How are Scala traits compiled into Java bytecode?

Question

I have played around with Scala for a while now, and I know that traits can act as the Scala equivalent of both interfaces and abstract classes. How exactly are traits compiled into Java bytecode?

I found some short explanations that stated traits are compiled exactly like Java interfaces when possible, and interfaces with an additional class otherwise. I still don't understand, however, how Scala achieves class linearization, a feature not available in Java.

Is there a good source explaining how traits compile to Java bytecode?

Answer 1

I'm not an expert, but here is my understanding:

Traits are compiled into an interface and corresponding class.

trait Foo {
  def bar = { println("bar!") }
}

becomes the equivalent of...

public interface Foo {
  public void bar();
}

public class Foo$class {
  public static void bar(Foo self) { println("bar!"); }
}

Which leaves the question: How does the static bar method in Foo$class get called? This magic is done by the compiler in the class that the Foo trait is mixed into.

class Baz extends Foo

becomes something like...

public class Baz implements Foo {
  public void bar() { Foo$class.bar(this); }
}

Class linearization just implements the appropriate version of the method (calling the static method in the Xxxx$class class) according to the linearization rules defined in the language specification.

Answer 2

For the sake of discussion, let's look the following Scala example using multiple traits with both abstract and concrete methods:

trait A {
  def foo(i: Int) = ???
  def abstractBar(i: Int): Int
}

trait B {
  def baz(i: Int) = ???
}

class C extends A with B {
  override def abstractBar(i: Int) = ???
}

At the moment (i.e. as of Scala 2.11), a single trait is encoded as:

• an interface containing abstract declarations for all the trait's methods (both abstract and concrete)
• an abstract static class containing static methods for all the trait's concrete methods, taking an extra parameter $this (in older versions of Scala, this class wasn't abstract, but it doesn't make sense to instantiate it)
• at every point in the inheritance hierarchy where the trait is mixed in, synthetic forwarder methods for all the concrete methods in the trait that forward to the static methods of the static class

The primary advantage of this encoding is that a trait without concrete members (which is isomorphic to an interface) actually is compiled to an interface.

interface A {
    int foo(int i);
    int abstractBar(int i);
}

abstract class A$class {
    static void $init$(A $this) {}
    static int foo(A $this, int i) { return ???; }
}

interface B {
    int baz(int i);
}

abstract class B$class {
    static void $init$(B $this) {}
    static int baz(B $this, int i) { return ???; }
}

class C implements A, B {
    public C() {
        A$class.$init$(this);
        B$class.$init$(this);
    }

    @Override public int baz(int i) { return B$class.baz(this, i); }
    @Override public int foo(int i) { return A$class.foo(this, i); }
    @Override public int abstractBar(int i) { return ???; }
}

However, Scala 2.12 requires Java 8, and thus is able to use default methods and static methods in interfaces, and the result looks more like this:

interface A {
    static void $init$(A $this) {}
    static int foo$(A $this, int i) { return ???; }
    default int foo(int i) { return A.foo$(this, i); };
    int abstractBar(int i);
}

interface B {
    static void $init$(B $this) {}
    static int baz$(B $this, int i) { return ???; }
    default int baz(int i) { return B.baz$(this, i); }
}

class C implements A, B {
    public C() {
        A.$init$(this);
        B.$init$(this);
    }

    @Override public int abstractBar(int i) { return ???; }
}

As you can see, the old design with the static methods and forwarders has been retained, they are just folded into the interface. The trait's concrete methods have now been moved into the interface itself as static methods, the forwarder methods aren't synthesized in every class but defined once as default methods, and the static $init$ method (which represents the code in the trait body) has been moved into the interface as well, making the companion static class unnecessary.

It could probably be simplified like this:

interface A {
    static void $init$(A $this) {}
    default int foo(int i) { return ???; };
    int abstractBar(int i);
}

interface B {
    static void $init$(B $this) {}
    default int baz(int i) { return ???; }
}

class C implements A, B {
    public C() {
        A.$init$(this);
        B.$init$(this);
    }

    @Override public int abstractBar(int i) { return ???; }
}

I'm not sure why this wasn't done. At first glance, the current encoding might give us a bit of forwards-compatibility: you can use traits compiled with a new compiler with classes compiled by an old compiler, those old classes will simply override the default forwarder methods they inherit from the interface with identical ones. Except, the forwarder methods will try to call the static methods on A$class and B$class which no longer exist, so that hypothetic forwards-compatibility doesn't actually work.

Answer 3

A very good explanation of this is in:

The busy Java developer's guide to Scala: Of traits and behaviors - Traits in the JVM

Quote:

In this case, it [the compiler] drops the method implementations and field declarations defined in the trait into the class that implements the trait

Answer 4

In the context of Scala 12 and Java 8, you can see another explanation in commit 8020cd6:

Better inliner support for 2.12 trait encoding

Some changes to the trait encoding came late in the 2.12 cycle, and the inliner was not adapted to support it in the best possible way.

In 2.12.0 concrete trait methods are encoded as

interface T {
  default int m() { return 1 }
  static int m$(T $this) { <invokespecial $this.m()> }
}
class C implements T {
  public int m() { return T.m$(this) }
}

If a trait method is selected for inlining, the 2.12.0 inliner would copy its body into the static super accessor T.m$, and from there into the mixin forwarder C.m.

This commit special-cases the inliner:

• We don't inline into static super accessors and mixin forwarders.
• Instead, when inlining an invocation of a mixin forwarder, the inliner also follows through the two forwarders and inlines the trait method body.