Oracle JDK 16迁移指南

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"JDK16-jdk-migration-guide.pdf" 这篇文档是Oracle JDK迁移指南，针对Java Platform, Standard Edition (Java SE) 的第16个版本。这份指南旨在帮助开发者从先前的JDK版本平滑过渡到JDK 16，以充分利用新版本带来的功能改进和性能提升。在JDK 16中，有几个重要的特性更新和变化值得注意： 1. **Records（记录类）**：JDK 16引入了Records的概念，这是一种新的类概念，用于简化无状态数据容器的表示。记录类自动提供了构造函数、equals()、hashCode() 和toString() 方法，从而减少了样板代码。 2. **Vector API增强**：虽然Vector类自Java 1.0以来就存在，但在JDK 16中，对Vector进行了性能优化和API扩展，使其更符合现代多线程编程的需求。 3. **JEP 395: Strongly-TypedHTTP Client API (Incubator)**：这个特性引入了一个新的HTTP客户端API，它提供了一种类型安全的方式来处理HTTP请求和响应，增强了Java在网络编程中的能力。 4. **JEP 384: Foreign-Memory Access API (Incubator)**：允许Java程序直接访问操作系统内存，从而提高与C/C++等语言的互操作性和高效数据处理。 5. **JEP 385: JEP 385: JLink: The Java Linker**：JLink工具使得构建模块化应用更加容易，允许开发者创建定制的运行时图像，只包含应用程序所需的模块和依赖。 6. **JEP 382: Pattern Matching for instanceof (Preview)**：这个特性扩展了`instanceof`关键字，支持模式匹配，使得代码更加简洁和可读。 7. **JEP 360: Switch Expressions (Second Preview)**：Switch表达式在JDK 14中首次作为预览特性引入，在JDK 16中继续作为预览，提供了更强大的控制流结构。 8. **JEP 371: macOS/AArch64 Port**：JDK 16支持macOS上的AArch64架构，即Apple M1芯片。 9. **JEP 373: JFR Event Streaming**：Java飞行记录（JFR）现在可以实时流式传输事件，这对于监控和诊断生产环境中的问题非常有用。 10. **JEP 378: ZGC: A Scalable Low-Latency Garbage Collector (Production)**：ZGC垃圾收集器进入生产阶段，它是一个低延迟的垃圾收集器，尤其适合大内存应用。迁移过程中，开发者需要考虑这些新特性的兼容性，以及可能对现有代码库的影响。例如，如果代码中大量使用了旧版的HTTP客户端，可能需要进行重构以利用新的HTTP客户端API。同时，对于使用了反射或JNI来访问内存的代码，Foreign-Memory Access API可能提供了更安全、高效的替代方案。为了确保顺利迁移，开发者应先进行详尽的测试，包括兼容性测试、性能测试以及功能验证。在升级前，了解新特性并评估其对项目潜在价值是至关重要的。此外，注意遵循Oracle提供的许可证条款，尊重知识产权，并遵循针对美国政府用户的特定条款和条件。

Significant Changes in the JDK

Before migrating your application to the latest JDK release, you must understand what

the updates and changes are between it and the previous JDK release. If you are

migrating from JDK 8, you should also be familiar with the differences between JDK 8

and later releases that are described in Migrating From JDK 8 to Later JDK Releases.

See the following sections to learn about some of the significant changes in latest JDK

releases.

Significant Changes in JDK 16 Release

See JDK 16 Release Notes for additional descriptions of the new features and

enhancements, and API specification in JDK 16.

The following are some of the updates in Java SE 16 and JDK 16:

• The Java programming language is enhanced with pattern matching for the

instanceof

operator. This feature allows common logic in a program, namely the

conditional extraction of components from objects, to be expressed more concisely

and safely. See JEP 394: Pattern Matching for instanceof and Pattern Matching for

instanceof in Java Platform, Standard Edition Java Language Updates guide.

• Records, first previewed in Java SE 14, is a permanent feature in this release.

The earlier restrictions have been relaxed whereby the inner classes can declare

members that are either explicitly or implicitly static. This includes record class

members, which are implicitly static. See JEP 395: Records and Record Classes

in Java Platform, Standard Edition Java Language Updates guide.

• By default, all internal elements of the JDK are strongly encapsulated, except for

critical internal APIs such as

sun.misc.Unsafe

. You can choose the relaxed strong

encapsulation that has been the default since JDK 9. See JEP 396: Strongly

Encapsulate JDK Internals by Default and Illegal Reflective Access.

• UNIX domain socket channels have been integrated into JDK 16. See JEP 380:

Unix-Domain Socket Channels and Internet Protocol and UNIX Domain Sockets

NIO Example in Java Platform, Standard Edition Core Libraries.

• The Z Garbage Collector processes the thread stacks concurrently. This allows all

roots in the JVM to be processed by ZGC in a concurrent phase. See JEP 376:

ZGC: Concurrent Thread-Stack Processing and The Z Garbage Collector in Java

Platform, Standard Edition HotSpot Virtual Machine Garbage Collection Tuning

Guide.

• The

jpackage

tool, which was incubated in JDK 14, is now a permanent feature.

The tool packages a Java application into a platform-specific package that

includes the necessary dependencies. See JEP 392: Packaging Tool and Java

Platform, Standard Edition Packaging Tool User's Guide.

• Elastic metaspace overhauls the VM-internal metaspace- and class-space-

implementation. The unused HotSpot class-metadata (that is metaspace) memory

is returned to the operating system. It reduces the metaspace footprint and

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simplify the metaspace code in order to reduce maintenance costs. See JEP 387:

Elastic Metaspace.

Preview Features and Incubator Modules

See Java Language Preview Feature for more information about preview features.

• Sealed classes, a preview feature from JDK 15, is re-previewed in this release.

Sealed classes and interfaces restrict which other classes or interfaces may

extend or implement them. There has been several refinements in this release,

including the introduction of character sequences

sealed

non-sealed

, and

permits

as contextual keywords. See JEP 397: Sealed Classes (Second Preview)

and Sealed Classes in Java Platform, Standard Edition Java Language Updates

guide.

• Initial iteration of an incubator module,

jdk.incubator.vector

, is provided to

express vector computations. It reliably compiles at runtime to optimal vector

hardware instructions on supported CPU architectures and thus achieve superior

performance to equivalent scalar computations. See JEP 338: Vector API

(Incubator).

• Foreign Linker API is introduced that offers statically-typed, pure-Java access to

native code. This API, along with the Foreign-Memory Access API (JEP 393), will

simplify the otherwise error-prone process of binding to a native library. See JEP

389: Foreign Linker API (Incubator).

• Foreign-Memory Access API allows Java programs to safely and efficiently access

foreign memory outside of the Java heap. See JEP 393: Foreign-Memory Access

API (Third Incubator).

Removed APIs, Tools, and Components

See:

• API Removed in Java SE 16

• Tools and Components Removed in JDK 16

In addition, there are security related updates that you need to be aware of. See:

Security Updates in JDK 16.

Significant Changes in JDK 15 Release

See JDK 15 Release Notes for the complete list of new features and enhancements in

JDK 15.

The following are some of the updates in Java SE 15 and JDK 15:

• Text Blocks, first previewed in Java SE 13, is a permanent feature in this release

and can be used without enabling preview features.

Text blocks are multiline string literals that avoid the need for most escape

sequences, automatically format the string in a predictable way, and give the

developer control over the format when desired. See JEP 378: Text Blocks and

Programmer's Guide to Text Blocks.

• The Z Garbage Collector (ZGC) is ready to use in production and no longer

an experimental feature. Enable ZGC by using the command-line option

XX:+UseZGC

. See JEP 377: ZGC: A Scalable Low-Latency Garbage Collector

(Production).

Chapter 2

Significant Changes in JDK 15 Release

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