[jakewins]

Someone apparently had the exact same question in 2020: https://stackoverflow.com/questions/62298929/what-is-an-anal...

Answer is pretty vague though, but sounds like it’s about not trying to “reverse” what the compiler did, but rather try and “analytically” work put what source code would likely have yielded the byte code it’s looking at?

[rhdunn]

Yes, that's what it is doing.

If you have a block of code a compiler will compile a language expression or statement into a particular set of assembly/bytecode instructions. For example converting `a + b` to `ADD a b`.

A reversing decompiler will look at the `ADD a b` and produce `a + b` as the output. This is the simplest approach as it is effectively just a collection of these types of mapping. While this works, it can be harder to read and noisier than the actual code. This is because:

1. it does not handle annotations like @NotNull correctly -- these are shown as `if (arg == null) throw ...` instead of the annotation because the if/throw is what the compiler generated for that annotation;

2. it doesn't make complex expressions readable;

3. it doesn't detect optimizations like unrolling loops, reordering expressions, etc.

For (1) an analytical decompiler can recognize the `if (arg == null) throw` expression at the start of the function and map that to a @NotNull annotation.

Likewise, it could detect other optimizations like loop unrolling and produce better code for that.

[vbezhenar]

I'm not sure that @NotNull example is appropriate. Java compiler does not add any checks for @NotNull annotations. Those annotations exist for IDE and linting tools, compiler doesn't care. May be there are Java-like languages like Lombok or non-standard compilers which do add those checks, but I think that Java decompiler shouldn't do assumptions of these additional tools.

[rhdunn]

I was trying to think of examples.

A better example for Java would be something like lambda expressions on functional interfaces. There, the compiler is creating an anonymous object that implements the interface. A reversable decompiler will just see the anonymous class instance whereas an analytical decompiler can detect that it is likely a lambda expression due to it being an anonymous class object implementing a single method interface and is being passed to a function argument that takes that interface as a parameter.

In C# yield is implemented as a state machine, so an analytical decompiler could recognise that construct.

And yes, for JVM decompilers it could have language heuristics to detect (or be specifically for) Lombok, Scala, Groovy, Kotlin, etc.

[1] https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaex...

[rhdunn]

https://www.jetbrains.com/help/idea/annotating-source-code.h...

> When you compile your project with IntelliJ IDEA build tool, the IDE adds assertions to all code elements annotated with @NotNull. These assertions will throw an error if the elements happen to be null at runtime.

[vbezhenar]

That's not java compiler. That's intellij compiler. I'd say that's very weird anti-feature, because your build in IDE and maven will work differently.

[wokkel]

When using Lombok it will use a compiler plugin for this so maven builds have @nonnull generated as if-statements. I dont know if intellij uses their own plugin but they do support Lombok in maven projects, so maybe thats where this is coming from. Afaik intellij has no built in compiler but relies on java .

[Bjartr]

Lombok hijacks the compiler to do it's own thing, and violates the contract Java compiler plugins are supposed to follow.

See this comment by an OpenJDK tech lead: https://news.ycombinator.com/item?id=37666793