Mybatis随笔(十) 聊聊NameSpace

在我刚接触Mybatis那会，有位先生说到这个NameSpace时，说这个东西不一定就要写对应Mapper接口的全限定类名，我就来试了试。

原来当年先生少说了一句，或是我走神漏听了一句，才有了现在的Mybatis系列。

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首先我的目录结构

package

• 注意 Mapper.java 与 Mapper.xml 文件不在同一目录下就行了

先看几种用法及其效果

1. NameSpace 使用全限定类名 & mapper 配置 resource

• 配置文件 mybatis-config.xml

<mapper resource="resources/mapper/AccountMapper.xml" />

• sql映射文件 AccountMaper.xml

<mapper namespace="com.hy.test.dao.AccountMapper" >

• 使用

@Test
public void queryAccountTest() {

    basicMethod("queryAccountTest", k -> {
        AccountMapper accountMapper = k.getMapper(AccountMapper.class);
        Account account = accountMapper.selectByPrimaryKey(31);
        log.info("---- account: " + account.toString() + " -----------");
        return SUCCESS;
    });
}

/**
 * SqlSessionFactory -> SqlSession
 */
private void basicMethod(String methodName, Function<SqlSession, String> mappingFunction) {
    String config = "resources/mybatis-config.xml";
    // 测试代码
    SqlSession sqlSession = null;
    try (InputStream is = Resources.getResourceAsStream(config)) {
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(is);
        sqlSession = factory.openSession();
        log.error("function: " + methodName + ", result: " + mappingFunction.apply(sqlSession));
        sqlSession.commit();
    } catch (Exception ignore) {
        log.error(ignore);
        ignore.printStackTrace();
    } finally {
        if (null != sqlSession ) sqlSession.close();
    }
}

输出

==>  Preparing: select id, name, balance from account where id = ? 
==> Parameters: 31(Integer)
Account: class com.hy.test.po.Account
object test: com.hy.test.po.Account@6a1aab78
<==      Total: 1
---- account: Account{id=31, name='java', balance=2} -----------

这种方式无疑是可以正确读取的

2. NameSpace 不使用全限定类名 & mapper 配置 resource

修改一下NameSpace，其他不变
sql映射文件 AccountMaper.xml

<mapper namespace="lalalala" >

执行结果

org.apache.ibatis.binding.BindingException: Type interface com.hy.test.dao.AccountMapper is not known to the MapperRegistry.
    at org.apache.ibatis.binding.MapperRegistry.getMapper(MapperRegistry.java:47)

    at org.apache.ibatis.session.Configuration.getMapper(Configuration.java:779)
    at org.apache.ibatis.session.defaults.DefaultSqlSession.getMapper(DefaultSqlSession.java:291)
    at com.hy.test.main.MainTest.lambda$queryAccountTest$1(MainTest.java:52)
    at com.hy.test.main.MainTest.basicMethod(MainTest.java:93)
    at com.hy.test.main.MainTest.queryAccountTest(MainTest.java:51)
    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.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

报错 MapperRegistry 没找到接口，前面的源码解析我们知道这个 MapperRegistry 是 Configuration 的一个属性，其中有一个 Map类型的knownMappers 用于存储加载过的 *Mapper.java 接口类型与对应的JDK动态代理工厂的映射。

那我们是不是可以手动将 Mapper接口注入到这个 MapperRegistry 呢？

3. NameSpace 不使用全限定类名 & mapper 配置 resource/class

配置文件 mybatis-config.xml

<mapper resource="resources/mapper/AccountMapper.xml" />

sql映射文件 AccountMaper.xml

<mapper namespace="com.hy.test.dao.AccountMapper" >
<mapper class="com.hy.test.dao.AccountMapper"/>

• 添加了 class 属性就可以指定加载 AccountMapper 接口类了

执行结果

org.apache.ibatis.binding.BindingException: Invalid bound statement (not found): com.hy.test.dao.AccountMapper.selectByPrimaryKey
    at org.apache.ibatis.binding.MapperMethod.<init>(MapperMethod.java:53)

    at org.apache.ibatis.binding.MapperProxy.lambda$cachedInvoker$0(MapperProxy.java:107)
    at java.util.concurrent.ConcurrentHashMap.computeIfAbsent(ConcurrentHashMap.java:1660)
    at org.apache.ibatis.binding.MapperProxy.cachedInvoker(MapperProxy.java:94)
    at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:85)
    at com.sun.proxy.$Proxy6.selectByPrimaryKey(Unknown Source)
    at com.hy.test.main.MainTest.lambda$queryAccountTest$1(MainTest.java:53)
    at com.hy.test.main.MainTest.basicMethod(MainTest.java:93)
    at com.hy.test.main.MainTest.queryAccountTest(MainTest.java:51)
    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.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

这个报错是告诉我们，你想要执行的SQL方法在 Configuration 的mappedStatements 中没有对应的 MapperStatement#id

Configuration.java 类源码

protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection")
      .conflictMessageProducer((savedValue, targetValue) ->
          ". please check " + savedValue.getResource() + " and " + targetValue.getResource());

public void addMappedStatement(MappedStatement ms) {
    mappedStatements.put(ms.getId(), ms);
}

其中 id 可以查询调用轨迹

public String applyCurrentNamespace(String base, boolean isReference) {
    if (base == null) {
        return null;
    }
    if (isReference) {
        // is it qualified with any namespace yet?
        if (base.contains(".")) {
            return base;
        }
    } else {
        // is it qualified with this namespace yet?
        if (base.startsWith(currentNamespace + ".")) {
            return base;
        }
        if (base.contains(".")) {
            throw new BuilderException("Dots are not allowed in element names, please remove it from " + base);
        }
    }
    return currentNamespace + "." + base;
}

• id 的值是 namespace + "." + 方法名

这就很明显了

• 配置接口类加载进去只是为了获取到对应的动态代理对象，可是执行的时候是从 mappedStatements 中取对应的 MapperStatement 执行的
• 而 mappedStatements 中的id是 lalalala.selectByPrimaryKey，而你代理对象用的是com.hy.test.dao.AccountMapper.selectByPrimaryKey 自然取不到

public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) {
    final String methodName = method.getName();
    final Class<?> declaringClass = method.getDeclaringClass();
    MappedStatement ms = resolveMappedStatement(mapperInterface, methodName, declaringClass, configuration);
    if (ms == null) {
        if (method.getAnnotation(Flush.class) != null) {
            name = null;
            type = SqlCommandType.FLUSH;
        } else {
            // here
            // here
            // here
            throw new BindingException("Invalid bound statement (not found): " + mapperInterface.getName() + "." + methodName);
        }
    } else {
        name = ms.getId();
        type = ms.getSqlCommandType();
        if (type == SqlCommandType.UNKNOWN) {
            throw new BindingException("Unknown execution method for: " + name);
        }
    }
}

private MappedStatement resolveMappedStatement(Class<?> mapperInterface, String methodName, Class<?> declaringClass, Configuration configuration) {
    // 接口名 + 方法名 作为 statementId
    String statementId = mapperInterface.getName() + "." + methodName;
    if (configuration.hasStatement(statementId)) {
        return configuration.getMappedStatement(statementId);
    } else if (mapperInterface.equals(declaringClass)) {
        return null;
    }
    // 允许继承
    for (Class<?> superInterface : mapperInterface.getInterfaces()) {
        if (declaringClass.isAssignableFrom(superInterface)) {
            MappedStatement ms = resolveMappedStatement(superInterface, methodName,
            declaringClass, configuration);
            if (ms != null) {
                return ms;
            }
        }
    }
    return null;
}

归根究底，就是 MapperStatement 的 id 不一致，也就是 NameSpace 没有写接口类的全限定类名。

那么 NameSpace 一定要写接口类的全限定类名吗？

4. 换种思路

Configuration类在解析时有两个属性很重要

1. mapperRegistry

• 判断是否加载过 Mapper.java
• 内有属性 Configuration configuration
• Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();
• if (configuration.hasMapper(Class<T> type)) {}

2. loadedResources

• 通过 XML 的命名空间判断是否解析过 Mapper.xml
• Set<String> loadedResources = new HashSet<>();
• 命名空间是否有效取决于是否能被类加载器加载
• if (configuration.isResourceLoade(String resource)) {}

由于mappedStatements存储的是具体的SQL实现，那么mappedStatements属性的构建可以有两个来源，一个就是通过配置文件的形式，也就是 XML；一个就是通过 Mapper.java 接口方法上的注解，比如

@SelectKey(keyProperty = "account",
        before = false,
        statementType = StatementType.STATEMENT,
        statement = "select * from account where id = #{id}",
        resultType = Account.class)
Account selectByPrimaryKey(@Param("id") Integer id);

既然我们使用的是 XML 方式，那么mappedStatements中的key就只能是 namespace + 方法ID，而代理对象用于执行的SqlId只能是 mapperInterface.getName + 方法ID，这么来看好像我们只能把 XML 的 NameSpace 写成接口类的全限定类名了。

不妨换种思路。

既然配置文件我们已经没办法再修改，试试修改执行方式呢

@Test
public void Three() {
    try {
        SqlSessionFactory sqlSessionFactory =
                new SqlSessionFactoryBuilder().build(Resources.getResourceAsReader("resources/mybatis-config.xml"));
        SqlSession session = sqlSessionFactory.openSession();
        Account a = session.selectOne("selectByPrimaryKey", 31);
        System.out.println(a.toString());
    } catch (IOException e) {
        e.printStackTrace();
    }
}

执行结果

==>  Preparing: select id, name, balance from account where id = ? 
==> Parameters: 31(Integer)
Account: class com.hy.test.po.Account
object test: Account{id=null, name='null', balance=null}
<==      Total: 1
Account{id=31, name='java', balance=2}

ok，这种方式是可以成功的。

来分析下这种方式为什么可以
直接找到对应SqlSession的具体执行

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
        MappedStatement ms = configuration.getMappedStatement(statement);
        return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    } catch (Exception e) {
        throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
        ErrorContext.instance().reset();
    }
}

这里也是直接从configuration的mapperStatements中取MapperStatement的，不过id就换成了selectByPrimaryKey，为什么这里可以直接取到值呢？

很简单，往mapperStatements中塞值的时候，不仅会塞一个长id，还会再加一个短id，我们再看下mapperStatements的实现

protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection")
      .conflictMessageProducer((savedValue, targetValue) ->
          ". please check " + savedValue.getResource() + " and " + targetValue.getResource());

注意到这不是一个常见的 HashMap，而是一个 StrictMap
其重写了put方法

protected static class StrictMap<V> extends HashMap<String, V> {

    @Override
    @SuppressWarnings("unchecked")
    public V put(String key, V value) {
        if (containsKey(key)) {
            throw new IllegalArgumentException(name + " already contains value for " + key + (conflictMessageProducer == null ? "" : conflictMessageProducer.apply(super.get(key), value)));
        }
        if (key.contains(".")) {
            final String shortKey = getShortName(key);
            if (super.get(shortKey) == null) {
                super.put(shortKey, value);
            } else {
                super.put(shortKey, (V) new Ambiguity(shortKey));
            }
        }
        return super.put(key, value);
    }

    private String getShortName(String key) {
        final String[] keyParts = key.split("\\.");
        return keyParts[keyParts.length - 1];
    }
}

• 所以debug可以看到mapperStatements的长度是接口方法的两倍，就是因为加了 shortName

mapperStatements AccountMapper.java

• 6个接口方法，12的mapperStatements长度
• 再仔细看下 mapperStatements 里面的值都是一长key带一短key

所以通过 SqlSession 直接调用的方式是可以忽略 NameSpace 的校验的，只要唯一就行了，不然多个 xml 使用同一个 NameSpace 则只会加载一个。

总结

1. NameSpace的值并不一定是要配置成接口的全限定类名，前提是不使用类型安全的代理映射方式来执行SQL，而是直接使用 SqlSession 来直接使用方法ID调用指定方法，这种方式就不怎么优雅
2. 配置class- Mapper.java或package - name 指定加载接口类，解析时会去尝试加载同目录下同名的xml文件（支持 classPath）
3. 配置resource - Mapper.xml 指定加载 XML 文件，解析时会尝试将其中 NameSpace 当做接口Mapper进行加载，前提是NameSpace 可以被类加载器加载

private void bindMapperForNamespace() {
    String namespace = builderAssistant.getCurrentNamespace();
    if (namespace != null) {
        Class<?> boundType = null;
        try {
            boundType = Resources.classForName(namespace);
        } catch (ClassNotFoundException e) {
            //ignore, bound type is not required
        }
        if (boundType != null) {
            // 先判断是否已经加载
            if (!configuration.hasMapper(boundType)) {
                // Spring may not know the real resource name so we set a flag
                // to prevent loading again this resource from the mapper interface
                // look at MapperAnnotationBuilder#loadXmlResource
                configuration.addLoadedResource("namespace:" + namespace);
                configuration.addMapper(boundType);
            }
        }
    }
}

• 先判断是否已经加载，若是已经记载再直接加载会报错

if (hasMapper(type)) {
    throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}

看个配置实例，在NameSpace写了全限定类名时，下面三种配置方式

<mapper resource="resources/mapper/AccountMapper.xml" />

<mapper class="com.hy.test.dao.AccountMapper"/>
<mapper resource="resources/mapper/AccountMapper.xml" />

<mapper resource="resources/mapper/AccountMapper.xml" />
<mapper class="com.hy.test.dao.AccountMapper"/>

1与2 都能成功且效果是一样的，而3 会报错

### Cause: org.apache.ibatis.builder.BuilderException: Error parsing SQL Mapper Configuration. 
Cause: org.apache.ibatis.binding.BindingException: 

Type interface com.hy.test.dao.AccountMapper is already known to the MapperRegistry.

注意我的代码目录，Mapper.java 与 Mapper.xml 不在同一级目录下

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谢谢当年先生没说完的后面半句话。