Explain the execution flow of a polyglot call between Java and JavaScript on GraalVM?

A polyglot call walks through several layers before returning a result back to Java host code.

sequenceDiagram
  participant Host as Java Host Code
  participant Ctx as polyglot.Context
  participant JS as GraalJS (Truffle)
  participant Graal as Graal Compiler
  Host->>Ctx: context.eval("js", source)
  Ctx->>JS: parse source into Truffle AST
  JS->>JS: interpret AST nodes, specialize on types
  JS->>Graal: hot nodes handed off for partial evaluation
  Graal-->>JS: optimized machine code installed
  JS-->>Ctx: result value produced
  Ctx-->>Host: wrapped as org.graalvm.polyglot.Value

1. Java host code calls context.eval("js", source), handing the JavaScript source string to the Context.
2. GraalJS, a Truffle interpreter, parses it into an AST and begins tree-walking execution, with nodes specializing themselves based on observed operand types.
3. Once a code path is hot, the Graal compiler partially evaluates the interpreter against that specific AST, producing optimized machine code.
4. The final result is wrapped in a Value object and handed back to the Java host, which can convert it to a native Java type or a proxying interface.

Calls in the opposite direction (JavaScript calling into Java objects) work symmetrically, going through the same Context's host-access policy.