遇到一件怪事，今天在测试一段代码时老时报NullPointException

代码：

---  ObjectUtil.java  
    
        JdOrder jdOrder = new JdOrder();
        jdOrder.setSkuId(132323324l);
        jdOrder.setCommissionRate(BigDecimal.valueOf(0.53));
        jdOrder.setEstimateCosPrice(BigDecimal.valueOf(0.43));
        ShopguideOrder order = new ShopguideOrder();

        BeanUtils.copyProperties(jdOrder,order);

Exception in thread "main" java.lang.NullPointerException
    at org.springframework.core.BridgeMethodResolver.findBridgedMethod(BridgeMethodResolver.java:69)
    at org.springframework.beans.GenericTypeAwarePropertyDescriptor.<init>(GenericTypeAwarePropertyDescriptor.java:71)
    at org.springframework.beans.CachedIntrospectionResults.buildGenericTypeAwarePropertyDescriptor(CachedIntrospectionResults.java:353)
    at org.springframework.beans.CachedIntrospectionResults.<init>(CachedIntrospectionResults.java:307)
    at org.springframework.beans.CachedIntrospectionResults.forClass(CachedIntrospectionResults.java:188)
    at org.springframework.beans.BeanUtils.getPropertyDescriptors(BeanUtils.java:360)
    at org.springframework.beans.BeanUtils.copyProperties(BeanUtils.java:608)
    at org.springframework.beans.BeanUtils.copyProperties(BeanUtils.java:543)
    at com.davidhu.shopguide.api.utils.ObjectUtil.main(ObjectUtil.java:128)

看源代码看了不少时间，只时知道类里有一个PropertyDescriptor propertyName = class，然后这个属性没有writemethod，然后就报错。但这个方法是copy对象属性的常用方法，不应该有问题呀。然后我把代码拷贝到

ThirdApiTest里又跑了一下，奇怪，在这里又没问题了

google了一下，果然还是找到了

https://github.com/spring-projects/spring-framework/issues/24177

Issue:
Failure in BeanUtils.copyProperties(source, target)

Stack Trace:
Exception in thread "main" java.lang.NullPointerException at org.springframework.core.BridgeMethodResolver.findBridgedMethod(BridgeMethodResolver.java:69) at org.springframework.beans.GenericTypeAwarePropertyDescriptor.(GenericTypeAwarePropertyDescriptor.java:70) at org.springframework.beans.CachedIntrospectionResults.buildGenericTypeAwarePropertyDescriptor(CachedIntrospectionResults.java:359) at org.springframework.beans.CachedIntrospectionResults.(CachedIntrospectionResults.java:316) at org.springframework.beans.CachedIntrospectionResults.forClass(CachedIntrospectionResults.java:186) at org.springframework.beans.BeanUtils.getPropertyDescriptors(BeanUtils.java:360) at org.springframework.beans.BeanUtils.copyProperties(BeanUtils.java:608) at org.springframework.beans.BeanUtils.copyProperties(BeanUtils.java:579)

Initial Analysis:

findBridgedMethod(Method bridgeMethod) method of BridgeMethodResolver was refactored as part of Spring 5.0 release.
Whenever this method is invoked for getting writeMethodToUse for propertyName = class, it results in Null Pointer exception as isBridge() method of bridgeMethod is invoked which is null.

回答：

 
Contributor
jhoeller commented on Dec 10, 2019
It seems that you're running spring-beans-4.3.x against spring-core-5.x.x there. We don't support that, the Spring Framework module versions need to be in sync. Please make sure that your classpath is consistent in that respect.

spring-beans-5.x.x has corresponding non-null guards at that position so bypasses the BridgeMethodResolver call upfront.

焕然大悟，原来在ObjectUtil.java用到的spring-beans-4.0.9，而运行在ThirdApiTest 里跑的是spring-beans-5.3.1,因为ObjectUtil.java所在工程是api，而ThirdApiTest所在工程是admin。

那为什么api用的是spring-beans-4.0.9？在api下面有个idea生成的文件api.iml,上面有

    <orderEntry type="library" name="Maven: org.springframework:spring-beans:4.0.9.RELEASE" level="project" />

删掉重新生成，还是依赖了4.0.9包。分析了api的pom.xml,发现这个spring是依赖于swagger的，因为api没有单独引入spring的任何jar包。到此，真相大白了。