JSONObject和JSONArray区别及注意事项

阳光穿透心脏的1/2处 2024-03-02 10:46 62阅读 0赞

### 1、JSONObject和JSONArray的数据表示形式 ###

JSONObject的数据是用 \{ \} 来表示的，

例如：   {"name":"John","age":30,"city":"New York"}

而JSONArray，顾名思义是由JSONObject构成的数组，用 \[ \{ \} , \{ \} , … , \{ \} \] 来表示

例如：   [{"name":"John","age":30,"city":"New York"}] ；

### 2、解析json及可能遇到的问题 ###

String str1 = "{\"name\":\"John\",\"age\":30,\"city\":\"New York\"}";
            String str2 = "[{\"name\":\"John\",\"age\":30,\"city\":\"New York\"}]";
            JSONObject jsonObject = JSONObject.parseObject(str1);
            JSONArray jsonArray = JSONArray.parseArray(str2);

jsonObject对象如下  
![在这里插入图片描述][c909b0751a574d3fb75633b1146a57ef.png]  
解析jsonArray的对象如下：  
![在这里插入图片描述][bad23912c5d3438180e24ba6866ba63c.png]  
如果执行下面的代码

String str1 = "{\"name\":\"John\",\"age\":30,\"city\":\"New York\"}";
            String str2 = "[{\"name\":\"John\",\"age\":30,\"city\":\"New York\"}]";
            JSONObject jsonObject = JSONObject.parseObject(str2);
            JSONArray jsonArray = JSONArray.parseArray(str1);

则会抛异常，即对象形式的json不能用parseArray进行解析，数组形式的json也不能用parseObject进行解析。

### 3、解决方案 ###

可以使用JSON.parse()方法，该方法会自动判断json的类型，然后调不同的方法进行反解析。若需要需要知道具体对象，通过如下的方式进一步判断就可以了

Object obj = JSON.parse(jsonString);
           if (obj instanceof JSONObject) {
               JSONObject jsonObject = (JSONObject) obj;  
           } else if (obj instanceof JSONArray) {
               JSONArray jsonArray = (JSONArray) obj;
               }
           }

JSON.parse()方法中判断json串是什么类型的代码如下：

public DefaultJSONParser(Object input, JSONLexer lexer, ParserConfig config) {
        
            this.resolveStatus = 0;
            this.derializer = new DefaultObjectDeserializer();
            this.contextArray = new ParseContext[8];
            this.contextArrayIndex = 0;
            this.resolveTaskList = new ArrayList();
            this.dateFormatPattern = JSON.DEFFAULT_DATE_FORMAT;
            this.input = input;
            this.lexer = lexer;
            this.config = config;
            this.symbolTable = config.getSymbolTable();
            lexer.nextToken(12);
        }
    
    public void nextToken(int expect) {
        
            while(true) {
        
                switch (expect) {
        
                    case 2:
                        if (this.ch >= '0' && this.ch <= '9') {
        
                            this.sp = 0;
                            this.pos = this.bp;
                            this.scanNumber();
                            return;
                        }
    
                        if (this.ch == '"') {
        
                            this.sp = 0;
                            this.pos = this.bp;
                            this.scanString();
                            return;
                        }
    
                        if (this.ch == '[') {
        
                            this.token = 14;
                            this.ch = this.buf[++this.bp];
                            return;
                        }
    
                        if (this.ch == '{') {
        
                            this.token = 12;
                            this.ch = this.buf[++this.bp];
                            return;
                        }
                    case 3:
                    case 5:
                    case 6:
                    case 7:
                    case 8:
                    case 9:
                    case 10:
                    case 11:
                    case 13:
                    case 17:
                    case 18:
                    case 19:
                    default:
                        break;
                    case 4:
                        if (this.ch == '"') {
        
                            this.sp = 0;
                            this.pos = this.bp;
                            this.scanString();
                            return;
                        }
    
                        if (this.ch >= '0' && this.ch <= '9') {
        
                            this.sp = 0;
                            this.pos = this.bp;
                            this.scanNumber();
                            return;
                        }
    
                        if (this.ch == '[') {
        
                            this.token = 14;
                            this.ch = this.buf[++this.bp];
                            return;
                        }
    
                        if (this.ch == '{') {
        
                            this.token = 12;
                            this.ch = this.buf[++this.bp];
                            return;
                        }
                        break;
                    case 12:  //默认传进来的的expect就是12，所以会走到这个分支里边，会根据json串的情况给token赋值不同的值
                        if (this.ch == '{') {
         
                            this.token = 12;
                            this.ch = this.buf[++this.bp];
                            return;
                        }
    
                        if (this.ch == '[') {
        
                            this.token = 14;
                            this.ch = this.buf[++this.bp];
                            return;
                        }
                        break;
                    case 14:
                        if (this.ch == '[') {
        
                            this.token = 14;
                            this.ch = this.buf[++this.bp];
                            return;
                        }
    
                        if (this.ch == '{') {
        
                            this.token = 12;
                            this.ch = this.buf[++this.bp];
                            return;
                        }
                        break;
                    case 15:
                        if (this.ch == ']') {
        
                            this.token = 15;
                            this.ch = this.buf[++this.bp];
                            return;
                        }
                    case 20:
                        if (this.ch == 26) {
        
                            this.token = 20;
                            return;
                        }
                        break;
                    case 16:
                        if (this.ch == ',') {
        
                            this.token = 16;
                            this.ch = this.buf[++this.bp];
                            return;
                        }
    
                        if (this.ch == '}') {
        
                            this.token = 13;
                            this.ch = this.buf[++this.bp];
                            return;
                        }
    
                        if (this.ch == ']') {
        
                            this.token = 15;
                            this.ch = this.buf[++this.bp];
                            return;
                        }
    
                        if (this.ch == 26) {
        
                            this.token = 20;
                            return;
                        }
                }
    
                if (this.ch != ' ' && this.ch != '\n' && this.ch != '\r' && this.ch != '\t' && this.ch != '\f' && this.ch != '\b') {
        
                    this.nextToken();
                    return;
                }
    
                this.ch = this.buf[++this.bp];
            }
        }

在解析的主代码如下

public Object parse(Object fieldName) {
        
            JSONLexer lexer = this.getLexer();
            switch (lexer.token()) {
        
                case 2:
                    Number intValue = lexer.integerValue();
                    lexer.nextToken();
                    return intValue;
                case 3:
                    Object value = lexer.decimalValue(this.isEnabled(Feature.UseBigDecimal));
                    lexer.nextToken();
                    return value;
                case 4:
                    String stringLiteral = lexer.stringVal();
                    lexer.nextToken(16);
                    if (lexer.isEnabled(Feature.AllowISO8601DateFormat)) {
        
                        JSONScanner iso8601Lexer = new JSONScanner(stringLiteral);
                        if (iso8601Lexer.scanISO8601DateIfMatch()) {
        
                            return iso8601Lexer.getCalendar().getTime();
                        }
                    }
    
                    return stringLiteral;
                case 6:
                    lexer.nextToken();
                    return Boolean.TRUE;
                case 7:
                    lexer.nextToken();
                    return Boolean.FALSE;
                case 8:
                    lexer.nextToken();
                    return null;
                case 9:
                    lexer.nextToken(18);
                    if (lexer.token() != 18) {
        
                        throw new JSONException("syntax error");
                    }
    
                    lexer.nextToken(10);
                    this.accept(10);
                    long time = lexer.integerValue().longValue();
                    this.accept(2);
                    this.accept(11);
                    return new Date(time);
                case 12:   //token就用在这里，可以看到这里就是调的parseObject
                    JSONObject object = new JSONObject();
                    return this.parseObject(object, fieldName);
                case 14: //这里就是调的parseArray
                    JSONArray array = new JSONArray();
                    this.parseArray(array, fieldName);
                    return array;
                case 20:
                    if (lexer.isBlankInput()) {
        
                        return null;
                    }
                case 5:
                case 10:
                case 11:
                case 13:
                case 15:
                case 16:
                case 17:
                case 18:
                case 19:
                default:
                    throw new JSONException("TODO " + lexer.tokenName() + " " + lexer.stringVal());
                case 21:
                    lexer.nextToken();
                    HashSet<Object> set = new HashSet();
                    this.parseArray(set, fieldName);
                    return set;
                case 22:
                    lexer.nextToken();
                    TreeSet<Object> treeSet = new TreeSet();
                    this.parseArray(treeSet, fieldName);
                    return treeSet;
            }
        }

[c909b0751a574d3fb75633b1146a57ef.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/02/f22778b93d1e482fba858b854d8c15a9.png
[bad23912c5d3438180e24ba6866ba63c.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/02/3439658870ff4956a6c746a726ee46e2.png