问题

在使用 shardingsphere 时,在执行sql查询语句时报了一个异常：

### Cause: java.lang.NullPointerException
    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 java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:433)
    ... 110 common frames omitted
Caused by: java.lang.NullPointerException: null
    at org.apache.shardingsphere.core.parse.antlr.rule.registry.ParsingRuleRegistry.findSQLStatementRule(ParsingRuleRegistry.java:117)
    at org.apache.shardingsphere.core.parse.antlr.parser.SQLParserEngine.parse(SQLParserEngine.java:55)
    at org.apache.shardingsphere.core.parse.antlr.AntlrParsingEngine.parse(AntlrParsingEngine.java:67)
    at org.apache.shardingsphere.core.parse.SQLParsingEngine.parse(SQLParsingEngine.java:61)
    at org.apache.shardingsphere.core.route.router.sharding.ParsingSQLRouter.parse(ParsingSQLRouter.java:78)
    at org.apache.shardingsphere.core.route.PreparedStatementRoutingEngine.route(PreparedStatementRoutingEngine.java:64)
    at org.apache.shardingsphere.core.PreparedQueryShardingEngine.route(PreparedQueryShardingEngine.java:60)
    at org.apache.shardingsphere.core.BaseShardingEngine.shard(BaseShardingEngine.java:64)
    at org.apache.shardingsphere.shardingjdbc.jdbc.core.statement.ShardingPreparedStatement.shard(ShardingPreparedStatement.java:224)
    at org.apache.shardingsphere.shardingjdbc.jdbc.core.statement.ShardingPreparedStatement.execute(ShardingPreparedStatement.java:170)

根据异常信息，定位到ParsingRuleRegistry的代码，实际为集合parserRuleDefinitions 为空：

public Optional<SQLStatementRule> findSQLStatementRule(final DatabaseType databaseType, final String contextClassName) {
        return Optional.fromNullable(parserRuleDefinitions.get(DatabaseType.H2 == databaseType ? DatabaseType.MySQL : databaseType).getSqlStatementRuleDefinition().getRules().get(contextClassName));
    }

那么看集合 parserRuleDefinitions 是如何加载的，为何为空？ 依次继续定位到 ShardingParsingRuleRegistry 的 init 方法

public static ParsingRuleRegistry getInstance() {
        if (null == instance) {
            synchronized (ShardingParsingRuleRegistry.class) {
                if (null == instance) {
                    instance = new ShardingParsingRuleRegistry();
                    instance.init();
                }
            }
        }
        return instance;
}

其中，在 instance.init() 方法中进行了来个类的数据初始化，即：

 protected final void init() {
        initCommonParserRuleDefinition();
        initParserRuleDefinition();
    }

其中 initParserRuleDefinition 就是我们报空指针的集合的初始化的地方，于是在这两个通过打断点方式启动调试，调试时发现，initParserRuleDefinition 没有调用就结束了，所以初步判定在 initCommonParserRuleDefinition 时发生了异常。

于是重点放在 initParserRuleDefinition方法的追踪，最终定位到一个 Method threw 'java.lang.ClassNotFoundException' exception. 异常信息，具体代码为：

public final class FillerRuleDefinitionEntityLoader implements RuleDefinitionEntityLoader {
    
    @Override
    @SneakyThrows
    public FillerRuleDefinitionEntity load(final String fillerRuleDefinitionFile) {
        InputStream inputStream = FillerRuleDefinitionEntityLoader.class.getClassLoader().getResourceAsStream(fillerRuleDefinitionFile);
        Preconditions.checkNotNull(inputStream, "Cannot load SQL filler rule definition file :%s, ", fillerRuleDefinitionFile);
        return (FillerRuleDefinitionEntity) JAXBContext.newInstance(FillerRuleDefinitionEntity.class).createUnmarshaller().unmarshal(inputStream);//此行异常  ClassNotFoundException
    }
}

这个地方比较坑人的时 这个异常并没有在控制台输出，而是被shardingsphere吃掉了，所以比较难以定位。

解决

• 方法一：将 jdk的运行版本将为 1.8，我的默认是1.9所以出现了异常。

• 方法二：将 JAXB 相关jar包重新引入，具体maven

 <dependency>
    <groupId>javax.xml.bind</groupId>
    <artifactId>jaxb-api</artifactId>
    <version>2.3.1</version>
</dependency>
<dependency>
    <groupId>com.sun.xml.bind</groupId>
    <artifactId>jaxb-impl</artifactId>
    <version>2.3.1</version>
</dependency>
<dependency>
    <groupId>com.sun.xml.bind</groupId>
    <artifactId>jaxb-core</artifactId>
    <version>2.3.0</version>
</dependency>
<dependency>
    <groupId>javax.activation</groupId>
    <artifactId>activation</artifactId>
    <version>1.1.1</version>
</dependency>