巨坑系列：Java Bean 转 Map 的坑要注意！

点击关注 ? Java面试那些事儿 文章源自JAVA秀-https://www.javaxiu.com/71239.html

收录于合集#Java面试那些事儿438个文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

来源：juejin.cn/post/7118073840999071751文章源自JAVA秀-https://www.javaxiu.com/71239.html

# 一、背景文章源自JAVA秀-https://www.javaxiu.com/71239.html

有些业务场景下需要将 Java Bean 转成 Map 再使用。文章源自JAVA秀-https://www.javaxiu.com/71239.html

本以为很简单场景，但是坑很多。文章源自JAVA秀-https://www.javaxiu.com/71239.html

# 二、那些坑文章源自JAVA秀-https://www.javaxiu.com/71239.html

2.0 测试对象文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

import lombok.Data;import java.util.Date;@Datapublic class MockObject extends  MockParent{    private Integer aInteger;    private Long aLong;    private Double aDouble;private Date aDate;}

文章源自JAVA秀-https://www.javaxiu.com/71239.html

父类文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

import lombok.Data;@DatapublicclassMockParent{privateLong parent;}

文末福利文章源自JAVA秀-https://www.javaxiu.com/71239.html

文末领取：651页Java面试题库文章源自JAVA秀-https://www.javaxiu.com/71239.html

2.1 JSON 反序列化了类型丢失文章源自JAVA秀-https://www.javaxiu.com/71239.html

2.1.1 问题复现文章源自JAVA秀-https://www.javaxiu.com/71239.html

将 Java Bean 转 Map 最常见的手段就是使用 JSON 框架，如 fastjson 、 gson、jackson 等。但使用 JSON 将 Java Bean 转 Map 会导致部分数据类型丢失。如使用 fastjson ，当属性为 Long 类型但数字小于 Integer 最大值时，反序列成 Map 之后，将变为 Integer 类型。文章源自JAVA秀-https://www.javaxiu.com/71239.html

maven 依赖：文章源自JAVA秀-https://www.javaxiu.com/71239.html

<!-- https://mvnrepository.com/artifact/com.alibaba/fastjson --><dependency><groupId>com.alibaba</groupId><artifactId>fastjson</artifactId><version>2.0.8</version></dependency>

文章源自JAVA秀-https://www.javaxiu.com/71239.html

示例代码：文章源自JAVA秀-https://www.javaxiu.com/71239.html

import com.alibaba.fastjson.JSON;import com.alibaba.fastjson.TypeReference;import java.util.Date;import java.util.Map;public class JsonDemo { public staticvoid main(String[] args) { MockObject mockObject = new MockObject(); mockObject.setAInteger(1); mockObject.setALong(2L); mockObject.setADate(newDate()); mockObject.setADouble(3.4D); mockObject.setParent(3L);       String json = JSON.toJSONString(mockObject);       Map<String,Object> map = JSON.parseObject(json, new TypeReference<Map<String,Object>>(){}); System.out.println(map); }}

文章源自JAVA秀-https://www.javaxiu.com/71239.html

结果打印：文章源自JAVA秀-https://www.javaxiu.com/71239.html

{"parent":3,"ADouble":3.4,"ALong":2,"AInteger":1,"ADate":1657299916477}

文章源自JAVA秀-https://www.javaxiu.com/71239.html

调试截图：文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

通过 Java Visualizer 插件进行可视化查看：文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

2.2.2 问题描述文章源自JAVA秀-https://www.javaxiu.com/71239.html

存在两个问题 文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

（1） 通过 fastjson 将 Java Bean 转为 Map ，类型会发生转变。如 Long 变成 Integer ，Date 变成 Long, Double 变成 Decimal 类型等。文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

（2）在某些场景下，Map 的 key 并非和属性名完全对应，像是通过 get set 方法“推断”出来的属性名。文章源自JAVA秀-https://www.javaxiu.com/71239.html

2.2 BeanMap 转换属性名错误文章源自JAVA秀-https://www.javaxiu.com/71239.html

2.2.1 commons-beanutils 的 BeanMap文章源自JAVA秀-https://www.javaxiu.com/71239.html

maven 版本：文章源自JAVA秀-https://www.javaxiu.com/71239.html

<!-- https://mvnrepository.com/artifact/commons-beanutils/commons-beanutils --><dependency><groupId>commons-beanutils</groupId><artifactId>commons-beanutils</artifactId><version>1.9.4</version></dependency>

文章源自JAVA秀-https://www.javaxiu.com/71239.html

代码示例：文章源自JAVA秀-https://www.javaxiu.com/71239.html

import org.apache.commons.beanutils.BeanMap;import third.fastjson.MockObject;import java.util.Date;publicclassBeanUtilsDemo {publicstaticvoidmain(String[] args){ MockObject mockObject = new MockObject(); mockObject.setAInteger(1); mockObject.setALong(2L); mockObject.setADate(new Date()); mockObject.setADouble(3.4D); mockObject.setParent(3L); BeanMap beanMap = new BeanMap(mockObject); System.out.println(beanMap); }}

文章源自JAVA秀-https://www.javaxiu.com/71239.html

调试截图：文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

存在和 cglib 一样的问题，虽然类型没问题但是属性名还是不对。文章源自JAVA秀-https://www.javaxiu.com/71239.html

原因分析：文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

/** * Constructs a new <code>BeanMap</code> that operates on the * specified bean. If the given bean is <code>null</code>, then * this map will be empty. * * @param bean the bean for this map to operate on */publicBeanMap(final Object bean){this.bean = bean; initialise(); }

文章源自JAVA秀-https://www.javaxiu.com/71239.html

关键代码：文章源自JAVA秀-https://www.javaxiu.com/71239.html

privatevoidinitialise(){if(getBean() == null) {return; }final Class<? extends Object> beanClass = getBean().getClass();try {//BeanInfo beanInfo = Introspector.getBeanInfo( bean, null );final BeanInfo beanInfo = Introspector.getBeanInfo( beanClass );final PropertyDescriptor[] propertyDescriptors = beanInfo.getPropertyDescriptors();if ( propertyDescriptors != null ) {for (final PropertyDescriptor propertyDescriptor : propertyDescriptors) {if ( propertyDescriptor != null ) {final String name = propertyDescriptor.getName();final Method readMethod = propertyDescriptor.getReadMethod();final Method writeMethod = propertyDescriptor.getWriteMethod();final Class<? extends Object> aType = propertyDescriptor.getPropertyType();if ( readMethod != null ) { readMethods.put( name, readMethod ); }if ( writeMethod != null ) { writeMethods.put( name, writeMethod ); } types.put( name, aType ); } } } }catch ( final IntrospectionException e ) { logWarn( e ); } }

文章源自JAVA秀-https://www.javaxiu.com/71239.html

调试一下就会发现，问题出在 BeanInfo 里面 PropertyDescriptor 的 name 不正确。文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

经过分析会发现 java.beans.Introspector#getTargetPropertyInfo 方法是字段解析的关键文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

对于无参的以 get 开头的方法名从 index =3 处截取，如 getALong 截取后为 ALong， 如 getADouble 截取后为 ADouble。文章源自JAVA秀-https://www.javaxiu.com/71239.html

然后去构造 PropertyDescriptor:文章源自JAVA秀-https://www.javaxiu.com/71239.html

/** * Creates <code>PropertyDescriptor</code> for the specified bean * with the specified name and methods to read/write the property value. * * @param bean the type of the target bean * @param base the base name of the property (the rest of the method name) * @param read the method used for reading the property value * @param write the method used for writing the property value * @exception IntrospectionException if an exception occurs during introspection * * @since 1.7 */ PropertyDescriptor(Class<?> bean, Stringbase, Methodread, Methodwrite) throwsIntrospectionException{if (bean == null) {thrownew IntrospectionException("Target Bean class is null"); } setClass0(bean); setName(Introspector.decapitalize(base)); setReadMethod(read); setWriteMethod(write); this.baseName = base; }

文章源自JAVA秀-https://www.javaxiu.com/71239.html

底层使用 java.beans.Introspector#decapitalize 进行解析：文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

/** * Utility method to take a string and convert it to normal Java variable * name capitalization. This normally means converting the first * character from upper case to lower case, but in the (unusual) special * case when there is more than one character and both the first and * second characters are upper case, we leave it alone. * <p> * Thus "FooBah" becomes "fooBah" and "X" becomes "x", but "URL" stays * as "URL". * * @param name The string to be decapitalized. * @return The decapitalized version of the string. */publicstaticString decapitalize(String name) {if (name == null || name.length() == 0) {return name; }if (name.length() > 1 && Character.isUpperCase(name.charAt(1)) && Character.isUpperCase(name.charAt(0))){return name; } char chars[] = name.toCharArray(); chars[0] = Character.toLowerCase(chars[0]);returnnewString(chars); }

文章源自JAVA秀-https://www.javaxiu.com/71239.html

从代码中我们可以看出 (1) 当 name 的长度 > 1，且第一个字符和第二个字符都大写时，直接返回参数作为PropertyDescriptor name。(2) 否则将 name 转为首字母小写文章源自JAVA秀-https://www.javaxiu.com/71239.html

这种处理本意是为了不让属性为类似 URL 这种缩略词转为 uRL ，结果“误伤”了我们这种场景。文章源自JAVA秀-https://www.javaxiu.com/71239.html

2.2.2 使用 cglib 的 BeanMap文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

cglib 依赖文章源自JAVA秀-https://www.javaxiu.com/71239.html

<!-- https://mvnrepository.com/artifact/cglib/cglib --><dependency><groupId>cglib</groupId><artifactId>cglib-nodep</artifactId><version>3.2.12</version></dependency>

文章源自JAVA秀-https://www.javaxiu.com/71239.html

代码示例：文章源自JAVA秀-https://www.javaxiu.com/71239.html

import net.sf.cglib.beans.BeanMap;import third.fastjson.MockObject;import java.util.Date;publicclassBeanMapDemo {publicstaticvoidmain(String[] args){ MockObject mockObject = new MockObject(); mockObject.setAInteger(1); mockObject.setALong(2L); mockObject.setADate(new Date()); mockObject.setADouble(3.4D); mockObject.setParent(3L); BeanMap beanMapp = BeanMap.create(mockObject); System.out.println(beanMapp); }}

结果展示：文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

我们发现类型对了，但是属性名依然不对。文章源自JAVA秀-https://www.javaxiu.com/71239.html

关键代码：net.sf.cglib.core.ReflectUtils#getBeanGetters 底层也会用到 java.beans.Introspector#decapitalize 所以属性名存在一样的问题就不足为奇了。文章源自JAVA秀-https://www.javaxiu.com/71239.html

# 三、解决办法文章源自JAVA秀-https://www.javaxiu.com/71239.html

3.1 解决方案文章源自JAVA秀-https://www.javaxiu.com/71239.html

解决方案有很多，本文提供一个基于 dubbo的解决方案。文章源自JAVA秀-https://www.javaxiu.com/71239.html

maven 依赖：文章源自JAVA秀-https://www.javaxiu.com/71239.html

<!-- https://mvnrepository.com/artifact/org.apache.dubbo/dubbo --><dependency><groupId>org.apache.dubbo</groupId><artifactId>dubbo</artifactId><version>3.0.9</version></dependency>

文章源自JAVA秀-https://www.javaxiu.com/71239.html

示例代码：文章源自JAVA秀-https://www.javaxiu.com/71239.html

import org.apache.dubbo.common.utils.PojoUtils;import third.fastjson.MockObject;import java.util.Date;publicclass DubboPojoDemo {publicstaticvoid main(String[] args) { MockObject mockObject = new MockObject(); mockObject.setAInteger(1); mockObject.setALong(2L); mockObject.setADate(newDate()); mockObject.setADouble(3.4D); mockObject.setParent(3L);Object generalize = PojoUtils.generalize(mockObject); System.out.println(generalize); }}

调试效果：文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

Java Visualizer 效果：文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

3.2 原理解析文章源自JAVA秀-https://www.javaxiu.com/71239.html

大家可以下载源码来简单研究下：GitHub - apache/dubbo: Apache Dubbo is a high-performance, java based, open source RPC framework.文章源自JAVA秀-https://www.javaxiu.com/71239.html

核心代码：org.apache.dubbo.common.utils.PojoUtils#generalize(java.lang.Object)文章源自JAVA秀-https://www.javaxiu.com/71239.html

publicstatic Object generalize(Object pojo){eturn generalize(pojo, new IdentityHashMap());}

关键代码：文章源自JAVA秀-https://www.javaxiu.com/71239.html

// pojo 待转换的对象// history 缓存 Map，提高性能 private staticObject generalize(Object pojo, Map<Object, Object> history) {if (pojo == null) {returnnull; }// 枚举直接返回枚举名if (pojo instanceof Enum<?>) {return ((Enum<?>) pojo).name(); }// 枚举数组，返回枚举名数组if (pojo.getClass().isArray() && Enum.class.isAssignableFrom(pojo.getClass().getComponentType())) { int len = Array.getLength(pojo);String[] values = newString[len];for (int i = 0; i < len; i++) { values[i] = ((Enum<?>) Array.get(pojo, i)).name(); }return values; }// 基本类型返回 pojo 自身if (ReflectUtils.isPrimitives(pojo.getClass())) {return pojo; }// Class 返回 nameif (pojo instanceof Class) {return ((Class) pojo).getName(); }Object o = history.get(pojo);if (o != null) {return o; } history.put(pojo, pojo);// 数组类型，递归if (pojo.getClass().isArray()) { int len = Array.getLength(pojo);Object[] dest = newObject[len]; history.put(pojo, dest);for (int i = 0; i < len; i++) {Object obj = Array.get(pojo, i); dest[i] = generalize(obj, history); }return dest; }// 集合类型递归if (pojo instanceof Collection<?>) { Collection<Object> src = (Collection<Object>) pojo; int len = src.size(); Collection<Object> dest = (pojo instanceof List<?>) ? new ArrayList<Object>(len) : new HashSet<Object>(len); history.put(pojo, dest);for (Object obj : src) { dest.add(generalize(obj, history)); }return dest; }// Map 类型，直接 对 key 和 value 处理if (pojo instanceofMap<?, ?>) {Map<Object, Object> src = (Map<Object, Object>) pojo;Map<Object, Object> dest = createMap(src); history.put(pojo, dest);for (Map.Entry<Object, Object> obj : src.entrySet()) { dest.put(generalize(obj.getKey(), history), generalize(obj.getValue(), history)); }return dest; }Map<String, Object> map = new HashMap<String, Object>(); history.put(pojo, map);// 开启生成 class 则写入 pojo 的classif (GENERIC_WITH_CLZ) { map.put("class", pojo.getClass().getName()); }// 处理 get 方法 for (Method method : pojo.getClass().getMethods()) {if (ReflectUtils.isBeanPropertyReadMethod(method)) { ReflectUtils.makeAccessible(method);try { map.put(ReflectUtils.getPropertyNameFromBeanReadMethod(method), generalize(method.invoke(pojo), history)); } catch (Exception e) {thrownew RuntimeException(e.getMessage(), e); } } }// 处理公有属性for (Field field : pojo.getClass().getFields()) {if (ReflectUtils.isPublicInstanceField(field)) {try {Object fieldValue = field.get(pojo);// 对象已经解析过，直接从缓存里读提高性能if (history.containsKey(pojo)) {Object pojoGeneralizedValue = history.get(pojo);// 已经解析过该属性则跳过（如公有属性，且有 get 方法的情况）if (pojoGeneralizedValue instanceofMap && ((Map) pojoGeneralizedValue).containsKey(field.getName())) {continue; } }if (fieldValue != null) { map.put(field.getName(), generalize(fieldValue, history)); } } catch (Exception e) {thrownew RuntimeException(e.getMessage(), e); } } }return map; }

文章源自JAVA秀-https://www.javaxiu.com/71239.html

关键截图文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

org.apache.dubbo.common.utils.ReflectUtils#getPropertyNameFromBeanReadMethod文章源自JAVA秀-https://www.javaxiu.com/71239.html

public static String getPropertyNameFromBeanReadMethod(Method method) {if (isBeanPropertyReadMethod(method)) {// get 方法，则从 index =3 的字符小写 + 后面的字符串if (method.getName().startsWith("get")) {return method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4); }// is 开头方法， index =2 的字符小写 + 后面的字符串if (method.getName().startsWith("is")) {return method.getName().substring(2, 3).toLowerCase() + method.getName().substring(3); } }returnnull; }

因此， getALong 方法对应的属性名被解析为 aLong。文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

同时，这么处理也会存在问题。如当属性名叫 URL 时，转为 Map 后 key 就会被解析成 uRL。文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

从这里看出，当属性名比较特殊时也很容易出问题，但 dubbo 这个工具类更符合我们的预期。更多细节，大家可以根据 DEMO 自行调试学习。文章源自JAVA秀-https://www.javaxiu.com/71239.html

如果想严格和属性保持一致，可以使用反射获取属性名和属性值，加缓存机制提升解析的效率。文章源自JAVA秀-https://www.javaxiu.com/71239.html

# 四、总结文章源自JAVA秀-https://www.javaxiu.com/71239.html

Java Bean 转 Map 的坑很多，最常见的就是类型丢失和属性名解析错误的问题。大家在使用 JSON 框架和 Java Bean 转 Map 的框架时要特别小心。平时使用某些框架时，多写一些 DEMO 进行验证，多读源码，多调试，少趟坑。文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

文章源自JAVA秀-https://www.javaxiu.com/71239.html

技术交流群文章源自JAVA秀-https://www.javaxiu.com/71239.html

最后，D哥也建了一个技术群，主要探讨一些新的技术和开源项目值不值得去研究及IDEA使用的“骚操作”，有兴趣入群的同学，可长按扫描下方二维码，一定要备注：城市+昵称+技术方向，根据格式备注，可更快被通过且邀请进群。▲长按扫描

热门推荐：史上最全的 IDEA Debug 调试技巧（超详细案例）淦，为什么 "???" .length !== 3 ？？Ubuntu 最新版发布 ！手里的系统瞬间不香了…《最全Java面试题库》总共651页Java面试题！！！包含Java 集合、JVM、多线程、并发编程、设计模式、Spring全家桶、Java、MyBatis、ZooKeeper、Dubbo、Elasticsearch、Memcached、MongoDB、Redis、MySQL、RabbitMQ、Linux等面试题！??点击阅读原文领取！！！