Get a list of classes lambdas

Question

In Java 8 it looks like the lambdas of a class are kept in an array. For example, lets say we have this class:

public class LambdaFactory {
    public Supplier<Integer> getOne(){
        return () -> 42;
    }

    public Supplier<Integer> getTwo(){
        return () -> 128;
    }

    public Supplier<Integer> getThree(){
         return () -> 3;
    }
}

and then I print it out like so:

    System.out.println(factory.getOne());
    System.out.println(factory.getOne());
    System.out.println(factory.getTwo());
    System.out.println(factory.getThree());

the output will be something like

examples.LambdaFactory$$Lambda$1@4e515669
examples.LambdaFactory$$Lambda$1@4e515669
examples.LambdaFactory$$Lambda$2@1b9e1916
examples.LambdaFactory$$Lambda$3@ba8a1dc

So we can see two thing here. The same lambda called twice gives us the same lambda object (this is not the same as with anon inner classes where we could get a new one every time). We also see that they look like they are being kept in some kind of "Lambda" structure that is part of the class

My question is, can I get ahold of the lambdas in a class? I don't have any reason to do so, I just like dissecting things

Answer 1

The Java Language Specification states

At run time, evaluation of a lambda expression is similar to evaluation of a class instance creation expression, insofar as normal completion produces a reference to an object. [...]

Either a new instance of a class with the properties below is allocated and initialized, or an existing instance of a class with the properties below is referenced.

[...]

These rules are meant to offer flexibility to implementations of the Java programming language, in that:

• A new object need not be allocated on every evaluation.
• [...]

As such, it is up to a compiler or run time environment to decide what should be returned when a lambda expression is evaluated.

My question is, can I get ahold of the lambdas in a class? I don't have any reason to do so, I just like dissecting things

You can think of a lambda expression as any other class constant, a String, an integer literal, etc. These are constants that appear in the constant pool of a .class file. These are references to objects that are created and exist at run time. There is no way to refer to the actual objects from a class' constant pool.

In the case of a lambda, it wouldn't be helpful anyway because it might not actually be the same object.

Answer 2

The lambdas are created by the JRE and the way they are created is controlled by the JRE and might vary between different JRE vendors and might change in future versions.

If you want to have fun you can create a lambda at runtime which has no corresponding information within the class file:

import java.lang.invoke.*;

public class ManualLambda {
  public static void main(String[] args) throws Throwable {
    MethodHandles.Lookup me=MethodHandles.lookup();
    MethodType t=MethodType.methodType(void.class);
    MethodType rt=MethodType.methodType(Runnable.class);
    CallSite site = LambdaMetafactory.metafactory(
      me, "run", rt, t, me.findStatic(ManualLambda.class, "sayHello", t), t);
    MethodHandle factory=site.getTarget();
    Runnable r=(Runnable)factory.invoke();

    System.out.println("created lambda: "+r);
    r.run();
  }
  private static void sayHello() {
      System.out.println("hello world");
  }
}

The code above retraces what happens when a lambda is created. But for compile-time (“real”) lambda expressions the entire thing is triggered by a single invokedynamic byte code instruction. The LambdaMetafactory.metafactory(…) method is the bootstrap method which is called when the invokedynamic instruction is executed the first time. The returned CallSite object is permanently associated with the invokedynamic instruction. If the CallSite is a ConstantCallSite and its MethodHandle returns the same lambda object on every execution, the invokedynamic instruction will “produce” the same lambda instance forever.