Java异常架构与异常关键字

Java异常简介

Java异常架构

1. Throwable

2. Error（错误）

3. Exception（异常）

4. 受检异常与非受检异常

Java异常关键字

Java异常处理

声明异常

捕获异常

如何选择异常类型

常见异常处理方式

直接抛出异常

private static void readFile(String filePath) throws IOException {
    File file = new File(filePath);
    String result;
    BufferedReader reader = new BufferedReader(new FileReader(file));
    while((result = reader.readLine())!=null) {
        System.out.println(result);
    }
    reader.close();
}

封装异常再抛出

private static void readFile(String filePath) throws MyException {    
    try {
        // code
    } catch (IOException e) {
        MyException ex = new MyException("read file failed.");
        ex.initCause(e);
        throw ex;
    }
}

捕获异常

private static void readFile(String filePath) {
    try {
        // code
    } catch (FileNotFoundException e) {
        // handle FileNotFoundException
    } catch (IOException e){
        // handle IOException
    }
}

private static void readFile(String filePath) {
    try {
        // code
    } catch (FileNotFoundException | UnknownHostException e) {
        // handle FileNotFoundException or UnknownHostException
    } catch (IOException e){
        // handle IOException
    }
}

自定义异常

public class MyException extends Exception {
    public MyException(){ }
    public MyException(String msg){
        super(msg);
    }
    // ...
}

try-catch-finally

private static void readFile(String filePath) throws MyException {
    File file = new File(filePath);
    String result;
    BufferedReader reader = null;
    try {
        reader = new BufferedReader(new FileReader(file));
        while((result = reader.readLine())!=null) {
            System.out.println(result);
        }
    } catch (IOException e) {
        System.out.println("readFile method catch block.");
        MyException ex = new MyException("read file failed.");
        ex.initCause(e);
        throw ex;
    } finally {
        System.out.println("readFile method finally block.");
        if (null != reader) {
            try {
                reader.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

catch (IOException e) {
    System.out.println("readFile method catch block.");
    return;
}

readFile method catch block.
readFile method finally block.

try-with-resource

private  static void tryWithResourceTest(){
    try (Scanner scanner = new Scanner(new FileInputStream("c:/abc"),"UTF-8")){
        // code
    } catch (IOException e){
        // handle exception
    }
}

Java异常常见面试题

1. Error 和 Exception 区别是什么？

2. 运行时异常和一般异常(受检异常)区别是什么？

3. JVM 是如何处理异常的？

4. throw 和 throws 的区别是什么？

5. final、finally、finalize 有什么区别？

6. NoClassDefFoundError 和 ClassNotFoundException 区别？

7. try-catch-finally 中哪个部分可以省略？

8. try-catch-finally 中，如果 catch 中 return 了，finally 还会执行吗？

public static int getInt() {
    int a = 10;
    try {
        System.out.println(a / 0);
        a = 20;
    } catch (ArithmeticException e) {
        a = 30;
        return a;
        /*
         * return a 在程序执行到这一步的时候，这里不是return a 而是 return 30；这个返回路径就形成了
         * 但是呢，它发现后面还有finally，所以继续执行finally的内容，a=40
         * 再次回到以前的路径,继续走return 30，形成返回路径之后，这里的a就不是a变量了，而是常量30
         */
    } finally {
        a = 40;
    }
 return a;
}

public static int getInt() {
    int a = 10;
    try {
        System.out.println(a / 0);
        a = 20;
    } catch (ArithmeticException e) {
        a = 30;
        return a;
    } finally {
        a = 40;
        //如果这样，就又重新形成了一条返回路径，由于只能通过1个return返回，所以这里直接返回40
        return a; 
    }

}

9. 类 ExampleA 继承 Exception，类 ExampleB 继承ExampleA。

try {
 throw new ExampleB("b")
} catch（ExampleA e）{
 System.out.println("ExampleA");
} catch（Exception e）{
 System.out.println("Exception");
}

class Annoyance extends Exception {
}
class Sneeze extends Annoyance {
}
class Human {
 public static void main(String[] args)
 throws Exception {
  try {
   try {
    throw new Sneeze();
   } catch ( Annoyance a ) {
    System.out.println("Caught Annoyance");
    throw a;
   }
  } catch ( Sneeze s ) {
   System.out.println("Caught Sneeze");
   return ;
  } finally {
   System.out.println("Hello World!");
  }
 }
}

Caught Annoyance
Caught Sneeze
Hello World!

10. 常见的 RuntimeException 有哪些？

11. Java常见异常有哪些

Java异常处理最佳实践

1. 在 finally 块中清理资源或者使用 try-with-resource 语句

public void doNotCloseResourceInTry() {
    FileInputStream inputStream = null;
    try {
        File file = new File("./tmp.txt");
        inputStream = new FileInputStream(file);
        // use the inputStream to read a file
        // do NOT do this
        inputStream.close();
    } catch (FileNotFoundException e) {
        log.error(e);
    } catch (IOException e) {
        log.error(e);
    }
}

1.1 使用 finally 代码块

public void closeResourceInFinally() {
    FileInputStream inputStream = null;
    try {
        File file = new File("./tmp.txt");
        inputStream = new FileInputStream(file);
        // use the inputStream to read a file
    } catch (FileNotFoundException e) {
        log.error(e);
    } finally {
        if (inputStream != null) {
            try {
                inputStream.close();
            } catch (IOException e) {
                log.error(e);
            }
        }
    }
}

1.2 Java 7 的 try-with-resource 语法

public void automaticallyCloseResource() {
    File file = new File("./tmp.txt");
    try (FileInputStream inputStream = new FileInputStream(file);) {
        // use the inputStream to read a file
    } catch (FileNotFoundException e) {
        log.error(e);
    } catch (IOException e) {
        log.error(e);
    }
}

2. 优先明确的异常

public void doNotDoThis() throws Exception {
    ...
}
public void doThis() throws NumberFormatException {
    ...
}

3. 对异常进行文档说明

public void doSomething(String input) throws MyBusinessException {
    ...
}

4. 使用描述性消息抛出异常

try {
    new Long("xyz");
} catch (NumberFormatException e) {
    log.error(e);
}

5. 优先捕获最具体的异常

public void catchMostSpecificExceptionFirst() {
    try {
        doSomething("A message");
    } catch (NumberFormatException e) {
        log.error(e);
    } catch (IllegalArgumentException e) {
        log.error(e)
    }
}

6. 不要捕获 Throwable 类

public void doNotCatchThrowable() {
    try {
        // do something
    } catch (Throwable t) {
        // don't do this!
    }
}

7. 不要忽略异常

public void doNotIgnoreExceptions() {
    try {
        // do something
    } catch (NumberFormatException e) {
        // this will never happen
    }
}

public void logAnException() {
    try {
        // do something
    } catch (NumberFormatException e) {
        log.error("This should never happen: "   e);
    }
}

8. 不要记录并抛出异常

try {
    new Long("xyz");
} catch (NumberFormatException e) {
    log.error(e);
    throw e;
}

17:44:28,945 ERROR TestExceptionHandling:65 - java.lang.NumberFormatException: For input string: "xyz"
Exception in thread "main" java.lang.NumberFormatException: For input string: "xyz"
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
at java.lang.Long.parseLong(Long.java:589)
at java.lang.Long.(Long.java:965)
at com.stackify.example.TestExceptionHandling.logAndThrowException(TestExceptionHandling.java:63)
at com.stackify.example.TestExceptionHandling.main(TestExceptionHandling.java:58)

public void wrapException(String input) throws MyBusinessException {
    try {
        // do something
    } catch (NumberFormatException e) {
        throw new MyBusinessException("A message that describes the error.", e);
    }
}

9. 包装异常时不要抛弃原始的异常

public void wrapException(String input) throws MyBusinessException {
    try {
        // do something
    } catch (NumberFormatException e) {
        throw new MyBusinessException("A message that describes the error.", e);
    }
}

10. 不要使用异常控制程序的流程

11. 使用标准异常

12. 异常会影响性能

13. 总结

private int x = 0;
public int checkReturn() {
    try {
        // x等于1，此处不返回
        return   x;
    } finally {
        // 返回的结果是2
        return   x;
    }
}