jolt/docs/host-interop.md

Host interop and JVM standard-library shims

Jolt runs on Chez Scheme, not the JVM, so there are no real Java classes behind interop forms. Instead the runtime ships shims for the slice of the JVM standard library that portable Clojure code reaches for, so libraries written against clojure.core and common java.* classes run unchanged. The Clojure interop syntax works against these shims:

(Math/sqrt 2)                  ; static call
Math/PI                        ; static field
(StringBuilder.)               ; constructor
(.append sb "x")               ; instance method
(instance? String "hi")        ; class token

A class token (String, java.util.UUID, …) resolves to a name; there is no reflection and no class hierarchy. (class x) returns the JVM class name for the scalar/collection types Clojure programs compare against ("java.lang.Long", "java.lang.String", and so on).

What's shimmed

This is the surface today, not the whole JVM. Methods not listed generally aren't implemented; a few are accepted but no-ops (noted inline).

Numbers and language

• java.lang.Math — sqrt cbrt pow exp log log10 floor ceil round abs max min sin cos tan asin acos atan signum random; fields PI, E. (clojure.math mirrors these as functions.)
• Long / Integer — parseLong/parseInt/valueOf (optional radix), MAX_VALUE, MIN_VALUE; (Integer. x).
• Double / Float — parseDouble, valueOf, toString, isNaN, isInfinite, the *_VALUE/*_INFINITY/NaN fields; (Double. s).
• Boolean — parseBoolean, TRUE, FALSE.
• Character — isUpperCase isLowerCase isDigit isWhitespace (ASCII).
• Boxed-number methods — every number answers .intValue .longValue .doubleValue .floatValue .byteValue .shortValue .toString .hashCode (integer projections wrap modulo their width, as on the JVM).
• java.lang.System — currentTimeMillis nanoTime exit getProperty setProperty clearProperty getProperties getenv.
• java.lang.Thread — sleep (real), yield/interrupted (no-ops), currentThread.
• java.lang.Object — (Object.) as a fresh-identity sentinel; .toString .hashCode .equals .getClass work on any value.
• java.lang.Class — forName.

Strings and text

• java.lang.String statics — valueOf, format (the clojure.core/format engine; String/format with a leading locale is accepted). Instance methods go through clojure.string / the native string ops.
• StringBuilder — append toString length charAt setLength.
• java.text.NumberFormat — getInstance getNumberInstance getIntegerInstance; .format, .setGroupingUsed, .setMinimum/MaximumFractionDigits.
• java.util.StringTokenizer — hasMoreTokens countTokens nextToken.
• java.util.regex.Pattern — compile (with Pattern/MULTILINE), quote; .split, .pattern. (#"…" literals and clojure.string regex fns are the usual entry points.)

Collections (mutable)

• java.util.ArrayList — add get set size isEmpty remove clear contains toArray iterator.
• java.util.HashMap — put get getOrDefault containsKey containsValue size isEmpty remove clear putAll keySet values entrySet.

I/O

• java.io.File — (File. path) / (File. parent child); getPath getName getAbsolutePath getCanonicalPath toURI toURL exists isDirectory isFile listFiles getParent.
• java.io.StringReader / StringWriter / PushbackReader — the read/readLine/mark/reset/unread/write/append/toString surface the reader and with-out-str rely on.
• java.lang.ClassLoader — getSystemClassLoader, .getResource, .getResourceAsStream (resolved against the source roots).

Time and date

• java.util.Date — (Date.) / (Date. ms); getTime toInstant toLocalDate(Time) before after equals toString (RFC 3339).
• java.time — Instant (now, ofEpochMilli, toEpochMilli, atZone), LocalDateTime, ZoneId, DateTimeFormatter (ofPattern, ISO_LOCAL_*, localized styles), FormatStyle.
• java.text.SimpleDateFormat — (SimpleDateFormat. pattern); parse format toPattern applyPattern (setTimeZone/setLenient accepted but ignored — formatting is UTC).
• java.util.TimeZone / java.util.Locale — constructed and passed through; only UTC is honored for formatting.

Net, encoding, misc

• java.net.URL — (URL. spec); toString toExternalForm getProtocol getPath getFile.
• java.net.URI — full component accessors (getScheme getHost getPort getPath getQuery getFragment, raw variants, isAbsolute).
• java.util.Base64 — getEncoder/getDecoder with encode, encodeToString, decode.
• java.nio.charset.Charset — forName.
• java.util.UUID — randomUUID, fromString; (UUID. s).
• Exceptions — Throwable Exception RuntimeException IllegalArgumentException IllegalStateException IOException NumberFormatException ArithmeticException NullPointerException ClassCastException IndexOutOfBoundsException FileNotFoundException UnsupportedOperationException and the common network exceptions, each with the (E.) / (E. msg) / (E. msg cause) / (E. cause) constructors.

What's deliberately absent: STM (clojure.lang.LockingTransaction/isRunning returns false), reflection, gen-class/proxy of Java classes, and BigDecimal.

Adding your own shim from a library

The built-in shims above are baked into the seed. A library or project can register its own host classes at load time — no seed re-mint, no host edits. Put the registration calls at the top level of a namespace your code requires. Four functions (in clojure.core) plus the tagged-table seam (in jolt.host) cover it.

__register-class-ctor! makes (Name. …) work; __register-class-statics! makes Name/field and (Name/method …) work; __register-class-methods! attaches instance methods to a tagged value; __register-instance-check! teaches instance? about your class. Method and static names are strings (they match the literal name in the interop form).

A stateful object is a tagged table — jolt.host/tagged-table creates one, ref-put!/ref-get set and read its fields. Read the tag back with jolt.host/ref-get (or test it with jolt.host/table?); a plain get / keyword lookup deliberately can't see a wrapper's own :jolt/type.

(ns mylib.greeter
  (:require [jolt.host :as host]))

;; (Greeter. name) -> a tagged value carrying its name
(__register-class-ctor! "Greeter"
  (fn [name] (-> (host/tagged-table :greeter)
                 (host/ref-put! :name name))))

;; (.hello g) -> instance method, keyed by the literal method name
(__register-class-methods! :greeter
  {"hello" (fn [self] (str "hi " (host/ref-get self :name)))})

;; Greeter/VERSION (field) and (Greeter/make x) (static method)
(__register-class-statics! "Greeter"
  {"VERSION" "1.0"
   "make"    (fn [name] (Greeter. name))})

;; (instance? Greeter x)
(__register-instance-check!
  (fn [class-name v]
    (when (= class-name "Greeter")
      (and (host/table? v) (= :greeter (host/ref-get v :jolt/type))))))

(.hello (Greeter. "ada"))            ;=> "hi ada"
Greeter/VERSION                      ;=> "1.0"
(.hello (Greeter/make "bob"))        ;=> "hi bob"
(instance? Greeter (Greeter. "x"))   ;=> true

An instance-check predicate returns true/false to decide, or nil to defer to the next registered check and the built-ins — so several libraries can register checks without clobbering each other. This is the mechanism jolt's HTTP client library uses to emulate java.net.URL and HttpURLConnection so clj-http-lite runs unchanged.

Extending a built-in class instead (adding a method to core's String shim, say) means editing the relevant host/chez/*.ss file and running make remint — see building-and-deps.md.