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寫在前面
這個(gè)demo來說明怎么排查一個(gè)@Transactional引起的NullPointerException。
https://github.com/hengyunabc/spring-boot-inside/tree/master/demo-Transactional-NullPointerException
定位 NullPointerException 的代碼
Demo是一個(gè)簡單的spring事務(wù)例子,提供了下面一個(gè)StudentDao,并用@Transactional來聲明事務(wù):
@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); } }
應(yīng)用啟動(dòng)后,會(huì)依次調(diào)用selectStudentById和finalSelectStudentById:
@PostConstruct public void init() { studentDao.selectStudentById(1); studentDao.finalSelectStudentById(1); }
用mvn spring-boot:run 或者把工程導(dǎo)入IDE里啟動(dòng),拋出來的異常信息是:
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)
為什么應(yīng)用代碼里執(zhí)行selectStudentById沒有問題,而執(zhí)行finalSelectStudentById就拋出NullPointerException?
同一個(gè)bean里,明明SqlSession sqlSession已經(jīng)被注入了,在selectStudentById里它是非null的。為什么finalSelectStudentById函數(shù)里是null?
獲取實(shí)際運(yùn)行時(shí)的類名
當(dāng)然,我們對(duì)比兩個(gè)函數(shù),可以知道是因?yàn)閒inalSelectStudentById的修飾符是final。但是具體原因是什么呢?
我們先在拋出異常的地方打上斷點(diǎn),調(diào)試代碼,獲取到具體運(yùn)行時(shí)的class是什么:
System.err.println(studentDao.getClass());
打印的結(jié)果是:
class sample.mybatis.dao.StudentDao$$EnhancerBySpringCGLIB$$210b005d
可以看出是一個(gè)被spring aop處理過的類,但是它的具體字節(jié)碼內(nèi)容是什么呢?
dumpclass分析
我們使用dumpclass工具來把jvm里的類dump出來:
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j(luò)ava進(jìn)程pid:
$ jps 5907 DemoNullPointerExceptionApplication
把相關(guān)的類都dump下來:
sudo java -jar dumpclass.jar 5907 'sample.mybatis.dao.StudentDao*' /tmp/dumpresult
反匯編分析
用javap或者圖形化工具jd-gui來反編繹sample.mybatis.dao.StudentDao$$EnhancerBySpringCGLIB$$210b005d。
反編繹后的結(jié)果是:
class StudentDao$$EnhancerBySpringCGLIB$$210b005d extends StudentDao
StudentDao$$EnhancerBySpringCGLIB$$210b005d
里沒有finalSelectStudentById相關(guān)的內(nèi)容
selectStudentById實(shí)際調(diào)用的是this.CGLIB$CALLBACK_0,即MethodInterceptor tmp4_1,等下我們實(shí)際debug,看具體的類型
public final Student selectStudentById(long paramLong) { try { MethodInterceptor tmp4_1 = this.CGLIB$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); } }
再來實(shí)際debug,盡管StudentDao$$EnhancerBySpringCGLIB$$210b005d的代碼不能直接看到,但是還是可以單步執(zhí)行的。
在debug時(shí),可以看到
1. StudentDao$$EnhancerBySpringCGLIB$$210b005d里的所有field都是null
2. this.CGLIB$CALLBACK_0的實(shí)際類型是CglibAopProxy$DynamicAdvisedInterceptor,在這個(gè)Interceptor里實(shí)際保存了原始的target對(duì)象
3. CglibAopProxy$DynamicAdvisedInterceptor在經(jīng)過TransactionInterceptor處理之后,最終會(huì)用反射調(diào)用自己保存的原始target對(duì)象
拋出異常的原因
所以整理下整個(gè)分析:
1.在使用了@Transactional之后,spring aop會(huì)生成一個(gè)cglib代理類,實(shí)際用戶代碼里@Autowired注入的StudentDao也是這個(gè)代理類的實(shí)例
2.cglib生成的代理類StudentDao$$EnhancerBySpringCGLIB$$210b005d繼承自StudentDao
3.StudentDao$$EnhancerBySpringCGLIB$$210b005d里的所有field都是null
4.StudentDao$$EnhancerBySpringCGLIB$$210b005d在調(diào)用selectStudentById,實(shí)際上通過CglibAopProxy$DynamicAdvisedInterceptor,最終會(huì)用反射調(diào)用自己保存的原始target對(duì)象
5.所以selectStudentById函數(shù)的調(diào)用沒有問題
那么為什么finalSelectStudentById函數(shù)里的SqlSession sqlSession會(huì)是null,然后拋出NullPointerException?
1.StudentDao$$EnhancerBySpringCGLIB$$210b005d里的所有field都是null
2.finalSelectStudentById函數(shù)的修飾符是final,cglib沒有辦法重寫這個(gè)函數(shù)
3.當(dāng)執(zhí)行到finalSelectStudentById里,實(shí)際執(zhí)行的是原始的StudentDao里的代碼
4.但是對(duì)象是StudentDao$$EnhancerBySpringCGLIB$$210b005d的實(shí)例,它里面的所有field都是null,所以會(huì)拋出NullPointerException
解決問題辦法
1.最簡單的當(dāng)然是把finalSelectStudentById函數(shù)的final修飾符去掉
2.還有一種辦法,在StudentDao里不要直接使用sqlSession,而通過getSqlSession()函數(shù),這樣cglib也會(huì)處理getSqlSession(),返回原始的target對(duì)象
總結(jié)
1.排查問題多debug,看實(shí)際運(yùn)行時(shí)的對(duì)象信息
2。對(duì)于cglib生成類的字節(jié)碼,可以用dumpclass工具來dump,再反編繹分析
總結(jié)
以上所述是小編給大家介紹的深入學(xué)習(xí)Spring Boot排查 @Transactional 引起的 NullPointerException問題,希望對(duì)大家有所幫助,如果大家有任何疑問請(qǐng)給我留言,小編會(huì)及時(shí)回復(fù)大家的。在此也非常感謝大家對(duì)億速云網(wǎng)站的支持!
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