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MyBatis中的@SelectProvider注解源码分析

作者:叶长风

这篇文章主要介绍了MyBatis中的@SelectProvider注解源码分析,@SelectProvider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,需要的朋友可以参考下

@SelectProvider注解用法

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//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。

@SelectProvider源码分析

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

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。

到此这篇关于MyBatis中的@SelectProvider注解源码分析的文章就介绍到这了,更多相关@SelectProvider注解源码内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家!

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