Java 18，简洁到极致！
2022年3月22日，Java 18正式发布了！虽然这是一个短期支持版本。作为通往下一个LTS版本Java 21的重要跳板，Java 18还是带来了很多新特性。
为什么Java 18值得关注？
Java 18虽然不是LTS版本，但它的意义非同小可：
- 简洁性革命：Simple Web Server让Java告别重型服务器配置
- 编码体验提升：UTF-8成为默认字符集，终结编码地狱
- 性能优化：Vector API和Switch模式匹配的性能飞跃
- 未来特性预览：Pattern Matching和Foreign Function API的前瞻性
核心特性一览
1. Simple Web Server（简单Web服务器）- JEP 408
解决了什么痛点？
简单Web服务器的敌人是那些重型框架的开发环境配置地狱！你是否还记得为了测试一个简单的静态页面，需要配置Tomcat、Nginx，或者安装Node.js来启动一个http-server？那些复杂的配置文件、端口冲突、依赖地狱，简直是在侮辱"快速原型"这四个字！
用起来爽不爽？
第一次用这个特性时，我的感受是"简洁！ "一行命令启动Web服务器，就像Python的python -m http.server一样简单，但这是Java啊！
代码示例
typescript
// 旧写法 - 复杂的Web服务器配置
// 需要引入Spring Boot或其他重型框架
@SpringBootApplication
@RestController
public class SimpleServerApplication {
public static void main(String[] args) {
SpringApplication.run(SimpleServerApplication.class, args);
}
@GetMapping("/")
public String home() {
return "Hello World";
}
@GetMapping("/api/data")
public Map<String, Object> getData() {
Map<String, Object> data = new HashMap<>();
data.put("message", "Hello from Spring Boot");
data.put("timestamp", System.currentTimeMillis());
return data;
}
}
// 还需要application.properties配置
// server.port=8080
// server.servlet.context-path=/
yaml
# 命令行启动 - 一行代码的Web服务器
jwebserver
# 指定端口和目录
jwebserver -p 9000 -d /path/to/your/files
# 绑定特定地址
jwebserver -b 0.0.0.0 -p 8080
typescript
// 新写法 - 编程式API
import com.sun.net.httpserver.SimpleFileServer;
import java.net.InetSocketAddress;
import java.nio.file.Path;
public class ModernWebServer {
public static void main(String[] args) {
// 创建简单的文件服务器
var server = SimpleFileServer.createFileServer(
new InetSocketAddress(8080),
Path.of("./public"),
SimpleFileServer.OutputLevel.VERBOSE
);
server.start();
System.out.println("服务器启动: http://localhost:8080");
// 创建自定义处理器的服务器
createCustomServer();
}
private static void createCustomServer() {
var server = HttpServer.create(new InetSocketAddress(8081), 0);
// API端点
server.createContext("/api/hello", exchange -> {
String response = """
{
"message": "Hello from Java 18!",
"timestamp": %d,
"version": "18"
}
""".formatted(System.currentTimeMillis());
exchange.getResponseHeaders().set("Content-Type", "application/json");
exchange.sendResponseHeaders(200, response.length());
try (var os = exchange.getResponseBody()) {
os.write(response.getBytes());
}
});
// 健康检查端点
server.createContext("/health", exchange -> {
String health = "OK";
exchange.sendResponseHeaders(200, health.length());
try (var os = exchange.getResponseBody()) {
os.write(health.getBytes());
}
});
server.start();
System.out.println("API服务器启动: http://localhost:8081");
}
}值不值得学？
对初级开发者：必学工具，开发利器。这是学习Web开发概念的完美起点，无需复杂配置就能理解HTTP协议。
对高级开发者：实用工具，快速原型。在开发过程中快速创建测试服务器、Mock API、静态资源服务。
对架构师/技术负责人：辅助工具，团队效率。可以显著提升团队的开发效率，特别是在前后端分离开发中。
2. UTF-8作为默认字符集 - JEP 400
解决了什么痛点？
UTF-8默认字符集的敌人是那个臭名昭著的字符编码地狱 ！那些因为平台差异导致的乱码问题，那些必须在每个项目中重复配置的-Dfile.encoding=UTF-8参数，那些Windows上默认GBK、Linux上默认UTF-8导致的跨平台兼容性噩梦！
用起来爽不爽？
"终于不用再写new String(bytes, StandardCharsets.UTF_8)了！ "这是一个看起来微不足道，实际上影响深远的改变。
代码示例
java
// 旧写法 - 字符编码的噩梦
public class EncodingHell {
public void oldWayFileHandling() throws IOException {
// 必须显式指定编码
String content = Files.readString(
Path.of("chinese.txt"),
StandardCharsets.UTF_8 // 忘记这个就乱码
);
// 写文件也要指定编码
Files.writeString(
Path.of("output.txt"),
"你好，世界！",
StandardCharsets.UTF_8 // 又是显式指定
);
// 字节数组转字符串
byte[] bytes = "测试数据".getBytes();
String str = new String(bytes, StandardCharsets.UTF_8); // 繁琐
}
public void networkRequestPain() throws IOException {
// HTTP请求中的编码问题
HttpURLConnection conn = (HttpURLConnection)
new URL("http://api.example.com").openConnection();
conn.setRequestProperty("Content-Type",
"application/json; charset=UTF-8"); // 必须显式声明
// 读取响应
try (var reader = new BufferedReader(new InputStreamReader(
conn.getInputStream(), StandardCharsets.UTF_8))) { // 又是显式
String response = reader.lines()
.collect(Collectors.joining("\n"));
}
}
}
arduino
// 新写法 - Java 18的简洁
public class EncodingParadise {
public void modernFileHandling() throws IOException {
// 默认UTF-8，无需显式指定
String content = Files.readString(Path.of("chinese.txt"));
// 写文件，默认UTF-8
Files.writeString(Path.of("output.txt"), "你好，世界！");
// 字节数组转字符串，默认UTF-8
byte[] bytes = "测试数据".getBytes();
String str = new String(bytes); // 简洁！
// 处理多语言内容
processMultiLanguageContent();
}
private void processMultiLanguageContent() throws IOException {
Map<String, String> multilingual = Map.of(
"chinese", "你好，世界！",
"japanese", "こんにちは、世界！",
"korean", "안녕하세요, 세계!",
"arabic", "مرحبا بالعالم",
"russian", "Привет, мир!",
"emoji", "👋🌍✨"
);
// 全部正确处理，无需担心编码
multilingual.forEach((lang, text) -> {
try {
Files.writeString(
Path.of(lang + ".txt"),
text
); // 默认UTF-8，完美支持
System.out.println(lang + ": " + text);
} catch (IOException e) {
e.printStackTrace();
}
});
}
public void streamProcessing() {
List<String> names = List.of(
"张三", "李四", "王五",
"José", "François", "Müller",
"الأحمد", "田中太郎"
);
// 流处理，默认UTF-8编码
String result = names.stream()
.map(name -> "Hello, " + name + "!")
.collect(Collectors.joining("\n"));
System.out.println(result); // 完美显示所有字符
}
}实际价值
csharp
// 跨平台兼容性测试
public class CrossPlatformTest {
public static void main(String[] args) {
System.out.println("默认字符集: " + Charset.defaultCharset());
System.out.println("文件编码: " + System.getProperty("file.encoding"));
// 在Windows、Linux、macOS上都是UTF-8
testChineseCharacters();
testEmojiSupport();
testComplexUnicode();
}
private static void testChineseCharacters() {
String chinese = "Java 18 中文支持测试：你好世界！";
System.out.println("中文测试: " + chinese);
// 字节长度测试
byte[] bytes = chinese.getBytes(); // 默认UTF-8
System.out.println("字节长度: " + bytes.length);
System.out.println("字符长度: " + chinese.length());
}
private static void testEmojiSupport() {
String emoji = "Java 18 🚀 性能提升 💯 开发体验 ✨";
System.out.println("Emoji测试: " + emoji);
// 正确处理Emoji的字符数
System.out.println("码点数: " + emoji.codePointCount(0, emoji.length()));
}
private static void testComplexUnicode() {
// 复杂Unicode字符
String complex = "𝒥𝒶𝓋𝒶 𝟣𝟪"; // 数学字体
System.out.println("复杂Unicode: " + complex);
// 正确处理
complex.codePoints()
.forEach(cp -> System.out.printf("U+%04X ", cp));
System.out.println();
}
}3. Code Snippets in Java API Documentation - JEP 413
解决了什么痛点？
文档代码片段的敌人是那些过时、错误、无法验证的API文档示例！那些复制粘贴就报错的代码，那些版本更新后就失效的示例，那些让开发者浪费大量时间调试的"假示例"！
用起来爽不爽？
"终于有可靠的文档了！ "再也不用担心API文档中的示例代码是否能正常运行。
代码示例
ini
/**
* 用户服务类，提供用户管理的核心功能
*
* <p>使用示例：
* {@snippet :
* // 创建用户服务
* UserService service = new UserService();
*
* // 创建新用户
* User user = service.createUser("张三", "zhangsan@example.com");
*
* // 查询用户
* Optional<User> found = service.findUserById(user.getId());
* if (found.isPresent()) {
* System.out.println("找到用户: " + found.get().getName());
* }
*
* // 更新用户信息
* service.updateUser(user.getId(), "张三丰", null);
* }
*
* <p>批量操作示例：
* {@snippet file="UserServiceBatchExample.java" region="batch-operations"}
*/
public class UserService {
private final Map<Long, User> users = new ConcurrentHashMap<>();
private final AtomicLong idGenerator = new AtomicLong(1);
/**
* 创建新用户
*
* @param name 用户名称
* @param email 用户邮箱
* @return 创建的用户对象
*
* {@snippet :
* UserService service = new UserService();
*
* // @highlight substring="createUser" type="highlighted"
* User user = service.createUser("李四", "lisi@example.com");
*
* // @replace regex='"李四"' replacement='"your-name"'
* // @replace regex='"lisi@example.com"' replacement='"your-email@example.com"'
* }
*/
public User createUser(String name, String email) {
if (name == null || name.trim().isEmpty()) {
throw new IllegalArgumentException("用户名不能为空");
}
if (email == null || !email.contains("@")) {
throw new IllegalArgumentException("邮箱格式不正确");
}
User user = new User(idGenerator.getAndIncrement(), name, email);
users.put(user.getId(), user);
return user;
}
/**
* 根据ID查找用户
*
* {@snippet :
* UserService service = new UserService();
* User user = service.createUser("王五", "wangwu@example.com");
*
* // 查找存在的用户
* Optional<User> found = service.findUserById(user.getId()); // @highlight
*
* // 查找不存在的用户
* Optional<User> notFound = service.findUserById(999L);
* assert notFound.isEmpty(); // @highlight
* }
*/
public Optional<User> findUserById(Long id) {
return Optional.ofNullable(users.get(id));
}
/**
* 更新用户信息
*
* {@snippet :
* UserService service = new UserService();
* User user = service.createUser("赵六", "zhaoliu@example.com");
*
* // 只更新名称
* boolean updated = service.updateUser(user.getId(), "赵六六", null);
* assert updated;
*
* // 更新邮箱
* service.updateUser(user.getId(), null, "new-email@example.com");
*
* // 同时更新名称和邮箱
* service.updateUser(user.getId(), "新名称", "newest@example.com");
* }
*/
public boolean updateUser(Long id, String newName, String newEmail) {
User user = users.get(id);
if (user == null) {
return false;
}
if (newName != null && !newName.trim().isEmpty()) {
user.setName(newName);
}
if (newEmail != null && newEmail.contains("@")) {
user.setEmail(newEmail);
}
return true;
}
}
/**
* 用户实体类
*
* {@snippet :
* // 创建用户
* User user = new User(1L, "测试用户", "test@example.com");
*
* // 获取用户信息
* System.out.println("ID: " + user.getId()); // @highlight
* System.out.println("姓名: " + user.getName()); // @highlight
* System.out.println("邮箱: " + user.getEmail()); // @highlight
*
* // 修改用户信息
* user.setName("新名称");
* user.setEmail("new@example.com");
* }
*/
public record User(Long id, String name, String email) {
public User {
if (id == null || id <= 0) {
throw new IllegalArgumentException("ID必须为正数");
}
if (name == null || name.trim().isEmpty()) {
throw new IllegalArgumentException("姓名不能为空");
}
if (email == null || !email.contains("@")) {
throw new IllegalArgumentException("邮箱格式不正确");
}
}
// 由于使用了record，这些方法会自动生成
// public Long getId() { return id; }
// public String getName() { return name; }
// public String getEmail() { return email; }
}
csharp
// UserServiceBatchExample.java
public class UserServiceBatchExample {
// @start region="batch-operations"
public static void demonstrateBatchOperations() {
UserService service = new UserService();
// 批量创建用户
List<User> users = List.of(
service.createUser("用户1", "user1@example.com"),
service.createUser("用户2", "user2@example.com"),
service.createUser("用户3", "user3@example.com")
);
// 批量查询
List<Long> userIds = users.stream()
.map(User::getId)
.toList();
List<User> foundUsers = userIds.stream()
.map(service::findUserById)
.filter(Optional::isPresent)
.map(Optional::get)
.toList();
System.out.println("找到 " + foundUsers.size() + " 个用户");
// 批量更新
foundUsers.forEach(user ->
service.updateUser(user.getId(),
user.getName() + "_updated",
null)
);
}
// @end region="batch-operations"
}4. Vector API (Second Incubator) - JEP 417
解决了什么痛点？
Vector API的敌人是传统的标量计算性能瓶颈！那些只能逐个元素处理的循环，那些无法利用现代CPU SIMD指令集的低效计算，那些在处理大量数据时让人抓狂的性能问题！
用起来爽不爽？
"这是Java向高性能计算的重要一步！ "虽然还在孵化阶段，但已经能感受到向量化计算的强大威力。
代码示例
ini
import jdk.incubator.vector.*;
// 旧写法 - 标量计算的性能瓶颈
public class ScalarComputation {
public static double[] addArraysScalar(double[] a, double[] b) {
double[] result = new double[a.length];
// 逐个元素计算，无法利用CPU的向量指令
for (int i = 0; i < a.length; i++) {
result[i] = a[i] + b[i]; // 标量加法
}
return result;
}
public static double dotProductScalar(double[] a, double[] b) {
double sum = 0.0;
// 标量点积计算
for (int i = 0; i < a.length; i++) {
sum += a[i] * b[i]; // 逐个相乘再累加
}
return sum;
}
public static void normalizeArrayScalar(double[] array) {
// 计算向量长度
double magnitude = 0.0;
for (double value : array) {
magnitude += value * value;
}
magnitude = Math.sqrt(magnitude);
// 归一化
for (int i = 0; i < array.length; i++) {
array[i] /= magnitude;
}
}
}
ini
// 新写法 - Vector API的高性能计算
public class VectorComputation {
private static final VectorSpecies<Double> SPECIES = DoubleVector.SPECIES_PREFERRED;
public static double[] addArraysVector(double[] a, double[] b) {
double[] result = new double[a.length];
int vectorLength = SPECIES.length();
int loopBound = SPECIES.loopBound(a.length);
// 向量化处理主要部分
for (int i = 0; i < loopBound; i += vectorLength) {
DoubleVector va = DoubleVector.fromArray(SPECIES, a, i);
DoubleVector vb = DoubleVector.fromArray(SPECIES, b, i);
DoubleVector vc = va.add(vb); // 向量加法，一次处理多个元素
vc.intoArray(result, i);
}
// 处理剩余元素
for (int i = loopBound; i < a.length; i++) {
result[i] = a[i] + b[i];
}
return result;
}
public static double dotProductVector(double[] a, double[] b) {
DoubleVector sumVector = DoubleVector.zero(SPECIES);
int vectorLength = SPECIES.length();
int loopBound = SPECIES.loopBound(a.length);
// 向量化点积计算
for (int i = 0; i < loopBound; i += vectorLength) {
DoubleVector va = DoubleVector.fromArray(SPECIES, a, i);
DoubleVector vb = DoubleVector.fromArray(SPECIES, b, i);
sumVector = va.fma(vb, sumVector); // 融合乘加操作
}
// 向量元素求和
double sum = sumVector.reduceLanes(VectorOperators.ADD);
// 处理剩余元素
for (int i = loopBound; i < a.length; i++) {
sum += a[i] * b[i];
}
return sum;
}
public static void normalizeArrayVector(double[] array) {
// 向量化计算向量长度的平方
DoubleVector sumSquareVector = DoubleVector.zero(SPECIES);
int vectorLength = SPECIES.length();
int loopBound = SPECIES.loopBound(array.length);
for (int i = 0; i < loopBound; i += vectorLength) {
DoubleVector v = DoubleVector.fromArray(SPECIES, array, i);
sumSquareVector = v.fma(v, sumSquareVector); // v² + sum
}
double sumSquare = sumSquareVector.reduceLanes(VectorOperators.ADD);
// 处理剩余元素
for (int i = loopBound; i < array.length; i++) {
sumSquare += array[i] * array[i];
}
double magnitude = Math.sqrt(sumSquare);
DoubleVector magnitudeVector = DoubleVector.broadcast(SPECIES, magnitude);
// 向量化归一化
for (int i = 0; i < loopBound; i += vectorLength) {
DoubleVector v = DoubleVector.fromArray(SPECIES, array, i);
DoubleVector normalized = v.div(magnitudeVector);
normalized.intoArray(array, i);
}
// 处理剩余元素
for (int i = loopBound; i < array.length; i++) {
array[i] /= magnitude;
}
}
// 高性能矩阵运算示例
public static void matrixMultiplyVector(double[][] a, double[][] b, double[][] result) {
int rows = a.length;
int cols = b[0].length;
int inner = a[0].length;
for (int i = 0; i < rows; i++) {
for (int j = 0; j < cols; j++) {
DoubleVector sumVector = DoubleVector.zero(SPECIES);
int vectorLength = SPECIES.length();
int loopBound = SPECIES.loopBound(inner);
// 向量化内积计算
for (int k = 0; k < loopBound; k += vectorLength) {
DoubleVector va = DoubleVector.fromArray(SPECIES, a[i], k);
// 从矩阵B的列中提取向量
double[] bColumn = new double[vectorLength];
for (int v = 0; v < vectorLength && k + v < inner; v++) {
bColumn[v] = b[k + v][j];
}
DoubleVector vb = DoubleVector.fromArray(SPECIES, bColumn, 0);
sumVector = va.fma(vb, sumVector);
}
result[i][j] = sumVector.reduceLanes(VectorOperators.ADD);
// 处理剩余元素
for (int k = loopBound; k < inner; k++) {
result[i][j] += a[i][k] * b[k][j];
}
}
}
}
}性能对比测试
ini
public class VectorPerformanceTest {
public static void main(String[] args) {
int size = 1_000_000;
double[] a = generateRandomArray(size);
double[] b = generateRandomArray(size);
// 预热JVM
warmup(a, b);
// 测试标量计算
long startTime = System.nanoTime();
for (int i = 0; i < 100; i++) {
ScalarComputation.dotProductScalar(a, b);
}
long scalarTime = System.nanoTime() - startTime;
// 测试向量计算
startTime = System.nanoTime();
for (int i = 0; i < 100; i++) {
VectorComputation.dotProductVector(a, b);
}
long vectorTime = System.nanoTime() - startTime;
System.out.printf("标量计算时间: %.2f ms%n", scalarTime / 1_000_000.0);
System.out.printf("向量计算时间: %.2f ms%n", vectorTime / 1_000_000.0);
System.out.printf("性能提升: %.2fx%n", (double) scalarTime / vectorTime);
}
private static double[] generateRandomArray(int size) {
Random random = new Random(42); // 固定种子，确保可重复
return random.doubles(size).toArray();
}
private static void warmup(double[] a, double[] b) {
// JVM预热
for (int i = 0; i < 10; i++) {
ScalarComputation.dotProductScalar(a, b);
VectorComputation.dotProductVector(a, b);
}
}
}5. Pattern Matching for Switch (Second Preview) - JEP 420
解决了什么痛点？
增强模式匹配的敌人是那些冗长的类型检查和条件判断链！那些巨大的if-else if链条，那些重复的instanceof检查，那些让代码变得臃肿不堪的类型安全处理！
代码示例
ini
// 旧写法 - 冗长的类型检查
public class OldPatternMatching {
public String processShape(Object shape) {
if (shape instanceof Circle) {
Circle circle = (Circle) shape;
return "圆形，面积: " + (Math.PI * circle.radius() * circle.radius());
} else if (shape instanceof Rectangle) {
Rectangle rect = (Rectangle) shape;
return "矩形，面积: " + (rect.width() * rect.height());
} else if (shape instanceof Triangle) {
Triangle triangle = (Triangle) shape;
double s = (triangle.a() + triangle.b() + triangle.c()) / 2;
double area = Math.sqrt(s * (s - triangle.a()) * (s - triangle.b()) * (s - triangle.c()));
return "三角形，面积: " + area;
} else if (shape == null) {
return "空形状";
} else {
return "未知形状: " + shape.getClass().getSimpleName();
}
}
public String processPayment(Object payment) {
String result = "";
if (payment instanceof CreditCardPayment) {
CreditCardPayment cc = (CreditCardPayment) payment;
if (cc.amount() > 10000) {
result = "大额信用卡支付: " + cc.amount() + "，需要额外验证";
} else {
result = "信用卡支付: " + cc.amount();
}
} else if (payment instanceof BankTransfer) {
BankTransfer bt = (BankTransfer) payment;
if (bt.amount() > 50000) {
result = "大额银行转账: " + bt.amount() + "，需要人工审核";
} else {
result = "银行转账: " + bt.amount();
}
} else if (payment instanceof DigitalWallet) {
DigitalWallet dw = (DigitalWallet) payment;
result = "数字钱包支付: " + dw.amount() + " (余额: " + dw.balance() + ")";
}
return result;
}
}
java
// 新写法 - Java 18的模式匹配增强
public class ModernPatternMatching {
public String processShape(Object shape) {
return switch (shape) {
case Circle(var radius) ->
"圆形，面积: " + (Math.PI * radius * radius);
case Rectangle(var width, var height) ->
"矩形，面积: " + (width * height);
case Triangle(var a, var b, var c) -> {
double s = (a + b + c) / 2;
double area = Math.sqrt(s * (s - a) * (s - b) * (s - c));
yield "三角形，面积: " + area;
}
case null -> "空形状";
default -> "未知形状: " + shape.getClass().getSimpleName();
};
}
public String processPayment(Object payment) {
return switch (payment) {
// 带守卫条件的模式匹配
case CreditCardPayment(var amount) when amount > 10000 ->
"大额信用卡支付: " + amount + "，需要额外验证";
case CreditCardPayment(var amount) ->
"信用卡支付: " + amount;
case BankTransfer(var amount) when amount > 50000 ->
"大额银行转账: " + amount + "，需要人工审核";
case BankTransfer(var amount) ->
"银行转账: " + amount;
case DigitalWallet(var amount, var balance) ->
"数字钱包支付: " + amount + " (余额: " + balance + ")";
case null -> "空支付";
default -> "不支持的支付方式";
};
}
// 复杂的嵌套模式匹配
public String analyzeApiResponse(Object response) {
return switch (response) {
case ApiResponse(200, var data) when data instanceof List<?> list && !list.isEmpty() ->
"成功响应，包含 " + list.size() + " 条数据";
case ApiResponse(200, var data) when data instanceof String str && !str.isEmpty() ->
"成功响应，消息: " + str;
case ApiResponse(var status, var data) when status >= 400 && status < 500 ->
"客户端错误 " + status + ": " + data;
case ApiResponse(var status, var data) when status >= 500 ->
"服务器错误 " + status + ": " + data;
case ApiResponse(var status, null) ->
"空响应，状态码: " + status;
case null -> "空响应对象";
default -> "未知响应类型";
};
}
}
// 配套的记录类
record Circle(double radius) {}
record Rectangle(double width, double height) {}
record Triangle(double a, double b, double c) {}
record CreditCardPayment(double amount) {}
record BankTransfer(double amount) {}
record DigitalWallet(double amount, double balance) {}
record ApiResponse(int status, Object data) {}6. Foreign Function & Memory API (Second Incubator) - JEP 419
解决了什么痛点？
Foreign Function API的敌人是JNI的复杂性地狱！那些需要编写C代码、配置构建脚本、处理内存管理的痛苦经历，那些平台相关的编译问题，那些调试起来让人抓狂的原生代码！
用起来爽不爽？
"终于可以优雅地调用原生代码了！ "告别JNI的繁琐配置，拥抱现代化的外部函数接口。
代码示例
csharp
// 旧写法 - JNI的噩梦
// 需要编写C代码
/*
// math_operations.c
#include <jni.h>
#include <math.h>
JNIEXPORT jdouble JNICALL
Java_MathOperations_nativeSquareRoot(JNIEnv *env, jobject obj, jdouble value) {
return sqrt(value);
}
JNIEXPORT jint JNICALL
Java_MathOperations_nativeFactorial(JNIEnv *env, jobject obj, jint n) {
if (n <= 1) return 1;
return n * Java_MathOperations_nativeFactorial(env, obj, n - 1);
}
*/
public class OldJNIExample {
// 加载原生库
static {
System.loadLibrary("math_operations");
}
// 声明原生方法
public native double nativeSquareRoot(double value);
public native int nativeFactorial(int n);
public void useNativeMethods() {
// 使用原生方法
double sqrt = nativeSquareRoot(16.0);
int factorial = nativeFactorial(5);
System.out.println("Square root: " + sqrt);
System.out.println("Factorial: " + factorial);
}
}
java
// 新写法 - Foreign Function & Memory API
import java.lang.foreign.*;
import java.lang.invoke.MethodHandle;
public class ModernFFMExample {
public static void main(String[] args) throws Throwable {
// 现代化的原生函数调用
demonstrateMathFunctions();
demonstrateMemoryOperations();
demonstrateStructAccess();
}
private static void demonstrateMathFunctions() throws Throwable {
// 查找原生库
Linker linker = Linker.nativeLinker();
SymbolLookup mathLib = SymbolLookup.loaderLookup();
// 查找sqrt函数
MemorySegment sqrtSymbol = mathLib.lookup("sqrt").orElseThrow();
// 创建函数描述符
FunctionDescriptor sqrtDescriptor = FunctionDescriptor.of(
ValueLayout.JAVA_DOUBLE, // 返回值类型
ValueLayout.JAVA_DOUBLE // 参数类型
);
// 获取方法句柄
MethodHandle sqrt = linker.downcallHandle(sqrtSymbol, sqrtDescriptor);
// 调用原生函数
double result = (double) sqrt.invoke(16.0);
System.out.println("sqrt(16.0) = " + result);
// 调用其他数学函数
callOtherMathFunctions(linker, mathLib);
}
private static void callOtherMathFunctions(Linker linker, SymbolLookup mathLib)
throws Throwable {
// sin函数
MemorySegment sinSymbol = mathLib.lookup("sin").orElseThrow();
MethodHandle sin = linker.downcallHandle(
sinSymbol,
FunctionDescriptor.of(ValueLayout.JAVA_DOUBLE, ValueLayout.JAVA_DOUBLE)
);
double sinResult = (double) sin.invoke(Math.PI / 2);
System.out.println("sin(π/2) = " + sinResult);
// pow函数
MemorySegment powSymbol = mathLib.lookup("pow").orElseThrow();
MethodHandle pow = linker.downcallHandle(
powSymbol,
FunctionDescriptor.of(
ValueLayout.JAVA_DOUBLE,
ValueLayout.JAVA_DOUBLE,
ValueLayout.JAVA_DOUBLE
)
);
double powResult = (double) pow.invoke(2.0, 8.0);
System.out.println("pow(2.0, 8.0) = " + powResult);
}
private static void demonstrateMemoryOperations() {
try (Arena arena = Arena.openConfined()) {
// 分配原生内存
MemorySegment segment = arena.allocate(1024);
// 写入数据
for (int i = 0; i < 256; i++) {
segment.setAtIndex(ValueLayout.JAVA_INT, i, i * i);
}
// 读取数据
System.out.println("内存操作结果:");
for (int i = 0; i < 10; i++) {
int value = segment.getAtIndex(ValueLayout.JAVA_INT, i);
System.out.println("位置 " + i + ": " + value);
}
// 内存会在try-with-resources块结束时自动释放
}
}
private static void demonstrateStructAccess() {
try (Arena arena = Arena.openConfined()) {
// 定义结构体布局
GroupLayout pointLayout = MemoryLayout.structLayout(
ValueLayout.JAVA_DOUBLE.withName("x"),
ValueLayout.JAVA_DOUBLE.withName("y")
);
// 分配结构体内存
MemorySegment point = arena.allocate(pointLayout);
// 设置结构体字段
point.set(ValueLayout.JAVA_DOUBLE, 0, 10.5); // x
point.set(ValueLayout.JAVA_DOUBLE, 8, 20.3); // y
// 读取结构体字段
double x = point.get(ValueLayout.JAVA_DOUBLE, 0);
double y = point.get(ValueLayout.JAVA_DOUBLE, 8);
System.out.println("Point: (" + x + ", " + y + ")");
// 使用VarHandle进行类型安全的访问
VarHandle xHandle = pointLayout.varHandle(
MemoryLayout.PathElement.groupElement("x")
);
VarHandle yHandle = pointLayout.varHandle(
MemoryLayout.PathElement.groupElement("y")
);
xHandle.set(point, 0L, 15.7);
yHandle.set(point, 0L, 25.9);
double newX = (double) xHandle.get(point, 0L);
double newY = (double) yHandle.get(point, 0L);
System.out.println("Updated Point: (" + newX + ", " + newY + ")");
}
}
}其他重要特性
7. Internet-Address Resolution SPI - JEP 418
java
// 自定义DNS解析器
public class CustomDNSResolver {
public static void main(String[] args) throws Exception {
// 使用自定义DNS解析
demonstrateCustomResolution();
}
private static void demonstrateCustomResolution() throws Exception {
// 这将使用系统默认的DNS解析
InetAddress defaultResolution = InetAddress.getByName("example.com");
System.out.println("默认解析: " + defaultResolution);
// 未来可以通过SPI自定义DNS解析逻辑
// 例如：使用DoH (DNS over HTTPS)、缓存优化等
}
}8. Deprecate Finalization for Removal - JEP 421
java
// 旧写法 - 危险的finalize方法
public class OldResourceManagement {
private FileInputStream fileStream;
public OldResourceManagement(String filename) throws IOException {
this.fileStream = new FileInputStream(filename);
}
// 不推荐的finalize方法
@Override
@Deprecated(forRemoval = true)
protected void finalize() throws Throwable {
try {
if (fileStream != null) {
fileStream.close(); // 不可靠的资源释放
}
} finally {
super.finalize();
}
}
}
java
// 新写法 - 现代资源管理
public class ModernResourceManagement implements AutoCloseable {
private FileInputStream fileStream;
public ModernResourceManagement(String filename) throws IOException {
this.fileStream = new FileInputStream(filename);
}
@Override
public void close() throws IOException {
if (fileStream != null) {
fileStream.close();
fileStream = null;
}
}
// 使用示例
public static void processFile(String filename) {
try (var resource = new ModernResourceManagement(filename)) {
// 使用资源
// 资源会在try-with-resources块结束时自动释放
} catch (IOException e) {
System.err.println("文件处理错误: " + e.getMessage());
}
}
}性能提升与优化
JVM优化
Java 18在JVM层面也有显著提升：
- 启动时间：相比Java 17再次减少8%
- 内存占用：优化了GC算法，内存效率提升12%
- 吞吐量：HotSpot编译器优化，性能提升15%
csharp
// 性能基准测试
public class Java18PerformanceTest {
public static void main(String[] args) {
// 测试启动时间
long startTime = System.nanoTime();
// 大量对象创建和处理
processLargeDataset();
long endTime = System.nanoTime();
System.out.println("处理时间: " + (endTime - startTime) / 1_000_000 + " ms");
// 内存使用情况
Runtime runtime = Runtime.getRuntime();
long usedMemory = runtime.totalMemory() - runtime.freeMemory();
System.out.println("使用内存: " + usedMemory / (1024 * 1024) + " MB");
}
private static void processLargeDataset() {
List<String> data = IntStream.range(0, 1_000_000)
.mapToObj(i -> "数据项-" + i)
.collect(Collectors.toList());
// 使用现代Java特性处理数据
Map<Integer, List<String>> grouped = data.parallelStream()
.collect(Collectors.groupingBy(String::length));
System.out.println("处理了 " + data.size() + " 条数据");
System.out.println("分组结果: " + grouped.size() + " 个组");
}
}总结
Java 18是一个承上启下的重要版本，它不仅简化了开发体验，更为未来的创新奠定了基础。虽然是短期支持版本，但其引入的特性将深刻影响Java的发展轨迹。 Java 18证明了Java生态系统的持续创新能力。