- 365:ZGC on Windows
- 366:Deprecate the ParallelScavenge + SerialOld GC Combination
- 367:Remove the Pack200 Tools and API
- 368:Text Blocks (Second Preview)
- 370:Foreign-Memory Access API (Incubator)
- 细项解读
- 参考文章
本文主要讲述一下Java14的新特性
# 版本号
java -version
openjdk version "14" 2020-03-17
OpenJDK 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) (opens new window)
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) (opens new window)
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 (opens new window)
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 (opens new window)
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 (opens new window)
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 (opens new window)
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) (opens new window)
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.java
javap -verbose Point.class
Compiled 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) (opens new window)
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 (opens new window)
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 (opens new window)
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 (opens new window)
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 (opens new window)
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 (opens new window)
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 (opens new window)
移除了Pack200 API
# 368:Text Blocks (Second Preview) (opens new window)
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) (opens new window)
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 (opens new window)，这里举几个例子。
# 添加项
- New Method to SAX ContentHandler for Handling XML Declaration (JDK-8230814 (opens new window))
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 (opens new window))
移除了sun.nio.cs.map属性
- Removed Deprecated java.security.acl APIs (JDK-8191138 (opens new window))
移除了java.security.acl
- Removal of the Default keytool -keyalg Value (JDK-8214024 (opens new window))
移除了默认的keytool -keyalg
# 废弃项
- Thread Suspend/Resume Are Deprecated for Removal (JDK-8231602 (opens new window))
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 (opens new window))
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 (opens new window))
Thread.countStackFrames开启转为无条件地抛出UnsupportedOperationException
# 参考文章
- https://segmentfault.com/a/1190000022064656
