Use Java release option #2825

vasilmkd · 2022-02-16T09:57:02Z

Example 1

The easy example first, APIs that don't exist on JDK 8 (VarHandle):

package io.vasilev;

import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;

public class VarHandles {
    private volatile int n;

    private static final VarHandle N;

    static {
        try {
            N = MethodHandles.lookup().findVarHandle(VarHandles.class, "n", int.class);
        } catch (Exception e) {
            throw new ExceptionInInitializerError(e);
        }
    }
}

Compiling:

javac io/vasilev/VarHandles.java --release 8
io/vasilev/VarHandles.java:4: error: cannot find symbol
import java.lang.invoke.VarHandle;
                       ^
  symbol:   class VarHandle
  location: package java.lang.invoke
io/vasilev/VarHandles.java:9: error: cannot find symbol
    private static final VarHandle N;
                         ^
  symbol:   class VarHandle
  location: class VarHandles
io/vasilev/VarHandles.java:13: error: cannot find symbol
            N = MethodHandles.lookup().findVarHandle(VarHandles.class, "n", int.class);
                                      ^
  symbol:   method findVarHandle(Class<VarHandles>,String,Class<Integer>)
  location: class Lookup
3 errors

Example 2

Our favorite example, ByteBuffer#flip() on JDK 9+ overrides Buffer#flip() with a more specific type.

On JDK 8 we have:
1. Buffer.flip() has the JVM signature java/nio/Buffer.flip:()Ljava/nio/Buffer
2. ByteBuffer.flip() has the JVM signature java/nio/ByteBuffer.flip:()L/java/nio/Buffer
On JDK 9+ we have:
1. Buffer.flip() has the same JVM signature java/nio/Buffer.flip:()Ljava/nio/Buffer
2. ByteBuffer.flip() has the new JVM signature java/nio/ByteBuffer.flip:()L/java/nio/ByteBuffer and thus cannot be called on JDK 8.

Let's examine the output of javac --relase 8 in this case, when we have an explicit return type in the code.

package io.vasilev;

import java.nio.ByteBuffer;

public class Buffers {

    private static final ByteBuffer buffer = ByteBuffer.allocate(16);

    static {
        ByteBuffer flipped = buffer.flip();
    }
}

io/vasilev/Buffers.java:10: error: incompatible types: Buffer cannot be converted to ByteBuffer
        ByteBuffer flipped = buffer.flip();
                                        ^
1 error

Example 3

Final example, what if we do not have an explicit return type and let Java figure it out. In this case we have to examine the produced bytecode, because javac won't complain in this case, because we have a method in JDK 8 that syntactically matches the code.

package io.vasilev;

import java.nio.ByteBuffer;

public class Buffers {

    private static final ByteBuffer buffer = ByteBuffer.allocate(16);

    static {
        buffer.flip();
    }
}

Let's compile with JDK 17, without the --release 8 option.

javac io/vasilev/Buffers.java  

javap -p -c io.vasilev.Buffers 

Compiled from "Buffers.java"
public class io.vasilev.Buffers {
  private static final java.nio.ByteBuffer buffer;

  public io.vasilev.Buffers();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  static {};
    Code:
       0: bipush        16
       2: invokestatic  #7                  // Method java/nio/ByteBuffer.allocate:(I)Ljava/nio/ByteBuffer;
       5: putstatic     #13                 // Field buffer:Ljava/nio/ByteBuffer;
       8: getstatic     #13                 // Field buffer:Ljava/nio/ByteBuffer;
      11: invokevirtual #19                 // Method java/nio/ByteBuffer.flip:()Ljava/nio/ByteBuffer; <------- notice the new JDK 9+ method with overloaded return type, which cannot be called on JDK 8.
      14: pop
      15: return
}

Let's now compile with JDK 17 with the --release 8 option.

javac io/vasilev/Buffers.java --release 8

javap -p -c io.vasilev.Buffers

Compiled from "Buffers.java"
public class io.vasilev.Buffers {
  private static final java.nio.ByteBuffer buffer;

  public io.vasilev.Buffers();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  static {};
    Code:
       0: bipush        16
       2: invokestatic  #7                  // Method java/nio/ByteBuffer.allocate:(I)Ljava/nio/ByteBuffer;
       5: putstatic     #13                 // Field buffer:Ljava/nio/ByteBuffer;
       8: getstatic     #13                 // Field buffer:Ljava/nio/ByteBuffer;
      11: invokevirtual #19                 // Method java/nio/ByteBuffer.flip:()Ljava/nio/Buffer;  <------ notice that javac used the JDK 8 compatible method
      14: pop
      15: return
}

armanbilge · 2022-02-16T10:28:24Z

Seems like fs2 could benefit from this?

vasilmkd · 2022-02-16T10:39:41Z

Scala 3

Example 1

package io.vasilev

import java.lang.invoke.{MethodHandles, VarHandle}

object VarHandles {

  @volatile private[this] var n: Int = 0

  private[this] final val N: VarHandle = MethodHandles.lookup().findVarHandle(classOf[VarHandles.type], "n", classOf[Int])
}

scalac io/vasilev/VarHandles.scala --release 8
-- [E006] Not Found Error: io/vasilev/VarHandles.scala:9:29 --------------------
9 |  private[this] final val N: VarHandle = MethodHandles.lookup().findVarHandle(classOf[VarHandles.type], "n", classOf[Int])
  |                             ^^^^^^^^^
  |                             Not found: type VarHandle

longer explanation available when compiling with `-explain`
-- [E008] Not Found Error: io/vasilev/VarHandles.scala:9:64 --------------------
9 |  private[this] final val N: VarHandle = MethodHandles.lookup().findVarHandle(classOf[VarHandles.type], "n", classOf[Int])
  |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |value findVarHandle is not a member of java.lang.invoke.MethodHandles#Lookup
2 errors found

Example 2

package io.vasilev

import java.nio.ByteBuffer

object Buffers {
  private[this] final val buffer: ByteBuffer = ByteBuffer.allocate(16)

  val flipped: ByteBuffer = buffer.flip()
}

scalac io/vasilev/Buffers.scala --release 8 
-- [E007] Type Mismatch Error: io/vasilev/Buffers.scala:8:39 -------------------
8 |  val flipped: ByteBuffer = buffer.flip()
  |                            ^^^^^^^^^^^^^
  |                            Found:    java.nio.Buffer
  |                            Required: java.nio.ByteBuffer

longer explanation available when compiling with `-explain`
1 error found

Example 3

package io.vasilev

import java.nio.ByteBuffer

object Buffers {
  private[this] final val buffer: ByteBuffer = ByteBuffer.allocate(16)

  buffer.flip()
}

Compiling without -release 8 on JDK 17

scalac io/vasilev/Buffers.scala
javap -p -c io.vasilev.Buffers\$
Compiled from "Buffers.scala"
public final class io.vasilev.Buffers$ implements java.io.Serializable {
  public static final io.vasilev.Buffers$ MODULE$;

  private io.vasilev.Buffers$();
    Code:
       0: aload_0
       1: invokespecial #13                 // Method java/lang/Object."<init>":()V
       4: return

  public static {};
    Code:
       0: new           #2                  // class io/vasilev/Buffers$
       3: dup
       4: invokespecial #16                 // Method "<init>":()V
       7: putstatic     #18                 // Field MODULE$:Lio/vasilev/Buffers$;
      10: bipush        16
      12: invokestatic  #24                 // Method java/nio/ByteBuffer.allocate:(I)Ljava/nio/ByteBuffer;
      15: astore_0
      16: aload_0
      17: invokevirtual #28                 // Method java/nio/ByteBuffer.flip:()Ljava/nio/ByteBuffer; <----- Notice the new JVM signature that only works on JDK 9+
      20: pop
      21: return

  private java.lang.Object writeReplace();
    Code:
       0: new           #34                 // class scala/runtime/ModuleSerializationProxy
       3: dup
       4: ldc           #2                  // class io/vasilev/Buffers$
       6: invokespecial #37                 // Method scala/runtime/ModuleSerializationProxy."<init>":(Ljava/lang/Class;)V
       9: areturn
}

Compiling with -release 8 on JDK 17

scalac io/vasilev/Buffers.scala -release 8
javap -p -c io.vasilev.Buffers\$
Compiled from "Buffers.scala"
public final class io.vasilev.Buffers$ implements java.io.Serializable {
  public static final io.vasilev.Buffers$ MODULE$;

  private io.vasilev.Buffers$();
    Code:
       0: aload_0
       1: invokespecial #13                 // Method java/lang/Object."<init>":()V
       4: return

  public static {};
    Code:
       0: new           #2                  // class io/vasilev/Buffers$
       3: dup
       4: invokespecial #16                 // Method "<init>":()V
       7: putstatic     #18                 // Field MODULE$:Lio/vasilev/Buffers$;
      10: bipush        16
      12: invokestatic  #24                 // Method java/nio/ByteBuffer.allocate:(I)Ljava/nio/ByteBuffer;
      15: astore_0
      16: aload_0
      17: invokevirtual #28                 // Method java/nio/ByteBuffer.flip:()Ljava/nio/Buffer; <------ Notice the old JVM signature that works on JDK 8
      20: pop
      21: return

  private java.lang.Object writeReplace();
    Code:
       0: new           #34                 // class scala/runtime/ModuleSerializationProxy
       3: dup
       4: ldc           #2                  // class io/vasilev/Buffers$
       6: invokespecial #37                 // Method scala/runtime/ModuleSerializationProxy."<init>":(Ljava/lang/Class;)V
       9: areturn
}

vasilmkd · 2022-02-16T10:46:49Z

@mpilquist

vasilmkd · 2022-02-16T11:00:14Z

And with this, I will be deleting JDK 8 from my machine.

vasilmkd · 2022-02-16T11:57:30Z

scala/scala#6362

mpilquist · 2022-02-16T13:05:11Z

I don’t think this helps with fs2 where we use newer JDK APIs but want a JAR that’s 8 compatible. It should work for scodec though.

djspiewak

This is super impressive! Two concerns. First, can we sanity-check the ByteBuffer scenario on Scala 2 as well as Scala 3? Or did you already do that and I missed it? Second, I'd rather see this done as a one-off autoplugin rather than a settings val like this.

djspiewak · 2022-02-21T19:33:09Z

build.sbt

@@ -255,6 +255,32 @@ val jsProjects: Seq[ProjectReference] =
 val undocumentedRefs =
  jsProjects ++ Seq[ProjectReference](benchmarks, example.jvm, tests.jvm, tests.js)

+lazy val releaseSettings = Seq(


This can be achieved more easily by tossing a one-off autoplugin into project/

djspiewak · 2022-02-21T19:33:50Z

build.sbt

  .jvmSettings(
-    javacOptions ++= Seq("-source", "1.8", "-target", "1.8")
+    javacOptions ++= {


IMO we should add these to the aforementioned autoplugin and apply them to all projects just so we don't accidentally forget to add them in the future.

vasilmkd · 2022-02-21T19:36:20Z

@djspiewak Please read the first post carefully. ByteBuffer is covered.

djspiewak · 2022-02-21T19:37:29Z

@vasilmkd I was mostly concerned about Scala 2, since the OP output looks more like Scala 3's error messages and they have two different backends. If both have been sanity checked though I'm very strongly 👍

vasilmkd · 2022-02-21T19:37:52Z

I'll post Scala 2 too.

vasilmkd · 2022-02-21T19:45:24Z

Scala 2

Example 1

package io.vasilev

import java.lang.invoke.{ MethodHandles, VarHandle }

object VarHandles {

  @volatile private[this] var n: Int = 0

  private[this] final val N: VarHandle = MethodHandles.lookup().findVarHandle(classOf[VarHandles.type], "n", classOf[Int])
}

scalac io/vasilev/VarHandles.scala --release 8 
io/vasilev/VarHandles.scala:3: error: object VarHandle is not a member of package invoke
import java.lang.invoke.{ MethodHandles, VarHandle }
       ^
io/vasilev/VarHandles.scala:9: error: not found: type VarHandle
  private[this] final val N: VarHandle = MethodHandles.lookup().findVarHandle(classOf[VarHandles.type], "n", classOf[Int])
                             ^
io/vasilev/VarHandles.scala:9: error: value findVarHandle is not a member of java.lang.invoke.MethodHandles.Lookup
  private[this] final val N: VarHandle = MethodHandles.lookup().findVarHandle(classOf[VarHandles.type], "n", classOf[Int])
                                                                ^
3 errors

Example 2

package io.vasilev

import java.nio.ByteBuffer

object Buffers {
  private[this] final val buffer: ByteBuffer = ByteBuffer.allocate(16)

  val flipped: ByteBuffer = buffer.flip()
}

scalac io/vasilev/Buffers.scala --release 8
io/vasilev/Buffers.scala:8: error: type mismatch;
 found   : java.nio.Buffer
 required: java.nio.ByteBuffer
  val flipped: ByteBuffer = buffer.flip()
                                       ^
1 error

Example 3

package io.vasilev

import java.nio.ByteBuffer

object Buffers {
  private[this] final val buffer: ByteBuffer = ByteBuffer.allocate(16)

  buffer.flip()
}

Compiling without -release 8 on JDK 17

scalac io/vasilev/Buffers.scala
javap -p -c io.vasilev.Buffers\$
Compiled from "Buffers.scala"
public final class io.vasilev.Buffers$ {
  public static final io.vasilev.Buffers$ MODULE$;

  private static final java.nio.ByteBuffer buffer;

  public static {};
    Code:
       0: new           #2                  // class io/vasilev/Buffers$
       3: dup
       4: invokespecial #14                 // Method "<init>":()V
       7: putstatic     #16                 // Field MODULE$:Lio/vasilev/Buffers$;
      10: bipush        16
      12: invokestatic  #22                 // Method java/nio/ByteBuffer.allocate:(I)Ljava/nio/ByteBuffer;
      15: putstatic     #24                 // Field buffer:Ljava/nio/ByteBuffer;
      18: getstatic     #24                 // Field buffer:Ljava/nio/ByteBuffer;
      21: invokevirtual #28                 // Method java/nio/ByteBuffer.flip:()Ljava/nio/ByteBuffer; <------- notice the new JDK 9+ method with overloaded return type, which cannot be called on JDK 8.
      24: pop
      25: return

  private io.vasilev.Buffers$();
    Code:
       0: aload_0
       1: invokespecial #29                 // Method java/lang/Object."<init>":()V
       4: return
}

Compiling with -release 8 on JDK 17

scalac io/vasilev/Buffers.scala -release 8
javap -p -c io.vasilev.Buffers\$
Compiled from "Buffers.scala"
public final class io.vasilev.Buffers$ {
  public static final io.vasilev.Buffers$ MODULE$;

  private static final java.nio.ByteBuffer buffer;

  public static {};
    Code:
       0: new           #2                  // class io/vasilev/Buffers$
       3: dup
       4: invokespecial #14                 // Method "<init>":()V
       7: putstatic     #16                 // Field MODULE$:Lio/vasilev/Buffers$;
      10: bipush        16
      12: invokestatic  #22                 // Method java/nio/ByteBuffer.allocate:(I)Ljava/nio/ByteBuffer;
      15: putstatic     #24                 // Field buffer:Ljava/nio/ByteBuffer;
      18: getstatic     #24                 // Field buffer:Ljava/nio/ByteBuffer;
      21: invokevirtual #28                 // Method java/nio/ByteBuffer.flip:()Ljava/nio/Buffer; <------ notice that scalac used the JDK 8 compatible method
      24: pop
      25: return

  private io.vasilev.Buffers$();
    Code:
       0: aload_0
       1: invokespecial #29                 // Method java/lang/Object."<init>":()V
       4: return
}

djspiewak · 2022-02-21T19:56:29Z

Honestly outstanding! I think this means I can delete Java 8

armanbilge · 2022-02-21T19:57:10Z

Second, I'd rather see this done as a one-off autoplugin rather than a settings val like this.

cough sbt-typelevel cough 😝

djspiewak · 2022-02-21T20:01:17Z

cough sbt-typelevel cough 😝

:-D It's on my "to peruse" list. Still digging out from under everything else for the time being though.

vasilmkd · 2022-02-21T20:03:03Z

project/Java8Target.scala

+      // The release flag controls what JVM platform APIs are called. We only want JDK 8 APIs
+      // in order to maintain compability with JDK 8.
+      val releaseFlag =
+        if (version.startsWith("1.8"))


Let's factor this out as it is repeated:
val isJdk8: Boolean = System.getProperty("java.version").startsWith("1.8")

vasilmkd · 2022-03-09T18:16:57Z

Superseded by #2857.

Use Java release option

810535b

vasilmkd marked this pull request as draft February 16, 2022 10:20

Use Scala release option

cc0215b

armanbilge mentioned this pull request Feb 16, 2022

Add JDK target setting typelevel/sbt-typelevel#79

Closed

The Java runtime version is also needed for Scala

92e9fbe

Add target flag on Scala 2 and comments

e6ea24e

Guard with Java runtime version again

8877911

vasilmkd marked this pull request as ready for review February 16, 2022 12:32

Use the proper option syntax for Scala 2.12

b95665e

djspiewak requested changes Feb 21, 2022

View reviewed changes

Refactored releaseSettings into an autoplugin

8a4f854

vasilmkd commented Feb 21, 2022

View reviewed changes

armanbilge mentioned this pull request Mar 4, 2022

Migrate to sbt-typelevel #2857

Merged

vasilmkd closed this Mar 9, 2022

vasilmkd deleted the java-release branch March 9, 2022 18:17

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Use Java release option #2825

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Use Java release option #2825

Use Java release option #2825

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Example 1

Example 2

Example 3

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Scala 3

Example 1

Example 2

Example 3

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Scala 2

Example 1

Example 2

Example 3

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