7.2 捕捉异常

1 捕捉异常

try{    //这里写可能发生异常的代码    code    more code    more code}catch (ExceptionType e){    //一旦发生异常，就会跳转到这里对异常进行处理    handler for this type}

例如：

public void read(String filename){    try    {        InputStream in = new FileInputStream(filename);        int b;        while ((b = in.read()) != -1)        {            process input        }    }    catch (IOException exception)    {        exception.printStackTrace();    }}

当然也可以传递给方法的调用者，而不在本方法处理这个异常：

public void read(String filename) throws IOException{    InputStream in = new FileInputStream(filename);    int b;    while ((b = in.read()) != -1)    {        process input    }}

一般是这样考虑，能处理这个异常，那就不进行传递下去，如果处理不了，就传递给调用者处理。

注意：子类覆盖方法抛出的异常不能够超出父类该方法抛出的异常范围，否则，和权限修饰符一样，子类就不属于父类的定义了，而这是违背类的设计理念的。

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2 捕获多个异常

try{    code that might throw exceptions}catch (FileNotFoundException e){    emergency action for missing files}catch (UnknownHostException e){    emergency action for unknown hosts}catch (IOException e){    emergency action for all other I/O problems}

这样也可以：

try{    code that might throw exceptions}catch (FileNotFoundException | UnknownHostException e){    emergency action for missing files and unknown hosts}catch (IOException e){    emergency action for all other I/O problems}

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3 重抛异常和异常链

try{    access the database}catch (SQLException e){    //包含了更高级信息的异常    throw new ServletException("database error: " + e.getMessage());}

//这样重新获得那个被包装起来的异常，，通过这个方法可以把已检查异常包装成运行时异常Throwable e = se.getCause();

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4 finally语句

InputStream in = new FileInputStream(. . .);try{    // 1    code that might throw exceptions    // 2}catch (IOException e){    // 3    show error message    // 4}finally{    // 5    in.close();}// 6

1. 没有异常发生：1，2，5，6
2. 发生异常并捕捉到异常：1，3，4，5，6
3. 发生异常没有捕捉到：1，5

也可以没有catch块

InputStream in = . . .;try{    code that might throw exceptions}finally{    in.close();}

一般finally块用来关闭打开的资源。

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5 带资源的try语句

//一般是这样写的语句open a resourcetry{    work with the resource}finally{    close the resource}

//当try块结束时，会自动调用res.close()方法try (Resource res = ...){    work with res}

例如：

try (Scanner in = new Scanner(new FileInputStream("/usr/share/dict/words")), "UTF-8"){    while (in.hasNext())        System.out.println(in.next());}

也可以打开多个资源：

try (    Scanner in = new Scanner(new FileInputStream("/usr/share/dict/words"), "UTF-8");    PrintWriter out = new PrintWriter("out.txt")    ){    while (in.hasNext())    out.println(in.next().toUpperCase());}

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6 分析堆栈跟踪元素

就是分析方法调用层级顺序的东西。

import java.util.*;/** * A program that displays a trace feature of a recursive method call. * @version 1.01 2004-05-10 * @author Cay Horstmann */public class StackTraceTest{   /**    * Computes the factorial of a number    * @param n a non-negative integer    * @return n! = 1 * 2 * . . . * n    */   public static int factorial(int n)   {      System.out.println("factorial(" + n + "):");      Throwable t = new Throwable();      StackTraceElement[] frames = t.getStackTrace();      for (StackTraceElement f : frames)         System.out.println(f);      int r;      if (n <= 1) r = 1;      else r = n * factorial(n - 1);      System.out.println("return " + r);      return r;   }   public static void main(String[] args)   {      Scanner in = new Scanner(System.in);      System.out.print("Enter n: ");      int n = in.nextInt();      factorial(n);   }}