Mybatis的动态sql和StatementHandler

　　一年前为了在公司刷积分还写过关于mybatis的动态sql的原理，一年后就发现自己有点忘了，再写一次加深印象

　一初始化

　　还是先从MapperStatement说起

　　 XMLMapperBuilder.buildStatementFromContext

private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
    for (XNode context : list) {
      final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
      try {
        statementParser.parseStatementNode();
      } catch (IncompleteElementException e) {
        configuration.addIncompleteStatement(statementParser);
      }
    }
  }

每个XNode都是一个 select|update|delete|insert 的完整标签

<select resultMap="BaseResultMap" parameterType="java.lang.String" id="selectByPrimaryKey">
<include refid="Base_Column_List"/>
</select>

　　精简了代码，第8行表示把mapper文件中的一个 select|update|delete|insert 一种构造成一个MapperStatement并加入到configuration里

 1 public void parseStatementNode() {
 2     
 3     ...
 4     // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
 5     SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
 6     ...
 7 
 8     builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
 9         fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
10         resultSetTypeEnum, flushCache, useCache, resultOrdered, 
11         keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
12   }

　　而本篇重点分析第5行，获取SqlSource的过程

　　现在有一个带有动态标签的sql语句

<insert parameterType="me.gacl.domain.User" id="insertSelective">
<trim suffixOverrides="," prefix="(" suffix=")">
<if test="userId != null">
        user_id,
      </if>
<if test="userName != null">
        user_name,
      </if>
<if test="userBirthday != null">
        user_birthday,
      </if>
<if test="userSalary != null">
        user_salary,
      </if>
</trim>
<trim suffixOverrides="," prefix="values (" suffix=")">
<if test="userId != null">
        #{userId,jdbcType=CHAR},
      </if>
<if test="userName != null">
        #{userName,jdbcType=VARCHAR},
      </if>
<if test="userBirthday != null">
        #{userBirthday,jdbcType=DATE},
      </if>
<if test="userSalary != null">
        #{userSalary,jdbcType=DOUBLE},
      </if>
</trim>
</insert>

　　 XMLScriptBuilder

public SqlSource parseScriptNode() {
    List<SqlNode> contents = parseDynamicTags(context);
    MixedSqlNode rootSqlNode = new MixedSqlNode(contents);
    SqlSource sqlSource = null;
    if (isDynamic) {
      sqlSource = new DynamicSqlSource(configuration, rootSqlNode);
    } else {
      sqlSource = new RawSqlSource(configuration, rootSqlNode, parameterType);
    }
    return sqlSource;
  }

跟代码的结果就是经过 parseDynamicTags 的处理后， List<SqlNode> contents 的样子

　　再把这些SqlNode都汇聚成一个 MixedSqlNode ，再通过 MixedSqlNode 构造成SqlSource

　　总结，现在有了MappedStatement，在MappedStatement中有SqlSource，注意此时的SqlSource还是静态的，为什么呢？因为mybatis提供的动态sql功能，每次执行的sql是根据输入的入参

　　决定的，每次的sql是不同的。

　二执行阶段

　　先说一下mybatis的执行过程吧，sqlSession -> executor -> statementHandler

　　一个SqlSession有一个成员Executor，而每次调用executor的方法时，都是会新建一个StatementHandler。为啥会new出来一个StatementHandler呢？

　　因为mybatis的动态sql是根据入参的不同，每次执行的sql也不一定相同，所以每当调用executor的方法就只能new一个StatementHandler

public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.update(stmt);
    } finally {
      closeStatement(stmt);
    }
  }

　　那就来看看new一个statementHandler的逻辑，在new一个statementHandler的过程中就要获取BoundSql

protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    this.configuration = mappedStatement.getConfiguration();
    this.executor = executor;
    this.mappedStatement = mappedStatement;
    this.rowBounds = rowBounds;

    this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
    this.objectFactory = configuration.getObjectFactory();

    if (boundSql == null) { // issue #435, get the key before calculating the statement
      generateKeys(parameterObject);
      boundSql = mappedStatement.getBoundSql(parameterObject);
    }

    this.boundSql = boundSql;

　　最终还是调用到 DynamicSqlSource.public BoundSql getBoundSql(Object parameterObject)

 1 public BoundSql getBoundSql(Object parameterObject) {
 2     DynamicContext context = new DynamicContext(configuration, parameterObject);//parameterObject就是入参，也就是PO对象
 3     rootSqlNode.apply(context);//经过这一步之后，动态sql就会被确定下来了
 4     SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
 5     Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass();
 6     SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings());//sqlSource就长下面图片的样子
 7     BoundSql boundSql = sqlSource.getBoundSql(parameterObject);//BoundSql就是在SqlSource的基础上再加上实际的入参
 8     for (Map.Entry<String, Object> entry : context.getBindings().entrySet()) {
 9       boundSql.setAdditionalParameter(entry.getKey(), entry.getValue());
10     }
11     return boundSql;
12   }

　　　　SqlSource

三 #{}解析以及parameterMappings

　　BoundSql里有一个成员变量很重要，那就是parameterMappings

public BoundSql(Configuration configuration, String sql, List<ParameterMapping> parameterMappings, Object parameterObject) {
    this.sql = sql;
    this.parameterMappings = parameterMappings;
    this.parameterObject = parameterObject;
    this.additionalParameters = new HashMap<String, Object>();
    this.metaParameters = configuration.newMetaObject(additionalParameters);
  }

parameterMappings表示的意思是本次要执行的sql需要用到入参对象的哪些参数，比如上一次用到了id,name,salary,可能这次只是用到了name,salary。该参数也是很BoundSql一样是动态的

SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings());

　　 SqlSourceBuilder.parse

public SqlSource parse(String originalSql, Class<?> parameterType, Map<String, Object> additionalParameters) {
    ParameterMappingTokenHandler handler = new ParameterMappingTokenHandler(configuration, parameterType, additionalParameters);
    GenericTokenParser parser = new GenericTokenParser("#{", "}", handler);//这里ParameterMappingTokenHandler作为参数 执行构造方法
    String sql = parser.parse(originalSql);
    return new StaticSqlSource(configuration, sql, handler.getParameterMappings());
  }

通过跟代码此时的 originalSql 是这样的

insert into t_user
     ( user_id,
        user_name ) 
     values ( #{userId,jdbcType=CHAR},
        #{userName,jdbcType=VARCHAR} )

注意哦，这个时候还是带着#{}的

接着分析 GenericTokenParser.parse

public String parse(String text) {
    StringBuilder builder = new StringBuilder();
    if (text != null && text.length() > 0) {
      char[] src = text.toCharArray();
      int offset = 0;
      int start = text.indexOf(openToken, offset);//openToken就是 #{
      while (start > -1) {
        if (start > 0 && src[start - 1] == '\\') {
          // the variable is escaped. remove the backslash.
          builder.append(src, offset, start - offset - 1).append(openToken);
          offset = start + openToken.length();
        } else {
          int end = text.indexOf(closeToken, start);
          if (end == -1) {
            builder.append(src, offset, src.length - offset);
            offset = src.length;
          } else {
            builder.append(src, offset, start - offset); // insert into t_user ( user_id, user_name ) values ( 
            offset = start + openToken.length();
            String content = new String(src, offset, end - offset);// userId,jdbcType=CHAR
            builder.append(handler.handleToken(content));// insert into t_user ( user_id, user_name ) values ( ?
            offset = end + closeToken.length();
          }
        }
        start = text.indexOf(openToken, offset);
      }
      if (offset < src.length) {
        builder.append(src, offset, src.length - offset);
      }
    }
    return builder.toString();
  }

　　接着就要分析 ParameterMappingTokenHandler.handleToken

　　现在知道问号是哪里来得了吧

public String handleToken(String content) {
      parameterMappings.add(buildParameterMapping(content));
      return "?";
    }

　　还是要看看 buildParameterMapping

private ParameterMapping buildParameterMapping(String content) {
      Map<String, String> propertiesMap = parseParameterMapping(content);
      String property = propertiesMap.get("property"); // userId
      Class<?> propertyType;
      if (metaParameters.hasGetter(property)) { // issue #448 get type from additional params
        propertyType = metaParameters.getGetterType(property);
      } else if (typeHandlerRegistry.hasTypeHandler(parameterType)) {
        propertyType = parameterType;
      } else if (JdbcType.CURSOR.name().equals(propertiesMap.get("jdbcType"))) {
        propertyType = java.sql.ResultSet.class;
      } else if (property != null) {
        MetaClass metaClass = MetaClass.forClass(parameterType, configuration.getReflectorFactory());
        if (metaClass.hasGetter(property)) {
          propertyType = metaClass.getGetterType(property); // java.lang.String
        } else {
          propertyType = Object.class;
        }
      } else {
        propertyType = Object.class;
      }
      ParameterMapping.Builder builder = new ParameterMapping.Builder(configuration, property, propertyType);//这里得到了对象入参的属性名字和属性的类型
      Class<?> javaType = propertyType;
      String typeHandlerAlias = null;
      for (Map.Entry<String, String> entry : propertiesMap.entrySet()) {
        String name = entry.getKey();
        String value = entry.getValue();
        if ("javaType".equals(name)) {
          javaType = resolveClass(value);
          builder.javaType(javaType);
        } else if ("jdbcType".equals(name)) {
          builder.jdbcType(resolveJdbcType(value));
        } else if ("mode".equals(name)) {
          builder.mode(resolveParameterMode(value));
        } else if ("numericScale".equals(name)) {
          builder.numericScale(Integer.valueOf(value));
        } else if ("resultMap".equals(name)) {
          builder.resultMapId(value);
        } else if ("typeHandler".equals(name)) {
          typeHandlerAlias = value;
        } else if ("jdbcTypeName".equals(name)) {
          builder.jdbcTypeName(value);
        } else if ("property".equals(name)) {
          // Do Nothing
        } else if ("expression".equals(name)) {
          throw new BuilderException("Expression based parameters are not supported yet");
        } else {
          throw new BuilderException("An invalid property '" + name + "' was found in mapping #{" + content + "}.  Valid properties are " + parameterProperties);
        }
      }
      if (typeHandlerAlias != null) {
        builder.typeHandler(resolveTypeHandler(javaType, typeHandlerAlias));
      }
      return builder.build(); //最终我们拿到了第一个属性userId
    }

最终，构成一个BoundSql的要素就齐全了

public BoundSql(Configuration configuration, String sql, List<ParameterMapping> parameterMappings, Object parameterObject) {
    this.sql = sql;//已经把#{}替换成了?
    this.parameterMappings = parameterMappings;//本次执行要用到的对象中的属性列表
    this.parameterObject = parameterObject;//本次入参的对象
    this.additionalParameters = new HashMap<String, Object>();
    this.metaParameters = configuration.newMetaObject(additionalParameters);
  }

　总结

　　　　configuration中会缓存所有的MappedStatement，MappedStatement在解析mapper文件的过程中就会保留一个SqlSource，而这个SqlSource里有SqlNode的集合，对于一个Update标签来说，它

　　的每一个子节点比如if，都会形成一个SqlNode。这些信息相当于是静态的信息，缓存在MappedStatement中

　　　　当要执行某一个方法的时候，每次Executor都会new一个StatementHandler，在new的过程中就会通过真实的入参将这些SqlNode依次解析，最终构成本次要执行的SqlSource,把SqlSource和入参合并变成BoundSql交给StatementHandler，准备执行

posted on 2020-11-06 20:01 MaXianZhe 阅读(442) 评论(0) 编辑收藏举报

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