注:这个答案是在iOS 5发布之前给出的。

获取json-framework并执行以下操作:

#import "SBJsonWriter.h"

...

SBJsonWriter *jsonWriter = [[SBJsonWriter alloc] init];

NSString *jsonString = [jsonWriter stringWithObject:myDictionary];  

[jsonWriter release];

myDictionary就是你的字典。

苹果在iOS 5.0和Mac OS X 10.7中添加了JSON解析器和序列化器。看到NSJSONSerialization。

要从NSDictionary或NSArray中生成JSON字符串，你不再需要导入任何第三方框架。

以下是如何做到这一点:

NSError *error; 
NSData *jsonData = [NSJSONSerialization dataWithJSONObject:dictionaryOrArrayToOutput 
                                                   options:NSJSONWritingPrettyPrinted // Pass 0 if you don't care about the readability of the generated string
                                                     error:&error];

if (! jsonData) {
    NSLog(@"Got an error: %@", error);
} else {
    NSString *jsonString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
}

这里有NSArray和NSDictionary的类别，让这个超级简单。我添加了一个漂亮打印的选项(换行符和制表符使阅读更容易)。

@interface NSDictionary (BVJSONString)
-(NSString*) bv_jsonStringWithPrettyPrint:(BOOL) prettyPrint;
@end

.

@implementation NSDictionary (BVJSONString)

  -(NSString*) bv_jsonStringWithPrettyPrint:(BOOL) prettyPrint {
     NSError *error;
     NSData *jsonData = [NSJSONSerialization dataWithJSONObject:self
                                                   options:(NSJSONWritingOptions)    (prettyPrint ? NSJSONWritingPrettyPrinted : 0)
                                                     error:&error];

     if (! jsonData) {
        NSLog(@"%s: error: %@", __func__, error.localizedDescription);
        return @"{}";
     } else {
        return [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
     } 
 }
@end

.

@interface NSArray (BVJSONString)
- (NSString *)bv_jsonStringWithPrettyPrint:(BOOL)prettyPrint;
@end

.

@implementation NSArray (BVJSONString)
-(NSString*) bv_jsonStringWithPrettyPrint:(BOOL) prettyPrint {
    NSError *error;
    NSData *jsonData = [NSJSONSerialization dataWithJSONObject:self
                                                       options:(NSJSONWritingOptions) (prettyPrint ? NSJSONWritingPrettyPrinted : 0)
                                                         error:&error];

    if (! jsonData) {
        NSLog(@"%s: error: %@", __func__, error.localizedDescription);
        return @"[]";
    } else {
        return [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
    }
}
@end

现在不需要第三方类ios 5引入了Nsjsonserialization

NSString *urlString=@"Your url";
NSString *urlUTF8 = [urlString stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
NSURL *url=[[NSURL alloc]initWithString:urlUTF8];
NSURLRequest *request=[NSURLRequest requestWithURL:url];

NSURLResponse *response;

NSData *GETReply = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:nil];

NSError *myError = nil;

NSDictionary *res = [NSJSONSerialization JSONObjectWithData:GETReply options:NSJSONReadingMutableLeaves|| NSJSONReadingMutableContainers error:&myError];

Nslog(@"%@",res);

这段代码对于获取jsondata非常有用。

您也可以通过在调试器中输入以下内容即时执行此操作

po [[NSString alloc] initWithData:[NSJSONSerialization dataWithJSONObject:yourDictionary options:1 error:nil] encoding:4];

从ISO7开始，至少你可以用NSJSONSerialization轻松做到这一点。

将NSDictionary转换为NSString:

NSError * err;
NSData * jsonData = [NSJSONSerialization dataWithJSONObject:myDictionary options:0 error:&err]; 
NSString * myString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];

NSMutableDictionary *contentDictionary = [[NSMutableDictionary alloc]init];
        [contentDictionary setValue:@"a" forKey:@"b"];
        [contentDictionary setValue:@"c" forKey:@"d"];
        NSData *data = [NSJSONSerialization dataWithJSONObject:contentDictionary options:NSJSONWritingPrettyPrinted error:nil];
        NSString *jsonStr = [[NSString alloc] initWithData:data
                                                  encoding:NSUTF8StringEncoding];

在Swift中，我创建了以下helper函数:

class func nsobjectToJSON(swiftObject: NSObject) {
    var jsonCreationError: NSError?
    let jsonData: NSData = NSJSONSerialization.dataWithJSONObject(swiftObject, options: NSJSONWritingOptions.PrettyPrinted, error: &jsonCreationError)!

    if jsonCreationError != nil {
        println("Errors: \(jsonCreationError)")
    }
    else {
        // everything is fine and we have our json stored as an NSData object. We can convert into NSString
        let strJSON : NSString =  NSString(data: jsonData, encoding: NSUTF8StringEncoding)!
        println("\(strJSON)")
    }
}

在Swift 2.0版本中:

class func jsonStringWithJSONObject(jsonObject: AnyObject) throws -> String? {
    let data: NSData? = try? NSJSONSerialization.dataWithJSONObject(jsonObject, options: NSJSONWritingOptions.PrettyPrinted)

    var jsonStr: String?
    if data != nil {
        jsonStr = String(data: data!, encoding: NSUTF8StringEncoding)
    }

    return jsonStr
}

可以传递数组或字典。这里，我使用NSMutableDictionary。

NSMutableDictionary *contentDictionary = [[NSMutableDictionary alloc]init];
[contentDictionary setValue:@"a" forKey:@"b"];
[contentDictionary setValue:@"c" forKey:@"d"];

为了从NSDictionary或NSArray中生成JSON字符串，你不需要导入任何第三方框架。只需使用以下代码:-

NSError *error; 
NSData *jsonData = [NSJSONSerialization dataWithJSONObject:contentDictionary // Here you can pass array or dictionary
                    options:NSJSONWritingPrettyPrinted // Pass 0 if you don't care about the readability of the generated string
                    error:&error];
NSString *jsonString;
if (jsonData) {
    jsonString = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
    //This is your JSON String
    //NSUTF8StringEncoding encodes special characters using an escaping scheme
} else {
    NSLog(@"Got an error: %@", error);
    jsonString = @"";
}
NSLog(@"Your JSON String is %@", jsonString);

public func jsonPrint(_ o: NSObject, spacing: String = "", after: String = "", before: String = "") {
    let newSpacing = spacing + "    "
    if o.isArray() {
        print(before + "[")
        if let a = o as? Array<NSObject> {
            for object in a {
                jsonPrint(object, spacing: newSpacing, after: object == a.last! ? "" : ",", before: newSpacing)
            }
        }
        print(spacing + "]" + after)
    } else {
        if o.isDictionary() {
            print(before + "{")
            if let a = o as? Dictionary<NSObject, NSObject> {
                for (key, val) in a {
                    jsonPrint(val, spacing: newSpacing, after: ",", before: newSpacing + key.description + " = ")
                }
            }
            print(spacing + "}" + after)
        } else {
            print(before + o.description + after)
        }
    }
}

这个非常接近最初的Objective-C打印风格

这是Swift 4的版本

extension NSDictionary{

func toString() throws -> String? {
    do {
        let data = try JSONSerialization.data(withJSONObject: self, options: .prettyPrinted)
        return String(data: data, encoding: .utf8)
    }
    catch (let error){
        throw error
    }
}

}

使用的例子

do{
    let jsonString = try dic.toString()
    }
    catch( let error){
        print(error.localizedDescription)
    }

或者如果你确定它是有效的字典，那么你可以使用

let jsonString = try? dic.toString()

这将在swift4和swift5中工作。

let dataDict = "the dictionary you want to convert in jsonString" 

let jsonData = try! JSONSerialization.data(withJSONObject: dataDict, options: JSONSerialization.WritingOptions.prettyPrinted)

let jsonString = NSString(data: jsonData, encoding: String.Encoding.utf8.rawValue)! as String

print(jsonString)