这个demo来说明怎么排查一个@Transactional引起的NullPointerException。
https://github.com/hengyunabc/spring-boot-inside/tree/master/demo-Transactional-NullPointerException
Demo是一个简单的spring事务例子,提供了下面一个StudentDao,并用@Transactional来声明事务:
@Component @Transactional public class StudentDao { @Autowired private SqlSession sqlSession; public Student selectStudentById(long id) { return sqlSession.selectOne("selectStudentById", id); } public final Student finalSelectStudentById(long id) { return sqlSession.selectOne("selectStudentById", id); } }
应用启动后,会依次调用selectStudentById和finalSelectStudentById:
@PostConstruct public void init() { studentDao.selectStudentById(1); studentDao.finalSelectStudentById(1); }
用mvn spring-boot:run 或者把工程导入IDE里启动,抛出来的异常信息是:
Caused by: java.lang.NullPointerException at sample.mybatis.dao.StudentDao.finalSelectStudentById(StudentDao.java:27) at com.example.demo.transactional.nullpointerexception.DemoNullPointerExceptionApplication.init(DemoNullPointerExceptionApplication.java:30) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor$LifecycleElement.invoke(InitDestroyAnnotationBeanPostProcessor.java:366) at org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor$LifecycleMetadata.invokeInitMethods(InitDestroyAnnotationBeanPostProcessor.java:311)
为什么应用代码里执行selectStudentById没有问题,而执行finalSelectStudentById就抛出NullPointerException?
同一个bean里,明明SqlSession sqlSession已经被注入了,在selectStudentById里它是非null的。为什么finalSelectStudentById函数里是null?
当然,我们对比两个函数,可以知道是因为finalSelectStudentById的修饰符是final。但是具体原因是什么呢?
我们先在抛出异常的地方打上断点,调试代码,获取到具体运行时的class是什么:
System.err.println(studentDao.getClass());
打印的结果是:
class sample.mybatis.dao.StudentDao$$EnhancerBySpringCGLIB$$210b005d
可以看出是一个被spring aop处理过的类,但是它的具体字节码内容是什么呢?
我们使用dumpclass工具来把jvm里的类dump出来:
https://github.com/hengyunabc/dumpclass
wget http://search.maven.org/remotecontent?filepath=io/github/hengyunabc/dumpclass/0.0.1/dumpclass-0.0.1.jar -O dumpclass.jar
找到java进程pid:
$ jps 5907 DemoNullPointerExceptionApplication
把相关的类都dump下来:
sudo java -jar dumpclass.jar 5907 'sample.mybatis.dao.StudentDao*' /tmp/dumpresult
用javap或者图形化工具jd-gui来反编绎sample.mybatis.dao.StudentDao$$EnhancerBySpringCGLIB$$210b005d。
反编绎后的结果是:
public final Student selectStudentById(long paramLong) { try { MethodInterceptor tmp4_1 = this.CGLIB$CALLBACK_0; if (tmp4_1 == null) { tmp4_1; CGLIB$BIND_CALLBACKS(this); } MethodInterceptor tmp17_14 = this.CGLIB$CALLBACK_0; if (tmp17_14 != null) { Object[] tmp29_26 = new Object[1]; Long tmp35_32 = new java/lang/Long; Long tmp36_35 = tmp35_32; tmp36_35; tmp36_35.<init>(paramLong); tmp29_26[0] = tmp35_32; return (Student)tmp17_14.intercept(this, CGLIB$selectStudentById$0$Method, tmp29_26, CGLIB$selectStudentById$0$Proxy); } return super.selectStudentById(paramLong); } catch (RuntimeException|Error localRuntimeException) { throw localRuntimeException; } catch (Throwable localThrowable) { throw new UndeclaredThrowableException(localThrowable); } }
再来实际debug,尽管StudentDao$$EnhancerBySpringCGLIB$$210b005d的代码不能直接看到,但是还是可以单步执行的。
在debug时,可以看到
1. StudentDao$$EnhancerBySpringCGLIB$$210b005d里的所有field都是null
2. this.CGLIB$CALLBACK_0的实际类型是CglibAopProxy$DynamicAdvisedInterceptor,在这个Interceptor里实际保存了原始的target对象
3. CglibAopProxy$DynamicAdvisedInterceptor在经过TransactionInterceptor处理之后,最终会用反射调用自己保存的原始target对象
所以整理下整个分析:
那么为什么finalSelectStudentById函数里的SqlSession sqlSession会是null,然后抛出NullPointerException?