We Keep Coding

Take final parameters from interface to generated method stubs in implementation

I'm using the "final"-keyword for method-parameters like the "const"-keyword in C/C++ even if it's meaning in JAVA is not 100% the same as in C/C++. I do that mainly to easily distinguish between input- and output-parameters. Like here:
interface Test
{
public void myMethod(final int input1, final float input2, SomeResults output);
}
When I then create an implemention of the interface (or abstract class) and let eclipse generate the overloaded methods eclipse will ommit all "final"-keywords which is really bad. There is an option within the SaveActions-Part of the Java-Editor settings but there I can just enforce eclipse to use "final" everywhere where it is possible which is definitely not my intention. How can I force eclipse to NOT IGNORE my "final"-keywords in interface methods or abstract methods and put them in generated method stubs of implementions and child classes instead?

How can I force eclipse to NOT IGNORE my final keywords in interface methods or abstract methods and put them in generated method stubs of implementations and child classes instead?
I don't think it is possible ... unless you are willing to modify Eclipse.
I am afraid that what you are doing doesn't have any effect.
As stated I use the final keyword only to clarify that a parameter is a "pure input" variable. Regarding that primitives will always be passed by value is known to me.
That is not what final means.
All parameters are "pure input" ... in the sense that they are passed by value. There is no such thing as an "out" parameter in Java.
Conversely, if a parameter's type is a reference type, then declaring it final does not stop the method body from mutating the actual parameter. In other words, it doesn't stop the method body from using the parameter to simulate an "out" parameter.
In Java, declaring a formal parameter as final in an abstract method declarator doesn't have any meaning. It only has meaning if the method declarator is followed by a method body. (Then it means that the variable cannot be assigned to.) Therefore, it would be suspect if a tool (e.g. an Eclipse stub generator) were to place any meaning on the final in the the former context.
My advice would be not to place this unintended (by the Java designers) meaning on final, in either interfaces or in classes. If you want to express "this is not an "out" parameter, either do it in the javadocs, or invent a custom annotation to express it. (In the latter case, you could potentially implement a static checker to ensure that the method body does not mutate the parameter object.)

Can I insert instructions in constructors before calling this() / super() and before initialising any final fields?

Preface
I have been experimenting with ByteBuddy and ASM, but I am still a beginner in ASM and between beginner and advanced in ByteBuddy. This question is about ByteBuddy and about JVM bytecode limitations in general.
Situation
I had the idea of creating global mocks for testing by instrumenting constructors in such a way that instructions like these are inserted at the beginning of each constructor:
if (GlobalMockRegistry.isMock(getClass()))
return;
FYI, the GlobalMockRegistry basically wraps a Set<Class<?>> and if that set contains a certain class, then isMock(Class<?>> clazz) would return true. The advantage of that concept is that I can (de)activate global mocking for each class during runtime because if multiple tests run in the same JVM process, one test might need a certain global mock, the next one might not.
What the if(...) return; instructions above want to achieve is that if mocking is active, the constructor should not do anything:
no this() or super() calls, → update: impossible
no field initialisations, → update: possible
no other side effects. → update: might be possible, see my update below
The result would be an object with uninitialised fields that did not create any (possibly expensive) side effects such as resource allocation (database connection, file creation, you name it). Why would I want that? Could I not just create an instance with Objenesis and be happy? Not if I want a global mock, i.e. mock objects I cannot inject because they are created somewhere inside methods or field initialisers I do not have control over. Please do not worry about what method calls on such an object would do if its instance fields are not properly initialised. Just assume I have instrumented the methods to return stub results, too. I know how to do that already, the problem are only constructors in the context of this question.
Questions / problems
Now if I try to simulate the desired result in Java source code, I meet the following limitations:
I cannot insert any code before this() or super(). I could mitigate that by also instrumenting the super class hierarchy with the same if(...) return;, but would like to know if I could in theory use ASM to insert my code before this() or super() using a method visitor. Or would the byte code of the instrumented class somehow be verified during loading or retransformation and then rejected because the byte code is "illegal"? I would like to know before I start learning ASM because I want to avoid wasting time for an idea which is not feasible.
If the class contains final instance fields, I also cannot enter a return before all of those fields have been initialised in the constructor. That might happen at the very end of a complex constructor which performs lots of side effects before actually initialising the last field. So the question is similar to the previous one: Can I use ASM to insert my if(...) return; before any fields (including final ones) are initialised and produce a valid class which I could not produce using javac and will not be rejected when loaded or retransformed?
BTW, if it is relevant, we are talking about Java 8+, i.e. at the time of writing this that would be Java versions 8 to 14.
If anything about this question is unclear, please do not hesitate to ask follow-up questions, so I can improve it.
Update after discussing Antimony's answer
I think this approach could work and avoid side effects, calling the constructor chain but avoiding any side effects and resulting in a newly initialised instance with all fields empty (null, 0, false):
In order to avoid calling this.getClass(), I need to hard-code the mock target's class name directly into all constructors up the parent chain. I.e. if two "global mock" target classes have the same parent class(es), multiple of the following if blocks would be woven into each corresponding parent class, one for each hard-coded child class name.
In order to avoid any side effects from objects being created or methods being called, I need to call a super constructor myself, using null/zero/false values for each argument. That would not matter because the next parent class up the chain would have a similar code block so that the arguments given do not matter anyway.
// Avoid accessing 'this.getClass()'
if (GlobalMockRegistry.isMock(Sub.class)) {
// Identify and call any parent class constructor, ideally a default constructor.
// If none exists, call another one using default values like null, 0, false.
// In the class derived from Object, just call 'Object.<init>'.
super(null, 0, false);
return;
}
// Here follows the original byte code, i.e. the normal super/this call and
// everything else the original constructor does.
Note to myself: Antimony's answer explains "uninitialised this" very nicely. Another related answer can be found here.
Next update after evaluating my new idea
I managed to validate my new idea with a proof of concept. As my JVM byte code knowledge is too limited and I am not used to the way of thinking it requires (stack frames, local variable tables, "reverse" logic of first pushing/popping variables, then applying an operation on them, not being able to easily debug), I just implemented it in Javassist instead of ASM, which in comparison was a breeze after failing miserably with ASM after hours of trial & error.
I can take it from here and I want to thank user Antimony for his very instructive answer + comments. I do know that theoretically the same solution could be implemented using ASM, but it would be exceedingly difficult in comparison because its API is too low level for the task at hand. ByteBuddy's API is too high level, Javassist was just right for me in order to get quick results (and easily maintainable Java code) in this case.

Yes and no. Java bytecode is much less restrictive than Java (source) in this regard. You can put any bytecode you want before the constructor call, as long as you don't actually access the uninitialized object. (The only operations allowed on an uninitialized this value are calling a constructor, setting private fields declared in the same class, and comparing it against null).
Bytecode is also more flexible in where and how you make the constructor call. For example, you can call one of two different constructors in an if statement, or you can wrap the super constructor call in a "try block", both things that are impossible at the Java language level.
Apart from not accessing the uninitialized this value, the only restriction* is that the object has to be definitely initialized along any path that returns from the constructor call. This means the only way to avoid initializing the object is to throw an exception. While being much laxer than Java itself, the rules for Java bytecode were still very deliberately constructed so it is impossible to observe uninitialized objects. In general, Java bytecode is still required to be memory safe and type safe, just with a much looser type system than Java itself. Historically, Java applets were designed to run untrusted code in the JVM, so any method of bypassing these restrictions was a security vulnerability.
* The above is talking about traditional bytecode verification, as that is what I am most familiar with. I believe stackmap verification behaves similarly though, barring implementation bugs in some versions of Java.
P.S. Technically, Java can have code execute before the constructor call. If you pass arguments to the constructor, those expressions are evaluated first, and hence the ability to place bytecode before the constructor call is required in order to compile Java code. Likewise, the ability to set private fields declared in the same class is used to set synthetic variables that arise from the compilation of nested classes.
If the class contains final instance fields, I also cannot enter a return before all of those fields have been initialised in the constructor.
This, however, is eminently possible. The only restriction is that you call some constructor or superconstructor on the uninitialized this value. (Since all constructors recursively have this restriction, this will ultimately result in java.lang.Object's constructor being called). However, the JVM doesn't care what happens after that. In particular, it only cares that the fields have some well typed value, even if it is the default value (null for objects, 0 for ints, etc.) So there is no need to execute the field initializers to give them a meaningful value.
Is there any other way to get the type to be instantiated other than this.getClass() from a super class constructor?
Not as far as I am aware. There's no special opcode for magically getting the Class associated with a given value. Foo.class is just syntactic sugar which is handled by the Java compiler.

Does Java 8's Method Reference use Reflection? [duplicate]

The invokedynamic instruction is used to help the VM determine the method reference at runtime instead hardwiring it at compile time.
This is useful with dynamic languages where the exact method and argument types aren't known until runtime. But that isn't the case with Java lambdas. They are translated to a static method with well defined arguments. And this method can be invoked using invokestatic.
So then what is the need of invokedynamic for lambdas, especially when there is a performance hit?

Lambdas are not invoked using invokedynamic, their object representation is created using invokedynamic, the actual invocation is a regular invokevirtual or invokeinterface.
For example:
// creates an instance of (a subclass of) Consumer
// with invokedynamic to java.lang.invoke.LambdaMetafactory
something(x -> System.out.println(x));
void something(Consumer<String> consumer) {
// invokeinterface
consumer.accept("hello");
}
Any lambda has to become an instance of some base class or interface. That instance will sometimes contain a copy of the variables captured from the original method and sometimes a pointer to the parent object.
This can be implemented as an anonymous class.
Why invokedynamic
The short answer is: to generate code in runtime.
The Java maintainers chose to generate the implementation class in runtime.
This is done by calling java.lang.invoke.LambdaMetafactory.metafactory.
Since the arguments for that call (return type, interface, and captured parameters) can change, this requires invokedynamic.
Using invokedynamic to construct the anonymous class in runtime, allows the JVM to generate that class bytecode in runtime. The subsequent calls to the same statement use a cached version. The other reason to use invokedynamic is to be able to change the implementation strategy in the future without having to change already compiled code.
The road not taken
The other option would be the compiler creating an innerclass for each lambda instantiation, equivalent to translating the above code into:
something(new Consumer() {
public void accept(x) {
// call to a generated method in the base class
ImplementingClass.this.lambda$1(x);
// or repeating the code (awful as it would require generating accesors):
System.out.println(x);
}
);
This requires creating classes in compile time and having to load then during runtime. The way jvm works those classes would reside in the same directory as the original class. And the first time you execute the statement that uses that lambda, that anonymous class would have to be loaded and initialized.
About performance
The first call to invokedynamic will trigger the anonymous class generation. Then the opcode invokedynamic is replaced with code that's equivalent in performance to the writing manually the anonymous instantiation.

Brain Goetz explained the reasons for the lambda translation strategy in one of his papers which unfortunately now seem unavailable. Fortunately I kept a copy:
Translation strategy
There are a number of ways we might represent a lambda expression in
bytecode, such as inner classes, method handles, dynamic proxies, and
others. Each of these approaches has pros and cons. In selecting a
strategy, there are two competing goals: maximizing flexibility for
future optimization by not committing to a specific strategy, vs
providing stability in the classfile representation. We can achieve
both of these goals by using the invokedynamic feature from JSR 292 to
separate the binary representation of lambda creation in the bytecode
from the mechanics of evaluating the lambda expression at runtime.
Instead of generating bytecode to create the object that implements
the lambda expression (such as calling a constructor for an inner
class), we describe a recipe for constructing the lambda, and delegate
the actual construction to the language runtime. That recipe is
encoded in the static and dynamic argument lists of an invokedynamic
instruction.
The use of invokedynamic lets us defer the selection of a translation
strategy until run time. The runtime implementation is free to select
a strategy dynamically to evaluate the lambda expression. The runtime
implementation choice is hidden behind a standardized (i.e., part of
the platform specification) API for lambda construction, so that the
static compiler can emit calls to this API, and JRE implementations
can choose their preferred implementation strategy. The invokedynamic
mechanics allow this to be done without the performance costs that
this late binding approach might otherwise impose.
When the compiler encounters a lambda expression, it first lowers
(desugars) the lambda body into a method whose argument list and
return type match that of the lambda expression, possibly with some
additional arguments (for values captured from the lexical scope, if
any.) At the point at which the lambda expression would be captured,
it generates an invokedynamic call site, which, when invoked, returns
an instance of the functional interface to which the lambda is being
converted. This call site is called the lambda factory for a given
lambda. The dynamic arguments to the lambda factory are the values
captured from the lexical scope. The bootstrap method of the lambda
factory is a standardized method in the Java language runtime library,
called the lambda metafactory. The static bootstrap arguments capture
information known about the lambda at compile time (the functional
interface to which it will be converted, a method handle for the
desugared lambda body, information about whether the SAM type is
serializable, etc.)
Method references are treated the same way as lambda expressions,
except that most method references do not need to be desugared into a
new method; we can simply load a constant method handle for the
referenced method and pass that to the metafactory.
So, the idea here seemed to be to encapsulate the translation strategy and not commit to a particular way of doing things by hiding those details. In the future when type erasure and lack of value types have been solved and maybe Java supports actual function types, they might just as well go there and change that strategy for another one without causing any problems in the users' code.

Current Java 8's lambda implementation is a compound decision:
Compile the lambda expression to a static method in the enclosing class; instead of compiling lambdas to separate inner class files (Scala compiles this way, which generates many $$$ class files)
Introduce a constant pool: BootstrapMethods, which wraps the static method invocation to callsite object (can be cached for later use)
So to answer your question,
the current lambda implementation using invokedynamic is a little bit faster than the separate inner class way, because no need to load these inner class files, but instead create the inner class byte[] on the fly (to satisfy for example the Function interface), and cached for later use.
JVM team may still choose to generate separate inner class (by referencing the enclosing class's static methods) files: it's flexible

Two methods with the same name in java

I noticed that if I have two methods with the same name, the first one accepts SomeObject and the second one accepts an object extending SomeObject when I call the method with SomeOtherObject, it automatically uses the one that only accepts SomeObject. If I cast SomeOtherObject to SomeObject, the method that accepts SomeObject is used, even if the object is an instanceof SomeOtherObject. This means the method is selected when compiling. Why?

That's how method overload resolution in Java works: the method is selected at compile time.
For all of the ugly details, see the Java Language Specification §15.12.

This means the method is selected when compiling.
Yes you are correct. That is what it means.
Why?
I can think of four reasons why they designed Java this way:
This is consistent with the way that other statically typed OO languages that support overloading work. It is what people who come / came from the C++ world expect. (This was particularly important in the early days of Java ... though not so much now.). It is worth noting that C# handles overloading the same way.
It is efficient. Resolving method overloads at runtime (based on actual argument types) would make overloaded method calls expensive.
It gives more predictable (and therefore more easy to understand) behaviour.
It avoids the Brittle Base Class problem, where adding adding a new overloaded method in a base class causes unexpected problems in existing derived classes.
References:
http://blogs.msdn.com/b/ericlippert/archive/2004/01/07/virtual-methods-and-brittle-base-classes.aspx

Yes the function to be executed is decided at compile time! So JVM has no idea of the actual type of the Object at compile time. It only knows the type of the reference that points to the object given as argument to the function.
For more details you can look into Choosing the Most Specific Method in Java Specification.

What is the internal identification of a Java method?

As we know, in Java, method name is not sufficient to distinguish different methods.
I think (may be wrong), to distinguish a method, it needs the following info:
(className, methodName, methodParameters)
Further,
how to identify a method more efficiently internally?
I heard of "method id". Does it mean there is a mapping between the above triple and an integer, so JVM use only method id after parsing?
If so, is it resided in symbol table?
Thanks!

It's a CONSTANT_NameAndType_info Structure pointing at a method descriptor.
It pretty much consists of the method name, the parameter types, and (somewhat surprisingly) the return type.

I do not understand very well what you are trying to do but I think there are some possible answers nonetheless:
You may be interested in the JNI Method Descriptors, one of the various string formats used internally by the JVM (and by JNI libraries) for identifying Java elements.
It is difficult to know about what you are talking about. The "method id" can be a reference for a java.lang.reflect.Method object, or can be the method descriptor mentioned below, or any other thing. Where did you read about it?
I doubt there is such table inside the JVM. I mean, I doubt there is a global table, because almost always you retrieve a method from a class, even when dealing with it inside the JVM, so it is reasonable to believe the method is stored in the class. It is likewhen we use reflection to retrieve a method:
Class clazz = String.class;
Method method = clazz.getDeclaredMethod("charAt", Integer.TYPE);
System.out.println(method.getName());
Note that I ask the class String for the method, instead of asking some util class to give me the method charAt, which receives an int and is from the class String.
In other words, your identification tuple is almost correct - it just does not have a class:
(methodName, methodParameters)
and, instead of retrieving the method from the JVM passing the class and then the method name and then the parameter types, you retrieve the method directly from the class, giving the class the method name and the parameter types. A subtle difference, for sure, but I think it is what you are wondering about.
This is evident even in the JNI descriptors I mentioned below. For example, the method
long f(int i, Class c);
is represented by the following descriptor:
"(ILjava/lang/Class;)J"
Note that there is no reference to the class of the method.
The excellent documentation on the class file format (already pointed by #Lawence) may give you some insights. I recommend you to read it fully.

1) How to identify a method more efficiently internally?
Internally to what? There are many places where a method might need to be "identified" "internally". In the bytecode compiler, the JIT compiler, the classloader / linker, the classfile representation, reflection API, a debugger and so on. They each have different efficiency concerns.
2) I heard of "method id". Does it mean there is a mapping between the above triple and an integer, so JVM use only method id after parsing?
A method id is used in the classfile representation, and could be used by anything based on that, including the class loader / linker, the JIT compiler and the debugger.
The JVM doesn't parse Java code.
3) If so, is it resided in symbol table?
It might do. It depends on what you mean by "the symbol table". Bear in mind that there are lots of places where method identification is required, throughout the lifecycle of a class. For instance, the Java reflection APIs require method information to implement methods such as getDeclaredMethod(...) and various methods of Method.

Java always differentiate its language elements by their fully qualified names.
Suppose you have a method myMethod(int a, int b) in class MyClass which lies in the package com.mypackage then java will identify the method with the name com.mypackage.MyClass.myMethod(int a , int b).
Just to give you some more insight, it also takes the Class Loader into consideration when there is a need to resolve two identical elements.
It does consider, which class loader was used to load the particular class containing the method to which you are referring. There are four types of class loaders in java. You can read the documention for java.lang.Thread class for this.