Java18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

本文主要讲述一下Java18的新特性

版本号

java -version

openjdk version "18" 2022-03-22

OpenJDK Runtime Environment (build 18+36-2087)

OpenJDK 64-Bit Server VM (build 18+36-2087, mixed mode, sharing)

从version信息可以看出是build 18+36

特性列表

JEP 400: UTF-8 by Default

java18以前Charset.defaultCharset()是根据操作系统、user locale等来决定的,导致不同操作系统的默认charset是不一样,这次统一改为了UTF-8
java18要统一为UTF-8则需要-Dfile.encoding=UTF-8来设置
如果还想沿用以前的判断方式则可以通过-Dfile.encoding=COMPAT来设置

JEP 408: Simple Web Server

提供了一个类似python的SimpleHTTPServer(python -m SimpleHTTPServer [port])的开箱即用的HTTP文件服务器
可以通过jwebserver -p 9000启动
jwebserver -p 9000

Binding to loopback by default. For all interfaces use "-b 0.0.0.0" or "-b ::".

Serving /tmp and subdirectories on 127.0.0.1 port 9000

URL http://127.0.0.1:9000/

也可以在代码里定制并启动
jshell> var server = SimpleFileServer.createFileServer(new InetSocketAddress(8080),

...> Path.of("/some/path"), OutputLevel.VERBOSE);

jshell> server.start()

JEP 413: Code Snippets in Java API Documentation

以前要在通过javadoc展示代码可以使用@code如下
<pre>{@code

lines of source code

}</pre>

但是它的缺点就是得用pre包装,导致该片段不能包含html标签,而且缩进不太灵活

而这次给javaDoc引入了@snippet标签,无需对html标签再进行转义

/**

* The following code shows how to use {@code Optional.isPresent}:

* {@snippet :

* if (v.isPresent()) {

* System.out.println("v: " + v.get());

* }

* }

*/

也可以直接引用源代码,避免javadoc的代码与实际代码脱节
/**

* The following code shows how to use {@code Optional.isPresent}:

* {@snippet file="ShowOptional.java" region="example"}

*/

JEP 416: Reimplement Core Reflection with Method Handles

通过Method Handles重新实现java.lang.reflect.Method, Constructor及Field来替代字节码生成的Method::invoke, Constructor::newInstance, Field::get, and Field::set的实现
方便支持Project Valhalla,为以后减少扩展成本

JEP 417: Vector API (Third 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进行改进并作为第三轮的incubator

JEP 418: Internet-Address Resolution SPI

给解析网络地址提供了SPI,即java.net.spi包的InetAddressResolverProvider
方便给project loom做准备(目前InetAddress的API会阻塞在系统调用),也方便定制化及testing

JEP 419: Foreign Function & Memory API (Second Incubator)

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
JDK17引入JEP 412: Foreign Function & Memory API (Incubator)作为第一轮incubator
JDK18则作为第二轮的incubator

JEP 420: Pattern Matching for switch (Second 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
static String formatterPatternSwitch(Object o) {

return switch (o) {

case Integer i -> String.format("int %d", i);

case Long l -> String.format("long %d", l);

case Double d -> String.format("double %f", d);

case String s -> String.format("String %s", s);

default -> o.toString();

};

}

JDK17引入JEP 406: Pattern Matching for switch (Preview)
JDK18则作为第二轮的preview

JEP 421: Deprecate Finalization for Removal

废弃finalize方法方便后续移除

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 18 Release Notes,这里举几个例子。

添加项

  • SerialGC、ParallelGC、ZGC支持String Deduplication

    可使用-XX:+UseStringDeduplication开启
  • Map from an Element to its JavaFileObject

    新增Elements.getFileObjectOf(Element)来映射为JavaFileObject
  • Configurable Card Table Card SizeJDK-8272773

    可以使用-XX:GCCardSizeInBytes来设置card table大小
  • Allow G1 Heap Regions up to 512MBJDK-8275056

    允许G1的heap regions的最大值从之前的32MB到512MB
  • JDK Flight Recorder Event for FinalizationJDK-8266936

    新增jdk.FinalizerStatistics

移除项

  • Removal of Google's GlobalSign Root CertificateJDK-8225083

    移除了google的GlobalSign根证书
  • Removal of Empty finalize() Methods in java.desktop ModuleJDK-8273102

    移除java.desktop模块里头的空finalize()方法
  • Removal of impl.prefix JDK System Property Usage From InetAddressJDK-8274227

    移除impl.prefix属性,转而使用InetAddressResolver这个spi
  • Removal of Support for Pre JDK 1.4 DatagramSocketImpl ImplementationsJDK-8260428

    移除jdk1.4之前的DatagramSocketImpl
  • Removal of Legacy PlainSocketImpl and PlainDatagramSocketImpl ImplementationsJDK-8253119

    移除java.net.SocketImpl及java.net.DatagramSocketImpl的老实现PlainSocketImpl、PlainDatagramSocketImpl
    jdk.net.usePlainDatagramSocketImpl属性也一并移除

废弃项

完整列表见deprecated-list

  • Deprecated Subject::doAs for RemovalJDK-8267108

    废弃javax.security.auth.Subject::doAs为移除做准备
  • Deprecated sun.misc.Unsafe Methods That Return OffsetsJDK-8277863

    sun.misc.Unsafe中objectFieldOffset, staticFieldOffset, staticFieldBase方法被废弃
  • Terminally Deprecated Thread.stopJDK-8277861

    废弃Thread.stop为后续移除做准备
  • Obsoleted Product Options -XX:G1RSetRegionEntries and -XX:G1RSetSparseRegionEntriesJDK-8017163

    废弃-XX:G1RSetRegionEntries-XX:G1RSetSparseRegionEntries

已知问题

  • Extended Delay Before JDK Executable Installer Starts From Network DriveJDK-8274002

    在 Windows 11 和 Windows Server 2022 上,从映射的网络驱动器启动时,临时安装文件的提取可能会有些缓慢。安装程序仍然可以工作,但可能会有暂时的延迟。

小结

Java18主要有如下几个特性

  • JEP 400: UTF-8 by Default
  • JEP 408: Simple Web Server
  • JEP 413: Code Snippets in Java API Documentation
  • JEP 416: Reimplement Core Reflection with Method Handles
  • JEP 417: Vector API (Third Incubator)
  • JEP 418: Internet-Address Resolution SPI
  • JEP 419: Foreign Function & Memory API (Second Incubator)
  • JEP 420: Pattern Matching for switch (Second Preview)
  • JEP 421: Deprecate Finalization for Removal

doc

  • JDK 18 Features
  • JDK 18 Release Notes
  • Consolidated JDK 18 Release Notes
  • Java SE 18 deprecated-list
  • The Arrival of Java 18
  • JDK 18 G1/Parallel/Serial GC changes

以上是 Java18的新特性 的全部内容, 来源链接: utcz.com/z/267408.html

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