Java12的新特性

刺骨的言语ヽ痛彻心扉 2022-10-01 08:54 165阅读 0赞

> Java语言特性系列

*  [Java5的新特性][Java5]
 *  [Java6的新特性][Java6]
 *  [Java7的新特性][Java7]
 *  [Java8的新特性][Java8]
 *  [Java9的新特性][Java9]
 *  [Java10的新特性][Java10]
 *  [Java11的新特性][Java11]
 *  [Java12的新特性][Java12]
 *  [Java13的新特性][Java13]

## 序 ##

本文主要讲述一下Java12的新特性

## 版本号 ##

java -version
    openjdk version "12" 2019-03-19
    OpenJDK Runtime Environment (build 12+33)
    OpenJDK 64-Bit Server VM (build 12+33, mixed mode)
    复制代码

> 从version信息可以看出是build 12+33

## 特性列表 ##

*  [189: Shenandoah: A Low-Pause-Time Garbage Collector (Experimental)][189_ Shenandoah_ A Low-Pause-Time Garbage Collector _Experimental]

> Shenandoah GC是一个面向low-pause-time的垃圾收集器，它最初由Red Hat实现，支持aarch64及amd64 architecture；ZGC也是面向low-pause-time的垃圾收集器，不过ZGC是基于colored pointers来实现，而Shenandoah GC是基于brooks pointers来实现；如果要使用Shenandoah GC需要编译时--with-jvm-features选项带有shenandoahgc，然后启动时使用-XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC

*  [230: Microbenchmark Suite][230_ Microbenchmark Suite]

> 在jdk源码里头新增了一套基础的microbenchmarks suite

*  [325: Switch Expressions (Preview)][325_ Switch Expressions _Preview]

> 对switch进行了增强，除了使用statement还可以使用expression，比如原来的写法如下：

switch (day) {
        case MONDAY:
        case FRIDAY:
        case SUNDAY:
            System.out.println(6);
            break;
        case TUESDAY:
            System.out.println(7);
            break;
        case THURSDAY:
        case SATURDAY:
            System.out.println(8);
            break;
        case WEDNESDAY:
            System.out.println(9);
            break;
    }
    复制代码

现在可以改为如下写法：

switch (day) {
        case MONDAY, FRIDAY, SUNDAY -> System.out.println(6);
        case TUESDAY                -> System.out.println(7);
        case THURSDAY, SATURDAY     -> System.out.println(8);
        case WEDNESDAY              -> System.out.println(9);
    }
    复制代码

以及在表达式返回值

int numLetters = switch (day) {
        case MONDAY, FRIDAY, SUNDAY -> 6;
        case TUESDAY                -> 7;
        case THURSDAY, SATURDAY     -> 8;
        case WEDNESDAY              -> 9;
    };
    复制代码

对于需要返回值的switch expression要么正常返回值要么抛出异常，以下这两种写法都是错误的

int i = switch (day) {
        case MONDAY -> {
            System.out.println("Monday"); 
            // ERROR! Block doesn't contain a break with value
        }
        default -> 1;
    };
    i = switch (day) {
        case MONDAY, TUESDAY, WEDNESDAY: 
            break 0;
        default: 
            System.out.println("Second half of the week");
            // ERROR! Group doesn't contain a break with value
    };
    复制代码

*  [334: JVM Constants API][334_ JVM Constants API]

> 新增了JVM Constants API，具体来说就是java.base模块新增了java.lang.constant包，引入了ConstantDesc接口(`ClassDesc、MethodTypeDesc、MethodHandleDesc这几个接口直接继承了ConstantDesc接口`)以及Constable接口；ConstantDesc接口定义了resolveConstantDesc方法，Constable接口定义了describeConstable方法；String、Integer、Long、Float、Double均实现了这两个接口，而EnumDesc实现了ConstantDesc接口

*  [340: One AArch64 Port, Not Two][340_ One AArch64 Port_ Not Two]

> 64-bit Arm platform (arm64)，也可以称之为aarch64；之前JDK有两个关于aarch64的实现，分别是src/hotspot/cpu/arm以及open/src/hotspot/cpu/aarch64，它们的实现重复了，为了集中精力更好地实现aarch64，该特性在源码中删除了open/src/hotspot/cpu/arm中关于64-bit的实现，保留其中32-bit的实现，于是open/src/hotspot/cpu/aarch64部分就成了64-bit ARM architecture的默认实现

*  [341: Default CDS Archives][341_ Default CDS Archives]

> java10的新特性[JEP 310: Application Class-Data Sharing][JEP 310_ Application Class-Data Sharing]扩展了JDK5引入的Class-Data Sharing，支持application的Class-Data Sharing；Class-Data Sharing可以用于多个JVM共享class，提升启动速度，最早只支持system classes及serial GC，JDK9对其进行扩展以支持application classes及其他GC算法，并在JDK10中开源出来(`以前是commercial feature`)；JDK11将-Xshare:off改为默认-Xshare:auto，以更加方便使用CDS特性；JDK12的这个特性即在64-bit平台上编译jdk的时候就默认在$\{JAVA\_HOME\}/lib/server目录下生成一份名为classes.jsa的默认archive文件(`大概有18M`)方便大家使用

*  [344: Abortable Mixed Collections for G1][344_ Abortable Mixed Collections for G1]

> G1在garbage collection的时候，一旦确定了collection set(`CSet`)开始垃圾收集这个过程是without stopping的，当collection set过大的时候，此时的STW时间会过长超出目标pause time，这种情况在mixed collections时候比较明显。这个特性启动了一个机制，当选择了一个比较大的collection set，允许将其分为mandatory及optional两部分(`当完成mandatory的部分，如果还有剩余时间则会去处理optional部分`)来将mixed collections从without stopping变为abortable，以更好满足指定pause time的目标

*  [346: Promptly Return Unused Committed Memory from G1][346_ Promptly Return Unused Committed Memory from G1]

> G1目前只有在full GC或者concurrent cycle的时候才会归还内存，由于这两个场景都是G1极力避免的，因此在大多数场景下可能不会及时会还committed Java heap memory给操作系统。JDK12的这个特性新增了两个参数分别是G1PeriodicGCInterval及G1PeriodicGCSystemLoadThreshold，设置为0的话，表示禁用。当上一次garbage collection pause过去G1PeriodicGCInterval(`milliseconds`)时间之后，如果getloadavg()(`one-minute`)低于G1PeriodicGCSystemLoadThreshold指定的阈值，则触发full GC或者concurrent GC(`如果开启G1PeriodicGCInvokesConcurrent`)，GC之后Java heap size会被重写调整，然后多余的内存将会归还给操作系统

## 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 12 Release Notes][]，这里举几个例子。

### 添加项 ###

*  支持unicode 11
 *  支持Compact Number Formatting

> 使用实例如下

@Test
        public void testCompactNumberFormat(){
            var cnf = NumberFormat.getCompactNumberInstance(Locale.CHINA, NumberFormat.Style.SHORT);
            System.out.println(cnf.format(1_0000));
            System.out.println(cnf.format(1_9200));
            System.out.println(cnf.format(1_000_000));
            System.out.println(cnf.format(1L << 30));
            System.out.println(cnf.format(1L << 40));
            System.out.println(cnf.format(1L << 50));
        }
    复制代码

输出

1万
    2万
    100万
    11亿
    1兆
    1126兆
    复制代码

*  String支持transform、indent操作

@Test
        public void testStringTransform(){
            System.out.println("hello".transform(new Function<String, Integer>() {
                @Override
                public Integer apply(String s) {
                    return s.hashCode();
                }
            }));
        }
    
        @Test
        public void testStringIndent(){
            System.out.println("hello".indent(3));
        }
    复制代码

*  Files新增mismatch方法

@Test
        public void testFilesMismatch() throws IOException {
            FileWriter fileWriter = new FileWriter("/tmp/a.txt");
            fileWriter.write("a");
            fileWriter.write("b");
            fileWriter.write("c");
            fileWriter.close();
    
            FileWriter fileWriterB = new FileWriter("/tmp/b.txt");
            fileWriterB.write("a");
            fileWriterB.write("1");
            fileWriterB.write("c");
            fileWriterB.close();
    
            System.out.println(Files.mismatch(Path.of("/tmp/a.txt"),Path.of("/tmp/b.txt")));
        }
    复制代码

*  Collectors新增teeing方法用于聚合两个downstream的结果

@Test
        public void testCollectorTeeing(){
            var result = Stream.of("Devoxx","Voxxed Days","Code One","Basel One")
                    .collect(Collectors.teeing(Collectors.filtering(n -> n.contains("xx"),Collectors.toList()),
                                                Collectors.filtering(n -> n.endsWith("One"),Collectors.toList()),
                            (List<String> list1, List<String> list2) -> List.of(list1,list2)
                                                ));
    
            System.out.println(result.get(0));
            System.out.println(result.get(1));
        }
    复制代码

*  CompletionStage新增exceptionallyAsync、exceptionallyCompose、exceptionallyComposeAsync方法

@Test
        public void testExceptionallyAsync() throws ExecutionException, InterruptedException {
            LOGGER.info("begin");
            int result = CompletableFuture.supplyAsync(() -> {
                LOGGER.info("calculate");
                int i = 1/0;
                return 100;
            }).exceptionallyAsync((t) -> {
                LOGGER.info("error error:{}",t.getMessage());
                return 0;
            }).get();
    
            LOGGER.info("result:{}",result);
        }
    复制代码

*  JDK12之前CompletionStage只有一个exceptionally，该方法体在主线程执行，JDK12新增了exceptionallyAsync、exceptionallyComposeAsync方法允许方法体在异步线程执行，同时新增了exceptionallyCompose方法支持在exceptionally的时候构建新的CompletionStage
 *  Allocation of Old Generation of Java Heap on Alternate Memory Devices

> G1及Parallel GC引入experimental特性，允许将old generation分配在诸如NV-DIMM memory的alternative memory device

*  ZGC: Concurrent Class Unloading

> ZGC在JDK11的时候还不支持class unloading，JDK12对ZGC支持了Concurrent Class Unloading，默认是开启，使用-XX:-ClassUnloading可以禁用

*  新增-XX:+ExtensiveErrorReports

> \-XX:+ExtensiveErrorReports可以用于在jvm crash的时候收集更多的报告信息到hs\_err.log文件中，product builds中默认是关闭的，要开启的话，需要自己添加-XX:+ExtensiveErrorReports参数

*  新增安全相关的改进

> 支持java.security.manager系统属性，当设置为disallow的时候，则不使用SecurityManager以提升性能，如果此时调用System.setSecurityManager则会抛出UnsupportedOperationException keytool新增-groupname选项允许在生成key pair的时候指定一个named group 新增PKCS12 KeyStore配置属性用于自定义PKCS12 keystores的生成 Java Flight Recorder新增了security-related的event 支持ChaCha20 and Poly1305 TLS Cipher Suites

*  jdeps Reports Transitive Dependences

> jdeps的--print-module-deps, --list-deps, 以及--list-reduce-deps选项得到增强，新增--no-recursive用于non-transitive的依赖分析，--ignore-missing-deps用于suppress missing dependence errors

### 移除项 ###

*  移除com.sun.awt.SecurityWarnin
 *  移除FileInputStream、FileOutputStream、Java.util.ZipFile/Inflator/Deflator的finalize方法
 *  移除GTE CyberTrust Global Root
 *  移除javac的-source, -target对6及1.6的支持，同时移除--release选项

### 废弃项 ###

*  废弃的API列表见[deprecated-list][]
 *  废弃-XX:+/-MonitorInUseLists选项
 *  废弃Default Keytool的-keyalg值

### 已知问题 ###

*  Swing不支持GTK+ 3.20及以后的版本
 *  在使用JVMCI Compiler(`比如Graal`)的时候，JVMTI的can\_pop\_frame及can\_force\_early\_return的capabilities是被禁用的

### 其他事项 ###

*  如果用户没有指定user.timezone且从操作系统获取的为空，那么user.timezone属性的初始值为空变为null
 *  java.net.URLPermission的行为发生轻微变化，以前它会忽略url中的query及fragment部分，这次改动新增query及fragment部分，即`scheme : // authority [ / path ]`变动为`scheme : // authority [ / path ] [ ignored-query-or-fragment ]`
 *  javax.net.ssl.SSLContext API及Java Security Standard Algorithm Names规范移除了必须实现TLSv1及TLSv1.1的规定

## 小结 ##

*  java12不是LTS(`Long-Term Support`)版本(`oracle版本才有LTS`)，oracle对该版本的support周期为6个月。这个版本主要有几个更新点，一个是语法层更新，一个是API层面的更新，另外主要是GC方面的更新。
 *  语法层面引入了preview版本的Switch Expressions；API层面引入了JVM Constants API，引入CompactNumberFormat，让NumberFormat支持COMPACTSTYLE，对String、Files、Collectors、CompletionStage等新增方法；GC方面引入了experimental版本的Shenandoah GC，不过oracle build的openjdk没有enable Shenandoah GC support；另外主要对ZGC及G1 GC进行了改进
 *  其中JDK12对ZGC支持了Concurrent Class Unloading，默认是开启，使用-XX:-ClassUnloading可以禁用；对于G1 GC则新增支持Abortable Mixed Collections以及Promptly Return Unused Committed Memory特性

## doc ##

*  [openjdk 12][]
 *  [JDK 12 Release Notes][]
 *  [Java 12 Released with Experimental Switch Expressions and Shenandoah GC][]
 *  [Definitive Guide To Java 12][]
 *  [Definitive Guide To Switch Expressions In Java 12][]
 *  [JVM Class Data Sharing][]
 *  [JEP 310: Application Class-Data Sharing][JEP 310_ Application Class-Data Sharing]
 *  [Improve Launch Times On Java 10 With Application Class-Data Sharing][]
 *  [Make -Xshare:auto the default for server VM][Make -Xshare_auto the default for server VM]
 *  [Using application class-data sharing][]
 *  [Java Performance Tuning News February 2018][]
 *  [JDK 12 Security Enhancements][]

转载于:https://juejin.im/post/5c91fcc9e51d45563b62382c

[Java5]: https://link.juejin.im?target=https%3A%2F%2Fsegmentfault.com%2Fa%2F1190000004417288
[Java6]: https://link.juejin.im?target=https%3A%2F%2Fsegmentfault.com%2Fa%2F1190000004417536
[Java7]: https://link.juejin.im?target=https%3A%2F%2Fsegmentfault.com%2Fa%2F1190000004417830
[Java8]: https://link.juejin.im?target=https%3A%2F%2Fsegmentfault.com%2Fa%2F1190000004419611
[Java9]: https://link.juejin.im?target=https%3A%2F%2Fsegmentfault.com%2Fa%2F1190000013620826
[Java10]: https://link.juejin.im?target=https%3A%2F%2Fsegmentfault.com%2Fa%2F1190000014076481
[Java11]: https://link.juejin.im?target=https%3A%2F%2Fsegmentfault.com%2Fa%2F1190000016527932
[Java12]: https://link.juejin.im?target=https%3A%2F%2Fsegmentfault.com%2Fa%2F1190000018588495
[Java13]: https://link.juejin.im?target=%2560%25E5%25BE%2585%25E8%25A1%25A5%25E5%2585%2585%2560
[189_ Shenandoah_ A Low-Pause-Time Garbage Collector _Experimental]: https://link.juejin.im?target=http%3A%2F%2Fopenjdk.java.net%2Fjeps%2F189
[230_ Microbenchmark Suite]: https://link.juejin.im?target=http%3A%2F%2Fopenjdk.java.net%2Fjeps%2F230
[325_ Switch Expressions _Preview]: https://link.juejin.im?target=http%3A%2F%2Fopenjdk.java.net%2Fjeps%2F325
[334_ JVM Constants API]: https://link.juejin.im?target=http%3A%2F%2Fopenjdk.java.net%2Fjeps%2F334
[340_ One AArch64 Port_ Not Two]: https://link.juejin.im?target=http%3A%2F%2Fopenjdk.java.net%2Fjeps%2F340
[341_ Default CDS Archives]: https://link.juejin.im?target=http%3A%2F%2Fopenjdk.java.net%2Fjeps%2F341
[JEP 310_ Application Class-Data Sharing]: https://link.juejin.im?target=http%3A%2F%2Fopenjdk.java.net%2Fjeps%2F310
[344_ Abortable Mixed Collections for G1]: https://link.juejin.im?target=http%3A%2F%2Fopenjdk.java.net%2Fjeps%2F344
[346_ Promptly Return Unused Committed Memory from G1]: https://link.juejin.im?target=http%3A%2F%2Fopenjdk.java.net%2Fjeps%2F346
[JDK 12 Release Notes]: https://link.juejin.im?target=http%3A%2F%2Fjdk.java.net%2F12%2Frelease-notes
[deprecated-list]: https://link.juejin.im?target=http%3A%2F%2Fcr.openjdk.java.net%2F~iris%2Fse%2F12%2FlatestSpec%2Fapi%2Fdeprecated-list.html
[openjdk 12]: https://link.juejin.im?target=http%3A%2F%2Fopenjdk.java.net%2Fprojects%2Fjdk%2F12%2F
[Java 12 Released with Experimental Switch Expressions and Shenandoah GC]: https://link.juejin.im?target=https%3A%2F%2Fwww.infoq.com%2Fnews%2F2019%2F03%2Fjava12-released
[Definitive Guide To Java 12]: https://link.juejin.im?target=https%3A%2F%2Fblog.codefx.org%2Fjava%2Fjava-12-guide%2F
[Definitive Guide To Switch Expressions In Java 12]: https://link.juejin.im?target=https%3A%2F%2Fblog.codefx.org%2Fjava%2Fswitch-expressions%2F
[JVM Class Data Sharing]: https://link.juejin.im?target=https%3A%2F%2Fkupczynski.info%2F2018%2F05%2F29%2Fjvm-class-data-sharing.html
[Improve Launch Times On Java 10 With Application Class-Data Sharing]: https://link.juejin.im?target=https%3A%2F%2Fblog.codefx.org%2Fjava%2Fapplication-class-data-sharing%2F
[Make -Xshare_auto the default for server VM]: https://link.juejin.im?target=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8197967
[Using application class-data sharing]: https://link.juejin.im?target=https%3A%2F%2Fsubscription.packtpub.com%2Fbook%2Fapplication_development%2F9781789132359%2F1%2Fch01lvl1sec16%2Fusing-application-class-data-sharing
[Java Performance Tuning News February 2018]: https://link.juejin.im?target=http%3A%2F%2Fwww.javaperformancetuning.com%2Fnews%2Fnews219.shtml
[JDK 12 Security Enhancements]: https://link.juejin.im?target=https%3A%2F%2Fseanjmullan.org%2Fblog%2F2019%2F03%2F19%2Fjdk12