MyBatis源码分析之@SelectProvider注解使用详解

MyBatis源码分析之@SelectProvider注解使用详解

之前讲了MyBatis的配置、plugin、Select查询，还有@MapKey注解的使用与原理，还有返回@ResultMap等等，我原想直接从MyBatis的缓存开始说起，但是想想还是得说一下MyBatis中的@selectProvider，这个注解我也是在用了好久的MyBatis才用到，功能就是用来单独写一个class类与方法，用来提供一些xml或者注解中不好写的sql，今天就来说下这个注解的具体用法与源码。

@SelectProvider注解用法

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写一个简单的@SelectProvider的用法，新建class类，添加一个根据userId查询user的方法。

SelectSqlProvider:

public class SelectSqlProvider {

    public String selectByUserId(Long id) {
        StringBuffer buffer = new StringBuffer();
        buffer.append("SELECT * FROM user where ");
        buffer.append("id = ").append(id).append(";");
        return buffer.toString();
    }
}

SelectSqlProvider中提供了一个很简单的查询方法，根据userId返回user对象，里面就是用了一个StringBuffer对象来拼接一个SQL语句，我想更多的是想用MyBatis中的SQL Builder的写法，SQL Builder写法在官方网站地址为http://www.mybatis.org/mybatis-3/zh/statement-builders.html，不得不说SQL Builder的写法确实比较漂亮，很工整，不过也是看自己运用的熟练程度吧。

UserMapper:

@ResultMap("BaseResultMap")
@SelectProvider(type = SelectSqlProvider.class, method = "selectByUserId")
User getUserByUserId(long id);

mapper中的其他方法就不贴出来了，需要说的就是这一个，这一个方法在xml中没有对应的sql，在该方法上也没有@Select注解修饰，只有@SelectProvider注解，@SelectProvider中两个属性，type为提供sql的class类，method为指定方法。

对应Mapper的调用与结果在这就不再分析了，就是简单的返回user对象，下文将是对@SelectProvider注解作用的详解。

2. @SelectProvider源码分析

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说起Select查询，基本就又是回到我们先前那几篇文章说的了，@SelectProvider注解加载问题，之前的文章中说了如何在解析xml之后解析注解中的SQL，这一种无非换了种样式，从由注解提供改为了从class类中单独写方法提供SQL，我们来看下相关源码实现。
这里就还要回到mapper的解析处，回到开始的parseConfiguration方法中mapperElement。

mapperElement(root.evalNode("mappers"));

这一行在解析xml文件之后，最后进行了addMapper操作。

private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) {
          String mapperPackage = child.getStringAttribute("name");
          configuration.addMappers(mapperPackage);
        } else {
          String resource = child.getStringAttribute("resource");
          String url = child.getStringAttribute("url");
          String mapperClass = child.getStringAttribute("class");
          if (resource != null && url == null && mapperClass == null) {
            ErrorContext.instance().resource(resource);
            InputStream inputStream = Resources.getResourceAsStream(resource);
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
            mapperParser.parse();
          } else if (resource == null && url != null && mapperClass == null) {
            ErrorContext.instance().resource(url);
            InputStream inputStream = Resources.getUrlAsStream(url);
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
            mapperParser.parse();
          } else if (resource == null && url == null && mapperClass != null) {
            Class<?> mapperInterface = Resources.classForName(mapperClass);
            configuration.addMapper(mapperInterface);
          } else {
            throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
          }
        }
      }
    }
  }

但是从前文中我们知addMapper操作不仅将mapper保存进knownMappers中，并且还进行了注解Mapper的解析，从而实现了对注解sql的加载，同时**@SelectProvider**也是在这里进行加载的。

knownMappers.put(type, new MapperProxyFactory<T>(type));
        // It's important that the type is added before the parser is run
        // otherwise the binding may automatically be attempted by the
        // mapper parser. If the type is already known, it won't try.
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;

进入到parse方法中,parse方法最终转到parseStatement方法,在parseStatement方法中，在获取SqlSource对象时，对method方法进行了进一步的解析。

SqlSource sqlSource = getSqlSourceFromAnnotations(method, parameterTypeClass, languageDriver);

  private SqlSource getSqlSourceFromAnnotations(Method method, Class<?> parameterType, LanguageDriver languageDriver) {
    try {
      Class<? extends Annotation> sqlAnnotationType = getSqlAnnotationType(method);
      Class<? extends Annotation> sqlProviderAnnotationType = getSqlProviderAnnotationType(method);
      if (sqlAnnotationType != null) {
        if (sqlProviderAnnotationType != null) {
          throw new BindingException("You cannot supply both a static SQL and SqlProvider to method named " + method.getName());
        }
        Annotation sqlAnnotation = method.getAnnotation(sqlAnnotationType);
        final String[] strings = (String[]) sqlAnnotation.getClass().getMethod("value").invoke(sqlAnnotation);
        return buildSqlSourceFromStrings(strings, parameterType, languageDriver);
      } else if (sqlProviderAnnotationType != null) {
        Annotation sqlProviderAnnotation = method.getAnnotation(sqlProviderAnnotationType);
        return new ProviderSqlSource(assistant.getConfiguration(), sqlProviderAnnotation, type, method);
      }
      return null;
    } catch (Exception e) {
      throw new BuilderException("Could not find value method on SQL annotation.  Cause: " + e, e);
    }
  }

这里可以加上断点，对我们上面写的代码调试一下,如下图。


到这一步就是对@SelectProvider注解的解析，可以看到此时的method方法为getUserByUserId。type类型为UserMapper等等。我们继续进入到ProviderSqlSource中，看看是如何组装sql的。

public ProviderSqlSource(Configuration configuration, Object provider, Class<?> mapperType, Method mapperMethod) {
    String providerMethodName;
    try {
      this.configuration = configuration;
      this.sqlSourceParser = new SqlSourceBuilder(configuration);
      this.providerType = (Class<?>) provider.getClass().getMethod("type").invoke(provider);
      providerMethodName = (String) provider.getClass().getMethod("method").invoke(provider);

      for (Method m : this.providerType.getMethods()) {
        if (providerMethodName.equals(m.getName()) && CharSequence.class.isAssignableFrom(m.getReturnType())) {
          if (providerMethod != null){
            throw new BuilderException("Error creating SqlSource for SqlProvider. Method '"
                    + providerMethodName + "' is found multiple in SqlProvider '" + this.providerType.getName()
                    + "'. Sql provider method can not overload.");
          }
          this.providerMethod = m;
          this.providerMethodArgumentNames = new ParamNameResolver(configuration, m).getNames();
          this.providerMethodParameterTypes = m.getParameterTypes();
        }
      }
    } catch (BuilderException e) {
      throw e;
    } catch (Exception e) {
      throw new BuilderException("Error creating SqlSource for SqlProvider.  Cause: " + e, e);
    }
    if (this.providerMethod == null) {
      throw new BuilderException("Error creating SqlSource for SqlProvider. Method '"
          + providerMethodName + "' not found in SqlProvider '" + this.providerType.getName() + "'.");
    }
    for (int i = 0; i< this.providerMethodParameterTypes.length; i++) {
      Class<?> parameterType = this.providerMethodParameterTypes[i];
      if (parameterType == ProviderContext.class) {
        if (this.providerContext != null){
          throw new BuilderException("Error creating SqlSource for SqlProvider. ProviderContext found multiple in SqlProvider method ("
              + this.providerType.getName() + "." + providerMethod.getName()
              + "). ProviderContext can not define multiple in SqlProvider method argument.");
        }
        this.providerContext = new ProviderContext(mapperType, mapperMethod);
        this.providerContextIndex = i;
      }
    }
  }

此处对sqlSourceParser与providerType、providerMethodName等参数进行了实例化与赋值，最后返回sqlSource对象。
此处得到的可以说还不是原有的sql，所以在Select查询的时候，还要继续追踪看一下到底是如何执行sql的，这就要继续回到Select查询方法了，在前面很多文章中知最后查询调用基本都是调用的selectList方法，此处还是要从这里分析开始。

@Override
  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();
    }
  }

进入到executor.query方法中,executor的实现有两种，一种是BaseExecutor，一种是CacheingExecutor，而这种的初始化条件为openSession中的newExecutor方法。

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

这里一般就是实例化为Simple类型，但是如果cacheEnable字段为true的话，返回CachingExecutor对象。而cacheEnable字段算得上是之前漏说了的一个属性，这个是在loadSettings时进行初始化的，而如果没有设置cacheEnable字段时，默认设置为true，如下：

private void settingsElement(Properties props) throws Exception { configuration.setAutoMappingUnknownColumnBehavior(AutoMappingUnknownColumnBehavior.valueOf(props.getProperty("autoMappingUnknownColumnBehavior", "NONE")));
    configuration.setCacheEnabled(booleanValueOf(props.getProperty("cacheEnabled"), true));
    configuration.setProxyFactory((ProxyFactory) createInstance(props.getProperty("proxyFactory")))
  }

在说完BaseExecutor和CacheingExecutor之后，此处继续回到query方法。

@Override
  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameter);
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
 }

在query方法中获取到boundSql对象，此处可以调试一下代码，看看boundSql中有什么参数。

此处已经完成了sql的组装，继续getBoundSql看看进行了什么操作。

public BoundSql getBoundSql(Object parameterObject) {
    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings == null || parameterMappings.isEmpty()) {
      boundSql = new BoundSql(configuration, boundSql.getSql(), parameterMap.getParameterMappings(), parameterObject);
    }

    // check for nested result maps in parameter mappings (issue #30)
    for (ParameterMapping pm : boundSql.getParameterMappings()) {
      String rmId = pm.getResultMapId();
      if (rmId != null) {
        ResultMap rm = configuration.getResultMap(rmId);
        if (rm != null) {
          hasNestedResultMaps |= rm.hasNestedResultMaps();
        }
      }
    }

    return boundSql;
  }

还需要继续追溯sqlSource.getBoundSql(parameterObject)，此处SqlSource毫无疑问为ProviderSqlSource类。

@Override
  public BoundSql getBoundSql(Object parameterObject) {
    SqlSource sqlSource = createSqlSource(parameterObject);
    return sqlSource.getBoundSql(parameterObject);
  }
private SqlSource createSqlSource(Object parameterObject) {
    try {
      int bindParameterCount = providerMethodParameterTypes.length - (providerContext == null ? 0 : 1);
      String sql;
      if (providerMethodParameterTypes.length == 0) {
        sql = invokeProviderMethod();
      } else if (bindParameterCount == 0) {
        sql = invokeProviderMethod(providerContext);
      } else if (bindParameterCount == 1 &&
              (parameterObject == null || providerMethodParameterTypes[(providerContextIndex == null || providerContextIndex == 1) ? 0 : 1].isAssignableFrom(parameterObject.getClass()))) {
        sql = invokeProviderMethod(extractProviderMethodArguments(parameterObject));
      } else if (parameterObject instanceof Map) {
        @SuppressWarnings("unchecked")
        Map<String, Object> params = (Map<String, Object>) parameterObject;
        sql = invokeProviderMethod(extractProviderMethodArguments(params, providerMethodArgumentNames));
      } else {
        throw new BuilderException("Error invoking SqlProvider method ("
                + providerType.getName() + "." + providerMethod.getName()
                + "). Cannot invoke a method that holds "
                + (bindParameterCount == 1 ? "named argument(@Param)": "multiple arguments")
                + " using a specifying parameterObject. In this case, please specify a 'java.util.Map' object.");
      }
      Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
      return sqlSourceParser.parse(replacePlaceholder(sql), parameterType, new HashMap<String, Object>());
    } catch (BuilderException e) {
      throw e;
    } catch (Exception e) {
      throw new BuilderException("Error invoking SqlProvider method ("
          + providerType.getName() + "." + providerMethod.getName()
          + ").  Cause: " + e, e);
    }
  }

createSqlSource方法有些意思，对参数个数进行了校验，如果是没有参数，直接执行invokeProviderMethod()方法，如果是一个则进行传参，如果多个判断当前类型是否是Map类型，否则抛错，等会倒是可以测试一下，这里看下invokeProviderMethod方法。

  private String invokeProviderMethod(Object... args) throws Exception {
    Object targetObject = null;
    if (!Modifier.isStatic(providerMethod.getModifiers())) {
      targetObject = providerType.newInstance();
    }
    CharSequence sql = (CharSequence) providerMethod.invoke(targetObject, args);
    return sql != null ? sql.toString() : null;
  }

invokeProviderMethod方法其实就没多少可说的了,对当前方法、对象进行了一个反射获取值的操作，从而拿到对应sql。

在获取到sql之后剩下的执行就和常规的是一样的了，这里就不再继续说后面的东西了。