Java18的新特性

Java语言特性系列

本文主要讲述一下Java18的新特性

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

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

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。

• 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

• 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

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及-XX:G1RSetSparseRegionEntries

• 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。

Java18主要有如下几个特性