catch 关键字

概述

catch 关键字用于捕获和处理try块中抛出的异常，必须与try块一起使用。

语法格式

try {
    // 可能抛出异常的代码
} catch (ExceptionType e) {
    // 异常处理代码
} catch (AnotherExceptionType e) {
    // 处理另一种异常
}

基本用法

public class CatchExample {
    public static void main(String[] args) {
        // 捕获算术异常
        try {
            int result = 10 / 0;
            System.out.println("结果: " + result);
        } catch (ArithmeticException e) {
            System.out.println("除零错误: " + e.getMessage());
        }

        // 捕获数组越界异常
        try {
            int[] array = {1, 2, 3};
            System.out.println(array[5]);
        } catch (ArrayIndexOutOfBoundsException e) {
            System.out.println("数组越界: " + e.getMessage());
        }

        System.out.println("程序继续执行");
    }
}

多个catch块

import java.io.*;

public class MultipleCatchExample {
    public static void readFile(String filename) {
        try {
            FileInputStream file = new FileInputStream(filename);
            byte[] data = new byte[100];
            file.read(data);
            file.close();

            // 可能的字符串转换
            String content = new String(data);
            int number = Integer.parseInt(content.trim());
            System.out.println("读取的数字: " + number);

        } catch (FileNotFoundException e) {
            System.out.println("文件未找到: " + e.getMessage());
        } catch (IOException e) {
            System.out.println("IO错误: " + e.getMessage());
        } catch (NumberFormatException e) {
            System.out.println("数字格式错误: " + e.getMessage());
        } catch (Exception e) {
            System.out.println("其他错误: " + e.getMessage());
        }
    }

    public static void main(String[] args) {
        readFile("nonexistent.txt");
        readFile("data.txt");
    }
}

多重异常捕获（Java 7+）

public class MultiCatchExample {
    public static void processData(String data) {
        try {
            // 可能抛出多种异常的操作
            int length = data.length();
            int number = Integer.parseInt(data);
            int result = 100 / number;

            System.out.println("处理结果: " + result);

        } catch (NullPointerException | NumberFormatException e) {
            System.out.println("数据格式错误: " + e.getMessage());
        } catch (ArithmeticException e) {
            System.out.println("算术错误: " + e.getMessage());
        }
    }

    public static void main(String[] args) {
        processData(null);      // NullPointerException
        processData("abc");     // NumberFormatException  
        processData("0");       // ArithmeticException
        processData("5");       // 正常执行
    }
}

异常信息获取

public class ExceptionInfoExample {
    public static void demonstrateExceptionInfo() {
        try {
            String str = null;
            int length = str.length();
        } catch (NullPointerException e) {
            System.out.println("异常类型: " + e.getClass().getSimpleName());
            System.out.println("异常消息: " + e.getMessage());
            System.out.println("异常字符串: " + e.toString());

            System.out.println("\n堆栈跟踪:");
            e.printStackTrace();

            System.out.println("\n堆栈跟踪元素:");
            StackTraceElement[] stack = e.getStackTrace();
            for (StackTraceElement element : stack) {
                System.out.println("  " + element.toString());
            }
        }
    }

    public static void main(String[] args) {
        demonstrateExceptionInfo();
    }
}

异常重新抛出

public class RethrowExample {
    public static void processOrder(String orderId) throws Exception {
        try {
            if (orderId == null || orderId.isEmpty()) {
                throw new IllegalArgumentException("订单ID不能为空");
            }

            // 模拟数据库操作
            if (orderId.equals("INVALID")) {
                throw new RuntimeException("数据库连接失败");
            }

            System.out.println("处理订单: " + orderId);

        } catch (IllegalArgumentException e) {
            System.out.println("参数错误，记录日志");
            throw e; // 重新抛出异常
        } catch (RuntimeException e) {
            System.out.println("运行时错误，记录错误日志");
            // 包装异常后重新抛出
            throw new Exception("订单处理失败: " + e.getMessage(), e);
        }
    }

    public static void main(String[] args) {
        try {
            processOrder("");
        } catch (Exception e) {
            System.out.println("最终捕获: " + e.getMessage());
        }

        try {
            processOrder("INVALID");
        } catch (Exception e) {
            System.out.println("最终捕获: " + e.getMessage());
            System.out.println("原因: " + e.getCause().getMessage());
        }
    }
}

资源管理中的catch

import java.io.*;

public class ResourceManagementExample {
    // 传统方式 - 需要手动关闭资源
    public static void readFileTraditional(String filename) {
        FileInputStream fis = null;
        try {
            fis = new FileInputStream(filename);
            // 读取文件操作
            byte[] buffer = new byte[1024];
            int bytesRead = fis.read(buffer);
            System.out.println("读取了 " + bytesRead + " 字节");

        } catch (FileNotFoundException e) {
            System.out.println("文件未找到: " + e.getMessage());
        } catch (IOException e) {
            System.out.println("读取错误: " + e.getMessage());
        } finally {
            if (fis != null) {
                try {
                    fis.close();
                } catch (IOException e) {
                    System.out.println("关闭文件失败: " + e.getMessage());
                }
            }
        }
    }

    // try-with-resources方式（推荐）
    public static void readFileWithResources(String filename) {
        try (FileInputStream fis = new FileInputStream(filename)) {
            byte[] buffer = new byte[1024];
            int bytesRead = fis.read(buffer);
            System.out.println("读取了 " + bytesRead + " 字节");

        } catch (FileNotFoundException e) {
            System.out.println("文件未找到: " + e.getMessage());
        } catch (IOException e) {
            System.out.println("读取错误: " + e.getMessage());
        }
        // 资源自动关闭
    }

    public static void main(String[] args) {
        readFileTraditional("test.txt");
        readFileWithResources("test.txt");
    }
}

最佳实践

public class CatchBestPractices {
    // 好的做法：具体异常处理
    public static void goodPractice() {
        try {
            // 业务逻辑
            performOperation();
        } catch (SpecificException e) {
            // 针对性处理
            handleSpecificException(e);
        } catch (AnotherException e) {
            // 另一种处理
            handleAnotherException(e);
        } catch (Exception e) {
            // 通用处理（最后的保障）
            handleGenericException(e);
        }
    }

    // 避免的做法：忽略异常
    public static void badPractice() {
        try {
            performOperation();
        } catch (Exception e) {
            // 什么都不做 - 这是不好的做法
        }
    }

    private static void performOperation() throws Exception {
        throw new Exception("示例异常");
    }

    private static void handleSpecificException(Exception e) {
        System.out.println("处理特定异常: " + e.getMessage());
    }

    private static void handleAnotherException(Exception e) {
        System.out.println("处理另一种异常: " + e.getMessage());
    }

    private static void handleGenericException(Exception e) {
        System.out.println("通用异常处理: " + e.getMessage());
        e.printStackTrace();
    }
}

catch关键字是Java异常处理机制的核心，正确使用它可以让程序更加健壮和用户友好。