Java 入门教程（九）：异常处理

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// throws：声明"本方法可能抛出这些异常，调用者负责"
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public static void setAge(int age) throws IllegalArgumentException {
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    if (age < 0 || age > 150) {
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        // throw：真正抛出异常对象
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        throw new IllegalArgumentException("年龄非法: " + age);
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    }
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}
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// 自定义受检异常：继承 Exception
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class InsufficientFundsException extends Exception {
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    private final double shortfall;
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    public InsufficientFundsException(double shortfall) {
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        super(String.format("余额不足，还差 %.2f 元", shortfall));
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        this.shortfall = shortfall;
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    }
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    public double getShortfall() { return shortfall; }
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}
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public class Account {
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    private double balance;
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    // 受检异常：调用方必须 try-catch 或继续 throws（编译器盯着）
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    public void withdraw(double amount) throws InsufficientFundsException {
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        if (amount > balance)
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            throw new InsufficientFundsException(amount - balance);
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        balance -= amount;
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    }
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}
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// 调用方
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public static void main(String[] args) {
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    Account acc = new Account();
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    try {
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        acc.withdraw(100);
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    } catch (InsufficientFundsException e) {
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        System.out.println(e.getMessage());           // 余额不足，还差 100.00 元
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        System.out.println(e.getShortfall());          // 100.0
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    }
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}

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// 异常链：底层异常包装成业务异常，保留根因
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public Config loadConfig(String path) {
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    try {
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        return parse(new String(java.nio.file.Files.readAllBytes(
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                java.nio.file.Path.of(path))));
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    } catch (IOException e) {
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        throw new ConfigException("配置加载失败: " + path, e);   // 传入原因
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    }
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}
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class ConfigException extends RuntimeException {
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    ConfigException(String msg, Throwable cause) {
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        super(msg, cause);           // cause 保留在堆栈里
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    }
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}
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// 打印时看到 Caused by: ... 链条，定位根因

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import java.io.IOException;
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import java.io.InputStream;
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import java.util.Properties;
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public class AppConfig {
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    private final Properties props = new Properties();
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    public static class ConfigException extends RuntimeException {
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        ConfigException(String msg, Throwable cause) { super(msg, cause); }
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    }
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    public static AppConfig load(String filename) {
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        // try-with-resources + 异常链 + 语义化包装
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        try (InputStream in = AppConfig.class
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                .getClassLoader().getResourceAsStream(filename)) {
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            if (in == null)
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                throw new ConfigException("找不到配置文件: " + filename, null);
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            AppConfig cfg = new AppConfig();
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            cfg.props.load(in);
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            return cfg;
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        } catch (IOException e) {
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            throw new ConfigException("读取 " + filename + " 失败", e);
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        }
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    }
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    public String get(String key, String def) {           // 安全读取
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        return props.getProperty(key, def);
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    }
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    public int getInt(String key, int def) {              // 类型安全读取
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        String v = props.getProperty(key);
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        try {
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            return v == null ? def : Integer.parseInt(v.trim());
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        } catch (NumberFormatException e) {
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            System.err.printf("配置 %s=%s 不是数字，用默认 %d%n", key, v, def);
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            return def;
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        }
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    }
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    public static void main(String[] args) {
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        AppConfig cfg = load("application.properties");
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        System.out.println("name = " + cfg.get("app.name", "未配置"));
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        System.out.println("port = " + cfg.getInt("app.port", 8080));
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    }
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}