JDK 14 新特性-你还跟得上吗

版本号

<code>java -version
openjdk version "13" 2019-12-17 

OpenJDK Runtime Environment (build 13+33)
OpenJDK 64-Bit Server VM (build 13+33, mixed mode, sharing)/<code>

从version信息可以看出是build 13+33

特性列表

350: Dynamic CDS Archives

JDK5引入了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
java10的新特性JEP 310: Application Class-Data Sharing扩展了JDK5引入的Class-Data Sharing，支持application的Class-Data Sharing并开源出来(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341: Default CDS Archives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
JDK13的这个特性支持在Java application执行之后进行动态archive

• 导出jsa

<code>java -XX:ArchiveClassesAtExit=hello.jsa -cp hello.jar Hello/<code>

• 使用jsa

<code>java -XX:SharedArchiveFile=hello.jsa -cp hello.jar Hello/<code>

351: ZGC: Uncommit Unused Memory

Java12的346: Promptly Return Unused Committed Memory from G1新增了两个参数分别是G1PeriodicGCInterval及G1PeriodicGCSystemLoadThreshold用于GC之后重新调整Java heap size，然后将多余的内存归还给操作系统
Java12的189: Shenandoah: A Low-Pause-Time Garbage Collector (Experimental)拥有参数-XX:ShenandoahUncommitDelay=<milliseconds>来指定ZPage的page cache的失效时间，然后归还内存

353: Reimplement the Legacy Socket API

本特性替换了java.net.Socket以及java.net.ServerSocket API的底层实现；它使用NioSocketImpl来替换JDK1.0的PlainSocketImpl；如果要继续使用旧版的Socket实现，可以使用-Djdk.net.usePlainSocketImpl参数来切换到旧版本

/Library/Java/JavaVirtualMachines/jdk-13.jdk/Contents/Home/lib/src.zip!/java.base/java/net/SocketImpl.java

<code>public abstract class SocketImpl implements SocketOptions {
    private static final boolean USE_PLAINSOCKETIMPL = usePlainSocketImpl();

    private static boolean usePlainSocketImpl() {
        PrivilegedAction<string> pa = () -> NetProperties.get("jdk.net.usePlainSocketImpl");
        String s = AccessController.doPrivileged(pa);
        return (s != null) && !s.equalsIgnoreCase("false");
    }

    /**
     * Creates an instance of platform's SocketImpl
     */
    @SuppressWarnings("unchecked")
    static  S createPlatformSocketImpl(boolean server) {
        if (USE_PLAINSOCKETIMPL) {
            return (S) new PlainSocketImpl(server);
        } else {
            return (S) new NioSocketImpl(server);
        }
    }

    //......
}/<string>/<code>

SocketImpl的USE_PLAINSOCKETIMPL取决于usePlainSocketImpl方法，而它会从NetProperties读取dk.net.usePlainSocketImpl配置，如果不为null且不为false，则usePlainSocketImpl方法返回true；createPlatformSocketImpl会根据USE_PLAINSOCKETIMPL来创建PlainSocketImpl或者NioSocketImpl

354: Switch Expressions (Preview)

本特性主要是使用yield替换了break来避免歧义，因为break可以用来进行跳转执行类似goto的操作

<code>    @Test
    public void testSwitchYield(){
        String dayOfWeek = switch(1){
            case 1 -> {
                String day = "Monday";
                yield day;
            }
            case 2 -> {
                String day = "Tuesday";
                yield day;
            }
            default -> "Unknown";
        };
        System.out.println(dayOfWeek);
    }/<code>

355: Text Blocks (Preview)

本特性主要引入了Text Blocks，使用"""来包围一段text block，可以内置占位符最后使用String.format来填充

<code>    @Test
    public void testTextBlock(){
        // Without Text Blocks
        String html = "\\n" +
                "   \\n" +
                "      
Hello, Escapes
\\n" +
                "   \\n" +
                "\\n";
        System.out.println(html);

        // With Text Blocks
        String html2 = """
            
                
                
Hello, %s

                 

            """;

        System.out.println(String.format(html2, "World"));

        String htmlWithNewLine = """
            
                
                
Hello World

                
            
            """;
        System.out.print(htmlWithNewLine);
        System.out.println("a new line");
    }/<code>

需要注意text blocks不能在一行，另外如果结尾的"""在新的一行则会输出新的一行

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 13 Release Notes，这里举几个例子。

添加项

• 添加FileSystems.newFileSystem(Path, Map<string>) Method/<string>
• 新的java.nio.ByteBuffer Bulk get/put Methods Transfer Bytes Without Regard to Buffer Position
• 支持Unicode 12.1
• 添加-XX:SoftMaxHeapSize Flag，目前仅仅对ZGC起作用
• ZGC的最大heap大小增大到16TB

移除项

• 移除awt.toolkit System Property
• 移除Runtime Trace Methods
• 移除-XX:+AggressiveOpts
• 移除Two Comodo Root CA Certificates、Two DocuSign Root CA Certificates
• 移除内部的com.sun.net.ssl包

废弃项

• 废弃-Xverify:none及-noverify
• 废弃rmic Tool并准备移除
• 废弃javax.security.cert并准备移除

已知问题

• 不再支持Windows 2019 Core Server
• 使用ZIP File System (zipfs) Provider来更新包含Uncompressed Entries的ZIP或JAR可能造成文件损坏

其他事项

• GraphicsEnvironment.getCenterPoint()及getMaximumWindowBounds()已跨平台统一
• 增强了JAR Manifest的Class-Path属性处理

jdk.net.URLClassPath.showIgnoredClassPathEntries属性设置为true可以用来帮助查看非法的Class-Path entries

• 针对Negatively Sized Argument，StringBuffer(CharSequence)及StringBuilder(CharSequence)会抛出NegativeArraySizeException
• linux的默认进程启动机制已经使用posix_spawn
• Lookup.unreflectSetter(Field)针对static final fields会抛出IllegalAccessException
• 使用了java.net.Socket.setSocketImplFactory及java.net.ServerSocket.setSocketFactory方法的要注意，要求客户端及服务端要一致，不能一端使用自定义的factory一端使用默认的factory
• SocketImpl的supportedOptions, getOption及setOption方法的默认实现发生了变化，默认的supportedOptions返回空，而默认的getOption,及setOption方法抛出UnsupportedOperationException
• JNI NewDirectByteBuffer创建的Direct Buffer为java.nio.ByteOrder.BIG_ENDIAN
• Base64.Encoder及Base64.Decoder可能抛出OutOfMemoryError
• 改进了Serial GC Young pause time report
• 改进了MaxRAM及UseCompressedOops参数的行为

小结

• Java13主要新增了如下特性350: Dynamic CDS Archives351: ZGC: Uncommit Unused Memory353: Reimplement the Legacy Socket API354: Switch Expressions (Preview)355: Text Blocks (Preview)
• 语法层面，改进了Switch Expressions，新增了Text Blocks，二者皆处于Preview状态；API层面主要使用NioSocketImpl来替换JDK1.0的PlainSocketImpl
• GC层面则改进了ZGC，以支持Uncommit Unused Memory

版本号

<code>java -version
openjdk version "14" 2020-03-17
OpenJDK Runtime Environment (build 14+36-1461)
OpenJDK 64-Bit Server VM (build 14+36-1461, mixed mode, sharing)/<code> 

从version信息可以看出是build 14+36

特性列表

305:Pattern Matching for instanceof (Preview)

JDK14引入了preview版本的针对instanceof的模式匹配，其用法示例如下

<code>    public boolean isBadRequestError(Exception ex) {
        return (ex instanceof HttpClientErrorException rce) &&
                HttpStatus.BAD_REQUEST == rce.getStatusCode();
    }/<code>

无需在自己进行类型强转

343:Packaging Tool (Incubator)

JDK14引入了jdk.incubator.jpackage.jmod，它基于JavaFX javapackager tool构建，目的在于创建一个简单的打包工具，可以用于构建exe、pkg、dmg、deb、rpm格式的安装文件；非模块化的app的构建示例如下

<code>jpackage --name myapp --input lib --main-jar main.jar/<code>

345:NUMA-Aware Memory Allocation for G1

实现了NUMA-aware的内存分配，以提升G1在大型机器上的性能

349:JFR Event Streaming

JDK11引入了JFR，使用的时候先dump到磁盘上然后再分析；而在JDK14则支持stream方式来进行持续性的监控，示例如下

<code>public class AgentMain implements Runnable {

    public static void premain(String agentArgs, Instrumentation inst) {
        try {
            Logger.getLogger("AgentMain").log(
                    Level.INFO, "Attaching JFR Monitor");
            new Thread(new AgentMain()).start();
        } catch (Throwable t) {
            Logger.getLogger("AgentMain").log(
                    Level.SEVERE,"Unable to attach JFR Monitor", t);
        }
    }

    @Override
    public void run() {
        var sender = new JfrStreamEventSender();
        try (var rs = new RecordingStream()) {
            rs.enable("jdk.CPULoad")
                    .withPeriod(Duration.ofSeconds(1));
            rs.enable("jdk.JavaMonitorEnter")
                    .withThreshold(Duration.ofMillis(10));
            rs.onEvent("jdk.CPULoad", sender);
            rs.onEvent("jdk.JavaMonitorEnter", sender);
            rs.start();
        }
    }
}/<code>

352:Non-Volatile Mapped Byte Buffers

该特性新增了java.base/jdk/internal/misc/ExtendedMapMode.java以支持MappedByteBuffer访问non-volatile memory (NVM)

358:Helpful NullPointerExceptions

该特性可以更好地提示哪个地方出现的空指针，需要通过-XX:+ShowCodeDetailsInExceptionMessages开启，示例如下

<code>public class NullPointerExample {

    public record City(String name){

    }

    public record Location(City city) {

    }

    public record User(String name, Location location) {

    }

    public static void main(String[] args){
        User user = new User("hello", new Location(null));
        System.out.println(user.location().city().name());
    }
}/<code>

输出如下

<code>Exception in thread "main" java.lang.NullPointerException: Cannot invoke "com.example.NullPointerExample$City.name()" because the return value of "com.example.NullPointerExample$Location.city()" is null
    at com.example.NullPointerExample.main(NullPointerExample.java:25)/<code>

359:Records (Preview)

JDK14引入了preview版本的record类型，示例如下

<code>public record Point(int x, int y) {

    public Point {
        System.out.println(String.format("(%d,%d)", x, y));
    }

    public Point(int value) {
        this(value, value);
    }

    public static Point of(int value) {
        return new Point(value, value);
    }
}/<code>

javap反编译如下

<code>javac --enable-preview -source 14 Point.java 

javap -verbose Point.class
Compiled from "Point.java"
public final class com.example.domain.Point extends java.lang.Record {
  public com.example.domain.Point(int, int);
  public static com.example.domain.Point of(int);
  public java.lang.String toString();
  public final int hashCode();
  public final boolean equals(java.lang.Object);
  public int x();
  public int y();
}/<code>

可以看见Point继承了java.lang.Record，而且通过invokedynamic提供了final的hashCode及equals

361:Switch Expressions (Standard)

JDK12引入的switch在JDK14变为正式版本，正式版主要是用->来替代以前的:+break；另外就是提供了yield来在block中返回值，示例如下

<code>    public void testSwitchWithArrowBlockAndYield() {
        int n = 3;
        String quantityString = switch (n) {
            case 1 -> "one";
            case 2 -> "two";
            default -> {
                System.out.println("default");
                yield "many";
            }
        };
        System.out.println(quantityString);
    }/<code>

362:Deprecate the Solaris and SPARC Ports

废弃了 Solaris/SPARC, Solaris/x64, and Linux/SPARC ports，以在未来的版本中移除

363:Remove the Concurrent Mark Sweep (CMS) Garbage Collector

移除了CMS垃圾收集器，如果在JDK14中使用-XX:+UseConcMarkSweepGC的话，会出现warning，但是不会exit而是以默认的垃圾收集器运行，如下

<code>OpenJDK 64-Bit Server VM warning: Ignoring option UseConcMarkSweepGC; support was removed in 14.0/<code>

364:ZGC on macOS

之前的ZGC只能在linux上使用，现在mac上也能使用ZGC了，示例如下

<code>-XX:+UnlockExperimentalVMOptions -XX:+UseZGC/<code>

365:ZGC on Windows

之前的ZGC只能在linux上使用，现在windows(不能低于1803版本)上也能使用ZGC了，示例如下

<code>-XX:+UnlockExperimentalVMOptions -XX:+UseZGC/<code>

366:Deprecate the ParallelScavenge + SerialOld GC Combination

废弃了parallel young generation GC与SerialOld GC的组合( -XX:+UseParallelGC与-XX:-UseParallelOldGC配合开启)，现在使用-XX:+UseParallelGC -XX:-UseParallelOldGC或者-XX:-UseParallelOldGC都会出现告警如下

<code>OpenJDK 64-Bit Server VM warning: Option UseParallelOldGC was deprecated in version 14.0 and will likely be removed in a future release./<code>

单独使用-XX:+UseParallelGC则开启parallel young and old generation GC algorithms

367:Remove the Pack200 Tools and API

移除了Pack200 API

368:Text Blocks (Second Preview)

JDK13引入的text blocks进行第二轮preview，JDK14的版本主要增加了两个escape sequences，分别是 \\<line-terminator>与\\s escape sequence，示例如下/<line-terminator>

<code>    @Test
    public void testTextBlockWithTwoNewEscapeSequences() {
        String inOneLine = """
                Lorem ipsum dolor sit amet, consectetur adipiscing \\
                elit, sed do eiusmod tempor incididunt ut labore \\
                et dolore magna aliqua.\\
                """;
        System.out.println(inOneLine);
 

        String singleSpace = """
                red  \\s
                green\\s
                blue \\s
                """;
        System.out.println(singleSpace);
    }/<code>

370:Foreign-Memory Access API (Incubator)

提供了incubator版本的API用于操纵堆外内存，使用示例如下

<code>    @Test
    public void testForeignMemoryAccessAPI() {
        SequenceLayout intArrayLayout
                = MemoryLayout.ofSequence(25,
                MemoryLayout.ofValueBits(32,
                        ByteOrder.nativeOrder()));

        VarHandle intElemHandle
                = intArrayLayout.varHandle(int.class,
                MemoryLayout.PathElement.sequenceElement());

        try (MemorySegment segment = MemorySegment.allocateNative(intArrayLayout)) {
            MemoryAddress base = segment.baseAddress();
            for (int i = 0; i < intArrayLayout.elementCount().getAsLong(); i++) {
                intElemHandle.set(base, (long) i, i);
            }
        }
    }/<code>

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 14 Release Notes，这里举几个例子。

添加项

• New Method to SAX ContentHandler for Handling XML Declaration (JDK-8230814)

给SAX ContentHandler新增了方法，如下

<code>    default void declaration(String version, String encoding, String standalone)
        throws SAXException
    {
        //no op
    }/<code>

移除项

• Removal of sun.nio.cs.map System Property (JDK-8229960)

移除了sun.nio.cs.map属性

• Removed Deprecated java.security.acl APIs (JDK-8191138)

移除了java.security.acl

• Removal of the Default keytool -keyalg Value (JDK-8214024)

移除了默认的keytool -keyalg

废弃项

• Thread Suspend/Resume Are Deprecated for Removal (JDK-8231602)

废弃了Thread的如下方法

<code>Thread.suspend()
Thread.resume()
ThreadGroup.suspend()
ThreadGroup.resume()
ThreadGroup.allowThreadSuspension(boolean)/<code>

已知问题

• Text Visibility Issues in macOS Dark Mode (JDK-8228555)

已知在macOS上的Dark Mode有Text Visibility Issues

其他事项

• Thread.countStackFrames Changed to Unconditionally Throw UnsupportedOperationException (JDK-8205132)

Thread.countStackFrames开启转为无条件地抛出UnsupportedOperationException

小结

Java14主要有如下几个特性

• 305:Pattern Matching for instanceof (Preview)
• 349:JFR Event Streaming
• 352:Non-Volatile Mapped Byte Buffers
• 358:Helpful NullPointerExceptions
• 359:Records (Preview)
• 361:Switch Expressions (Standard)
• 363:Remove the Concurrent Mark Sweep (CMS) Garbage Collector
• 364:ZGC on macOS
• 366:Deprecate the ParallelScavenge + SerialOld GC Combination
• 368:Text Blocks (Second Preview)
• 370:Foreign-Memory Access API (Incubator)

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