JDK 20 Release Candidate and Deprecation

[David Delabassee <da...@oracle.com>]

Welcome to the latest OpenJDK Quality Outreach update!

The first Release Candidates of JDK 20 have been released [1] as per the 
schedule [2]. At this stage, only P1 issues will be evaluated. And with 
the JDK 20 General Availability sets for March 21st, it is now time to 
fully focus on JDK 21. I'd like to thank those of you who have already 
provided feedback on the Early Builds of JDK 21. Feedback is always 
extremely useful, even more, when it comes early in the development cycle.

We are always thinking about the future but the future is not limited to 
new features (pun intended). Properly removing legacy features from the 
platform is also critical. Deprecation has always been an important, 
phased, and ongoing effort. To name just two recent examples, 
`Thread.stop()` is removed in JDK 20 [3], and the URL Public 
Constructors are deprecated in JDK 20 (see the related heads-up below). 
It is important to prepare your codebase for such upcoming evolutions 
sooner rather than later. To conclude on deprecation, I'll mention my 
colleague Nicolai who recently did a full video on this exact topic, 
i.e. "Prepare your Codebase for the Future Now!" [4].

[1] https://mail.openjdk.org/pipermail/jdk-dev/2023-February/007364.html
[2] https://openjdk.org/projects/jdk/20/
[3] https://inside.java/2022/11/09/quality-heads-up/
[4] https://inside.java/2023/02/02/newscast-41/


## Heads-Up - JDK 20 - Deprecate URL Public Constructors

The `java.net.URL` class, dating from Java SE 1.0, does not encode or 
decode any URL components according to the RFC2396 escaping mechanism. 
It is the responsibility of the caller to encode any fields, which need 
to be escaped prior to calling URL, and also to decode any escaped 
fields that are returned from URL. This has led to many usability 
issues, including some potential vulnerabilities when the calling code 
did not take this into consideration.

In Java SE 1.4, the `java.net.URI` class has been added to mitigate some 
of the `java.net.URL` shortcomings. It also offers methods to create an 
URL from an URI.

JDK 20 will deprecate all public constructors of `java.net.URL`. This 
will provide a strong warning and discourage developers from using them. 
To construct a URL, the `URI::toURL` alternative should instead be 
preferred. To construct a `file:` based URL, `Path::toURI` should be 
used prior to `URI::toURL`.

For more details, see https://bugs.openjdk.org/browse/JDK-8294241


## Heads-Up - JDK 20 - JMX Connections Use an ObjectInputFilter by Default

The default JMX agent now sets an ObjectInputFilter on the RMI 
connection to restrict the types that the server will deserialize. This 
should not affect normal usage of the MBeans in the JDK. Applications 
which register their own MBeans in the platform MBeanServer may need to 
extend the serialization filter to support any additional types that 
their custom MBeans accept as parameters. The default filter already 
covers any type that OpenMBeans and MXBeans might use.

The serialization filter pattern is set in 
`JDK/conf/management/management.properties` using the property 
`com.sun.management.jmxremote.serial.filter.pattern`. If additional Java 
types need to be passed, the default can be overridden by running with 
`-Dcom.sun.management.jmxremote.serial.filter.pattern=.`

Serialization Filtering and the filter pattern format are described in 
detail in the Core Libraries Guide [5].

[5] 
https://docs.oracle.com/en/java/javase/19/core/serialization-filtering1.html#GUID-55BABE96-3048-4A9F-A7E6-781790FF3480


## Heads-Up - Testing Loom: Scoped Values and Structured Concurrency

With one JEP in Preview (Virtual Threads - 2nd Preview) and two JEPs 
incubating (Scoped Values - Incubator & Structured Concurrency - 2nd 
Incubator) Loom made considerable progress in JDK 20. The Loom team is 
always eager to hear from developers experimenting with those APIs, 
especially given that both Scoped Values and Structured Concurrency 
might become Preview in JDK 21. Feedback should be reported to the 
loom-dev [6] mailing list.

[6] https://mail.openjdk.org/pipermail/loom-dev/


## JDK 20 Release Candidate builds

The Release Candidate builds (builds 36) are available [7] and are 
provided under the GNU General Public License v2, with the Classpath 
Exception. The Release Notes are available here [8].

[7] https://jdk.java.net/20/
[8] https://jdk.java.net/20/release-notes

### Changes in recent JDK 20 builds that may be of interest:

- JDK-8300623: Lambda deserialization regression involving Enum method 
reference
- JDK-8298400: Virtual thread instability when stack overflows
- JDK-8298377: JfrVframeStream causes deadlocks in ZGC


## JDK 21 Early-Access builds

The JDK 21 Early-Access (builds 9) are available [9], and are provided 
under the GNU General Public License v2, with the Classpath Exception. 
The related Javadocs are available here [10] and the Release Notes here 
[11].

[9] https://jdk.java.net/21/
[10] https://download.java.net/java/early_access/jdk21/docs/api/
[11] https://jdk.java.net/21/release-notes

### Changes in recent JDK 21 builds that may be of interest:

- JDK-8299891: JMX ObjectInputFilter additional classes needed [Reported 
by Apache Derby]
- JDK-8298478: (fs) Path.of should allow input to include long path prefix
- JDK-8300869: Make use of the Double.toString(double) algorithm in java.ut…
- JDK-8300891: Deprecate for removal 
javafx21x.swing.plaf.synth.SynthLookAndFee…
- JDK-8286907: keytool should warn about weak PBE algorithms
- JDK-8298445: Add LeakSanitizer support in HotSpot
- JDK-8288050: SunJCE provider now supports SHA-512/224 and SHA-512/256 
as digests for the PBES2 algorithms
- JDK-8301207: (jdeps) Deprecate jdeps -profile option
- JDK-8300247: Harden C1 xchg on AArch64 and PPC
- JDK-8208077: File::listRoots Changed To Return All Available Drives On 
Windows
- JDK-8300623: Lambda deserialization regression involving Enum method 
reference
- JDK-8294680: Refactor scaled border rendering
- JDK-8300584: Accelerate AVX-512 CRC32C for small buffers
- JDK-8299896: Reduce enum values of HtmlLinkInfo.Kind
- JDK-8300266: Detect Virtualization on Linux aarch64
- JDK-8298908: Instrument Metaspace for Asan


## Generational ZGC Early-Access Builds

The latest Early-Access builds (build 21-genzgc+1-8 - 2023/1/27) of 
Generational ZGC [12] are available [13]. Those builds are based on an 
incomplete version of JDK 21 and are provided under the GNU General 
Public License, version 2, with the Classpath Exception.

Feedback should be sent to the zgc-dev mailing list [14].

[12] https://bugs.openjdk.org/browse/JDK-8272979
[13] https://jdk.java.net/genzgc/
[14] https://mail.openjdk.org/pipermail/zgc-dev


## JavaFX 21 Early-Access Builds

The Early-Access builds of the JavaFX runtime, built from openjdk/jfx 
[15], allow JavaFX application developers to build and test their 
applications with JavaFX 21 on JDK 21. The latest Early-Access builds 
(build 2 2023/1/27) are now available [16] under the GNU General Public 
License, version 2, with the Classpath Exception. The related Javadocs 
are available here [17].

Feedback should be sent to the openjfx-dev mailing list [18].

[15] https://github.com/openjdk/jfx
[16] https://jdk.java.net/javafx21/
[17] 
https://download.java.net/java/early_access/javafx21/docs/api/index.html
[18] http://mail.openjdk.org/mailman/listinfo/openjfx-dev


## Topics of Interest:

- JDK 21 - Image Performance Improvements
https://minborgsjavapot.blogspot.com/2023/02/jdk-21-image-performance-improvements.html

- JDK 21 - Performance Improvements Revealed
https://minborgsjavapot.blogspot.com/2023/01/java-21-performance-improvements.html

- From Java Security with Love - Inside Java Newscast #42
https://inside.java/2023/02/14/newscast-42/

- ZGC - The Future of Low-Latency Garbage Collection Is Here
https://inside.java/2023/01/25/zgc/

- Draft JEP: JDK Packaging Guidelines
https://mail.openjdk.org/pipermail/jdk-dev/2023-February/007327.html

- Future Java - Prepare Your Codebase Now! - Inside Java Newscast #41
https://inside.java/2023/02/02/newscast-41/

- Java Modules in Real Life
https://inside.java/2023/01/29/java-modules-in-real-life/

~

Thanks for participating in the OpenJDK Quality Outreach program. And as 
always, if you find an issue, please let us know through the usual channels.

--
David