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99 lines
3.3 KiB
Markdown
99 lines
3.3 KiB
Markdown
---
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layout: developer-doc
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title: Polyglot Java
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category: polyglot
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tags: [polyglot, java]
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order: 3
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---
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# Polyglot Java
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This document deals with the implementation of polyglot interoperation with Java
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in the runtime. Please familiarise yourself with the general operation of
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[polyglot bindings](./polyglot-bindings.md).
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<!-- MarkdownTOC levels="2,3" autolink="true" -->
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- [Class Lookup](#class-lookup)
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- [Polyglot Library System](#polyglot-library-system)
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- [Polyglot Syntax System](#polyglot-syntax-system)
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<!-- /MarkdownTOC -->
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## Class Lookup
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In order for the Enso runtime to effectively find Java objects for working with
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in a polyglot fashion, it will look in the `polyglot/java` subdirectory of an
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Enso project. This directory has the following requirements placed on it.
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- The top level of the `java` directory should contain only `.jar` files and
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directories.
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- Each directory must provide a valid class-path structure, with `.class` files
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at the appropriate points.
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- Both `.jar` files and directories are added to the runtime class-path for
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Enso, and hence be made available to Enso programs.
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> The actionables for this section are:
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>
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> - In future, we want to expand this to support `.class` files directly, and
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> maybe even compiling Java code.
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## Polyglot Library System
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The dynamic polyglot system is a dynamic runtime lookup for Java objects,
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allowing Enso code to work with them through a runtime reflection-style
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mechanism. It is comprised of the following components:
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- `Java.lookup_class : Class.Path -> Maybe Class`: A function that lets users
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look up a class by a given name on the runtime classpath.
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- `Polyglot.instantiate : Class -> Object`: A function that lets users
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instantiate a class into an object.
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- A whole host of functions on the polyglot type that let you dynamically work
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with object bindings.
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An example can be found below:
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```ruby
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main =
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class = Java.lookup_class "org.enso.example.TestClass"
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instance = Polyglot.instantiate1 class (x -> x * 2)
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method = Polyglot.get_member instance "callFunctionAndIncrement"
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Polyglot.execute1 method 10
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```
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> The actionables for this section are:
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>
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> - Expand on the detail when there is time.
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## Polyglot Syntax System
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The static system, however, lets us do much better in terms of user experience.
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Instead of having to dynamically look things up at runtime, we can instead do
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the following:
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- Statically resolve imports of polyglot bindings within the project to make
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sure that they are available.
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- Create java-compatible object entities that dynamically look up and dispatch
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both static methods on classes (by name), and methods on objects (by name).
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This includes the constructor and field reads.
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- This invocation syntax is integrated into Enso as variadic methods, allowing
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us to deal with the inter-language impedance mismatch.
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- Due to different semantics of Java calls, currying and over-applying functions
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are necessarily disabled for such calls, instead expecting the exact arguments
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list to be passed.
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An example can be found below:
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```ruby
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polyglot java import com.example.MyClass as MyClassJava
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main =
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x = MyClassJava.foo 1 2 3
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inst = MyClassJava.new a b c
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bar = inst.methodName x y
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```
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> The actionables for this section are:
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>
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> - Expand on the detail as the implementation becomes clear.
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