Java18的新特性

朱雀 2023-09-23 18:18 18阅读 0赞

### 序 ###

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)
    复制代码

> 从version信息可以看出是build 18+36

### 特性列表 ###

#### [JEP 400: UTF-8 by Default][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][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/
    复制代码

> 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()
    复制代码

#### [JEP 413: Code Snippets in Java API Documentation][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>
    复制代码

> 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());
     * }
     * }
     */
    复制代码

> 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"}
     */
    复制代码

#### [JEP 416: Reimplement Core Reflection with Method Handles][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)][JEP 417_ Vector API _Third Incubator] ####

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

#### [JEP 418: Internet-Address Resolution SPI][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)][JEP 419_ Foreign Function _ Memory API _Second Incubator] ####

> JDK14的[JEP 370: Foreign-Memory Access API (Incubator)][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)][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)][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)][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)][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();
        };
    }
    复制代码

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

#### [JEP 421: Deprecate Finalization for Removal][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 Size[JDK-8272773][]

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

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

> 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][]

> 新增jdk.FinalizerStatistics

#### 移除项 ####

*  Removal of Google's GlobalSign Root Certificate[JDK-8225083][]

> 移除了google的GlobalSign根证书

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

> 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][]

> 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][]

> 移除jdk1.4之前的DatagramSocketImpl

*  Removal of Legacy PlainSocketImpl and PlainDatagramSocketImpl Implementations[JDK-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 Removal[JDK-8267108][]

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

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

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

*  Terminally Deprecated Thread.stop[JDK-8277861][]

> 废弃Thread.stop为后续移除做准备

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

> 废弃`-XX:G1RSetRegionEntries`及`-XX:G1RSetSparseRegionEntries`

#### 已知问题 ####

*  Extended Delay Before JDK Executable Installer Starts From Network Drive[JDK-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。

###  ###

[JEP 400_ UTF-8 by Default]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F400
[JEP 408_ Simple Web Server]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F408
[JEP 413_ Code Snippets in Java API Documentation]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F413
[JEP 416_ Reimplement Core Reflection with Method Handles]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F416
[JEP 417_ Vector API _Third Incubator]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F417
[JEP 338_ Vector API _Incubator]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F338
[JEP 414_ Vector API _Second Incubator]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F414
[JEP 418_ Internet-Address Resolution SPI]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F418
[JEP 419_ Foreign Function _ Memory API _Second Incubator]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F419
[JEP 370_ Foreign-Memory Access API _Incubator]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F370
[JEP 383_ Foreign-Memory Access API _Second Incubator]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F383
[JEP 393_ Foreign-Memory Access API _Third Incubator]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F393
[JEP 412_ Foreign Function _ Memory API _Incubator]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F412
[JEP 420_ Pattern Matching for switch _Second Preview]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F420
[JEP 406_ Pattern Matching for switch _Preview]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F406
[JEP 421_ Deprecate Finalization for Removal]: https://link.juejin.cn?target=https%3A%2F%2Fopenjdk.java.net%2Fjeps%2F421
[JDK 18 Release Notes]: https://link.juejin.cn?target=https%3A%2F%2Fjdk.java.net%2F18%2Frelease-notes
[JDK-8272773]: https://link.juejin.cn?target=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8272773
[JDK-8275056]: https://link.juejin.cn?target=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8275056
[JDK-8266936]: https://link.juejin.cn?target=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8266936
[JDK-8225083]: https://link.juejin.cn?target=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8225083
[JDK-8273102]: https://link.juejin.cn?target=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8273102
[JDK-8274227]: https://link.juejin.cn?target=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8274227
[JDK-8260428]: https://link.juejin.cn?target=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8260428
[JDK-8253119]: https://link.juejin.cn?target=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8253119
[deprecated-list]: https://link.juejin.cn?target=https%3A%2F%2Fcr.openjdk.java.net%2F~iris%2Fse%2F18%2FlatestSpec%2Fapi%2Fdeprecated-list.html
[JDK-8267108]: https://link.juejin.cn?target=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8267108
[JDK-8277863]: https://link.juejin.cn?target=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8277863
[JDK-8277861]: https://link.juejin.cn?target=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8277861
[JDK-8017163]: https://link.juejin.cn?target=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8017163
[JDK-8274002]: https://link.juejin.cn?target=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8274002