add-gc-test

name	add-gc-test
description	Add integration tests to TestGCMetrics.java for new GC collector support added to new-gc-default-jmx-metrics.yaml. Use when a PR adds a new garbage collector to the metrics YAML without a corresponding test.
allowed-tools	Read Edit Grep Bash(git diff *) Bash(git log *) Bash(./mvnw *)
argument-hint	["branch-or-commit"]

You are helping add an integration test to TestGCMetrics.java for new GC collector support.

Changes Introducing New GC Collectors

!`git diff master -- src/main/resources/org/datadog/jmxfetch/new-gc-default-jmx-metrics.yaml 2>/dev/null || git diff HEAD~1 -- src/main/resources/org/datadog/jmxfetch/new-gc-default-jmx-metrics.yaml 2>/dev/null || echo "(no diff found — check the branch or commit manually)"`

$ARGUMENTS

Your Task

1. Read src/main/resources/org/datadog/jmxfetch/new-gc-default-jmx-metrics.yaml — identify which new collector name: entries were added and what alias: metrics they map to.

2. Read src/test/java/org/datadog/jmxfetch/TestGCMetrics.java — understand existing test patterns and pick up on the startAndGetMetrics helper and both assertGCMetric overloads.

3. Read src/test/java/org/datadog/jmxfetch/util/server/JDKImage.java — confirm available JDK image constants (JDK_8, JDK_11, JDK_11_OPENJ9, JDK_17, JDK_21).

4. Add a new @Test method to TestGCMetrics.java following the patterns below.

Patterns

Standard GC with paired minor + major collectors

@Test
public void testDefaultNewGCMetricsUse<GCName>() throws IOException {
    try (final MisbehavingJMXServer server = new MisbehavingJMXServer.Builder().withJDKImage(
        <JDK_IMAGE>).appendJavaOpts("<-XX:+UseXxxGC>").build()) {
        final List<Map<String, Object>> actualMetrics = startAndGetMetrics(server, true);
        assertThat(actualMetrics, hasSize(13));
        assertGCMetric(actualMetrics,
            "jvm.gc.minor_collection_count", "<YoungCollectorName>", "counter");
        assertGCMetric(actualMetrics,
            "jvm.gc.minor_collection_time", "<YoungCollectorName>", "counter");
        assertGCMetric(actualMetrics,
            "jvm.gc.major_collection_count", "<OldCollectorName>", "counter");
        assertGCMetric(actualMetrics,
            "jvm.gc.major_collection_time", "<OldCollectorName>", "counter");
    }
}

GC with more than 2 active collectors (e.g., Generational ZGC with 4)

Use the assertGCMetric(actualMetrics, metric, List<String> gcGenerations) overload:

assertGCMetric(actualMetrics, "jvm.gc.major_collection_count",
    Arrays.asList("<Collector1>", "<Collector2>"));

Metric count formula

hasSize(9 + N*2) where N = number of distinct GC collector names active with that JVM flag.

• 9 = base JVM metrics (heap, threads, classes, etc.) emitted when collect_default_jvm_metrics: true + new_gc_metrics: true
• Each collector emits 2 metrics: count + time
• Examples: 2 collectors → hasSize(13), 4 collectors → hasSize(17)

JDK image guidance

GC flag	JDK image
-XX:+UseSerialGC, -XX:+UseParallelGC, -XX:+UseConcMarkSweepGC, -XX:+UseG1GC	JDK_11
-XX:+UseZGC (non-generational)	JDK_17
-XX:+UseZGC -XX:+ZGenerational	JDK_21
-XX:+UseShenandoahGC	JDK_17 or JDK_21
-Xgcpolicy:gencon, -Xgcpolicy:balanced	JDK_11_OPENJ9
GraalVM Native	no MisbehavingJMXServer support yet — note this limitation

Metric alias mapping

Look at the alias values in the YAML diff to determine which metric names to assert:

• jvm.gc.minor_collection_count / jvm.gc.minor_collection_time — young gen collectors
• jvm.gc.major_collection_count / jvm.gc.major_collection_time — old gen collectors
• Custom aliases (e.g., jvm.gc.zgc_cycles_collection_count) — assert those directly

Code Style Requirements

• 4-space indentation, no tabs
• Lines ≤ 100 characters — wrap appendJavaOpts(...) onto a second line if needed
• Method name in camelCase: testDefaultNewGCMetrics<Descriptive Suffix>
• No Javadoc required on test methods
• Imports: add any missing ones in the correct group (static → special → third-party → java → javax)

After Writing the Test

Run Checkstyle to verify formatting:

./mvnw checkstyle:check 2>&1 | grep -E "ERROR|WARNING|BUILD" | tail -20

If the new GC requires a JDK image not in JDKImage.java, note that it needs to be added there first.