22.1.异常介绍

  • Java语言中，将程序执行中发生的不正常情况称为 “异常” （开发过程中的语法错误和逻辑错误不是异常）
  • 执行过程中所发生的异常事件分为两大类：
  1. Error：Java虚拟机无法解决的严重问题。例如：JVM系统内部错误、资源耗尽等严重情况。比如：StackOverflowError [栈溢出] 和 OOM(out of memory) [内存不足] ，Error 是严重错误，程序会崩溃
  2. Exception：其它因编程错误或偶然的外在因素导致的一般性问题，可以使用针对性的代码进行处理。例如：空指针访问，试图读取不存在的文件，网络连接中断等等

Exception 分为两大类：运行时异常 [程序运行时，发生的异常] 和 编译时异常 [编程时，编译器检查出的异常]

22.2.异常体系图一览

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编译异常
运行异常
Java源程序
字节码文件
在内存中加载,运行类
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Throwable
Serializable
Object
Error
Exception
StackOverflowError
OutOfMemoryError
RuntimeException
FileNotFoundException
ClassNotFoundException
NullPointerException
ArithmeticException
ArrayIndexOutOfBoundsException
ClassCastException
NumberFormatException

22.3.常见的运行时异常

  1. NullPointerException：空指针异常
    当应用程序试图在需要对象的地方使用null时，抛出该异常
  2. ArithmeticException：数学运算异常
    当出现异常的运算条件时，抛出此异常。例如，一个整数 “除以零” 时，抛出此类的异常
  3. ArrayIndexOutOfBoundsException：数组下标越界异常
    用非法索引访问数组时抛出的异常。如果索引为负或大于等于数组大小，则该索引为非法索引
  4. ClassCastException：类型转换异常
    当试图将对象强制转换为不是实例的子类时，抛出该异常。例如：向下转型
  5. NumberFormatException：数字格式不正确异常
    当应用程序试图将字符串转换成一种数值类型，但该字符串不能转换为适当格式时，抛出该异常。例如：
String name = "奥利给，冲冲冲";
int num = Integer.parseInt(name);
// 抛出异常

22.4.编译异常

  • 介绍
    编译异常是指在编译期间，就必须处理的异常，否则代码不能通过编译
  • 常见的编译异常
    SQLException：操作数据库时，查询表可能发生异常
    IOException：操作文件时，发生的异常
    FileNotFoundException：当操作一个不存在的文件时，发生异常
    ClassNotFoundException：加载类，而该类不存在时，异常
    EOFException：操作文件，到文件末尾，发生异常
    IllegalArguementException：参数异常

22.5.异常处理

  • 基本介绍
    异常处理就是当异常发生时，对异常处理的方式
  • 异常处理的方式
  1. try-catch-finally
    程序员在代码中捕获发生的异常，自行处理
try {//代码/可能有异常
} catch(Exception e) {//捕获到异常
// 1.当异常发生时
// 2.系统将异常封装成Exception对象 e，传递给 catch
// 3.得到异常对象后，自定义处理
// 如果没有发生异常，catch代码块不执行
} finally {// 1.不管try代码块是否有异常发生，始终要执行fianlly
// 2.通常将释放资源的代码，放在finally
}
  1. throws
    将发生的异常抛出，交给调用者来处理，(可以一直往上层抛) 最顶级的处理者就是JVM
throws 处理机制图
  1. try-catch-finally 和 throws 二选一
  2. 如果程序员，没有显示是处理异常，默认throws
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throws
throws
throws
JVM //处理异常 1.输出异常信息 2.退出程序
main //try-catch-finally
f1方法 //try-catch-finally
f2方法 抛出异常

22.6.try-catch方式处理异常注意事项

  1. 如果异常发生了，则异常发生后面的代码不会执行，直接进入到catch块
  2. 如果异常没有发生，则顺序执行try的代码块，不会进入到catch
  3. 如果希望不管是否发生异常，都执行某段代码（比如关闭连接，释放资源等），使用 finally {}
  4. 可以有多个catch 语句，捕获不同的异常（进行不同的业务处理），要求父类异常在后，子类异常在前，比如 Exception 在后，NullPointerException 在前，如果发生异常，只会匹配一个catch
  5. 可以进行 try-finally 配合使用，这种用法相当于没有捕获异常，因此程序会直接崩掉/退出。
具体实例：

判断输入的数是否为整数

Scanner scanner = new Scanner(System.in);
int num = 0;
String inputStr = "";
while (true) {System.out.println("请输入一个整数:");inputStr = scanner.next();try {num = Integer.parseInt(inputStr);break;} catch (NumberFormatException e) {System.out.println("你输入的不是一个整数");}
}
System.out.println("你输入的值是="+ num);

输出结果

22.7.throws异常处理

  1. 如果一个方法（语句执行时）可能生成某种异常，但是并不能确定如何处理这种异常，则此方法应显示地声明抛出异常，表明该方法将不对这些异常进行处理，而由该方法的调用者负责处理
  2. 在方法声明中用 throws语句可以声明抛出异常的列表，throws后面的异常类型可以是方法中产生的异常类型，也可以是它的父类
注意事项和使用细节
  1. 对于编译异常，程序中必须处理，比如 try-catch 或者 throws
  2. 对于运行时异常，程序中如果没有处理，默认就是throws的方式处理
  3. 子类重写父类的方法时，对抛出异常的规定：子类重写的方法，所抛出的异常类型要么和父类抛出的异常一致，要么为父类抛出的异常的类型的子类型
  4. 在throws过程中，如果有方法 try-catch，就相当于处理异常，就可以不必throws

22.8.自定义异常

  • 基本概念
    当程序中出现了某些并没有在 Throwable 子类中描述处理的错误，这个时候可以自己设计异常类，用于描述该错误信息
  • 自定义异常的步骤
  1. 自定义：自定义异常类名继承 Exception 或 RuntimeException
  2. 如果继承 Exception，属于编译异常
  3. 如果继承 RuntimeException，属于运行异常
例子

自定义一个异常-运行异常

class AgeException extends RuntimeException {public AgeException(String message) {super(message);}
}

要求范围在 18-120 之间，否则抛出一个自定义异常

public static void main(String[] args) {int age = 180;if (!(age >= 18 && age <= 120)) {throw new AgeException("年龄需要在 18~120之间");}System.out.println("你的年龄范围正确");
}

输出结果：

一般情况下，我们自定义异常是继承 RuntimeException，即把自定义做成运行时异常，好处是可以使用默认的处理机制，比较方便

22.9.throws 和 throw 的区别

意义 位置 后面跟的东西
throws 异常处理的一种方式 方法声明处 异常类型
throw 手动生成异常对象的关键字 方法体中 异常对象

22.Java之异常处理（异常介绍，异常体系图一览，运行时异常，编译异常，try-catch方式处理异常，throws异常处理，自定义异常，throws 和 throw 的区别）相关推荐

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