Java 8 bytecode how are default methods in interfaces marked

Question

since Java 8 it is allowed to predefine methods in interfaces. Some of these standard implementations are already implemented on the "standard" interfaces such as CharSequence. If you try to read Java 8 ByteCode (e.g. the rt.jar of the Java home folder) with a JVM 7, an error occurs.

e.g. the type java.lang.CharSequence cannot be resolved.

This may not be the only difference between levels, but I would like to understand the structure of the new byte code.

public interface com.company.ITest {
public void HelloWorld();
Code:
   0: getstatic     #1                  // Field java/lang/System.out:Ljava/io/PrintStream;
   3: ldc           #2                  // String hello world
   5: invokevirtual #3                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8: return
}

This Bbytecode was produced by this Javacode:

public interface ITest {
    default void HelloWorld(){
        System.out.println("hello world");
    }
}

So here is my question: How does an default interface impact on the constant pool?

Is any of these flags relevant:

CONSTANT_MethodHandle CONSTANT_MethodType CONSTANT_InvokeDynamic

You could answer this by looking at some generated bytecode. — Oliver Charlesworth, Mar 31 '18 at 08:22
tryed it with javap... maybe i should add this result to my question. good point thanks — osanger, Mar 31 '18 at 08:25

score 5 · Answer 1 · answered Mar 31 '18 at 08:54

Take following example:

public interface A {
    default void foo(){
       System.out.println("Calling A.foo()");
    }
}

public class Clazz implements A {
}

From the client code perspective, default methods are just ordinary virtual methods. Hence the name – virtual extension methods. So in case of the example with the client code that invokes the default method will generate invokeinterface at the call site.

A clazz = new Clazz();
clazz.foo(); // invokeinterface foo()

Clazz clazz = new Clazz();
clazz.foo(); // invokevirtual foo()

In case of the default methods conflict resolution, when we override the default method and would like to delegate the invocation to one of the interfaces the invokespecial is inferred as we would call the implementation specifically:

public class Clazz implements A, B {
    public void foo(){
       A.super.foo(); // invokespecial foo()
    }
}

public void foo();
Code:
0: aload_0
1: invokespecial #2 // InterfaceMethod A.foo:()V
4: return

As you can see, invokespecial instruction is used to invoke the interface method foo(). This is also something new from the bytecode point of view as previously you would only invoke methods via super that points to a class (parent class), and not to an interface.

Great answer. But how does it affect the constant pool? – osanger Apr 03 '18 at 06:33 — osanger, Apr 03 '18 at 06:33

score 1 · Accepted Answer · edited Jun 20 '20 at 09:12

It depends on what you are actually doing, which obstacle you will stumble upon. You say

If you try to read Java 8 ByteCode (e.g. the rt.jar of the Java home folder) with a JVM 7, an error occurs.

e.g. the type java.lang.CharSequence cannot be resolved.

but this doesn’t even remotely match what will happen when you try to “read Java 8 ByteCode with a JVM 7”. If you try to load a Java 8 class with a Java 7 JVM, you usually get a VerifyError with a message telling you that the class file version is not supported.

Instead, the error message looks very much like the well known error message of the Eclipse compiler when it fails at reading the byte code of a class file, which has nothing to do with the JVM. As explained in this answer, Eclipse doesn’t seem to make a difference between classes it didn’t find or classes it failed to parse, just saying “cannot be resolved”. It also seems to ignore the class file version number, so it happens to work with newer class files when they are not using newer features like default methods.

On a technical level, the differences are tiny. There is no relationship between default methods and the constant pool. The pool entry types CONSTANT_MethodHandle, CONSTANT_MethodType, and CONSTANT_InvokeDynamic are not related to default methods and they are not even new—they are already part of the standard since Java 7 (which doesn’t stop some tool vendors from ignoring them as long as they do not encounter them). A default method simply is a method which is not abstract and not static, just like an ordinary public instance method, but in an interface. The new thing is that this is allowed now. A simple class file parser which doesn’t care whether it is reading a class or interface would have no difficulties with that. But depending on what the tool does with the data, it may struggle with such a class file (if it didn’t already stop at the version number), like older Eclipse compilers do.

If a JVM’s verifier didn’t already stop at the version number, it would simply throw a different VerifierError saying the the class file violates the constraint that all methods must be abstract.

score 0 · Answer 3 · answered Mar 31 '18 at 17:05

Anurag already explained a bit about the implementation of default methods, but I wanted to address one other point in your question:

If you try to read Java 8 ByteCode (e.g. the rt.jar of the Java home folder) with a JVM 7, an error occurs.

e.g. the type java.lang.CharSequence cannot be resolved.

This is because each classfile has a version code representing the version of Java it was compiled for. The JVM will reject any classfile with a version code higher than itself (lower is ok though, for backwards compatibility).

This means that even if literally nothing else about the bytecode changes, the JVM will still refuse to load classfiles from a future version of Java. You can compile "hello world" with Java 10, and the Java 9 JVM will refuse to load it, even though there are no new features or bytecode differences.

Java 8 bytecode how are default methods in interfaces marked

3 Answers3