TRJavalectureexception异常处理.ppt

Java程序设计 Java Programming Spring, 2009 Contents Why Exceptions? What are Exceptions? Handling Exceptions Creating Exception Types Why Exceptions? During execution, programs can run into many kinds of errors; What do we do when an error occurs? Java uses exceptions to provide the error-handling capabilities for its programs. Exceptions Error Class Critical error which is not acceptable in normal application program. Exception Class Possible exception in normal application program execution; Possible to handle by programmer. Exceptions An exception is an event that occurs during the execution of a program that disrupts(使中断) the normal flow of instructions. Treat exception as an object All exceptions are instances of a class extended from Throwable class or its subclass. Generally, a programmer makes new exception class to extend the Exception class which is a subclass of Throwable class. Classifying Java Exceptions Unchecked ExceptionsIt is not required that these types of exceptions be caught or declared on a method. Runtime exceptions can be generated by methods or by the JVM itself. Errors are generated from deep within the JVM, and often indicate a truly fatal state. Checked ExceptionsMust either be caught by a method or declared in its signature by placing exceptions in the method signature. Exception的分类 检查性异常(checked exception)： 一般程序中可预知的问题，其产生的例外可能会带来意想不到的结果，因此Java编译器要求Java程序必须捕获或声明所有的非运行时异常。以IOException为代表的一些类。如果代码中存在检查性异常，必须进行异常处理，否则编译不能通过。如：用户连接数据库SQLException、FileNotFoundException。 非检查性异常(unchecked exception)： 以RuntimeException为代表的一些类，编译时发现不了，只在能运行时才能发现。 Exception的分类 Runtime Exception： Java虚拟机在运行时生成的例外，如被0除等系统错误、数组下标超范围等，其产生比较频繁，处理麻烦，对程序可读性和运行效率影响太大。因此由系统检测, 用户可不做处理，系统将它们交给缺省的异常处理程序（当然，必要时，用户可对其处理）。 Java程序异常处理的原则是： 对于Error和RuntimeException，可以在程序中进行捕获和处理，但不是必须的。 对于IOException及其他异常，必须在程序进行捕获和处理。异常处理机制主要处理检查性异常。 System-Defined Exception(系统定义的异常) Raised implicitly by system because of illegal execution of program; When cannot continue program exe