Friday, December 20, 2019

General, Safe, and Deterministic Foreign Memory Access in JDK 14

In the post "JDK 14 Rampdown: Build 27," I summarized the numerous JDK 14-targeted features newly available with JDK 14 Early Access Build #27. There is already another JDK 14 Early Access Build available and this one [Build 28 (2019/12/18)] includes one particularly interesting feature: Enhancement JDK-8234049 ["Implementation of Memory Access API (Incubator)"]. This is the implementation of JEP 370 ["Foreign-Memory Access API (Incubator)"], which is summarized as, "Introduce an API to allow Java programs to safely and efficiently access foreign memory outside of the Java heap."

JEP 370 is intended to achieve a "foreign memory API" that provides "generality", "safety", and "determinism" (with each of these means described in further detail in the JEP). The JEP also states that this foreign memory API is intended as an alternative to currently used approaches (java.nio.ByteBuffer and sun.misc.Unsafe). Downsides and limitations of both of these currently available alternates are discussed in the JEP.

The JEP points out that libraries "access foreign memory" to "avoid the cost and unpredictability associated with garbage collection," to "share memory across multiple processes," and to "serialize and deserialize memory content by mapping files into memory." It adds that "the Java API does not provide a satisfactory solution for accessing foreign memory."

JEP 370's "Description" section introduces the implementation of the foreign memory access API. The section begins, "The foreign-memory access API introduces three main abstractions: MemorySegment, MemoryAddress and MemoryLayout." The section then describes these three main abstractions: "A MemorySegment is used to model a contiguous memory region with given spatial and temporal bounds. A MemoryAddress can be thought of as an offset within a segment. Finally, a MemoryLayout is a programmatic description of a memory segment's contents."

After several detailed examples of using the foreign memory access, the JEP's "Description" section states, "The foreign-memory access API will initially be provided as an incubating module, named jdk.incubator.foreign, in a package of the same name."

Additional background details related to JEP 370 are available in OpenJDK mailing list messages. One key thread starts with the message "RFR JDK-8234049: Implementation of Memory Access API (Incubator)." The CSR for this JEP [JDK-8234050: Implementation of Memory Access API (Incubator)] presents alternate useful descriptions. Javadoc-based documentation for the foreign memory access API is also temporarily available at

The JEP 370 implementation is included in the JDK 14 Early Access Build #28 and available to try.

Saturday, December 14, 2019

JDK 14 Rampdown: Build 27

Mark Reinhold's recent post "JDK 14 is now in Rampdown Phase One" announced that "we are now in Rampdown Phase One" and that "the overall feature set is frozen." JDK 14 Early Access Build #27 (2019/12/12) is a hefty build. In this post, I summarize some of the changes associated with JDK 14 Early Access Build 27 that are particularly interesting to me.

One might say that JDK 14 Early Access Build 27 is the "records build." This build is full of updates related to the implementation of the records preview feature (JEP 359). Here are some of the changes associated with this "records build" filled with JEP 359 implementation changes (note that there are already bugs written and fixed for records):

  • JEP 359 JDK-8222777: Records (Preview)
  • Sub-Task JDK-8227113: Specification for java.lang.Record
  • Sub-Task JDK-8225053: Preview APIs support for records
  • Sub-Task JDK-8233526: JVM support for records
  • Sub-Task JDK-8225054: Compiler implementation for records
  • Sub-Task JDK-8225057: Java Language Specification changes for Records
  • Sub-Task JDK-8225058: JVM Specification changes for Records
  • Sub-Task JDK-8225055: Javadoc for records
  • Sub-Task JDK-8225052: javax.lang.model support for records
  • Sub-Task JDK-8226314: com.sun.source support for records
  • Enhancement JDK-8235359: Simplify method Class.getRecordComponents()
  • Bug JDK-8235550: Clarify record reflective support specification
  • Bug JDK-8235369: Class.toGenericString need to be updated for records
  • Bug JDK-8235446: confusing error message for records with no parens
  • Bug JDK-8234101: Compilation error thrown when repeating annotation is used on record component
  • Bug JDK-8235474: JShell does not handle records properly

Although JEP 359/Records (Preview) will likely dominate interest of many Java developers in the JDK 14 Early Access Build 27, there are other interesting changes in this build that are summarized here (with links to more details and quotes from the change descriptions):

In addition to the changes that I called out above, other changes associated with JDK 14 Early Access Build 27 include changes to areas that have changes in many JDK 14 builds (Shenandoah garbage collector, Z garbage collector [ZGC], Java Flight Recorder, and the Java packaging tool).

The JDK 14 Early Access Build 27 Release Notes state that "SSLv2Hello and SSLv3 have been removed from the default enabled TLS protocols." The release notes add that "If a client or server still needs to use the SSLv3 protocol they can do so by enabling it via the jdk.tls.client.protocols or jdk.tls.server.protocols system properties or with the SSLSocket.setEnabledProtocols(), SSLServerSocket.setEnabledProtocols() and SSLEngine.setEnabledProtocols() APIs."

I am excited about Records in Java 14 and look forward to playing with them via the OpenJDK 14 Early Access Build 27.