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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[16]

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[17]

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[18]

以前要在通过javadoc展示代码可以使用@code如下

 {@code
    lines of source code
}  

 >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[19]

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)[20]

JDK16引入了 JEP 338: Vector API (Incubator)[21] 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incubator JEP 414: Vector API (Second Incubator)[22] JDK18进行改进并作为第三轮的incubator

JEP 418: Internet-Address Resolution SPI[23]

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)[24]

JDK14的 JEP 370: Foreign-Memory Access API (Incubator)[25] 引入了Foreign-Memory Access API作为incubator JDK15的 JEP 383: Foreign-Memory Access API (Second Incubator)[26] Foreign-Memory Access API作为第二轮incubator JDK16的 JEP 393: Foreign-Memory Access API (Third Incubator)[27] 作为第三轮，它引入了Foreign Linker API JDK17引入 JEP 412: Foreign Function & Memory API (Incubator)[28] 作为第一轮incubator JDK18则作为第二轮的incubator

JEP 420: Pattern Matching for switch (Second Preview)[29]

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)[30] JDK18则作为第二轮的preview

JEP 421: Deprecate Finalization for Removal[31]

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[32] ，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 Size JDK-8272773[33]

可以使用 -XX:GCCardSizeInBytes 来设置card table大小

•

Allow G1 Heap Regions up to 512MB JDK-8275056[34]

允许G1的heap regions的最大值从之前的32MB到512MB

•

JDK Flight Recorder Event for Finalization JDK-8266936[35]

新增jdk.FinalizerStatistics

移除项

•

Removal of Google’s GlobalSign Root Certificate JDK-8225083[36]

移除了google的GlobalSign根证书

•

Removal of Empty finalize() Methods in java.desktop Module JDK-8273102[37]

移除java.desktop模块里头的空finalize()方法

•

Removal of impl.prefix JDK System Property Usage From InetAddress JDK-8274227[38]

移除impl.prefix属性，转而使用InetAddressResolver这个spi

•

Removal of Support for Pre JDK 1.4 DatagramSocketImpl Implementations JDK-8260428[39]

移除jdk1.4之前的DatagramSocketImpl

•

Removal of Legacy PlainSocketImpl and PlainDatagramSocketImpl Implementations JDK-8253119[40]

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[41]

•

Deprecated Subject::doAs for Removal JDK-8267108[42]

废弃 javax.security.auth.Subject::doAs 为移除做准备

•

Deprecated sun.misc.Unsafe Methods That Return Offsets JDK-8277863[43]

sun.misc.Unsafe中objectFieldOffset, staticFieldOffset, staticFieldBase方法被废弃

•

Terminally Deprecated Thread.stop JDK-8277861[44]

废弃Thread.stop为后续移除做准备

•

Obsoleted Product Options -XX:G1RSetRegionEntries and -XX:G1RSetSparseRegionEntries JDK-8017163[45]

废弃 -XX:G1RSetRegionEntries 及 -XX:G1RSetSparseRegionEntries

已知问题

• Extended Delay Before JDK Executable Installer Starts From Network Drive JDK-8274002[46]

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[47]
• JEP 408: Simple Web Server[48]
• JEP 413: Code Snippets in Java API Documentation[49]
• JEP 416: Reimplement Core Reflection with Method Handles[50]
• JEP 417: Vector API (Third Incubator)[51]
• JEP 418: Internet-Address Resolution SPI[52]
• JEP 419: Foreign Function & Memory API (Second Incubator)[53]
• JEP 420: Pattern Matching for switch (Second Preview)[54]
• JEP 421: Deprecate Finalization for Removal[55]

doc

• JDK 18 Features[56]
• JDK 18 Release Notes[57]
• Consolidated JDK 18 Release Notes[58]
• Java SE 18 deprecated-list[59]
• The Arrival of Java 18[60]
• JDK 18 G1/Parallel/Serial GC changes[61]