第06章-异常处理

1、1面向对象程序设计（Java）异常的基本概念n异常(Exception)是程序执行过程中出现的非正常事件，即各种意外情况。比如说：n 用户输入出错n 所需文件找不到n 运行时磁盘空间不够n 内存不够2n 算术运算错 (数的溢出，被零除)n 数组下标越界n n 当Java程序出现以上的错误时，就会在所处的方法中产生一个异常对象。这个异常对象包括错误的类型，错误出现时程序的运行状态以及对该错误的详细描述。下面我们先看一个简单的例子。3npublic class ExceptionDemo n public static void main(String args) n int x=100;n Sy

2、stem.out.println(The result is+x/10);n System.out.println(Divided by zero: +x/0); 4n当我们对其编译后运行时，其对应的结果如下：nc:jbuilder3javabinjava Exception DemonThe result is10nException in thread main java.lang.Arithmetic Exception: / by zero at Exception Demo.main(Exception Demo.java:5)n 其意思是说，本程序执行到语句“System.out.

3、println (Divided by zero: +x/0)”时，系统会抛出一个例外，该例外在Java中定义为Arithmetic Exception (即属于算术运算例外)。5异常的处理机制n我们知道，一旦程序在执行的过程中出现异常，往往有两种处理方式：n (1) 当程序出现错误的时候，系统将终止程序的运行，如例5.1。在Java中这是属于运行异常，用户不加干预，完全交由系统对其进行处理。6n(2) 当程序出现错误时，采用捕捉抛出(catch-throw)的面向对象编程方式。该种方式是当程序运行出错时，系统和程序抛出各种标准类型的错误，程序捕捉该错误并进行相应处理。由于异常均以标准的形式提

4、供，使得程序员能以统一的方式对异常进行处理。n 通常，Java的出错与异常处理采用“try, catch, throws”语句来实现，下面我们分别加以介绍。781.异常和异常类2.异常处理3.创建异常第六章 异常处理9n软件程序肯定会发生错误/问题nwhat really matters is what happens after the error occurs. How is the error handled? Who handles it? Can the program recover, or should it just die? n从外部问题(硬盘、网络故障等)到编程错误(数组越

5、界、引用空对象等)n致命错误n内存空间不足等错误(Error)导致程序异常中断n程序不能简单地恢复执行n非致命错误n数组越界等异常(Exception)导致程序中断执行n程序在修正后可恢复执行异常(Exception)10nJava语言中已定义或用户定义的某个异常类的对象n一个异常类代表一种异常事件nJava语言利用异常来使程序获得处理错误的能力(error-handling) n异常事件(Exceptional Event)nAn exception is an event that occurs during the execution of a program that disrupts

6、 the normal flow of instructions. 异常异常类的类层次nJava语言采用继承的方式组织各种异常，所有的异常类都直接或间接地继承于类Throwable。我们可用图5.1描述异常的常用类层次。n 从图5.1中可以看出，Throwable类是所有异常类的父类。它分为两个子类：Error类和Exception类。Error类包括动态链接失败、虚拟机出错等异常，该类异常Java不要求捕获，同时系统也不会抛出该类异常1112ThrowableErrorExceptionLinkageErrorVirtualMachineErrorAWTErrorAWTExceptionIO

7、ExceptionRuntimeExceptionArithmeticExceptionInterruptedExceptionIndexOutOfBoundsExceptionFileNotFoundExceptionEOFException13nJava语言中用来处理异常的类n异常类的方法n构造方法npublic Exception()npublic Exception(String s)n常用方法npublic String toString()npublic String getMessage()npublic void printStackTrace()异常类14n对异常的定义nJa

8、va语言中定义的异常类: IOException/NullPointerExceptionn详细定义见Java文档中各个包的Exception Summaryn用户定义自已所需的异常类，描述程序中出现的异常结果第10讲 异常(Exception)class java.lang.Throwable class java.lang.Error (严重的问题，但不需程序捕捉的错误) class java.lang.LinkageError . . class java.lang.VirtualMachineError class java.lang.InternalError class java.

9、lang.OutOfMemoryError . . class java.lang.Exception(程序应该捕捉的错误) class java.lang.ClassNotFoundException class java.lang.RuntimeException (JVM正常运行时抛出的错误) class java.lang.ArithmeticException . . class java.io.IOException class java.awt.AWTException class java.sql.SQLException . .151.异常和异常类2.异常处理3.创建异常第六

10、章 异常处理16n当一个Java程序的方法产生一个错误，该方法创造一个异常对象并将其交给运行系统nIn Java terminology, creating an exception object and handing it to the runtime system is called throwing an exception(抛出异常)n运行系统从错误发生处开始寻找处理错误的程序段nThe exception handler chosen is said to catch the exception(捕获异常)n捕获异常的过程可以沿方法调用的逆向顺序寻找异常处理17n异常处理器(exc

11、eption handler)ntryncatchnfinallyn异常的抛出(throw)异常处理18n何时会出现异常?n方法中已定义了异常的抛出异常处理import java.io.IOException;class jex3_3 public static void main(String args) throws IOException char c;System.out.println(Please input a char: );c = (char)System.in.read();System.out.println(Received char= + c);public abst

12、ract int read() throws IOException在程序编译的时候必须完成异常的处理19n由于非预期的结果导致系统运行时产生异常异常处理class jex7_9 public static void main(String args) int a = 0;int b = 24/a;java jex7_9Exception in thread main“java.lang.ArithmeticException: / by zeroclass jex7_9 public static void main(String args) int i = Integer.parseInt

13、(args0);System.out.println(i);java jex7_9Exception in thread main“java.lang.ArrayIndexOutOfBoundsException: 0java jex7_9 aException in thread main“java.lang.NumberFormatException: For input string: “a”20n异常处理器(exception handler)ntryncatchnfinally异常处理21n异常处理器(exception handler)ntry程序块 n构造一个异常处理器，封装一些

14、抛出异常的语句try Java 语句块; /指一个或多个抛出异常的Java语句异常(Exception)class Test public static void main(String args) char c = (char)System.in.read(); System.out.println(c); import java.io.IOException;class Test public static void main(String args) try char c = (char)System.in.read(); System.out.println(c); catch (IO

15、Exception e) System.out.println(e); unreported exception java.io.IOException;must be caught or declared to be thrownchar c = (char)System.in.read();一个try语句必须带有至少一个catch语句块或一个finally语句块22n异常处理器(exception handler)ntry语句块定义了异常处理器的范围ncatch语句块捕捉try语句块抛出的异常try / Code that might generate exceptions catch(T

16、ype1 id1) / Handle exceptions of Type1 catch(Type2 id2) / Handle exceptions of Type2 catch(Type3 id3) / Handle exceptions of Type3/ etc .异常处理import java.io.IOException;class Test public static void main(String args) try char c = (char)System.in.read(); System.out.println(c); catch (IOException e) Sy

17、stem.out.println(e); try . . . catch (ArrayIndexOutOfBoundsException e) System.out.println(e); catch (IOException e) System.out.println(e);23n异常处理器(exception handler)nfinally语句块nTheres often some piece of code that you want to execute whether or not an exception is thrown within a try block.nfinally

18、语句块在异常处理中是必须执行的语句块n清理现场n关闭打开的文件n关闭网络连接异常处理try / The guarded region: Dangerous activities / that might throw A, B, or C catch(A a1) / Handler for situation A catch(B b1) / Handler for situation B catch(C c1) / Handler for situation C finally / Activities that happen every time24n异常的抛出n在一个方法中，抛出异常，同时捕

19、捉n当有多个方法调用时，由特定(适当)的方法捕捉异常n被调用的方法主动抛出异常(throws)异常处理import java.io.IOException;class Test static char getChar() throws IOException char c = (char)System.in.read(); return c; public static void main(String args) try char c = getChar(); System.out.println(c); catch (IOException e) System.out.println(e)

20、; 25n异常的抛出n主动抛出异常异常处理 void parseObj(String s) throws NullPointerException if (s = null) throw new NullPointerException(); ; 261.异常和异常类2.异常处理3.创建异常第六章 异常处理27n使用Java语言已有的异常异常的抛出/捕捉n创建自已的异常异常的抛出/捕捉创建异常28异常(Exception)class SimpleException extends Exception public class SimpleExceptionDemo public void f(

21、) throws SimpleException System.out.println(Throw SimpleException from f(); throw new SimpleException(); public static void main(String args) SimpleExceptionDemo sed = new SimpleExceptionDemo(); try sed.f(); catch(SimpleException e) System.out.println(e); System.out.println(Caught it!); 运行结果:Throw S

22、impleException from f()SimpleExceptionCaught it!29n自定义异常n自定义异常类必须是java.lang.Exception类的子类njava.lang.Exception类的两个构造方法nExceptionException() Constructs a new exception with null as its detail message.nExceptionException(Stringmessage) Constructs a new exception with the specified detail message.n自定义异常

23、类可以不定义构造方法nSimpleException() super(); n自定义异常类定义自已的构造方法创建异常30n定义自已的异常构造方法创建异常31class MyException extends Exception private int x; public MyException() public MyException(String msg) super(msg); public MyException(String msg, int x) super(msg);this.x = x; class ExtraFeatures public static void f() thr

24、ows MyException System.out.println(Throws MyException from f(); throw new MyException(); public static void g() throws MyException System.out.println(Throws MyException from g(); throw new MyException(Originated in g(); public static void h() throws MyException System.out.println(Throws MyException

25、from h(); throw new MyException(Originated in h(), 47); try f(); catch(MyException e) System.out.println(e);Throwing MyException from f()MyExceptiontry g(); catch(MyException e) System.out.println(e);Throwing MyException from g()MyException: Originated in g()Throwing MyException from h()MyException:

26、 Originated in h()try h(); catch(MyException e) System.out.println(e);32nQuiznQuestion: Is the following code legal?异常(Exception)try . finally .nAnswer: Yes, its legal. A try statement does not have to have a catch statement if it has a finally statement. If the code in the try statement has multiple exit points and no associated catch clauses, the code in the finally statement is executed no matter