Method Swizzling替换 NSURL 的initWithString: 奔溃

      我已经成功的替换了NSURL 的类方法URLWithString: 那就说明NSURL是支持Method Swizzling的，但是替换initWithString:却不成功，总是在url被释放的时候奔溃，报的是一个野指针的错误Thread 1: EXC_BAD_ACCESS (code=EXC_I386_GPFLT)。求大神解答。

 代码如下：

#import "NSURL+MethodSwizzing.h"

#import <objc/runtime.h>

@implementation NSURL (QQNSURLMethodSwizzing)

+ (void)load {

    Method system_Method = class_getClassMethod([self class], @selector(URLWithString:));

    Method my_Method = class_getClassMethod([self class], @selector(URLWithTESTDifferentEnviromentString:));

    method_exchangeImplementations(system_Method, my_Method);

    Method systemInitMethod = class_getInstanceMethod([self class], @selector(initWithString:));

    Method myMethod = class_getInstanceMethod([self class], @selector(initWithDifferentEnviromentString:));

    method_exchangeImplementations(systemInitMethod, myMethod);

}

+ (NSURL *)URLWithTESTDifferentEnviromentString:(NSString *)urlString {

    return [self URLWithTESTDifferentEnviromentString:urlString];

}

- (void)initWithDifferentEnviromentString:(NSString *)urlString {

    [self initWithDifferentEnviromentString:urlString];

}

@end