Java 17与Java 21之间的新特性是什么？

public class PlatformThreads {
 
    public static void main(String[] args) {
        testPlatformThreads(1000);
        testPlatformThreads(10_000);
        testPlatformThreads(100_000);
        testPlatformThreads(1_000_000);
    }
 
    private static void testPlatformThreads(int maximum) {
        long time = System.currentTimeMillis();
 
        try (var executor = Executors.newCachedThreadPool()) {
            IntStream.range(0, maximum).forEach(i -> {
                executor.submit(() -> {
                    Thread.sleep(Duration.ofSeconds(1));
                    return i;
                });
            });
        }
 
        time = System.currentTimeMillis() - time;
        System.out.println("Number of threads = " + maximum + ", Duration(ms) = " + time);
    }
 
}

Number of threads = 1000, Duration(ms) = 1094
Number of threads = 10000, Duration(ms) = 1625
Number of threads = 100000, Duration(ms) = 5292
[21,945s][warning][os,thread] Attempt to protect stack guard pages failed (0x00007f8525d00000-0x00007f8525d04000).
#
# A fatal error has been detected by the Java Runtime Environment:
# Native memory allocation (mprotect) failed to protect 16384 bytes for memory to guard stack pages
# An error report file with more information is saved as:
# /home/<user_dir>/MyJava21Planet/hs_err_pid8277.log
[21,945s][warning][os,thread] Attempt to protect stack guard pages failed (0x00007f8525c00000-0x00007f8525c04000).
[thread 82370 also had an error]
[thread 82371 also had an error]
[21,946s][warning][os,thread] Failed to start thread "Unknown thread" - pthread_create failed (EAGAIN) for attributes: stacksize: 1024k, guardsize: 0k, detached.
[21,946s][warning][os,thread] Failed to start the native thread for java.lang.Thread "pool-4-thread-32577"
...

try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
            IntStream.range(0, maximum).forEach(i -> {
                executor.submit(() -> {
                    Thread.sleep(Duration.ofSeconds(1));
                    return i;
                });
            });
        }

Number of threads = 1000, Duration(ms) = 1020
Number of threads = 10000, Duration(ms) = 1056
Number of threads = 100000, Duration(ms) = 1106
Number of threads = 1000000, Duration(ms) = 1806
Number of threads = 10000000, Duration(ms) = 22010

Collections.unmodifiableSequencedCollection(sequencedCollection)
Collections.unmodifiableSequencedSet(sequencedSet)
Collections.unmodifiableSequencedMap(sequencedMap)

interface SequencedCollection<E> extends Collection<E> {
    // new method
    SequencedCollection<E> reversed();
    // methods promoted from Deque
    void addFirst(E);
    void addLast(E);
    E getFirst();
    E getLast();
    E removeFirst();
    E removeLast();
}

private static void sequencedCollection() {
    List<String> sc = Stream.of("Alpha", "Bravo", "Charlie", "Delta").collect(Collectors.toCollection(ArrayList::new));
    System.out.println("Initial list: " + sc);
    System.out.println("Reversed list: " + sc.reversed());
    System.out.println("First item: " + sc.getFirst());
    System.out.println("Last item: " + sc.getLast());
    sc.addFirst("Before Alpha");
    sc.addLast("After Delta");
    System.out.println("Added new first and last item: " + sc);
}

Initial list: [Alpha, Bravo, Charlie, Delta]
Reversed list: [Delta, Charlie, Bravo, Alpha]
First item: Alpha
Last item: Delta
Added new first and last item: [Before Alpha, Alpha, Bravo, Charlie, Delta, After Delta]

interface SequencedSet<E> extends Set<E>, SequencedCollection<E> {
    SequencedSet<E> reversed();    // covariant override
}

private static void sequencedSet() {
    SortedSet<String> sortedSet = new TreeSet<>(Set.of("Charlie", "Alpha", "Delta", "Bravo"));
    System.out.println("Initial list: " + sortedSet);
    System.out.println("Reversed list: " + sortedSet.reversed());
    System.out.println("First item: " + sortedSet.getFirst());
    System.out.println("Last item: " + sortedSet.getLast());
    try {
        sortedSet.addFirst("Before Alpha");
    } catch (UnsupportedOperationException uoe) {
        System.out.println("addFirst is not supported");
    }
    try {
        sortedSet.addLast("After Delta");
    } catch (UnsupportedOperationException uoe) {
        System.out.println("addLast is not supported");
    }
}

Initial list: [Alpha, Bravo, Charlie, Delta]
Reversed list: [Delta, Charlie, Bravo, Alpha]
First item: Alpha
Last item: Delta
addFirst is not supported
addLast is not supported

interface SequencedMap<K,V> extends Map<K,V> {
    // new methods
    SequencedMap<K,V> reversed();
    SequencedSet<K> sequencedKeySet();
    SequencedCollection<V> sequencedValues();
    SequencedSet<Entry<K,V>> sequencedEntrySet();
    V putFirst(K, V);
    V putLast(K, V);
    // methods promoted from NavigableMap
    Entry<K, V> firstEntry();
    Entry<K, V> lastEntry();
    Entry<K, V> pollFirstEntry();
    Entry<K, V> pollLastEntry();
}

private static void sequencedMap() {
    LinkedHashMap<Integer,String> hm = new LinkedHashMap<Integer,String>();
    hm.put(1, "Alpha");
    hm.put(2, "Bravo");
    hm.put(3, "Charlie");
    hm.put(4, "Delta");
    System.out.println("== Initial List ==");
    printMap(hm);
    System.out.println("== Reversed List ==");
    printMap(hm.reversed());
    System.out.println("First item: " + hm.firstEntry());
    System.out.println("Last item: " + hm.lastEntry());
    System.out.println(" == Added new first and last item ==");
    hm.putFirst(5, "Before Alpha");
    hm.putLast(3, "After Delta");
    printMap(hm);
}

== Initial List ==
1 Alpha
2 Bravo
3 Charlie
4 Delta
== Reversed List ==
4 Delta
3 Charlie
2 Bravo
1 Alpha
First item: 1=Alpha
Last item: 4=Delta
 == Added new first and last item ==
5 Before Alpha
1 Alpha
2 Bravo
4 Delta
3 After Delta

record GrapeRecord(Color color, Integer nbrOfPits) {}

private static void singleRecordPatternOldStyle() {
    Object o = new GrapeRecord(Color.BLUE, 2);
    if (o instanceof GrapeRecord grape) {
        System.out.println("This grape has " + grape.nbrOfPits() + " pits.");
    }
}

private static void singleRecordPattern() {
    Object o = new GrapeRecord(Color.BLUE, 2);
    if (o instanceof GrapeRecord(Color color, Integer nbrOfPits)) {
        System.out.println("This grape has " + nbrOfPits + " pits.");
    }
}

record SpecialGrapeRecord(GrapeRecord grape, boolean special) {}

private static void nestedRecordPattern() {
    Object o = new SpecialGrapeRecord(new GrapeRecord(Color.BLUE, 2), true);
    if (o instanceof SpecialGrapeRecord(GrapeRecord grape, boolean special)) {
        System.out.println("This grape has " + grape.nbrOfPits() + " pits.");
    }
    if (o instanceof SpecialGrapeRecord(GrapeRecord(Color color, Integer nbrOfPits), boolean special)) {
        System.out.println("This grape has " + nbrOfPits + " pits.");
    }
}

private static void oldStylePatternMatching(Object obj) {
    if (obj instanceof Integer i) {
        System.out.println("Object is an integer:" + i);
    } else if (obj instanceof String s) {
        System.out.println("Object is a string:" + s);
    } else if (obj instanceof FruitType f) {
        System.out.println("Object is a fruit: " + f);
    } else {
        System.out.println("Object is not recognized");
    }
}

public class SnippetsExternal {
 
    public void example3(boolean success) {
        // @start region=example3
        if (success) {
            System.out.println("This is a success!");
        } else {
            System.out.println("This is a failure");
        }
        // @end
    }
 
    public void example4(boolean success) {
        // @start region=example4
        if (success) {
            System.out.println("This is a success!"); // @highlight substring="println"
        } else {
            System.out.println("This is a failure");
        }
        // @end
    }
 
}

/**
 * this is an example for external snippets
 * {@snippet file="SnippetsExternal.java" region="example3" }"
 *
 * @param success
 */
public void example3(boolean success) {
    if (success) {
        System.out.println("This is a success!");
    } else {
        System.out.println("This is a failure");
    }
}
 
/**
 * this is an example for highlighting
 * {@snippet file="SnippetsExternal.java" region="example4" }"
 *
 * @param success
 */
public void example4(boolean success) {
    if (success) {
        System.out.println("This is a success!");
    } else {
        System.out.println("This is a failure");
    }
}

src/com/mydeveloperplanet/myjava21planet/Snippets.java:48: error: file not found on source path or snippet path: SnippetsExternal.java
     * {@snippet file="SnippetsExternal.java" region="example3" }"
                 ^
src/com/mydeveloperplanet/myjava21planet/Snippets.java:62: error: file not found on source path or snippet path: SnippetsExternal.java
     * {@snippet file="SnippetsExternal.java" region="example4" }"

$ javadoc src/com/mydeveloperplanet/myjava21planet/Snippets.java -d javadoc --snippet-path=./src/com/mydeveloperplanet/myjava21planet/snippet-files

Welcome to Simple Web Server

private static void startFileServer() {
    var server = SimpleFileServer.createFileServer(new InetSocketAddress(8080),
            Path.of("/<absolute path>/MyJava21Planet/httpserver"),
            SimpleFileServer.OutputLevel.VERBOSE);
    server.start();
}

$ curl http://localhost:8080
Welcome to Simple Web Server

class MyHttpHandler implements com.sun.net.httpserver.HttpHandler {
 
    @Override
    public void handle(HttpExchange exchange) throws IOException {
        if ("GET".equals(exchange.getRequestMethod())) {
            OutputStream outputStream = exchange.getResponseBody();
            String response = "It works!";
            exchange.sendResponseHeaders(200, response.length());
            outputStream.write(response.getBytes());
            outputStream.flush();
            outputStream.close();
        }
    }
}

private static void customFileServerHandler() {
    try {
        var server = HttpServer.create(new InetSocketAddress(8081), 0);
        server.createContext("/custom", new MyHttpHandler());
        server.start();
    } catch (IOException ioe) {
        System.out.println("IOException occured");
    }
}