MyBatis中的@SelectProvider注解源碼分析

更新時(shí)間：2024年01月25日 09:34:31 作者：葉長風(fēng)

這篇文章主要介紹了MyBatis中的@SelectProvider注解源碼分析,@SelectProvider功能就是用來單獨(dú)寫一個(gè)class類與方法,用來提供一些xml或者注解中不好寫的sql,今天就來說下這個(gè)注解的具體用法與源碼,需要的朋友可以參考下

@SelectProvider注解用法

寫一個(gè)簡單的@SelectProvider的用法，新建class類，添加一個(gè)根據(jù)userId查詢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中提供了一個(gè)很簡單的查詢方法，根據(jù)userId返回user對象，里面就是用了一個(gè)StringBuffer對象來拼接一個(gè)SQL語句，我想更多的是想用MyBatis中的SQL Builder的寫法，SQL Builder寫法在官方網(wǎng)站地址為//www.mybatis.org/mybatis-3/zh/statement-builders.html，不得不說SQL Builder的寫法確實(shí)比較漂亮，很工整，不過也是看自己運(yùn)用的熟練程度吧。

UserMapper

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

mapper中的其他方法就不貼出來了，需要說的就是這一個(gè)，這一個(gè)方法在xml中沒有對應(yīng)的sql，在該方法上也沒有@Select注解修飾，只有@SelectProvider注解，@SelectProvider中兩個(gè)屬性，type為提供sql的class類，method為指定方法。

對應(yīng)Mapper的調(diào)用與結(jié)果在這就不再分析了，就是簡單的返回user對象，下文將是對@SelectProvider注解作用的詳解。

@SelectProvider源碼分析

說起Select查詢，基本就又是回到我們先前那幾篇文章說的了，@SelectProvider注解加載問題，之前的文章中說了如何在解析xml之后解析注解中的SQL，這一種無非換了種樣式，從由注解提供改為了從class類中單獨(dú)寫方法提供SQL，我們來看下相關(guān)源碼實(shí)現(xiàn)。

這里就還要回到mapper的解析處，回到開始的parseConfiguration方法中mapperElement。

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

這一行在解析xml文件之后，最后進(jìn)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.");
          }
        }
      }
    }
  }

但是從前文中我們知addMapper操作不僅將mapper保存進(jìn)knownMappers中，并且還進(jìn)行了注解Mapper的解析，從而實(shí)現(xiàn)了對注解sql的加載，同時(shí)**@SelectProvider**也是在這里進(jìn)行加載的。

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;

進(jìn)入到parse方法中,parse方法最終轉(zhuǎn)到parseStatement方法,在parseStatement方法中，在獲取SqlSource對象時(shí)，對method方法進(jìn)行了進(jìn)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);
    }
  }

這里可以加上斷點(diǎn)，對我們上面寫的代碼調(diào)試一下

到這一步就是對@SelectProvider注解的解析，可以看到此時(shí)的method方法為getUserByUserId。type類型為UserMapper等等。我們繼續(xù)進(jìn)入到ProviderSqlSource中，看看是如何組裝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;
      }
    }
  }

此處對sqlSourceParser與providerType、providerMethodName等參數(shù)進(jìn)行了實(shí)例化與賦值，最后返回sqlSource對象。此處得到的可以說還不是原有的sql，所以在Select查詢的時(shí)候，還要繼續(xù)追蹤看一下到底是如何執(zhí)行sql的，這就要繼續(xù)回到Select查詢方法了，在前面很多文章中知最后查詢調(diào)用基本都是調(diào)用的selectList方法，此處還是要從這里分析開始。

@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();
    }
  }

進(jìn)入到executor.query方法中,executor的實(shí)現(xiàn)有兩種，一種是BaseExecutor，一種是CacheingExecutor，而這種的初始化條件為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;
  }

這里一般就是實(shí)例化為Simple類型，但是如果cacheEnable字段為true的話，返回CachingExecutor對象。

而cacheEnable字段算得上是之前漏說了的一個(gè)屬性，這個(gè)是在loadSettings時(shí)進(jìn)行初始化的，而如果沒有設(shè)置cacheEnable字段時(shí)，默認(rèn)設(shè)置為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")))
  }

在說完BaseExecutor和CacheingExecutor之后，此處繼續(xù)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方法中獲取到boundSql對象，此處可以調(diào)試一下代碼，看看boundSql中有什么參數(shù)。

此處已經(jīng)完成了sql的組裝，繼續(xù)getBoundSql看看進(jìn)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;
  }

還需要繼續(xù)追溯sqlSource.getBoundSql(parameterObject)，此處SqlSource毫無疑問為ProviderSqlSource類。

@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，對參數(shù)個(gè)數(shù)進(jìn)行了校驗(yàn)，如果是沒有參數(shù)，直接執(zhí)行invokeProviderMethod()方法，如果是一個(gè)則進(jìn)行傳參，如果多個(gè)判斷當(dāng)前類型是否是Map類型，否則拋錯(cuò)，等會倒是可以測試一下，這里看下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方法其實(shí)就沒多少可說的了,對當(dāng)前方法、對象進(jìn)行了一個(gè)反射獲取值的操作，從而拿到對應(yīng)sql。

在獲取到sql之后剩下的執(zhí)行就和常規(guī)的是一樣的了，這里就不再繼續(xù)說后面的東西了。

到此這篇關(guān)于MyBatis中的@SelectProvider注解源碼分析的文章就介紹到這了,更多相關(guān)@SelectProvider注解源碼內(nèi)容請搜索腳本之家以前的文章或繼續(xù)瀏覽下面的相關(guān)文章希望大家以后多多支持腳本之家！

您可能感興趣的文章: