Java 20 release date: March 2023
Theme: More preview/incubator features focused on performance (Project Loom, Panama, Valhalla groundwork) and developer productivity.
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1) Virtual Threads (Second Preview) — Project Loom
What it is
Virtual threads are lightweight threads managed by the JVM (not 1:1 mapped to OS threads). They allow you to write classic “thread-per-request” style code but scale to massive concurrency with low overhead.
Why it matters
- Great for I/O-heavy apps (web calls, DB calls, messaging)
- Simplifies async code: fewer reactive chains, more readable code
Example: Start virtual threads
public class VirtualThreadsDemo {
public static void main(String[] args) throws Exception {
// 1) Start a single virtual thread
Thread vt = Thread.startVirtualThread(() -> {
System.out.println("Hello from virtual thread: " + Thread.currentThread());
});
vt.join();
// 2) Virtual thread per task using an executor
try (var executor = java.util.concurrent.Executors.newVirtualThreadPerTaskExecutor()) {
for (int i = 0; i < 5; i++) {
int taskId = i;
executor.submit(() -> {
System.out.println("Task " + taskId + " on " + Thread.currentThread());
return null;
});
}
}
}
}
Notes / Tips
- Virtual threads shine when threads spend time waiting (network, DB, file I/O).
- CPU-heavy workloads still need good parallelism strategy (ForkJoinPool, batching, etc.).
2) Scoped Values (Incubator) — safer alternative to ThreadLocal
What it is
ScopedValue lets you bind a value to a dynamic scope (a block of code), and it becomes accessible within that scope (and child calls). It’s designed to be more structured and safer than ThreadLocal—especially with virtual threads.
Why it matters
- Clean way to pass request context (requestId, userId, tenantId)
- Helps avoid
ThreadLocalleaks and unclear lifecycle
Example: Request ID propagation
import jdk.incubator.concurrent.ScopedValue;
public class ScopedValuesDemo {
private static final ScopedValue<String> REQUEST_ID = ScopedValue.newInstance();
public static void main(String[] args) throws Exception {
ScopedValue.where(REQUEST_ID, "REQ-2026-0001").run(() -> {
handleRequest();
});
}
static void handleRequest() {
System.out.println("Handling request: " + REQUEST_ID.get());
deeperLayer();
}
static void deeperLayer() {
System.out.println("Deeper layer sees: " + REQUEST_ID.get());
}
}
Key point
The value exists only inside the scope of where(...).run(...).
3) Structured Concurrency (Second Incubator) — treat tasks as a unit
What it is
Structured concurrency introduces a pattern/API to manage multiple concurrent subtasks as one operation:
- Start subtasks together
- Wait for them together
- If one fails, cancel the rest (depending on policy)
Why it matters
- Makes concurrent code more reliable and readable
- Great match with virtual threads
Example: Fetch profile + orders in parallel
import jdk.incubator.concurrent.StructuredTaskScope;
public class StructuredConcurrencyDemo {
record UserProfile(String name) {}
record Orders(int count) {}
public static void main(String[] args) throws Exception {
try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
var profileTask = scope.fork(StructuredConcurrencyDemo::loadProfile);
var ordersTask = scope.fork(StructuredConcurrencyDemo::loadOrders);
scope.join(); // wait for both
scope.throwIfFailed(); // propagate error if any task failed
UserProfile profile = profileTask.get();
Orders orders = ordersTask.get();
System.out.println("Profile=" + profile + ", Orders=" + orders);
}
}
static UserProfile loadProfile() throws InterruptedException {
Thread.sleep(200); // simulate I/O
return new UserProfile("Susil");
}
static Orders loadOrders() throws InterruptedException {
Thread.sleep(300); // simulate I/O
return new Orders(12);
}
}
Benefit
You avoid scattered Future.get() / try-catch / manual cancellation logic.
4) Foreign Function & Memory API (Second Preview) — Project Panama
What it is
A modern, safer way for Java to call native libraries (C/C++) and work with off-heap memory—aimed to replace JNI for many use cases.
Why it matters
- High-performance integration with native libs
- Safer, more ergonomic than raw JNI
Example: Allocate off-heap memory segment
import java.lang.foreign.Arena;
import java.lang.foreign.MemorySegment;
import java.lang.foreign.ValueLayout;
public class ForeignMemoryDemo {
public static void main(String[] args) {
try (Arena arena = Arena.ofConfined()) {
MemorySegment segment = arena.allocate(ValueLayout.JAVA_INT);
segment.set(ValueLayout.JAVA_INT, 0, 42);
int value = segment.get(ValueLayout.JAVA_INT, 0);
System.out.println("Value from off-heap segment: " + value);
}
}
}
Calling real native functions needs platform-specific libraries and more setup; the above shows the core memory concept in a blog-friendly way.
5) Record Patterns (Second Preview)
What it is
Record patterns let you deconstruct records directly in pattern matching, making code cleaner when extracting components.
Why it matters
- Less boilerplate when working with immutable data models
- Pairs nicely with sealed types + switch patterns (preview in other versions)
Example: Deconstruct a record in instanceof
public class RecordPatternsDemo {
record Point(int x, int y) {}
public static void main(String[] args) {
Object obj = new Point(10, 20);
if (obj instanceof Point(int x, int y)) {
System.out.println("x=" + x + ", y=" + y);
}
}
}
6) Pattern Matching for switch (Fourth Preview)
What it is
Enhances switch so it can use type patterns, guards, and handle complex branching more cleanly than nested if/else.
Example: Type-based switch
public class SwitchPatternsDemo {
static String format(Object o) {
return switch (o) {
case null -> "null";
case Integer i -> "int: " + i;
case String s -> "string: " + s;
default -> "other: " + o;
};
}
public static void main(String[] args) {
System.out.println(format(10));
System.out.println(format("hello"));
System.out.println(format(3.14));
System.out.println(format(null));
}
}
Note: This is still preview in JDK 20; syntax may evolve across releases.
7) Key Encapsulation Mechanism API (Incubator) — Post-Quantum crypto building blocks
What it is
An incubator API to support Key Encapsulation Mechanisms (KEMs) used in modern and post-quantum cryptography (for example, hybrid key exchange designs).
Why it matters
- Prepares Java security ecosystem for post-quantum transitions
- More structured than rolling your own key exchange patterns
For most application developers, this is “good to know” unless you’re working on security libraries or advanced crypto integrations.
Quick Summary Table
- Virtual Threads (Preview): massively scalable concurrency for I/O workloads
- Scoped Values (Incubator): structured context passing (better
ThreadLocal) - Structured Concurrency (Incubator): manage multiple tasks as one unit
- Foreign Function & Memory (Preview): safer native interop & off-heap memory
- Record Patterns (Preview): concise record deconstruction
- Pattern Matching for switch (Preview): cleaner branching by type/value
- KEM API (Incubator): foundations for post-quantum key exchange patterns