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本文主要讲述一下JAVA15的新特性

版本号

java -versionopenjdk version "15" 2020-09-15OpenJDK Runtime Environment (build 15+36-1562)OpenJDK 64-Bit Server VM (build 15+36-1562, mixed mode, sharing)

从version信息可以看出是build 15+36

特性列表

339:Edwards-Curve Digital Signature Algorithm (EdDSA)

新增rfc8032描述的Edwards-Curve Digital Signature Algorithm (EdDSA)实现 使用示例

// example: generate a key pair and signKeyPairGenerator kpg = KeyPairGenerator.getInstance("Ed25519");KeyPair kp = kpg.generateKeyPair();// algorithm is pure Ed25519Signature sig = Signature.getInstance("Ed25519");sig.initSign(kp.getPrivate());sig.update(msg);byte[] s = sig.sign();// example: use KeyFactory to contruct a public keyKeyFactory kf = KeyFactory.getInstance("EdDSA");boolean xOdd = ...BigInteger y = ...NamedParameterSpec paramSpec = new NamedParameterSpec("Ed25519");EdECPublicKeySpec pubSpec = new EdECPublicKeySpec(paramSpec, new EdPoint(xOdd, y));PublicKey pubKey = kf.generatePublic(pubSpec);

360:Sealed Classes (Preview)

JDK15引入了sealed classes and interfaces.用于限定实现类，限定父类的使用，为后续的pattern matching的exhaustive analysis提供便利 示例

package com.example.geometry;​public abstract sealed class Shape    permits Circle, Rectangle, Square {...}​public final class Circle extends Shape {...}​public sealed class Rectangle extends Shape     permits TransparentRectangle, FilledRectangle {...}public final class TransparentRectangle extends Rectangle {...}public final class FilledRectangle extends Rectangle {...}​public non-sealed class Square extends Shape {...}

这里使用了3个关键字，一个是sealed，一个是permits，一个是non-sealed；permits的这些子类要么使用final，要么使用sealed，要么使用non-sealed修饰；针对record类型，也可以使用sealed，因为record类型暗示这final

package com.example.expression;​public sealed interface Expr    permits ConstantExpr, PlusExpr, TimesExpr, NegExpr {...}​public record ConstantExpr(int i)       implements Expr {...}public record PlusExpr(Expr a, Expr b)  implements Expr {...}public record TimesExpr(Expr a, Expr b) implements Expr {...}public record NegExpr(Expr e)           implements Expr {...}

371:Hidden Classes

JDK15引入了Hidden Classes，同时废弃了非标准的sun.misc.Unsafe::defineAnonymousClass，目标是为frameworks提供在运行时生成内部的class

372:Remove the Nashorn JavaScript Engine

JDK15移除了Nashorn JavaScript Engine及jjs tool，它们在JDK11的被标记为废弃；具体就是jdk.scripting.nashorn及jdk.scripting.nashorn.shell这两个模块被移除了

373:Reimplement the Legacy DatagramSocket API

该特性使用更简单及更现代的方式重新实现了java.net.DatagramSocket及java.net.MulticastSocket以方便更好的维护及debug，新的实现将会更容易支持virtual threads

374:Disable and Deprecate Biased Locking

该特性默认禁用了biased locking(-XX:+UseBiasedLocking)，并且废弃了所有相关的命令行选型(BiasedLockingStartupDelay, BiasedLockingBulkRebiasThreshold, BiasedLockingBulkRevokeThreshold, BiasedLockingDecayTime, UseoptoBiasInlining, PrintBiasedLockingStatistics and PrintPreciseBiasedLockingStatistics)

375:Pattern Matching for instanceof (Second Preview)

instanceof的Pattern Matching在JDK15进行Second Preview，示例如下:

public boolean equals(Object o) {     return (o instanceof CaseInsensitiveString cis) &&         cis.s.equalsIgnoreCase(s); }

377:ZGC: A Scalable Low-Latency Garbage Collector

ZGC在JDK11被作为experimental feature引入，在JDK15变成Production，但是这并不是替换默认的GC，默认的GC仍然还是G1；之前需要通过-XX:+UnlockExperimentalVMOptions -XX:+UseZGC来启用ZGC，现在只需要-XX:+UseZGC就可以 相关的参数有ZAllocationSpikeTolerance、ZCollectionInterval、ZFragmentationLimit、ZMarkStackSpaceLimit、ZProactive、ZUncommit、ZUncommitDelay ZGC-specific JFR events(ZAllocationStall、ZPageAllocation、ZPageCacheFlush、ZRelocationSet、ZRelocationSetGroup、ZUncommit)也从experimental变为product

378:Text Blocks

Text Blocks在JDK13被作为preview feature引入，在JDK14作为Second Preview，在JDK15变为最终版

379:Shenandoah: A Low-Pause-Time Garbage Collector

Shenandoah在JDK12被作为experimental引入，在JDK15变为Production；之前需要通过-XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC来启用，现在只需要-XX:+UseShenandoahGC即可启用

381:Remove the Solaris and SPARC Ports

Solaris and SPARC Ports在JDK14被标记为废弃，在JDK15版本正式移除

383:Foreign-Memory Access API (Second Incubator)

Foreign-Memory Access API在JDK14被作为incubating API引入，在JDK15处于Second Incubator

384:Records (Second Preview)

Records在JDK14被作为preview引入，在JDK15处于Second Preview

385:Deprecate RMI Activation for Removal

JDK15废弃了RMI Activation，后续将被移除

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 15 Release Notes，这里举几个例子。

添加项

• Support for Unicode 13.0 (JDK-8239383)

升级了Unicode，支持Unicode 13.0

• Added isEmpty Default Method to CharSequence (JDK-8215401)

给CharSequence新增了isEmpty方法 java.base/java/lang/CharSequence.java

    /**     * Returns {@code true} if this character sequence is empty.     *     * @implSpec     * The default implementation returns the result of calling {@code length() == 0}.     *     * @return {@code true} if {@link #length()} is {@code 0}, otherwise     * {@code false}     *     * @since 15     */    default boolean isEmpty() {        return this.length() == 0;    }

• Added Support for SO_INCOMING_NAPI_ID Support (JDK-8243099)

jdk.net.ExtendedSocketOptions新增了SO_INCOMING_NAPI_ID选型

• Specialized Implementations of TreeMap Methods (JDK-8176894)

JDK15对TreeMap提供了putIfAbsent, computeIfAbsent, computeIfPresent, compute, merge方法提供了overriding实现

• New Option Added to jcmd for Writing a gzipped Heap Dump (JDK-8237354)

jcmd的GC.heap_dump命令现在支持gz选型，以dump出gzip压缩版的heap；compression level从1(fastest)到9(slowest, but best compression)，默认为1

• New System Properties to Configure the TLS Signature Schemes (JDK-8242141)

新增了jdk.tls.client.SignatureSchemes及jdk.tls.server.SignatureSchemes用于配置TLS Signature Schemes

• Support for certificate_authorities Extension (JDK-8206925)

支持certificate_authorities的扩展

移除项

• Obsolete -XX:UseAdaptiveGCBoundary (JDK-8228991)

淘汰了-XX:UseAdaptiveGCBoundary

废弃项

• Deprecated -XX:ForceNUMA Option (JDK-8243628)

废弃了ForceNUMA选项

• Disable Native SunEC Implementation by Default (JDK-8237219)

默认禁用了Native SunEC Implementation

已知问题

• java.net.HttpClient Does Not Override Protocols Specified in SSLContext Default Parameters (JDK-8239594)

HttpClient现在没有覆盖在SSLContext Default Parameters中指定的Protocols

其他事项

• DatagramPacket.getPort() Returns 0 When the Port Is Not Set (JDK-8237890)

当DatagramPacket没有设置port的时候，其getPort方法返回0

• Improved Ergonomics for G1 Heap Region Size (JDK-8241670)

优化了默认G1 Heap Region Size的计算

小结

Java15主要有如下几个特性

• 339:Edwards-Curve Digital Signature Algorithm (EdDSA)
• 360:Sealed Classes (Preview)
• 371:Hidden Classes
• 372:Remove the Nashorn JavaScript Engine
• 373:Reimplement the Legacy DatagramSocket API
• 374:Disable and Deprecate Biased Locking
• 375:Pattern Matching for instanceof (Second Preview)
• 377:ZGC: A Scalable Low-Latency Garbage Collector
• 378:Text Blocks
• 379:Shenandoah: A Low-Pause-Time Garbage Collector
• 381:Remove the Solaris and SPARC Ports
• 383:Foreign-Memory Access API (Second Incubator)
• 384:Records (Second Preview)
• 385:Deprecate RMI Activation for Removal

doc

• JDK 15 Features
• JDK 15 Release Notes
• Oracle JDK 15 Release Notes
• Java SE deprecated-list
• The Arrival of Java 15
• Java 15 and IntelliJ IDEA