Objective-C语法之异常处理

Objective-C的异常比较像Java的异常处理,也有@finally的处理,不管异常是否捕获都都要执行。
异常处理捕获的语法:
  @try {
        <#statements#>
    }
    @catch (NSException *exception) {
        <#handler#>
    }
    @finally {
        <#statements#>
    }
 @catch{} 块 对异常的捕获应该先细后粗,即是说先捕获特定的异常,再使用一些泛些的异常类型。
我们自定义两个异常类,看看异常异常处理的使用。

1、新建SomethingException,SomeOverException这两个类,都继承与NSException类。

SomethingException.h
#import <Foundation/Foundation.h>

@interface SomethingException : NSException

@end
SomethingException.m
#import "SomethingException.h"

@implementation SomethingException

@end
SomeOverException.h
#import <Foundation/Foundation.h>

@interface SomeOverException : NSException

@end
SomeOverException.m
#import "SomeOverException.h"

@implementation SomeOverException

@end

2、新建Box类,在某些条件下产生异常。

#import <Foundation/Foundation.h>

@interface Box : NSObject
{
    NSInteger number;
}
-(void) setNumber: (NSInteger) num;
-(void) pushIn;
-(void) pullOut;
-(void) printNumber;
@end

@implementation Box
-(id) init {
    self = [super init];
    
    if ( self ) {
        [self setNumber: 0];
    }
    
    return self;
}

-(void) setNumber: (NSInteger) num {
    number = num;
    
    if ( number > 10 ) {
        NSException *e = [SomeOverException
                          exceptionWithName: @"BoxOverflowException"
                          reason: @"The level is above 100"
                          userInfo: nil];
        @throw e;
    } else if ( number >= 6 ) {
        // throw warning
        NSException *e = [SomethingException
                          exceptionWithName: @"BoxWarningException"
                          reason: @"The level is above or at 60"
                          userInfo: nil];
        @throw e;
    } else if ( number < 0 ) {
        // throw exception
        NSException *e = [NSException
                          exceptionWithName: @"BoxUnderflowException"
                          reason: @"The level is below 0"
                          userInfo: nil];
        @throw e;
    }
}

-(void) pushIn {
    [self setNumber: number + 1];
}

-(void) pullOut {
    [self setNumber: number - 1];
}

-(void) printNumber {
    NSLog(@"Box number is: %d", number);
}
@end
这个类的作用是,初始化Box时,number数字是0,可以用pushIn 方法往Box里推入数字,每调用一次,number加1.当number数字大于等于6时产生SomethingException异常,告诉你数字达到或超过6了,超过10时产生 SomeOverException异常,小于1时产生普通的NSException异常。

这里写 [SomeOverException  exceptionWithName:@"BoxOverflowException"  reason:@"The level is above 100"异常的名称和理由,在捕获时可以获取。

3、使用Box,在适当添加下捕获Box类的异常

3.1、在没超过6时,没有异常

- (void)viewDidLoad
{
    [super viewDidLoad];    
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    Box *box = [[Box alloc]init];
    for (int i = 0; i < 5; i++) {
        [box pushIn];
        [box printNumber];
    }
}
打印结果:

Box number is: 1

Box number is: 2

Box number is: 3

Box number is: 4

Box number is: 5

3.2 超过6,产生异常

for (int i = 0; i < 11; i++) {
            [box pushIn];
            [box printNumber];
    }
2012-07-04 09:12:05.889 ObjectiveCTest[648:f803] Box number is: 1
2012-07-04 09:12:05.890 ObjectiveCTest[648:f803] Box number is: 2
2012-07-04 09:12:05.890 ObjectiveCTest[648:f803] Box number is: 3
2012-07-04 09:12:05.890 ObjectiveCTest[648:f803] Box number is: 4
2012-07-04 09:12:05.891 ObjectiveCTest[648:f803] Box number is: 5
2012-07-04 09:12:05.891 ObjectiveCTest[648:f803] *** Terminating app due to uncaught exception 'BoxWarningException', reason: 'The number is above or at 60'
这是时, 程序抛出异常崩溃了。那怎么使程序不崩溃呢,做异常处理。

3.3、加上异常处理

for (int i = 0; i < 11; i++) {
        @try {
            [box pushIn];
        }
        @catch (SomethingException *exception) {
            NSLog(@"%@ %@", [exception name], [exception reason]);
        }
        @catch (SomeOverException *exception) {
            NSLog(@"%@", [exception name]);
        }
        @finally {
            [box printNumber];
        }
    }
运行,程序没有崩溃,打印结果:
2012-07-04 09:14:35.165 ObjectiveCTest[688:f803] Box number is: 1
2012-07-04 09:14:35.167 ObjectiveCTest[688:f803] Box number is: 2
2012-07-04 09:14:35.167 ObjectiveCTest[688:f803] Box number is: 3
2012-07-04 09:14:35.167 ObjectiveCTest[688:f803] Box number is: 4
2012-07-04 09:14:35.167 ObjectiveCTest[688:f803] Box number is: 5
2012-07-04 09:14:35.167 ObjectiveCTest[688:f803] BoxWarningException The number is above or at 60
2012-07-04 09:14:35.168 ObjectiveCTest[688:f803] Box number is: 6
2012-07-04 09:14:35.168 ObjectiveCTest[688:f803] BoxWarningException The number is above or at 60
2012-07-04 09:14:35.168 ObjectiveCTest[688:f803] Box number is: 7
2012-07-04 09:14:35.168 ObjectiveCTest[688:f803] BoxWarningException The number is above or at 60
2012-07-04 09:14:35.168 ObjectiveCTest[688:f803] Box number is: 8
2012-07-04 09:14:35.168 ObjectiveCTest[688:f803] BoxWarningException The number is above or at 60
2012-07-04 09:14:35.169 ObjectiveCTest[688:f803] Box number is: 9
2012-07-04 09:14:35.169 ObjectiveCTest[688:f803] BoxWarningException The number is above or at 60
2012-07-04 09:14:35.169 ObjectiveCTest[688:f803] Box number is: 10
2012-07-04 09:14:35.169 ObjectiveCTest[688:f803] BoxOverflowException
2012-07-04 09:14:35.225 ObjectiveCTest[688:f803] Box number is: 11
超过10时,SomeOverException 异常抛出。

3.4 、小于0时的异常

在Box类的setNumber里,当number小于0时,我们抛出普通异常。
  @try {
        [box setNumber:-10];
    }
    @catch (NSException *exception) {
        NSLog(@"%@",[exception name]);
    }
    @finally {
        [box printNumber];
    }
打印结果:
2012-07-04 09:17:42.405 ObjectiveCTest[753:f803] BoxUnderflowException
2012-07-04 09:17:42.406 ObjectiveCTest[753:f803] Box number is: -10

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