• JSONObject.parseObject将_id赋值给id问题

        最近再写代码的时候发现一个JSONObject.parseObject使用的问题,会将json中_id的值赋值给对象id,最后得到一个错误的id,后来百度没有查到这个问题到底怎么解决,只好看下源码找出解决方式.由于业务代码没办法展开,这里写一个简单的Demo来重现一下问题.

    问题重现

    @Data
public class Student {

    private String id;
}

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    public class Demo {

    public static void main(String[] args) {
        String json = "{\"_id\":\"1\",\"id\":\"2\"}";
        Student student = JSON.parseObject(json, Student.class);
        // 1
        System.out.println(student.getId());
    }
}

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    我们期望student的id的值应该是2,但是在这里用代码输出的确是1

    源码研读

        源码太过复杂不一一展开,只针对为什么会把_id的值赋值给id代码;

    • com.alibaba.fastjson.parser.deserializer.FieldDeserializer#setValue(java.lang.Object, java.lang.Object)

    对属性进行赋值的关键代码;

        @SuppressWarnings({"rawtypes", "unchecked"})
    public void setValue(Object object, Object value) {
        if (value == null //
                && fieldInfo.fieldClass.isPrimitive()) {
            return;
        } else if (fieldInfo.fieldClass == String.class
                && fieldInfo.format != null
                && fieldInfo.format.equals("trim")) {
            value = ((String) value).trim();
        }

        try {
            Method method = fieldInfo.method;
            if (method != null) {
                if (fieldInfo.getOnly) {
                    if (fieldInfo.fieldClass == AtomicInteger.class) {
                        AtomicInteger atomic = (AtomicInteger) method.invoke(object);
                        if (atomic != null) {
                            atomic.set(((AtomicInteger) value).get());
                        } else {
                            degradeValueAssignment(fieldInfo.field, method, object, value);
                        }
                    } else if (fieldInfo.fieldClass == AtomicLong.class) {
                        AtomicLong atomic = (AtomicLong) method.invoke(object);
                        if (atomic != null) {
                            atomic.set(((AtomicLong) value).get());
                        } else {
                            degradeValueAssignment(fieldInfo.field, method, object, value);
                        }
                    } else if (fieldInfo.fieldClass == AtomicBoolean.class) {
                        AtomicBoolean atomic = (AtomicBoolean) method.invoke(object);
                        if (atomic != null) {
                            atomic.set(((AtomicBoolean) value).get());
                        } else {
                            degradeValueAssignment(fieldInfo.field, method, object, value);
                        }
                    } else if (Map.class.isAssignableFrom(method.getReturnType())) {
                        Map map = null;
                        try {
                            map = (Map) method.invoke(object);
                        } catch (InvocationTargetException e) {
                            degradeValueAssignment(fieldInfo.field, method, object, value);
                            return;
                        }
                        if (map != null) {
                            if (map == Collections.emptyMap()) {
                                return;
                            }

                            if (map.isEmpty() && ((Map) value).isEmpty()) {
                                return;
                            }

                            String mapClassName = map.getClass().getName();
                            if (mapClassName.equals("java.util.ImmutableCollections$Map1")
                                    || mapClassName.equals("java.util.ImmutableCollections$MapN")
                                    || mapClassName.startsWith("java.util.Collections$Unmodifiable")) {
                                // skip

                                return;
                            }

                            if (map.getClass().getName().equals("kotlin.collections.EmptyMap")) {
                                degradeValueAssignment(fieldInfo.field, method, object, value);
                                return;
                            }

                            map.putAll((Map) value);
                        } else if (value != null) {
                            degradeValueAssignment(fieldInfo.field, method, object, value);
                        }
                    } else {
                        Collection collection = null;
                        try {
                            collection = (Collection) method.invoke(object);
                        } catch (InvocationTargetException e) {
                            degradeValueAssignment(fieldInfo.field, method, object, value);
                            return;
                        }
                        if (collection != null && value != null) {
                            String collectionClassName = collection.getClass().getName();

                            if (collection == Collections.emptySet()
                                    || collection == Collections.emptyList()
                                    || collectionClassName == "java.util.ImmutableCollections$ListN"
                                    || collectionClassName == "java.util.ImmutableCollections$List12"
                                    || collectionClassName.startsWith("java.util.Collections$Unmodifiable")) {
                                // skip
                                return;
                            }

                            if (!collection.isEmpty()) {
                                collection.clear();
                            } else if (((Collection) value).isEmpty()) {
                                return; //skip
                            }


                            if (collectionClassName.equals("kotlin.collections.EmptyList")
                                    || collectionClassName.equals("kotlin.collections.EmptySet")) {
                                degradeValueAssignment(fieldInfo.field, method, object, value);
                                return;
                            }
                            collection.addAll((Collection) value);
                        } else if (collection == null && value != null) {
                            degradeValueAssignment(fieldInfo.field, method, object, value);
                        }
                    }
                } else {
                    method.invoke(object, value);
                }
            } else {
                final Field field = fieldInfo.field;
                
                if (fieldInfo.getOnly) {
                    if (fieldInfo.fieldClass == AtomicInteger.class) {
                        AtomicInteger atomic = (AtomicInteger) field.get(object);
                        if (atomic != null) {
                            atomic.set(((AtomicInteger) value).get());
                        }
                    } else if (fieldInfo.fieldClass == AtomicLong.class) {
                        AtomicLong atomic = (AtomicLong) field.get(object);
                        if (atomic != null) {
                            atomic.set(((AtomicLong) value).get());
                        }
                    } else if (fieldInfo.fieldClass == AtomicBoolean.class) {
                        AtomicBoolean atomic = (AtomicBoolean) field.get(object);
                        if (atomic != null) {
                            atomic.set(((AtomicBoolean) value).get());
                        }
                    } else if (Map.class.isAssignableFrom(fieldInfo.fieldClass)) {
                        Map map = (Map) field.get(object);
                        if (map != null) {
                            if (map == Collections.emptyMap()
                                    || map.getClass().getName().startsWith("java.util.Collections$Unmodifiable")) {
                                // skip
                                return;
                            }
                            map.putAll((Map) value);
                        }
                    } else {
                        Collection collection = (Collection) field.get(object);
                        if (collection != null && value != null) {
                            if (collection == Collections.emptySet()
                                    || collection == Collections.emptyList()
                                    || collection.getClass().getName().startsWith("java.util.Collections$Unmodifiable")) {
                                // skip
                                return;
                            }

                            collection.clear();
                            collection.addAll((Collection) value);
                        }
                    }
                } else {
                    if (field != null) {
                        field.set(object, value);
                    }
                }
            }
        } catch (Exception e) {
            throw new JSONException("set property error, " + clazz.getName() + "#" + fieldInfo.name, e);
        }
    }

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    在这里插入图片描述
    首次进行setValue的是json中_id的值对应的fieldInfo的fileId却是id,到最后使用fieldInfo的method也就是Student.setId(java.lang.String);通过执行setId就将_id的值赋值给了student中的id属性,为什么_id对应的fieldInfo是id呢?

    • com.alibaba.fastjson.parser.deserializer.JavaBeanDeserializer#smartMatch(java.lang.String, int[])
      通过json中的key和int[]获取当前key的FieldDeserializer,FieldDeserializer中有个属性fieldInfo,这个属性就决定了json中key对应的值赋值给对象中的那个属性.
    public FieldDeserializer smartMatch(String key, int[] setFlags) {
        if (key == null) {
            return null;
        }
        
        FieldDeserializer fieldDeserializer = getFieldDeserializer(key, setFlags);

        if (fieldDeserializer == null) {
            if (this.smartMatchHashArray == null) {
                long[] hashArray = new long[sortedFieldDeserializers.length];
                for (int i = 0; i < sortedFieldDeserializers.length; i++) {
                    hashArray[i] = sortedFieldDeserializers[i].fieldInfo.nameHashCode;
                }
                Arrays.sort(hashArray);
                this.smartMatchHashArray = hashArray;
            }

            // smartMatchHashArrayMapping
            long smartKeyHash = TypeUtils.fnv1a_64_lower(key);
            int pos = Arrays.binarySearch(smartMatchHashArray, smartKeyHash);
            if (pos < 0) {
                long smartKeyHash1 = TypeUtils.fnv1a_64_extract(key);
                pos = Arrays.binarySearch(smartMatchHashArray, smartKeyHash1);
            }

            boolean is = false;
            if (pos < 0 && (is = key.startsWith("is"))) {
                smartKeyHash = TypeUtils.fnv1a_64_extract(key.substring(2));
                pos = Arrays.binarySearch(smartMatchHashArray, smartKeyHash);
            }

            if (pos >= 0) {
                if (smartMatchHashArrayMapping == null) {
                    short[] mapping = new short[smartMatchHashArray.length];
                    Arrays.fill(mapping, (short) -1);
                    for (int i = 0; i < sortedFieldDeserializers.length; i++) {
                        int p = Arrays.binarySearch(smartMatchHashArray, sortedFieldDeserializers[i].fieldInfo.nameHashCode);
                        if (p >= 0) {
                            mapping[p] = (short) i;
                        }
                    }
                    smartMatchHashArrayMapping = mapping;
                }

                int deserIndex = smartMatchHashArrayMapping[pos];
                if (deserIndex != -1) {
                    if (!isSetFlag(deserIndex, setFlags)) {
                        fieldDeserializer = sortedFieldDeserializers[deserIndex];
                    }
                }
            }

            if (fieldDeserializer != null) {
                FieldInfo fieldInfo = fieldDeserializer.fieldInfo;
                if ((fieldInfo.parserFeatures & Feature.DisableFieldSmartMatch.mask) != 0) {
                    return null;
                }

                Class fieldClass = fieldInfo.fieldClass;
                if (is && (fieldClass != boolean.class && fieldClass != Boolean.class)) {
                    fieldDeserializer = null;
                }
            }
        }


        return fieldDeserializer;
    }

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    匹配哈希数组映射

                long smartKeyHash = TypeUtils.fnv1a_64_lower(key);
            int pos = Arrays.binarySearch(smartMatchHashArray, smartKeyHash);
            if (pos < 0) {
                long smartKeyHash1 = TypeUtils.fnv1a_64_extract(key);
                pos = Arrays.binarySearch(smartMatchHashArray, smartKeyHash1);
            }

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    先调用TypeUtils.fnv1a_64_lower参数传入key也就是_id获取smartKeyHash ,然后检查smartKeyHash是否存在,也就是当前key再对象中是否有对应的属性;_id肯定是没有的,会进入到if逻辑里面,然后调用TypeUtils.fnv1a_64_extract参数传入的还是key(_id)获取smartKeyHash1,然后检查smartKeyHash1是否是否存在,也就是当前key再对象中是否有对应的属性;这次却能通过检查,主要原因就是因为TypeUtils中的两个方法

    • com.alibaba.fastjson.util.TypeUtils#fnv1a_64_lower
        public static long fnv1a_64_lower(String key) {
        long hashCode = fnv1a_64_magic_hashcode;
        for (int i = 0; i < key.length(); ++i) {
            char ch = key.charAt(i);
            if (ch >= 'A' && ch <= 'Z') {
                ch = (char) (ch + 32);
            }
            hashCode ^= ch;
            hashCode *= fnv1a_64_magic_prime;
        }
        return hashCode;
    }

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    • com.alibaba.fastjson.util.TypeUtils#fnv1a_64_extract
        public static long fnv1a_64_extract(String key) {
        long hashCode = fnv1a_64_magic_hashcode;
        for (int i = 0; i < key.length(); ++i) {
            char ch = key.charAt(i);
            if (ch == '_' || ch == '-') {
                continue;
            }
            if (ch >= 'A' && ch <= 'Z') {
                ch = (char) (ch + 32);
            }
            hashCode ^= ch;
            hashCode *= fnv1a_64_magic_prime;
        }
        return hashCode;
    }

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    上面两个方法相似度是比较高的,唯一不同的是fnv1a_64_extract再对key循环的过程中过滤掉了_和-字符,这样的话相当于将_id变成了id所以返回的FieldDeserializer是id对应的FieldDeserializer,所以到最后会将json中_id的值赋值给对象中的id
    等到json中真正的id返回的就是null而不是id对应的FieldDeserializer,因为setFlags的int数组,当我们对一个属性进行赋值以后,会把setFlags数组中这个属性对应的位置设置为1(之前是0),所以等到真正的id进来就不会再进行赋值了,为了证明这里直接使用代码验证一下

        public static void main(String[] args) {
        // 赋值时_id先进去赋值给student的id,id不会赋值
        String json = "{\"_id\":\"1\",\"id\":\"2\"}";
        Student student = JSON.parseObject(json, Student.class);
        // 1
        System.out.println(student.getId());
        // 赋值时id先进去赋值给student的id,_id不会赋值
        String json1 = "{\"id\":\"2\",\"_id\":\"1\"}";
        Student student1 = JSON.parseObject(json1, Student.class);
        // 2
        System.out.println(student1.getId());
    }

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    解决方法

    添加Feature.DisableFieldSmartMatch参数

        public static void main(String[] args) {
        String json = "{\"_id\":\"1\",\"id\":\"2\"}";
        Student student = JSON.parseObject(json, Student.class, Feature.DisableFieldSmartMatch);
        // 2
        System.out.println(student.getId());
    }

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    使用这种方式将不会使用com.alibaba.fastjson.parser.deserializer.JavaBeanDeserializer#smartMatch(java.lang.String, int[])来获取key对应的FieldDeserializer,转而会使用com.alibaba.fastjson.parser.deserializer.JavaBeanDeserializer#getFieldDeserializer(java.lang.String)来获取key对应的FieldDeserializer,有兴趣的可以自己了解下

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  • 原文地址：https://blog.csdn.net/qq_43135259/article/details/127617916