Unlocking JDK 17: Key Features, ZGC Benefits, and Upgrade Strategies

1. Main Features of JDK 17

JDK 17 introduces language enhancements such as local variable type inference (var), sealed classes, record classes, switch expressions, text blocks, and new APIs like HttpClient, jpackage, and an improved Process API.

1.1 Language Features

Local variable type inference

String str = "Hello world"; // JDK8
var str = "Hello world";   // JDK17

Sealed classes

public sealed class Shape permits Circle, Rectangle, Triangle { }

Record classes

Records provide immutable data carriers with automatically generated equals(), hashCode(), toString(), and constructors.

record Rectangle(double length, double width) { }

Switch expression

int num = switch (day) {
    case MONDAY, FRIDAY, SUNDAY -> 6;
    case TUESDAY -> 7;
    case THURSDAY, SATURDAY -> 8;
    case WEDNESDAY -> 9;
    default -> throw new IllegalStateException("Invalid day: " + day);
};

Text blocks

String sql = """
    SELECT * FROM users
    WHERE status = 'ACTIVE'
    ORDER BY created_at DESC
    """;

1.2 New APIs and Tools

HttpClient

HttpClient client = HttpClient.newBuilder()
    .version(HttpClient.Version.HTTP_1_1)
    .followRedirects(HttpClient.Redirect.NORMAL)
    .connectTimeout(Duration.ofSeconds(20))
    .build();

jpackage

Creates native installers that bundle the application and a JDK runtime image, eliminating the need for a separate JDK installation.

Process API

ProcessBuilder pb = new ProcessBuilder("echo", "Hello World!");
Process p = pb.start();

2. JDK 17 + ZGC in Security Practice

Meituan’s security team migrated core services from JDK 8 to JDK 17 with ZGC, achieving up to 30 % throughput improvement, 99 % pause‑time reduction, and roughly 10 % cost savings.

Key scenarios for ZGC include large‑memory servers (>16 GB heap), high CPU utilization, and services where GC dominates latency.

Performance Results

TP9999 latency reduced by 18 %–74 % (220‑380 ms).

TP999 latency reduced by 10 %–63 % (60‑125 ms).

TP99 latency reduced by up to 25 % (3‑20 ms).

Two production cases (intelligent decision system and content‑security core service) show lower CPU busy, stable long‑term latency, fewer error spikes, and reduced Metaspace usage after the upgrade.

3. JDK 17 Upgrade Process

Installation & Compatibility

Common errors stem from Java module encapsulation; they are resolved by adding --add-opens JVM options for required packages.

--add-opens java.base/java.lang=ALL-UNNAMED \
--add-opens java.base/java.io=ALL-UNNAMED \
--add-opens java.base/java.util.concurrent=ALL-UNNAMED ...

Performance Testing

Baseline: JDK 8 + CMS. Metrics collected: peak/average CPU, TP9999, error count, GC time/count, heap and Metaspace usage.

JVM Parameters

-Xmx12g -Xms12g \
-XX:+UseZGC \
-XX:+UnlockExperimentalVMOptions -XX:+UnlockDiagnosticVMOptions \
-XX:ConcGCThreads=3 -XX:ParallelGCThreads=8 \
-XX:ZCollectionInterval=130 -XX:ZAllocationSpikeTolerance=1 \
-XX:MaxDirectMemorySize=460m -XX:MetaspaceSize=330m -XX:MaxMetaspaceSize=330m \
-XX:ReservedCodeCacheSize=256m -XX:InitialCodeCacheSize=256m \
-Xlog:safepoint,class+load=info,class+unload=info,classhisto*=trace,age*,gc*=info:file=/opt/logs/logs/gc-%t.log:time,tid,tags:filecount=5,filesize=50m

4. Conclusion

ZGC delivers significant latency and cost benefits.

Spring AI SDK requires JDK 17 or higher.

Direct migration from JDK 8 to 17 may need an intermediate JDK 11 for compatibility.

AI assistants can help resolve upgrade issues efficiently.