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可以用dump 劳务派遣管理系统 class工具来dump

点击: 次  来源:劳务派遣管理系统 时间:2018-01-12

原文出处: hengyunabc

写在前面

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

定位 NullPointerException 的代码

Demo是一个简朴的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,会依次挪用selectStudentById和finalSelectStudentById:

    @PostConstruct
    public void init() {
        studentDao.selectStudentById(1);
        studentDao.finalSelectStudentById(1);
    }

用mvn spring-boot:run 可能把工程导入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,显着SqlSession sqlSession已经被注入了,在selectStudentById里它长短null的。为什么finalSelectStudentById函数里是null?

获取实际运行时的类名

虽然,我们比拟两个函数,可以知道是因为finalSelectStudentById的修饰符是final。可是详细原因是什么呢?

我们先在抛出异常的处所打上断点,调试代码,获取到详细运行时的class是什么:

System.err.println(studentDao.getClass());

打印的功效是:

class sample.mybatis.dao.StudentDao$$EnhancerBySpringCGLIB$$210b005d

可以看出是一个被spring aop处理惩罚过的类,可是它的详细字节码内容是什么呢?

dumpclass阐明

我们利用dumpclass东西来把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历程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

反汇编阐明

用javap可能图形化东西jd-gui来反编绎sample.mybatis.dao.StudentDao$$EnhancerBySpringCGLIB$$210b005d。

反编绎后的功效是:

  1. class StudentDao$$EnhancerBySpringCGLIB$$210b005d extends StudentDao
  2. StudentDao$$EnhancerBySpringCGLIB$$210b005d里没有finalSelectStudentById相关的内容
  3. selectStudentById实际挪用的是this.CGLIB$CALLBACK_0,即MethodInterceptor tmp4_1,等下我们实际debug,看详细的范例
  public final Student selectStudentById(long paramLong)
  {
    try
    {
      MethodInterceptor tmp4_1 = this.CGLIB$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,尽量StudentDao$$EnhancerBySpringCGLIB$$210b005d的代码不能直接看到,可是照旧可以单步执行的。

在debug时,昆山软件开发,可以看到