java-26

Java 26

To run each example use: java --enable-preview --source 26 <FileName.java>

JEPs

• 500 - Prepare to Make Final Mean Final
• 504 - Remove the Applet API
• 516 - Ahead-of-Time Object Caching with Any GC
• 517 - HTTP/3 for the HTTP Client API
• 522 - G1 GC: Improve Throughput by Reducing Synchronization
• 524 - PEM Encodings of Cryptographic Objects (Second Preview)
• 525 - Structured Concurrency (Sixth Preview)
• 526 - Lazy Constants (Second Preview)
• 529 - Vector API (Eleventh Incubator)
• 530 - Primitive Types in Patterns, instanceof, and switch (Fourth Preview)

Features

• Prepare to Make Final Mean Final
  • issue warnings about use of deep reflection to mutate final fields
  • prepare developers for a future release that ensures integrity by default by restricting final field mutation
  • goals:
    • prepare Java ecosystem for the future where, by default, disallows the mutation of final fields by deep reflection
    • align final fields of classes with the implicity declared fields of record classes, which cannot be mutated
    • allow serialization to continue working with Serializable classes, even classes with final fields
  • the mutation of final fields by deep reflection was introduced in JDK 5 to allow serialization to initialize the objects with final fields
    • Field.setAccessible(boolean)
  • deep reflection is disallowed in hidden and record classes
  • the JVM cannot consider a final field as really final, thus optimizations are not applied
  • by default, JVM will print a warning every time that a final field is mutated
    • in the future, an exception will be thrown
  • enabling final field mutation
    • to avoid the warning, and the exception in the future, we can pass inform the JVM with a new flag enable-final-field-mutation
      • we can enable all modules or a specific module
      • java --enable-final-field-mutation=ALL-UNNAMED
      • java --enable-final-field-mutation=M1,M2
    • to allow the mutation, we must enable with the flag and the module enabled also must be open to deep reflection
    • we have different options to pass this new flag:
      • passing the flag in the env JDK_JAVA_OPTIONS
      • passing the flag in the argument file
      • adding Enable-Final-Field-Mutation with value ALL-UNNAMED (only accepted option) in the manifest of a JAR file
      • passing the flag to jlink via the --add-options option
      • in JNI, passing the flag to JVM creation
  • final field mutation restrictions
    • it's illegal to mutate a final field if either:
      • the code is in a module for which final field mutation is not enabled
      • the code is in a module to which the field's package is not open
    • the behavior of the JVM when an illegal final field mutation occurs is controled by a new flag illegal-final-field-mutation
    • the options are allow, warn, debug and deny
    • in JDK 26 the default option is warn, in the future will be deny
  • a new event in JDK Flight Recorder was added: jdk.FinalFieldMutation
  • serialization libraries should use sun.reflect.ReflectionFactory
    • it initialize the class instance bypassing the constructor and setting its fields directly without having to mutate it
    • this is the only mechanism to mutate final field if final field mutation is not enabled
    • it adds a restriction because the target class need to implements java.io.Serializable interface
    • the JVM will treat the final field as immutable if it detects that a handle returned by ReflectionFactory is not used to mutate a final field
• Ahead-of-Time Object Caching with Any GC
  • enhance the AOT cache (improves startup and warmup time) to be used with any GC
  • this is done by making possible to load cached objects sequentially into memory
    • using a GC-agnostic format
    • previously the objects were mapped into memory using GC-specific format (bitwise heap format)
  • main goal is to allow all GC to work with AOT cache introduced by Project Leyden
    • this step will be optional though
  • the challenge is how to handle object references, because it depends on each GC and its policies
  • to achieve this the AOT cache will use logical indices in a streaming format to store the objects references
  • we can use the parameter -XX:+AOTStreamableObjects to create a cache in a GC-agnostic format
• HTTP/3 for the HTTP Client API
  • added support for HTTP/3 in the HTTP Client API
  • HTTP/3 was standardized in 2022
  • setting version in HttpClient builder:

    • var client = HttpClient.newBuilder()
          .version(HttpClient.Version.HTTP_3)
          .build();

  • setting version in HttpRequest builder:

    • var request = HttpRequest.newBuilder(URI.create("https://example.com"))
          .version(HttpClient.Version.HTTP_3)
          .GET()
          .build();

• G1 GC: Improve Throughput by Reducing Synchronization
  • increase application throughput when using G1 GC by reducing the amount of synchronization required between application threads and GC threads
  • goals:
    • reduce the G1 GC's synchronization overhead
    • reduce the size of the injected code for G1's write barriers
    • maintain the overall architecture of G1
  • G1 will use a second card table so that optimizer and application threads can run freely
  • the change will reduce overhead both while the application is running and during GC pauses
• Structured Concurrency
  • re-preview with minor changes
  • changes:
    • a new method Joiner.onTimeout to allow implementation to return a result when a timeout expires
    • Joiner.allSuccessfulOrThrow now returns a list of results rather than a stream of subtasks
    • Joiner.anySuccessfulResultOrThrow was renamed to anySuccessfulOrThrow
    • static method StructuredTaskScope.open that takes a Joiner and a function to modify the default configuration now takes a UnaryOperator instead of a Function
• Lazy Constants
  • re-preview with changes
  • Stable Values was renamed to Lazy Constants
  • changes from first preview:
    • changed the API to focus on high-level use cases
    • removed low-level methods like orElseSet, setOrThrow, trySet, function and intFunction
    • moved the factory methods for lazy lists and maps to List and Map interfaces
    • disallow null as a computed value to improve performance and align with constructs (unmodifiable collections, ScopedValue)
      • we must use Optional
  • now we always must create a lazy constant using of(Supplier), the previous unset state isn't allowed
  • get() will trigger the lazy constant initialization using the given supplier in factory method of(Supplier)
    • after calling the state will be initialized (isInitialized() will return true)
  • the lazy constant can be in an uninitialized state, when the method get() wasn't called yet
    • orElse(T) will return the given value in this case
• Primitive Types in Patterns, instanceof and switch
  • re-preview with two changes
  • enhance the definition of unconditional exactness
  • apply tighter dominance check in switch constructs by expanding the definition of dominance to also cover primitive type patterns
    • if a case label is float f, we cannot have a case label 42 because 42 will be captured by the first match

API

• removed Thread.stop method
• java.lang.Process now implements AutoCloseable and Closeable

Links

• JDK 26 - JEP Dashboard
• JDK 26 JEPs