Java语言特性系列
- Java5的新特性
- Java6的新特性
- Java7的新特性
- Java8的新特性
- Java9的新特性
- Java10的新特性
- Java11的新特性
- Java12的新特性
- Java13的新特性
- Java14的新特性
- Java15的新特性
- Java16的新特性
序
本文主要讲述一下Java15的新特性
版本号
java -version
openjdk version "15" 2020-09-15
OpenJDK 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 sign
KeyPairGenerator kpg = KeyPairGenerator.getInstance("Ed25519");
KeyPair kp = kpg.generateKeyPair();
// algorithm is pure Ed25519
Signature sig = Signature.getInstance("Ed25519");
sig.initSign(kp.getPrivate());
sig.update(msg);
byte[] s = sig.sign();
// example: use KeyFactory to contruct a public key
KeyFactory 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