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Add JsonPath ESM renderer: AST to ES2020 module codegen
Walk the JsonPath AST with exhaustive switch to emit JavaScript that evaluates the expression without interpretation overhead. Generated modules export a query(root) function that returns an array of matched values. Supports all segment types: property access, array index, slice, wildcard, recursive descent, filter, union, script. 18 tests verify structural validity and correct rendering for each segment type. To verify: mvn test -pl json-java21-jsonpath -Dtest=JsonPathEsmRendererTest
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‎json-java21-jsonpath/src/main/java/json/java21/jsonpath/JsonPathEsmRenderer.java‎

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package json.java21.jsonpath;
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import json.java21.jsonpath.JsonPathAst.*;
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import java.util.List;
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import java.util.logging.Logger;
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/// Renders a JsonPath AST into an ES2020 module that exports a `query(document)` function.
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///
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/// The generated JavaScript evaluates the JsonPath expression against a JSON document
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/// and returns an array of matched values. All dispatch decisions are resolved at
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/// render time - no interpretation at runtime.
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///
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/// Usage:
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/// ```java
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/// var ast = JsonPath.parse("$.store.book[*].title").ast();
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/// String esm = JsonPathEsmRenderer.render(ast, "$.store.book[*].title");
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/// // esm contains: export function query(root) { ... }
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/// ```
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public final class JsonPathEsmRenderer {
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private static final Logger LOG = Logger.getLogger(JsonPathEsmRenderer.class.getName());
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private JsonPathEsmRenderer() {}
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/// Renders a JsonPath AST into an ES2020 module string.
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///
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/// @param ast the parsed JsonPath AST root
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/// @param expression the original expression string (for comments)
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/// @return a complete ES2020 module string
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public static String render(Root ast, String expression) {
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final var sb = new StringBuilder();
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sb.append("// Generated JsonPath query: ").append(expression).append('\n');
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sb.append("// Do not edit - generated by JsonPathEsmRenderer\n\n");
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sb.append("export function query(root) {\n");
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sb.append(" const results = [];\n");
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emitSegmentChain(sb, ast.segments(), 0, "root", "root", " ");
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sb.append(" return results;\n");
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sb.append("}\n");
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LOG.fine(() -> "Rendered ESM for: " + expression);
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return sb.toString();
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}
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private static void emitSegmentChain(StringBuilder sb, List<Segment> segments,
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int segIdx, String currentVar, String rootVar, String indent) {
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if (segIdx >= segments.size()) {
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sb.append(indent).append("results.push(").append(currentVar).append(");\n");
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return;
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}
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final var segment = segments.get(segIdx);
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switch (segment) {
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case PropertyAccess prop ->
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emitPropertyAccess(sb, prop, segments, segIdx, currentVar, rootVar, indent);
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case ArrayIndex arr ->
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emitArrayIndex(sb, arr, segments, segIdx, currentVar, rootVar, indent);
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case ArraySlice slice ->
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emitArraySlice(sb, slice, segments, segIdx, currentVar, rootVar, indent);
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case Wildcard ignored ->
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emitWildcard(sb, segments, segIdx, currentVar, rootVar, indent);
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case RecursiveDescent desc ->
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emitRecursiveDescent(sb, desc, segments, segIdx, currentVar, rootVar, indent);
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case Filter filter ->
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emitFilter(sb, filter, segments, segIdx, currentVar, rootVar, indent);
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case Union union ->
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emitUnion(sb, union, segments, segIdx, currentVar, rootVar, indent);
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case ScriptExpression script ->
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emitScript(sb, script, segments, segIdx, currentVar, rootVar, indent);
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}
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}
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private static void emitPropertyAccess(StringBuilder sb, PropertyAccess prop,
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List<Segment> segments, int segIdx, String currentVar, String rootVar, String indent) {
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final var jsName = jsString(prop.name());
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sb.append(indent).append("if (").append(currentVar).append(" != null && typeof ")
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.append(currentVar).append(" === 'object' && !Array.isArray(").append(currentVar).append(")) {\n");
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final var valVar = "v" + segIdx;
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sb.append(indent).append(" const ").append(valVar).append(" = ").append(currentVar)
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.append("[").append(jsName).append("];\n");
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sb.append(indent).append(" if (").append(valVar).append(" !== undefined) {\n");
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emitSegmentChain(sb, segments, segIdx + 1, valVar, rootVar, indent + " ");
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sb.append(indent).append(" }\n");
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sb.append(indent).append("}\n");
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}
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private static void emitArrayIndex(StringBuilder sb, ArrayIndex arr,
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List<Segment> segments, int segIdx, String currentVar, String rootVar, String indent) {
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sb.append(indent).append("if (Array.isArray(").append(currentVar).append(")) {\n");
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final var idxExpr = arr.index() < 0
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? currentVar + ".length + " + arr.index()
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: String.valueOf(arr.index());
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final var iVar = "i" + segIdx;
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sb.append(indent).append(" const ").append(iVar).append(" = ").append(idxExpr).append(";\n");
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sb.append(indent).append(" if (").append(iVar).append(" >= 0 && ").append(iVar)
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.append(" < ").append(currentVar).append(".length) {\n");
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final var elemVar = "e" + segIdx;
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sb.append(indent).append(" const ").append(elemVar).append(" = ").append(currentVar)
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.append("[").append(iVar).append("];\n");
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emitSegmentChain(sb, segments, segIdx + 1, elemVar, rootVar, indent + " ");
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sb.append(indent).append(" }\n");
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sb.append(indent).append("}\n");
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}
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private static void emitArraySlice(StringBuilder sb, ArraySlice slice,
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List<Segment> segments, int segIdx, String currentVar, String rootVar, String indent) {
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sb.append(indent).append("if (Array.isArray(").append(currentVar).append(")) {\n");
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final var step = slice.step() != null ? slice.step() : 1;
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final var iVar = "i" + segIdx;
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final var lenVar = "len" + segIdx;
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sb.append(indent).append(" const ").append(lenVar).append(" = ").append(currentVar).append(".length;\n");
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if (step > 0) {
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final var startExpr = slice.start() != null
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? "Math.max(0, " + normalizeIdx(slice.start(), lenVar) + ")"
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: "0";
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final var endExpr = slice.end() != null
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? "Math.min(" + lenVar + ", " + normalizeIdx(slice.end(), lenVar) + ")"
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: lenVar;
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sb.append(indent).append(" for (let ").append(iVar).append(" = ").append(startExpr)
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.append("; ").append(iVar).append(" < ").append(endExpr)
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.append("; ").append(iVar).append(" += ").append(step).append(") {\n");
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} else {
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final var startExpr = slice.start() != null
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? "Math.min(" + lenVar + " - 1, " + normalizeIdx(slice.start(), lenVar) + ")"
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: lenVar + " - 1";
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final var endExpr = slice.end() != null
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? normalizeIdx(slice.end(), lenVar)
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: "-1";
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sb.append(indent).append(" for (let ").append(iVar).append(" = ").append(startExpr)
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.append("; ").append(iVar).append(" > ").append(endExpr)
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.append(" && ").append(iVar).append(" >= 0 && ").append(iVar).append(" < ").append(lenVar)
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.append("; ").append(iVar).append(" += ").append(step).append(") {\n");
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}
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final var elemVar = "e" + segIdx;
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sb.append(indent).append(" const ").append(elemVar).append(" = ")
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.append(currentVar).append("[").append(iVar).append("];\n");
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emitSegmentChain(sb, segments, segIdx + 1, elemVar, rootVar, indent + " ");
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sb.append(indent).append(" }\n");
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sb.append(indent).append("}\n");
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}
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private static String normalizeIdx(int idx, String lenVar) {
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return idx < 0 ? lenVar + " + " + idx : String.valueOf(idx);
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}
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private static void emitWildcard(StringBuilder sb,
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List<Segment> segments, int segIdx, String currentVar, String rootVar, String indent) {
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final var valVar = "w" + segIdx;
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sb.append(indent).append("if (Array.isArray(").append(currentVar).append(")) {\n");
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sb.append(indent).append(" for (const ").append(valVar).append(" of ").append(currentVar).append(") {\n");
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emitSegmentChain(sb, segments, segIdx + 1, valVar, rootVar, indent + " ");
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sb.append(indent).append(" }\n");
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sb.append(indent).append("} else if (").append(currentVar).append(" != null && typeof ")
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.append(currentVar).append(" === 'object') {\n");
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sb.append(indent).append(" for (const ").append(valVar).append(" of Object.values(")
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.append(currentVar).append(")) {\n");
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emitSegmentChain(sb, segments, segIdx + 1, valVar, rootVar, indent + " ");
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sb.append(indent).append(" }\n");
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sb.append(indent).append("}\n");
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}
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private static void emitRecursiveDescent(StringBuilder sb, RecursiveDescent desc,
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List<Segment> segments, int segIdx, String currentVar, String rootVar, String indent) {
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final var target = desc.target();
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final var fnName = "_descent" + segIdx;
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final var nodeVar = "n" + segIdx;
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// Emit a local recursive function
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sb.append(indent).append("function ").append(fnName).append("(").append(nodeVar).append(") {\n");
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// Match target at current level
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switch (target) {
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case PropertyAccess prop -> {
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sb.append(indent).append(" if (").append(nodeVar).append(" != null && typeof ")
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.append(nodeVar).append(" === 'object' && !Array.isArray(").append(nodeVar).append(")) {\n");
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sb.append(indent).append(" const _m = ").append(nodeVar).append("[")
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.append(jsString(prop.name())).append("];\n");
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sb.append(indent).append(" if (_m !== undefined) {\n");
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emitSegmentChain(sb, segments, segIdx + 1, "_m", rootVar, indent + " ");
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sb.append(indent).append(" }\n");
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sb.append(indent).append(" }\n");
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}
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case Wildcard ignored2 -> {
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sb.append(indent).append(" if (Array.isArray(").append(nodeVar).append(")) {\n");
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sb.append(indent).append(" for (const _m of ").append(nodeVar).append(") {\n");
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emitSegmentChain(sb, segments, segIdx + 1, "_m", rootVar, indent + " ");
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sb.append(indent).append(" }\n");
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sb.append(indent).append(" } else if (").append(nodeVar).append(" != null && typeof ")
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.append(nodeVar).append(" === 'object') {\n");
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sb.append(indent).append(" for (const _m of Object.values(").append(nodeVar).append(")) {\n");
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emitSegmentChain(sb, segments, segIdx + 1, "_m", rootVar, indent + " ");
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sb.append(indent).append(" }\n");
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sb.append(indent).append(" }\n");
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}
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default -> {
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// Unsupported target in recursive descent
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}
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}
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// Recurse into children
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sb.append(indent).append(" if (Array.isArray(").append(nodeVar).append(")) {\n");
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sb.append(indent).append(" for (const _c of ").append(nodeVar).append(") ").append(fnName).append("(_c);\n");
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sb.append(indent).append(" } else if (").append(nodeVar).append(" != null && typeof ")
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.append(nodeVar).append(" === 'object') {\n");
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sb.append(indent).append(" for (const _c of Object.values(").append(nodeVar).append(")) ").append(fnName).append("(_c);\n");
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sb.append(indent).append(" }\n");
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sb.append(indent).append("}\n");
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sb.append(indent).append(fnName).append("(").append(currentVar).append(");\n");
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}
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private static void emitFilter(StringBuilder sb, Filter filter,
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List<Segment> segments, int segIdx, String currentVar, String rootVar, String indent) {
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sb.append(indent).append("if (Array.isArray(").append(currentVar).append(")) {\n");
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final var elemVar = "f" + segIdx;
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sb.append(indent).append(" for (const ").append(elemVar).append(" of ").append(currentVar).append(") {\n");
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sb.append(indent).append(" if (");
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emitFilterExpression(sb, filter.expression(), elemVar);
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sb.append(") {\n");
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emitSegmentChain(sb, segments, segIdx + 1, elemVar, rootVar, indent + " ");
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sb.append(indent).append(" }\n");
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sb.append(indent).append(" }\n");
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sb.append(indent).append("}\n");
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}
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private static void emitFilterExpression(StringBuilder sb, FilterExpression expr, String elemVar) {
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switch (expr) {
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case ExistsFilter exists -> {
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emitPropertyPathAccess(sb, exists.path(), elemVar);
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sb.append(" !== undefined");
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}
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case ComparisonFilter comp -> {
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sb.append("(");
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emitFilterOperand(sb, comp.left(), elemVar);
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sb.append(" ").append(jsOp(comp.op())).append(" ");
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emitFilterOperand(sb, comp.right(), elemVar);
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sb.append(")");
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}
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case LogicalFilter logical -> {
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switch (logical.op()) {
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case AND -> {
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sb.append("(");
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emitFilterExpression(sb, logical.left(), elemVar);
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sb.append(" && ");
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if (logical.right() != null) {
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emitFilterExpression(sb, logical.right(), elemVar);
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} else {
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sb.append("true");
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}
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sb.append(")");
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}
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case OR -> {
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sb.append("(");
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emitFilterExpression(sb, logical.left(), elemVar);
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sb.append(" || ");
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if (logical.right() != null) {
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emitFilterExpression(sb, logical.right(), elemVar);
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} else {
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sb.append("false");
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}
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sb.append(")");
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}
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case NOT -> {
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sb.append("!(");
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emitFilterExpression(sb, logical.left(), elemVar);
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sb.append(")");
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}
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}
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}
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case CurrentNode cn -> sb.append("true");
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case PropertyPath path -> {
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emitPropertyPathAccess(sb, path, elemVar);
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sb.append(" !== undefined");
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}
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case LiteralValue lv -> sb.append("true");
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}
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}
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private static void emitFilterOperand(StringBuilder sb, FilterExpression expr, String elemVar) {
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switch (expr) {
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case PropertyPath path -> emitPropertyPathAccess(sb, path, elemVar);
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case LiteralValue lit -> {
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if (lit.value() == null) {
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sb.append("null");
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} else if (lit.value() instanceof String s) {
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sb.append(jsString(s));
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} else if (lit.value() instanceof Number n) {
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sb.append(n);
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} else if (lit.value() instanceof Boolean b) {
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sb.append(b);
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} else {
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sb.append("null");
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}
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}
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case CurrentNode cn2 -> sb.append(elemVar);
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default -> sb.append("null");
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}
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}
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private static void emitPropertyPathAccess(StringBuilder sb, PropertyPath path, String elemVar) {
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sb.append(elemVar);
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for (final var prop : path.properties()) {
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sb.append("?.[").append(jsString(prop)).append("]");
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}
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}
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private static void emitUnion(StringBuilder sb, Union union,
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List<Segment> segments, int segIdx, String currentVar, String rootVar, String indent) {
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for (final var selector : union.selectors()) {
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switch (selector) {
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case PropertyAccess prop ->
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emitPropertyAccess(sb, prop, segments, segIdx, currentVar, rootVar, indent);
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case ArrayIndex arr ->
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emitArrayIndex(sb, arr, segments, segIdx, currentVar, rootVar, indent);
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default -> {}
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}
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}
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}
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private static void emitScript(StringBuilder sb, ScriptExpression script,
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List<Segment> segments, int segIdx, String currentVar, String rootVar, String indent) {
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if ("@.length-1".equals(script.script())) {
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sb.append(indent).append("if (Array.isArray(").append(currentVar)
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.append(") && ").append(currentVar).append(".length > 0) {\n");
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final var elemVar = "s" + segIdx;
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sb.append(indent).append(" const ").append(elemVar).append(" = ")
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.append(currentVar).append("[").append(currentVar).append(".length - 1];\n");
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emitSegmentChain(sb, segments, segIdx + 1, elemVar, rootVar, indent + " ");
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sb.append(indent).append("}\n");
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}
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}
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private static String jsString(String s) {
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return "\"" + s.replace("\\", "\\\\").replace("\"", "\\\"") + "\"";
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}
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private static String jsOp(ComparisonOp op) {
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return switch (op) {
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case EQ -> "===";
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case NE -> "!==";
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case LT -> "<";
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case LE -> "<=";
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case GT -> ">";
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case GE -> ">=";
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};
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}
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}

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