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本文主要讲述一下Java16的新特性
版本号
java -version
openjdk version "16" 2021-03-16
OpenJDK Runtime Environment (build 16+36-2231)
OpenJDK 64-Bit Server VM (build 16+36-2231, mixed mode, sharing)
从version信息可以看出是build 16+36
特性列表
JEP 338: Vector API (Incubator)
提供了jdk.incubator.vector来用于矢量计算，实例如下
static final VectorSpecies<Float> SPECIES = FloatVector.SPECIES_256;
void vectorComputation(float[] a, float[] b, float[] c) {
for (int i = 0; i < a.length; i += SPECIES.length()) {
var m = SPECIES.indexInRange(i, a.length);
// FloatVector va, vb, vc;
var va = FloatVector.fromArray(SPECIES, a, i, m);
var vb = FloatVector.fromArray(SPECIES, b, i, m);
var vc = va.mul(va).
add(vb.mul(vb)).
neg();
vc.intoArray(c, i, m);
}
}
JEP 347: Enable C++14 Language Features
在JDK C++的源码中允许使用C++14的语言特性
JEP 357: Migrate from Mercurial to Git
OpenJDK源码的版本控制从Mercurial (hg) 迁移到git
JEP 369: Migrate to GitHub
将OpenJDK源码的版本控制迁移到github上
JEP 376: ZGC: Concurrent Thread-Stack Processing
实现了并发thread-stack处理来降低GC safepoints的负担
JEP 380: Unix-Domain Socket Channels
对socket channel及server socket channel的api提供对unix domain socket的支持
JEP 386: Alpine Linux Port
将glibc的jdk移植到使用musl的alpine linux上
JEP 387: Elastic Metaspace
支持不再使用的class metadata归还内存给操作系统，降低内存占用
JEP 388: Windows/AArch64 Port
移植JDK到Windows/AArch64
JEP 389: Foreign Linker API (Incubator)
提供jdk.incubator.foreign来简化native code的调用
JEP 390: Warnings for Value-Based Classes
提供@jdk.internal.ValueBased来用于标注作为value-based的类，实例如下
@jdk.internal.ValueBased
public final class SomeVbc {
public SomeVbc() {}
final String ref = "String";
void abuseVbc() {
synchronized(ref) { // OK
synchronized (this) { // WARN
}
}
}
}
final class AuxilliaryAbuseOfVbc {
void abuseVbc(SomeVbc vbc) {
synchronized(this) { // OK
synchronized (vbc) { // WARN
}
}
}
}
JEP 392: Packaging Tool
jpackage在JDK14引入，JDK15作为incubating工具，在JDK16转正，从
jdk.incubator.jpackage转为jdk.jpackage。它支持Linux: deb and rpm、macOS: pkg and dmg、Windows: msi and exe
JEP 393: Foreign-Memory Access API (Third Incubator)
Foreign-Memory Access API在JDK14首次引入作为incubating API，在JDK15处于第二轮incubating，在JDK16处于第三轮incubating
JEP 394: Pattern Matching for instanceof
instanceof的模式匹配在JDK14作为preview，在JDK15作为第二轮的preview，在JDK16转正
JEP 395: Records
Record类型在JDK14作为preview，在JDK15处于第二轮preview，在JDK16转正
JEP 396: Strongly Encapsulate JDK Internals by Default
对内部的api进行更多的封装，鼓励开发者从使用内部的方法迁移到标准的API，但是
sun.misc.Unsafe还是继续保留
JEP 397: Sealed Classes (Second Preview)
Sealed Classes在JDK15作为preview引入，在JDK16作为第二轮preview
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 16 Release Notes，这里举几个例子。
添加项
- Add InvocationHandler::invokeDefault Method for Proxy's Default Method Support (JDK-8159746)
给InvocationHandler添加invokeDefault方法
- Day Period Support Added to java.time Formats (JDK-8247781)
java.time支持Day Period
- Add Stream.toList() Method (JDK-8180352) (JDK-8180352)
Stream新增toList方法
- Concurrently Uncommit Memory in G1 (JDK-8236926)
针对G1提供了并发归还内存给操作系统
移除项
- Removal of Experimental Features AOT and Graal JIT (JDK-8255616)
移除jaotc工具
- Deprecated Tracing Flags Are Obsolete and Must Be Replaced With Unified Logging Equivalents (JDK-8256718)
使用
-Xlog:class+load=info替代-XX:+TraceClassLoading；使用-Xlog:class+unload=info替代-XX:+TraceClassUnloading；使用-Xlog:exceptions=info替代-XX:+TraceExceptions
废弃项
- Terminally Deprecated ThreadGroup stop, destroy, isDestroyed, setDaemon and isDaemon (JDK-8256643)
废弃ThreadGroup的stop, destroy, isDestroyed, setDaemon, isDaemon方法
- Deprecated the java.security.cert APIs That Represent DNs as Principal or String Objects (JDK-8241003)
废弃了java.security.cert.X509Certificate的getIssuerDN()、getSubjectDN()方法
已知问题
- Incomplete Support for Unix Domain Sockets in Windows 2019 Server (JDK-8259014)
Unix Domain Sockets对Windows 2019 Server的支持还不完善
- TreeMap.computeIfAbsent Mishandles Existing Entries Whose Values Are null (JDK-8259622)
TreeMap.computeIfAbsent方法针对null的处理与规范有偏差
其他事项
- Enhanced Support of Proxy Class (JDK-8236862)
对Proxy Class进行了增强，支持
jdk.serialProxyInterfaceLimit属性
- Support Supplementary Characters in String Case Insensitive Operations (JDK-8248655)
对compareToIgnoreCase、equalsIgnoreCase、regionMatches方法的Case Insensitive语义进行了增强
- HttpClient.newHttpClient and HttpClient.Builder.build Might Throw UncheckedIOException (JDK-8248006)
HttpClient.newHttpClient及HttpClient.Builder.build方法可能抛出UncheckedIOException
- The Default HttpClient Implementation Returns Cancelable Futures (JDK-8245462)
默认的HttpClient实现返回Cancelable Futures
小结
Java16主要有如下几个特性
- JEP 338: Vector API (Incubator)
- JEP 347: Enable C++14 Language Features
- JEP 357: Migrate from Mercurial to Git
- JEP 369: Migrate to GitHub
- JEP 376: ZGC: Concurrent Thread-Stack Processing
- JEP 380: Unix-Domain Socket Channels
- JEP 386: Alpine Linux Port
- JEP 387: Elastic Metaspace
- JEP 388: Windows/AArch64 Port
- JEP 389: Foreign Linker API (Incubator)
- JEP 390: Warnings for Value-Based Classes
- JEP 392: Packaging Tool
- JEP 393: Foreign-Memory Access API (Third Incubator)
- JEP 394: Pattern Matching for instanceof
- JEP 395: Records
- JEP 396: Strongly Encapsulate JDK Internals by Default
- JEP 397: Sealed Classes (Second Preview)
