feat(alerting): MustacheRenderer with literal fallback on missing vars
Sentinel-substitution approach: unresolved {{x.y.z}} tokens are replaced
with a unique NUL-delimited sentinel before Mustache compilation, rendered
as opaque text, then post-replaced with the original {{x.y.z}} literal.
Malformed templates (unclosed {{) are caught and return the raw template.
Never throws. 9 unit tests.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
This commit is contained in:
hsiegeln
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package com.cameleer.server.app.alerting.notify;
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import com.samskivert.mustache.Mustache;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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import org.springframework.stereotype.Component;
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import java.util.LinkedHashMap;
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import java.util.Map;
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import java.util.regex.Matcher;
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import java.util.regex.Pattern;
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/**
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* Renders Mustache templates against a context map.
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* <p>
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* Contract:
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* <ul>
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* <li>Unresolved {@code {{x.y.z}}} tokens render as the literal {@code {{x.y.z}}} and log WARN.</li>
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* <li>Malformed templates (e.g. unclosed {@code {{}) return the original template string and log WARN.</li>
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* <li>Never throws on template content.</li>
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* </ul>
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*/
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@Component
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public class MustacheRenderer {
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private static final Logger log = LoggerFactory.getLogger(MustacheRenderer.class);
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/** Matches {{path}} tokens, capturing the trimmed path. Ignores triple-mustache and comments. */
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private static final Pattern TOKEN = Pattern.compile("\\{\\{\\s*([^#/!>{\\s][^}]*)\\s*\\}\\}");
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/** Sentinel prefix/suffix to survive Mustache compilation so we can post-replace. */
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private static final String SENTINEL_PREFIX = "\u0000TPL\u0001";
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private static final String SENTINEL_SUFFIX = "\u0001LPT\u0000";
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public String render(String template, Map<String, Object> ctx) {
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if (template == null) return "";
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try {
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// 1) Walk all {{path}} tokens. Those unresolved get replaced with a unique sentinel.
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Map<String, String> literals = new LinkedHashMap<>();
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StringBuilder pre = new StringBuilder();
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Matcher m = TOKEN.matcher(template);
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int sentinelIdx = 0;
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boolean anyUnresolved = false;
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while (m.find()) {
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String path = m.group(1).trim();
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if (resolvePath(ctx, path) == null) {
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anyUnresolved = true;
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String sentinelKey = SENTINEL_PREFIX + sentinelIdx++ + SENTINEL_SUFFIX;
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literals.put(sentinelKey, "{{" + path + "}}");
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m.appendReplacement(pre, Matcher.quoteReplacement(sentinelKey));
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}
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}
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m.appendTail(pre);
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if (anyUnresolved) {
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log.warn("MustacheRenderer: unresolved template variables; rendering as literals. template={}",
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template.length() > 200 ? template.substring(0, 200) + "..." : template);
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}
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// 2) Compile & render the pre-processed template (sentinels are plain text — not Mustache tags).
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String rendered = Mustache.compiler()
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.defaultValue("")
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.escapeHTML(false)
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.compile(pre.toString())
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.execute(ctx);
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// 3) Restore the sentinel placeholders back to their original {{path}} literals.
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for (Map.Entry<String, String> e : literals.entrySet()) {
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rendered = rendered.replace(e.getKey(), e.getValue());
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}
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return rendered;
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} catch (Exception e) {
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log.warn("MustacheRenderer: template render failed, returning raw template: {}", e.getMessage());
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return template;
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}
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}
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/**
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* Resolves a dotted path like "alert.state" against a nested Map context.
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* Returns null if any segment is missing or the value is null.
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*/
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@SuppressWarnings("unchecked")
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Object resolvePath(Map<String, Object> ctx, String path) {
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if (ctx == null || path == null || path.isBlank()) return null;
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String[] parts = path.split("\\.");
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Object current = ctx.get(parts[0]);
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for (int i = 1; i < parts.length; i++) {
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if (!(current instanceof Map)) return null;
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current = ((Map<String, Object>) current).get(parts[i]);
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}
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return current;
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}
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}
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@@ -0,0 +1,77 @@
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package com.cameleer.server.app.alerting.notify;
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import org.junit.jupiter.api.BeforeEach;
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import org.junit.jupiter.api.Test;
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import java.util.Map;
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import static org.assertj.core.api.Assertions.assertThat;
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class MustacheRendererTest {
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private MustacheRenderer renderer;
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@BeforeEach
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void setUp() {
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renderer = new MustacheRenderer();
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}
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@Test
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void simpleVariable_rendersValue() {
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var ctx = Map.<String, Object>of("name", "production");
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assertThat(renderer.render("Env: {{name}}", ctx)).isEqualTo("Env: production");
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}
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@Test
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void nestedPath_rendersValue() {
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var ctx = Map.<String, Object>of(
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"alert", Map.of("state", "FIRING"));
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assertThat(renderer.render("State: {{alert.state}}", ctx)).isEqualTo("State: FIRING");
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}
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@Test
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void missingVariable_rendersLiteralMustache() {
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var ctx = Map.<String, Object>of("known", "yes");
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String result = renderer.render("{{known}} and {{missing.path}}", ctx);
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assertThat(result).isEqualTo("yes and {{missing.path}}");
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}
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@Test
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void missingVariable_exactLiteralNoPadding() {
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// The rendered literal must be exactly {{x}} — no surrounding whitespace or delimiter residue.
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String result = renderer.render("{{unknown}}", Map.of());
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assertThat(result).isEqualTo("{{unknown}}");
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}
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@Test
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void malformedTemplate_returnsRawTemplate() {
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String broken = "Hello {{unclosed";
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String result = renderer.render(broken, Map.of());
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assertThat(result).isEqualTo(broken);
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}
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@Test
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void nullTemplate_returnsEmptyString() {
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assertThat(renderer.render(null, Map.of())).isEmpty();
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}
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@Test
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void emptyTemplate_returnsEmptyString() {
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assertThat(renderer.render("", Map.of())).isEmpty();
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}
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@Test
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void mixedResolvedAndUnresolved_rendersCorrectly() {
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var ctx = Map.<String, Object>of(
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"env", Map.of("slug", "prod"),
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"alert", Map.of("id", "abc-123"));
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String tmpl = "{{env.slug}} / {{alert.id}} / {{alert.resolvedAt}}";
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String result = renderer.render(tmpl, ctx);
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assertThat(result).isEqualTo("prod / abc-123 / {{alert.resolvedAt}}");
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}
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@Test
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void plainText_noTokens_returnsAsIs() {
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assertThat(renderer.render("No tokens here.", Map.of())).isEqualTo("No tokens here.");
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}
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}
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