2022-01-06

前言

    对绝大数Java开发者而言mybatis并不陌生，从经典的SSM(Spring，spring-mvc，mybatis)框架，到现在流行的Springboot，随处可见mybatis的身影。mybatis作为比较主流的orm框架，支持用户定制sql，灵活又方便，颇受开发者喜爱。我们在使用mybatis难免会遇到各种坑，其中SqlSession的线程安全性问题也总会遇到。

  SqlSession作为一个接口，其并没有线程安全性的问题，我们常说的线程安全问题是SqlSession的一个实现类DefaultSqlSession，mybatis的作者也对此类加以"Note that this class is not Thread-Safe"的注释。此外SqlSession还有两个实现类SqlSessionManager和SqlSessionTemplate，这两个实现类是线程安全的。

线程不安全的DefaultSqlSession

    我们都知道DefaultSqlSession是线程不安全的，也会有很多博主讲解"SqlSessionTemplate是如何保证DefaultSqlSession线程安全的"，但是DefaultSqlSession不安全的体现是什么？不安全产生的原因在哪？今天作者通过一个例子给读者演示下并发情况下DefaultSqlSession线程不安全的表现，以及源码追踪产生线程安全问题的源头。

样例代码如下：

配置文件mybatis-config.xml，简单配置保证能正常运行

<configuration>

    <!-- 自行配置db.properties -->

    <properties resource="db.properties"/>

    <environments default="test">

        <environment id="test">

            <transactionManager type="JDBC"></transactionManager>

            <dataSource type="POOLED">

                <property name="driver" value="${driverClassName}"/>

                <property name="url" value="${url}"/>

                <property name="username" value="${username}"/>

                <property name="password" value="${password}"/>

            </dataSource>

        </environment>

    </environments>

    <mappers>

        <!-- mapper映射文件和mapper接口在同一个路径 -->

    <mapper class="idin.sun.study.mapper.StudentMapper"/>

    </mappers>

</configuration>

mapper映射文件

<?xml version="1.0" encoding="UTF-8" ?>

<!DOCTYPE mapper

    PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"

    "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<resultMap id="baseResultMap" type="idin.sun.study.model.Student">

    <id property="stdId" column="std_id"/>

    <result property="stdName" column="std_name"/>

    <result property="stdBirth" column="std_birth"/>

    <result property="stdGrade" column="std_grade"/>

    <result property="stdSex" column="std_sex"/>

</resultMap>

<select id="getStudents" resultMap="baseResultMap">

    SELECT *

    FROM sm_student

</select>

</mapper>

mapper接口

public interface StudentMapper {

List<Student> getStudents();

}

实体类

@Getter

@Setter

@ToString

public class Student {

private Integer stdId;

private String stdName;

private Date stdBirth;

private int stdGrade;

private int stdSex;

}

测试类

public class MybatisApp {

private static final int COUNT = 10;

// 使用CountDownLatch 来模拟并发，并发量10个

private static CountDownLatch cdl = new CountDownLatch(COUNT);

private SqlSession sqlSession;

//初始工作，用于初始化sqlSession此处获得是DefaultSqlSession的实例

@Before

public void init() {

    ClassLoader loader = Thread.currentThread().getContextClassLoader();

    InputStream inputStream = loader.getResourceAsStream("mybatis-config.xml");

    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder()

                                                .build(inputStream);

    sqlSession = sqlSessionFactory.openSession();

}

@Test

public void testThreadSafe() throws InterruptedException {

    for (int i = 0; i < COUNT; i++) {

        new Thread(() -> {

            try {

                cdl.await();

            } catch (Exception e) {

                e.printStackTrace();

            }

            // 调用查询方法

            getStudents();

        }).start();

        cdl.countDown();

    }

    Thread.sleep(5000);

}



public void getStudents() {

    // 使用statementId方法调用

    String statementId = "idin.sun.study.mapper.StudentMapper.getStudents";

    List<Student> students = sqlSession.selectList(statementId);

}

}

运行testThreadSafe()方法，控制台输出如下内容

Exception in thread "Thread-8" Exception in thread "Thread-4" org.apache.ibatis.exceptions.PersistenceException:

Error querying database. Cause: java.lang.ClassCastException: org.apache.ibatis.executor.ExecutionPlaceholder cannot be cast to java.util.List

The error may exist in idin/sun/study/mapper/StudentMapper.xml

The error may involve idin.sun.study.mapper.StudentMapper.getStudents

The error occurred while executing a query

Cause: java.lang.ClassCastException: org.apache.ibatis.executor.ExecutionPlaceholder cannot be cast to java.util.List

at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30)

at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:150)

at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:141)

at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:136)

at app.MybatisApp.getStudents1(MybatisApp.java:133)

at app.MybatisApp.lambda $testThreadSafe$ 0(MybatisApp.java:99)

at java.lang.Thread.run(Thread.java:748)

Caused by: java.lang.ClassCastException: org.apache.ibatis.executor.ExecutionPlaceholder cannot be cast to java.util.List

at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:152)

at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:109)

at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:83)

at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:148)

... 5 more

org.apache.ibatis.exceptions.PersistenceException:

Error querying database. Cause: java.lang.ClassCastException: org.apache.ibatis.executor.ExecutionPlaceholder cannot be cast to java.util.List

The error may exist in idin/sun/study/mapper/StudentMapper.xml

The error may involve idin.sun.study.mapper.StudentMapper.getStudents

The error occurred while executing a query

Cause: java.lang.ClassCastException: org.apache.ibatis.executor.ExecutionPlaceholder cannot be cast to java.util.List

at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:30)

at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:150)

at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:141)

at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:136)

at app.MybatisApp.getStudents1(MybatisApp.java:133)

at app.MybatisApp.lambda $testThreadSafe$ 0(MybatisApp.java:99)

at java.lang.Thread.run(Thread.java:748)

Caused by: java.lang.ClassCastException: org.apache.ibatis.executor.ExecutionPlaceholder cannot be cast to java.util.List

at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:152)

at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:109)

at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:83)

at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:148)

... 5 more

    通过控制台的输出报错，说明在并发情况下使用同一个DefaultSqlSession的实例做查询是有问题的(读者可以尝试编写增加、删除、修改方法的并发)，抛出的异常是类型转换异常(java.lang.ClassCastException：org.apache.ibatis.executor.ExecutionPlaceholder cannot be cast to java.util.List)，从上述报错信息来看报错发生在BaseExecutor类中第152行，作者将该涉及这行的方法全粘贴过来，一起研究下：

///方法来自<BaseExecutor>类

@SuppressWarnings("unchecked")

@Override

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {

ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());

if (closed) {

    throw new ExecutorException("Executor was closed.");

}

if (queryStack == 0 && ms.isFlushCacheRequired()) {

    clearLocalCache();

}

List<E> list;

try {

    queryStack++;

    // 下面这行代码即是源码中的第152行

    list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;

    if (list != null) {

        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);

    } else {

        // 调用queryFromDatabase，queryFromDatabase方法见下文

        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);

    }

} finally {

    queryStack--;

}

if (queryStack == 0) {

    for (DeferredLoad deferredLoad : deferredLoads) {

        deferredLoad.load();

    }

    // issue #601

    deferredLoads.clear();

    if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {

        // issue #482

        clearLocalCache();

    }

}

return list;

}

// queryFromDatabase方法

private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {

List<E> list;

// 缓存值

localCache.putObject(key, EXECUTION_PLACEHOLDER);

try {

    list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);

} finally {

    localCache.removeObject(key);

}

localCache.putObject(key, list);

if (ms.getStatementType() == StatementType.CALLABLE) {

    localOutputParameterCache.putObject(key, parameter);

}

return list;

}

BaseExecutor类中第152行,是一个三元表达式，用于判断resultHandler 是否是null，这里给出结论：此处的resultHandler =null（作者的测试用例中使用selectList方法，而selectList方法传入的resultHandler就是个null，读者可翻阅mybatis源码验证 ），所以三元表单式会成为：list = (List<E>) localCache.getObject(key) ，这里出现了强制类型装换，说明问题出在localCache取得值。

这里的localCache是我们常说的mybatis一级缓存，其原理作者在此不讲，也是给出结论：

localCache的key值生成策略，与查询方法、参数等有关，完全一样的查询，生成的key是一样的。读者可以查看源码

BaseExecutor#createCacheKey

    查询时，由于第一次查询是不存在缓存的，此时"list = (List<E>) localCache.getObject(key) "中"list=null"，代码会进入" list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);"，在queryFromDatabase方法中第二句代码"localCache.putObject(key, EXECUTION_PLACEHOLDER)"，先给localCache存了一个EXECUTION_PLACEHOLDER，而EXECUTION_PLACEHOLDER是枚举类ExecutionPlaceholder的一个枚举项。由于BaseExecutor中的方法都不是同步方法，在并发的情况下，就会出现这样的场景：

    Thread1进入query方法，用key取缓存localCache数据不存在，则进入了queryFromDatabase，并执行了"localCache.putObject(key, EXECUTION_PLACEHOLDER)"，而此时Thread2进入了query方法，用key取缓存localCache数据，此时取出来的是Thread1刚缓存的EXECUTION_PLACEHOLDER，然后执行类型转换，由于EXECUTION_PLACEHOLDER不是list类型，所以转换抛出异常(java.lang.ClassCastException：org.apache.ibatis.executor.ExecutionPlaceholder cannot be cast to java.util.List)。

  通过上述场景，读者应该明白了吧，是BaseExecutor中缓存机制（mybatis的一级缓存）导致了并发问题。这种并发问题，产生原因：并发操作使用了同一个DefaultSqlSession的实例，而同一个DefaultSqlSession的实例使用的是同一个Executor对象，当缓存命中时就会出现异常或者数据不完整的情况。

总结

    关于SqlSessionTemplate如何保证线程安全性的博文太多，而且很多博主讲的都特别详细，作者不再老生常谈。我们已经知道DefaultSqlSession线程安全问题的产生原因，故避免线程安全问题就得避免多个线程并发使用同一个DefaultSqlSession的实例。SqlSessionTemplate中也是这么做的，这也是SqlSessionTemplate中一级缓存失效的原因，因为一级缓存是基于同一个DefaultSqlSession实例实现的。

————————————————

原文链接：https://blog.csdn.net/Zhs901026/article/details/103997750

2022-01-06

Error querying database. Cause: java.lang.ClassCastException: org.apache.ibatis.executor.ExecutionPlaceholder cannot be cast to java.util.List

The error may exist in idin/sun/study/mapper/StudentMapper.xml

The error may involve idin.sun.study.mapper.StudentMapper.getStudents

The error occurred while executing a query

Cause: java.lang.ClassCastException: org.apache.ibatis.executor.ExecutionPlaceholder cannot be cast to java.util.List

Error querying database. Cause: java.lang.ClassCastException: org.apache.ibatis.executor.ExecutionPlaceholder cannot be cast to java.util.List

The error may exist in idin/sun/study/mapper/StudentMapper.xml

The error may involve idin.sun.study.mapper.StudentMapper.getStudents

The error occurred while executing a query

Cause: java.lang.ClassCastException: org.apache.ibatis.executor.ExecutionPlaceholder cannot be cast to java.util.List

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