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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.

Constructor put by jvm or compliler?

I read it somewhere that calling a constructor is work of JVM, so i created a Class named Hello and did not put anything at all, and just compiled it, after compiling when i open the byte code there is constructor created inside a class which is default constructor.
So is it compiler's duty to put a default constructor.
I thought it's jvm who checks and calls constructor.
Ps: I haven't run that code.

I thought it's jvm who checks and calls constructor.
Wrong assumption. The JVM reads compiled classes (.class) files. It doesn't modify or add them.
Of course, the JVM executes code, and thus calls/invokes methods and constructors.
But the java compiler is responsible for "adding" such things a default constructor, see here for more details.
Having said that, of course there is the JIT (just in time compiler) that is part of the JVM. But the JIT translates byte code into machine code, and its job is again, not to add things such as additional constructors.

I am trying to work out what you mean by:
I thought it's jvm who checks and calls constructor.
The "call" makes sense.
The "check" .... not sure. If you mean that the JVM's classloader checks that the required constructors are present when it loads1 the class, that is correct. But if the JVM finds that a (default or otherwise) constructor is missing, it doesn't just add one. Instead the JVM marks the class and its dependents as unusable, throws an Error exception, and typically exits.
(Note that the kind of checks described above are done to deal with cases where there is a binary compatibility mismatch between versions of classes used at compile time and at runtime. Typically the you compiled a class against one version of an API and used and put an incompatible version on the runtime classpath.)
The checking that you are probably thinking about is done by the bytecode compiler.
If there is no constructor in the source code of a class, the compiler defines a default constructor, and includes it the .class file. This is in conformance to what the JLS says.
If the source code contains a new that uses any constructor that hasn't been defined, the compiler treats this as a compilation error.
By the time the JVM sees any bytecode file for a Java class, it will contain at least one constructor.
1 - I am deliberately leaving out some details here.

A default constructor is automatically generated by the compiler if you do not explicitly define at least one constructor in your class. You've defined two, so your class does not have a default constructor.A default constructor is created if you don't define any constructors in your class. It simply is a no argument constructor which does nothing. Edit: Except call super()
public Module(){
}

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

Overriding Javadoc comment on java.lang.Enum.values()

I have a very specific problem about the method java.lang.Enum.values().
I would like to override its javadoc. Very precisely, the current javadoc for this is, after I created my own enum:
public static MyClass.MyEnum[] values()
...
This method may be used to iterate over the constants as follows:
for (MyClass.MyEnum c : MyClass.MyEnum.values())
System.out.println(c);
Returns:
...
But in my company System.out calls are considered bad practice so I would like it not to be shown. My first try was to override values() but it is apparently not possible. Is there another way I can do this? Or is the only possibility to update the generated doc ?
I am also curious about why values() is not overridable. I read on other questions that "it is generated by the compiler". But can someone be more precise? It seems that it's generated from the enum's name, but it does not explain why.

values is a static method and is not subject to overriding. You cannot provide your own method to replace the generated one, and this is by specification.
There is no standard mechanism to replace the Javadoc of a method whose source code you don't control, but you could probably mess around with either the build tool, or, if all else fails, the final Javadoc HTML.

I don't think this is possible, but you could file a JDK issue and maybe provide an OpenJDK fix, if you like.

From the Oracle Java Tutorials:
The enum declaration defines a class (called an enum type). The enum class body can include methods and other fields. The compiler automatically adds some special methods when it creates an enum. For example, they have a static values method that returns an array containing all of the values of the enum in the order they are declared.
So, the method values cannot be overridden, since it's a special method created by the compiler. The Eclipse IDE generates this error when you try to do so:
The enum (your enum) already defines the method values() implicitly

Java implicit "this" parameter in method?

Within the programming language Java do method invocations on an object, work by implicitly passing a reference to the object to act on and working as static methods?

Details on how method invocation works can be found in the Java SE 7 JVM specification, section 3.7. For an instance method the this reference is passed as the first parameter. This reference is also used to select which method to invoke, since it might be overridden in a subclass, so it is a bit more complicated than a static method.

In short, no. That is how C++ was originally written, back when it was just a system of macros, but that was only because nothing existed (in C) like classes or static functions.
Java simply calls methods on objects. It has a shared piece of code that is the method, so in that sense it's static conceptually, but there is a bit that tells the modifiers of a method, and static is one of the bits, and it is not set for normal methods.